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A Critical Assessment of the Microstructure and Mechanical Properties of Friction Stir Welded Reduced Activation Ferritic-Martensitic Steel
Abstract
Bead-on-plate friction stir welding was conducted on 6 mm thick plate of Reduced Activation Ferritic–Martensitic Steel employing polycrystalline cubic boron nitride tool with rotational speeds of 200, 300, 500 and 700 rpm and traverse speed of 30 mm/min. The interface temperature between shoulder bottom and top surface of the plate was monitored by non-contact in-line thermography which served to identify the peak temperature attained in the stir zone (SZ). This temperature for 200, 300 and 500, and 700 rpm was respectively below Ac1, between Ac1 and Ac3, and above Ac3. In the base metal (BM), the prior austenite grain and martensite lath boundaries were decorated with chromium and tungsten rich M23C6 precipitates while intra-lath regions revealed Ta and V rich MX type carbides. Rotational speeds greater than 300 rpm led to martensite formation and simultaneous recovery, recrystallization and grain growth in SZs with wide distribution in grain size whereas SZ of 200 rpm and BM possessed similar distribution. The grain boundary M23C6 dissolved and very fine needles of Fe3C precipitated in all SZs. The hardness of all SZs was unacceptably higher compared to the BM. The 200 rpm weld exhibited higher impact toughness in the absence of martensite in SZ.
... Therefore, FSW is promising to eliminate the IV crack premature failure related to the welded joint of RAFM steels. Up to now, several researchers have studied the possibility of using the FSW technique for joining RAFM steels [8][9][10][11][12]. The friction stir welded joint of RAFM steel generally can be divided into four different subregions, namely stir zone (SZ), thermo-mechanically affected zone (TMAZ), heat-affected zone (HAZ), and BM [10]. ...
... Up to now, several researchers have studied the possibility of using the FSW technique for joining RAFM steels [8][9][10][11][12]. The friction stir welded joint of RAFM steel generally can be divided into four different subregions, namely stir zone (SZ), thermo-mechanically affected zone (TMAZ), heat-affected zone (HAZ), and BM [10]. The existence of the SZ is one of the primary characteristics that are significantly different from the fusion-welded joint of RAFM steels. ...
... The existence of the SZ is one of the primary characteristics that are significantly different from the fusion-welded joint of RAFM steels. Different from HAZ, which is only influenced by the weld thermal cycle, in SZ, a combination between severe plastic deformation and high-temperature thermal exposure results in the microstructure evolution process including grain refinement caused by dynamic recrystallization, martensitic phase transformation, and dissolution of the precipitates [8][9][10][11][12]. Moreover, the SZ occupies a relatively large proportion of the entire welded joint, of which the dimension of the welding tool controls the width of the SZ. ...
Reduced activation ferritic/martensitic (RAFM) steels are among the most competitive candidates of structural materials for nuclear fusion reactors, due to their superior comprehensive properties. Friction stir welding (FSW) was investigated in joining RAFM steel, considering its potential advantages in obtaining an optimal microstructure and mechanical properties of welded joint. To evaluate the feasibility of FSW in joining RAFM steel, an in-depth understanding of the microstructure-property relationships for friction stir welded joints of RAFM steel is necessary. In this research, the quantitative relationships between microstructural evolution and tensile properties in the stir zone (SZ) of friction stir welded RAFM steel after post-weld tempering treatment (PWTT) were systematically studied. Three different post-weld tempering temperatures namely 720 °C, 760 °C, and 800 °C were adopted. Then the uniaxial tensile properties were tested at room temperature and 550 °C, respectively. Electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and the Thermo-Calc Calphad software were adopted to systematically investigate the microstructural evolution. Martensite lath width, precipitate number density, equilibrium solid solubility of alloying elements in the matrix, and geometrically necessary dislocation (GND) density were analyzed quantitatively. With the results obtained, we assessed the contribution of each strengthening mechanism to the 0.2% offset yield strength. According to the effective inter-barrier spacing theory, a microstructure-sensitive yield strength model was obtained to well predict the change in yield strength at different conditions. Finally, the results calculated by equivalent strengthening effect indicated that the crucial microstructure determining the yield strength of the SZ for RAFM steel after PWTT is the high density of dislocation substructures.
... In full austenitic region which is composed of CGHAZ and FGHAZ, all of the tempered martensite has transformed to austenite which decomposes upon cooling and different kinds of products are formed, depending on the cooling rate and chemical composition of steels in question, or hardenability. In CGHAZ the temperature experienced is relatively high so that the redissolution of large amount of stable carbides reduce the pinning effect of the second phase on dislocation movement and grain growth, leading to an increase in size of austenite grain [36,37]. High carbon concentration and addition of austenite stablizers significantly increase the hardenability, with the results being that large amount of martensite and small amount of bainite are present in this region even with air cooling as indicated in Fig. 8a. ...
... However, it is worth noting that the polygonal ferrite in this region may form via two different mechanisms. One comes from the nucleation of austenite at '␣' grain boundaries when being reheated to the intercritical temperature regime that lies between Ac1 to Ac3 and transformation to martensite, bainite and polygonal ferrite upon cooling [37]. The other is a result of elimination of the fine martensitic crystals, say laths, and formation of equiaxed ferritic grains during high temperature tempering, a variation in microstructure ascribed to recovery and recrystallization mechanisms associated with the migration of the low angle boundaries that separated parallel martensite crystals with the same orientation as well as the rearrangement of the large angle parallel boundaries to form equiaxed ferritic grains in order to minimize grain boundary energy [32,39]. ...
... In terms of grain size, it is known that increased number of grain boundaries helps to block crack propagation and increase the energy required for crack to propagate as a result [57]. It was also reported that finer grain size is conductive to minimizing stress resulting from dislocation pile-up and raise the resistance to cleavage fracture [37]. Therefore, The larger prior austenite grain size in weld metal and inherited larger packet size associated with high heat input compared with those in base metal increases the possibility of initiated cleavage crack propagation and deteriorate the toughness as a result. ...
Motivated by significant loss of mechanical properties during conventional fusion arc welding processes owing to under matching filler materials used and low efficiency associated with multipass welding, the viability of keyhole tungsten inert gas (K-TIG) welding for joining armour grade quenched and tempered (Q&T) steel was presented. Single pass full penetration was achieved on 9 mm thick plates at a speed of 28 cm/min⁻¹ without using any filler materials and edge preparation. In-depth investigation into the weld was conducted by optical microscope, scanning electron microscope, electron back-scattered diffraction, microhardness and tensile test. The results show that the weld metal consists of dendritic structure and predominantly bainitic microstructure and is dominated by low angle grain boundaries. Hardness distribution across the weld is higher than current practice, which would lead to improved ballistic performance. Although the joint efficiency of the weld is 65% due to reduction in weld metal hardness, it is still much higher than that produced via conventional fusion welding, which is not surpassing 50%. It has been demonstrated that the K-TIG welding process offers a new way to weld medium thick armour grade Q&T steel with high efficiency and low cost, while maintaining the mechanical properties at a high level.
... In order to overcome the aforementioned limitations, an attempt has been made in the present study to join the P9 steel with friction stir welding. In recent years, the friction stir technology is developed to a stage where welding can be successfully achieved on the high strength and high-temperature materials such as high strength low alloy (HSLA) steel (Ramesh et al., 2017), reduced activated ferritic /martensitic (RAFM) steel (Manugula et al., 2016(Manugula et al., .a, 2017(Manugula et al., . ...
... In addition to that, the SEM micrograph of as-welded specimen shows (Fig. 9) the significant amount of finer precipitates within the α-grains and martensite lath. Manugula et al. (2016) studied this kind of precipitates in the SZ evolved during FSW of RAFM steel with the help of transmission electron microscopy (TEM), and found out that these typical precipitates are needle morphology and confirmed that the precipitates are iron-rich Fe 3 C. As the RAFM steel composition was derived from the P9 steel, both the steels undergo similar solid-state phase transformations. ...
... This will result in the type-IV cracking at the interface of parent metal and HAZ during creep exposure. Serizawa et al. (2014) and Manugula et al. (2016) showed that these types of cracking failures are often noticed in the ferritic-martensitic steel welded joints fabricated by the fusion welding processes with high heat input. Manugula et al. (2016) also indicated that this issue could be minimized to a certain degree in a smaller HAZ by using the welding technologies with lowheat input. ...
... Serizawa et al. (2014) and Manugula et al. (2016) showed that these types of cracking failures are often noticed in the ferritic-martensitic steel welded joints fabricated by the fusion welding processes with high heat input. Manugula et al. (2016) also indicated that this issue could be minimized to a certain degree in a smaller HAZ by using the welding technologies with lowheat input. ...
... Researchers have conducted experimental research on FSW process of F/M steel in recent years [23][24][25][26]. Yano et al. [23] studied the mechanical properties of friction-stir-welded 11Cr F/M steel. ...
... Noh et al. [25] reported that the FSW joints of F82H steel possessed excellent tensile properties, which could be comparable with the BM when the temperature is lower than 500°C. Manugula et al. [26] conducted bead-on-plate FSW in Indian Reduced Activation F/M Steel. The results indicated that there was no martensite formation in the stir zone (SZ) of 200 rpm due to the peak temperature below Ac 1 . ...
The modified 9Cr–1Mo steel was selected to conduct friction stir welding experiment. Defect-free welded joints were successfully obtained with the welding parameters of 300 rpm–50 mm/min and 400 rpm–50 mm/min. The microstructures of the welded joint were observed by scanning electron microscopy, electron backscatter diffraction and transmission electron microscope. The main microstructural characteristics of the weld zone (WZ) and the high-temperature heat affected zone are the formation of fresh quenched martensite, dissolution of M23C6 carbides, grain refinement and increase in kernel average misorientation. The WZ is significantly hardened, and the maximum hardness of this region is about twice that of the base material (BM). The ultimate tensile strength of the welded joint is up to 98% that of BM. The absorbed impact energies of the SZ and HAZ reach 77.8% and 87.4% that of the BM, respectively. Besides, the influence of post weld heat treatment on the microstructure and mechanical properties of the welded joint was also investigated.
... In recent years, several efforts have been directed on Reduced Activation Ferritic-Martensitic steels (F82H, Eurofer-97, INRAFM) towards assessing the effects of tool rotational and traverse speeds on evolving microstructures and mechanical properties [8][9][10]. It has been shown that the increase in tool rotational speed increases the heat input and also leads to wider stir zone (SZ) and thermo-mechanically affected zone (TMAZ). ...
... It shall be pointed out that at a rotational speed of 500 rev min ?1 the INRAFM steel developed a heat input of 5.02 kJ mm ?1 in case of 6 mm thick plate, [9] against 11.49 kJ mm ?1 observed for 12 mm thick plate in this study. In case of FSW of thin sections, con- duction can easily transmit the heat throughout the thickness of the material leading to no or very small temperature gradient. ...
Full penetration friction stir welding was conducted on 12 mm thick reduced activation ferritic–martensitic steel at tool rotational speeds of 500 and 900 rev min⁻¹. Comparator welds at 500 rev min⁻¹ were also produced in 6 mm thick reduced activation ferritic–martensitic steel plate to evaluate section thickness effects. Increase in section thickness led to an increase in heat input, which strongly influenced the microstructure evolution in stir zone (SZ), thermo-mechanical affected zone and the overall hardness in the SZ of this steel. In the as-welded condition, the base metal microstructure was significantly altered and resulted in carbide-free grain boundaries. The desirable microstructure and mechanical properties were achieved by subjecting the as-welded joints to appropriate post-weld heat treatments.
... The higher GS value for the MCA intermedium is confirmed by the coarser grains observed in Fig. 1d. For common friction weld process, it is well recognized that severe thermal deformation occurs inside the weld zone, due to the presence of significant stress and temperature fields induced by the high-speed friction [24][25][26]. Accordingly, a large number of dislocation segments are produced in the severely deformed regions of the weld zone, and their local accumulation consequently leads to the generation of LAGBs there [27,28]. This fact results in a high fraction of LAGBs in the weld zone, as shown in Fig. 1f. ...
A strategy of using multicomponent alloy (MCA) intermedium for inertia friction welding of IN718 alloys was proposed in this work. Exceptional metallurgic bonding was attained between the IN718 and non-equiatomic CoCrNi-AlTa MCA layer, and the as-received weld joint was absent of any defects like micro-cracks or voids. Compared with the counterpart without an intermedium layer (~530 MPa), the yield strength of the IN718-MCA weld joint was increased to ~610 MPa. Moreover, by conducting a two-step ageing treatment, the yield strength of the weld joint was further improved to over 1360 MPa with prominent strain hardening behavior. Noteworthily , the fracture along the interface boundary between the two materials initially observed in the unaged weld joint was fully eliminated, indicating that the ageing treatment efficiently improved the bonding of the interfacial region and further enhanced the reliability of the weld zone. Moreover, the significant strengthening effect of the aged dissimilar weld joint was found to originate from the enhanced atomic diffusion and extra back-stress hardening from geometric necessary dislocations (GNDs) produced within the interfacial regions, as well as the high density of nano-sized dispersive precipitates (for IN718) and ordered structures (for MCA) generated in the alloy matrix at elevated temperatures. The result verifies that the present strategy of adding an MCA intermedium layer can efficiently improve the mechanical performance of the weld joint for IN718 alloys, which is promising for real applications in aerospace engineering.
... The results reveal that a tool causing maximum material movement can provide a better bonding surface and join steel with soft metals like aluminium. The steel grains should be minimized to obtain sound joint strength [205,206]. ...
Friction Stir Welding (FSW) is the most promising solid-state metals joining method introduced in this era. Compared to the conventional fusion welding methods, this FSW can produce joints with higher mechanical and metallurgical properties. Formerly, FSW was adopted for low melting metals like aluminum alloys. In recent years it has made significant progress in friction stir welding of steels since unfavourable phase transformations occurred in welds due to the melting of the parent and filler metals in fusion welding can be eliminated. The main advantage of FSW over traditional fusion welding is the reduction in the heat-affected zone (HAZ), and the joints exhibit excellent mechanical and corrosion resistance properties. This article reviews the progress in the relevant issues such as the FSW tool materials and tool profiles for joining steels, microstructure and mechanical properties of steels joints, special problems in joining dissimilar steels. Moreover, in-situ heating sources was used to overcome the main limitations in FSW of hard metals and their alloys, i.e., tool damages and insufficient heat generation. Different in-situ heating sources like laser, induction heat, gas tungsten arc welding assisted FSW for various types of steels are introduced in this review. On the basis of the up-to-date status, some problems that need further investigation are put forward.
... Friction stir welding of RAFM steels is under intense investigation in our laboratory and some reports have already been published [17][18][19][20]. FSW of thick section IN-RAFM steels, avoiding the formation of deleterious/undesired microstructural features is still a challenge and has been taken up in the present study. ...
... The peak temperature evolved during FSW of P9 steel was close to Ac 3 temperature (1106 < 1169 K) and thereby martensite transformation was found in the SZ and TMAZ (Figure 4) [10]. Similar peak temperature values were reported for RAFM steel during FSW [16]. The HAZ peak temperature might have crossed just above Ac 1 temperature (P9). ...
The consequence of friction stir welding (FSW) and activated-gas tungsten arc welding (A-GTAW) processes on the evolution of microstructure and mechanical properties of 9Cr-1Mo (P9) steel to 316LN stainless steel dissimilar weld joint is investigated. The FSW specimen shows considerably higher tensile strength (∼ 652 MPa) compared to A-GTAW specimen (∼ 595 MPa) as well as its base metal of P9 (∼ 642 MPa) and 316LN (∼ 608 MPa) owing to the formation of tempered martensite and refined austenite in P9 and 316LN weld portion, respectively. The cross-weld tensile test revealed that the specimens failed in the base metal of 316LN SS for both FSW and A-GTAW process with ductile mode fracture. This study proves that FSW could be an alternate joining technique. ARTICLE HISTORY
... In 9-12% Cr steels, the presence of primary particles, spherical MX particles, are important to hinder the coarsening of austenite grains during austinitization treatment via pinning the grain boundaries. The evolution of primary particles that were not dissolved during austinitization [24] is discussed in two categories: large and small primary particles. ...
Compositionally complex Z-phase strengthened 12% Chromium steels are considered as potentially viable materials for components used in highly-demanding environments in steam power plants, operating at a target temperature of 650 °C. To date, however, the transformation processes of various phases into the desired precipitate, i.e., Z-phase CrTaN, are not fully understood. In this research, we first designed and produced three different alloys and then studied the microstructure in the as-tempered and aged conditions (for up to 10,000 h at 650 °C) using advanced electron microscopy, X-ray diffraction, and atom probe tomography. We report on the evolution of the densely distributed MX (Ta(C, N)) and M2X ((Cr, Ta)2N) precipitates into blade-like and bulky Z-phase, respectively. The blade-like precipitates benefit from a smaller size compared to the bulky ones, providing precipitation hardening for creep resistance. We discuss an interactive role of carbon and nitrogen content in the formation of the Z-phase. Our findings pave the way towards designing new alloys with improved properties to serve in harsh environments at 650 °C.
... Recent experimental progress has mainly focused on the fabrication, manufacturing, mechanical properties (precipitation behavior, fracture toughness, creep, fatigue, and thermal aging), effects of irradiation, and corrosion analysis of RAFM steels [14,[16][17][18][19][20]. Irradiation damage on the microstructure and mechanical properties, including irradiation hardening and embrittlement by neutrons and helium, fatigue and creep after irradiation, were intensively investigated [14,17,19,21]. ...
Reduced activation ferritic/martensitic (RAFM) steels are structural materials with potential application in Generation-IV fission and fusion reactors. We use density-functional theory to scrutinize the micro-mechanical properties of the main alloy phases of three RAFM steels based on the body-centered cubic FeCrWVMn solid solution. We assess the lattice parameters and elastic properties of ferromagnetic -Fe and FeCr, which are the main building blocks of the RAFM steels, and present a detailed analysis of the calculated alloying effects of V, Cr, Mn, and W on the mechanical properties of FeCr. The composition dependence of the elastic parameters is decomposed into electronic and volumetric contributions and studied for alloying levels that cover the typical intervals in RAFM steels. A linear superposition of the individual solute effects on the properties of FeCr is shown to provide an excellent approximation for the \emph{ab initio} values obtained for the RAFM steels. The intrinsic ductility is evaluated through Rice's phenomenological theory using the surface and unstable stacking fault energies, and the predictions are contrasted with those obtained by empirical criteria. Alloying with V or W is found to enhance the ductility, whereas additional Cr or Mn turns the RAFM base alloys more brittle.
... The high toughness for 200 rpm condition is related to refinement of grain size, fine scale distribution of undissolved carbides and absence of phase transformation. All these findings are comprehensively discussed in ref. [5]. ...
Traditional welding techniques result in the creation of large heat affected zones (HAZ) as well as several filler materials need to be used. In addition, evolution of several unwanted gases into the atmosphere as well as solidification of some unwanted phases in the materials being welded demands for the alternate welding routes by which all these problems could be avoided. As an attempt in this direction, friction stir welding has been carried out on 6 mm thick reduced activation ferritic martensitic steel at various rotational speeds. The resultant microstructures and the observed properties are discussed in this paper.
... A combination of W thimble with a W tile is joined to a low activation ferritic/martensitic steel transition piece (EUROFER), which is one of the reference structural materials proposed to DEMO. EUROFER contains 9-12% Cr and the usual alloy elements used in conventional steels (Ni, Mo or Nb) have been substituted by W, V or Ta, in order to control its activation capability [3]. ...
The [Formula: see text]-Fe(100)/Cr(100) interface has an important effect on the precipitation of the Cr-rich phase in ferritic stainless steel, and alloying elements affect the stability of the [Formula: see text]-Fe(100)/Cr(100) interface. However, it is difficult to clearly understand the effect of alloying elements on the [Formula: see text]-Fe(100)/Cr(100) interface behavior experimentally. Therefore, the first-principles calculation is employed to address this problem in this paper. First of all, the segregation behaviors of alloy elements (Mo, Nb, and Si) on the [Formula: see text]-Fe(100)/Cr(100) interface were systematically investigated. And then, the effects of these alloying elements on the property of the [Formula: see text]-Fe(100)/Cr(100) interface were studied. Based on the density functional theory, the work of adhesion (W ad ), interfacial energy (γ int ), doping energy ([Formula: see text][Formula: see text]), and electronic properties of the [Formula: see text]-Fe(100)/Cr(100) interface were analyzed. The results show that the body-centered [Formula: see text]-Fe(100)/Cr(100) interface is constructed by using seven layers of [Formula: see text]-Fe(100) and Cr(100) surface models. The W ad is 6.115 J.m[Formula: see text], and the γ int is −0.12 J.m [Formula: see text]. The doping atoms tend to segregate on the surface of [Formula: see text]-Fe(100). When the Si atom doped in the interface, the W ad increases, and the γ int reduces. The doping of the Si strengthens the bonding strength of the interface and promotes the nucleation of the Cr-rich phase. While the addition of Mo and Nb atoms decreases the W ad of [Formula: see text]-Fe(100)/Cr(100) interface, increases the γ int . The bonding strength of the interface is weakened and the nucleation of the Cr phase is inhibited. Based on this result, it is clearer to understand the effect of Mo, Nb, and Si atoms on the precipitation of the Cr-rich phase in ferritic stainless steel.
Point defects and dislocation can be induced in dual-phase stainless steels in the circumstance of irradiation. These defects affect the phase separation and the morphology evolution of Fe-Cr based alloys, increasing the hardening and damage of alloys. The influence of dislocation on the evolution of Cr-enriched α′ nanophase accompanying the vacancy and interstitial atoms is studied by the multi-phase-field model. Multi-dislocation loops are introduced to investigate the dragging effect of dislocation stress field on the solute atoms, and the dragging effect of solute atoms on the point defects. Tensile stress of dislocation drags the Cr atoms to accelerate the local phase separation, and the migration of Cr atoms drags the vacancies to form a two-dimensional vacancy loop around the α/α′ phase boundary. Large diffusion potential difference drives the atomic diffusion rapidly and accelerates the growth and coarsening of the α′ phase. The superposition of tensile and compressive stress field of multi-dislocation loops makes the redissolution of the α′ phase. The multi-interactions of stress–atoms–point defects are clearly presented in this work, and the dynamic effects are clarified for the dragging on solute atoms and point defects under dislocation stress.
Reduced Activation Ferritic-Martensitic (RAFM) steels are currently being considered for test blanket modules of International Thermonuclear Experimental Reactor. This study is aimed at understanding the microstructure and mechanical properties of Indian RAFM steel during friction stir welding (FSW) of 6 mm thick plates. The full penetration bead-on-plate welds were fabricated employing PcBN tool at a rotational speed of 200 rpm. The FSW joint consisted of stir zone, thermomechanically affected zone and BM. Microstructure, hardness and tensile properties were evaluated in as-received (base metal, BM), as-welded and post weld normalised + tempered states. The as-received microstructure was composed of Cr-rich M23C6 on prior austenite grain and tempered lath martensite boundaries with intra-lath V-and Ta -rich monocarbides. Substantial changes occurred during welding leading to destruction and dissolution of M23C6 and precipitation of Fe3C in SZ. The microstructure was restored to the BM level by giving a post weld normalising and tempering treatment. Comparative study on tensile properties has been carried out at room and elevated temperatures (up to 550 °C) in all the three states and the observed variations have been explained on the basis of initial microstructure and evolving substructures. The as-welded samples showed higher strength and low percentage elongation values. The ductility was minimum in all the states at an intermediate temperature and ascribed to the dynamic strain ageing. All the states revealed falling strength with increasing temperature up to 550 °C. Failure occurred in AW and PWNT states at the interface between TMAZ and BM. Irrespective of material state and tensile test temperature, the transgranular ductile fracture prevailed. The contribution of the different strengthening mechanisms to the yield strength of the SZ region has been estimated theoretically and it matches with the experimental results. The influence of low angle grain boundaries (LAGB) and high angle grain boundaries (HAGB) on the tensile behaviour of the SZ was well explained using the Electron Back Scattered Diffraction (EBSD) maps of these regions.
In this paper, an attempt has been made to explore the feasibility of employing friction stir welding (FSW) of 12-mm-thick reduced activation ferritic–martensitic (RAFM) steel plates using a polycrystalline cubic boron nitride (PCBN) tool at a rotation speed of 200 rpm. Deep penetration bead-on-plate welds were successfully produced. Microstructures evolved in stir zone (SZ), thermomechanically affected zone (TMAZ), heat-affected zone (HAZ) and unaltered base metal zones, and their impact on microhardness was assessed. The SZ and TMAZ revealed high hardness, while HAZ became much softer than the base metal. Several grain boundaries in SZ and TMAZ were found to be free from carbides, and SZ contained martensite laths, Fe3C needles and very high dislocation density. HAZ displayed coarse and coagulated carbides. The inhomogeneous microstructure and hardness variation across the weld joint have been rectified by appropriate post-weld normalising and tempering (PWNT) treatment. Tensile tests were conducted at room and elevated temperatures (450 °C, 500 °C, 550 °C) on transverse samples profiled from as-welded and PWNT states; and the displayed stress–strain behaviour and mechanical properties were compared with those obtained concurrently on base metal at various temperatures. The observed tensile hardening/softening as a function of temperature has been explained based on the substructure evolution. It has been noticed that the variation of the fraction of low-angle grain boundaries and high-angle grain boundaries played a significant role in influencing the tensile deformation behaviour in the as-welded and PWNT states. The low ductility observed at 450 °C has been ascribed to dynamic strain ageing. Alloy displayed ductile fracture, in spite of variation in tensile properties depending upon the state of the material and tensile test temperature. This is the first ever investigation dealing with the tensile behaviour of friction stir welded RAFM steel under different states and conditions.
In this paper, laser weld technology was applied to produce the welded joints of CLF-1 steel, which is the primary material for manufacturing test blanket module (TBM). The microstructures and mechanical properties of the joints were studied at as-welded state, post-weld heat treatment (PWHT) of 710 °C/2 h/air cooling (PWDT), and PWHT of 980 °C/1 h/air cooling (AC) followed by tempering at 710 °C/2 h/AC (PWNT). The results showed that abundant lath martensite phase with width 800 nm and small amount of δ-Fe phase were present in the weld at as-welded state. Its impact toughness was about 40 J. The microstructures of the weld at PWDT state included a large amount of tempered lath martensite with width 650 nm, few δ-Fe, M23C6 carbide (distributed along the boundary of martensite) and MX carbide (precipitated inside the martensite). Its impact toughness was about 237 J (equivalent to the base metal). The microstructures of the weld at PWNT state included a large amount of tempered lath martensite with width 350 nm, M23C6 carbide and MX carbide. Even though the δ-Fe phase disappeared, the impact toughness (about 156 J) was lower than that of the weld at PWDT state due to the aggregated growth of M23C6 and MX carbides.
Thick tempered martensitic steel plates were friction stir processed at different tool traverse and rotational speeds. Friction stir processing led to formation of finer martensite phase, ferrite, and higher angle grain boundaries, which resulted in higher hardness of the processed samples. However, the macro mechanical results indicated that the tensile strength and elongation were reduced after friction stir processing, was due to the formation of ferritic bands and WC debris in the processed zones. The brittle fractured surfaces of processed samples confirmed the negative effect of friction stir processing on tensile behavior, which contained more cleavage than dimples compared to base material. The outcome of this study can be used for surface modification of thick martensitic materials for industrial applications.
The focus of this research is the low activation ferrite/martensitic steel that is used for manufacturing nuclear fusion reactor experimental blanket modules. A fiber laser was used to weld on one side, realizing double-sided forming, and the organization and properties of the as-welded metal were systematically studied. The Ta mass fraction varied in the order of 0%, 0.26%, 0.54%, and 0.81%. The results showed that the weld seam had been well formed and did not have other welding metallurgical defects. With an increase in the Ta mass fraction in the weld, the number and size of lath martensite and δ-Fe in the weld gradually decreased; at the same time, the number and size of ferrite and pearlite gradually increased. When the mass fraction of Ta increased from 0% to 0.26%, the impact toughness of the weld increased from about 40 J to about 82.3 J, which is equivalent to an increase of about 2.06 times. This increase is because the nanoscale Ta-rich MX carbides that precipitated from the martensitic lath in the as-welded metal had an effect on dispersion strengthening. This was because the mass fraction of Ta corresponded to 0.26%. However, when the mass fraction of Ta increased from 0.26% to 0.81%, the coarsened TaC precipitated from the as-welded metal and was accompanied by the lath martensite and δ-Fe gradually disappearing while the content and size of ferrite and pearlite gradually increased. This resulted in a decline of the impact toughness.
Vacuum electron beam welding was adopted for the reduced activation ferritic martensitic steel (CLF-1 steel) with a thickness of 32 mm, and a well-formed butt joint without metallurgical defects was obtained under the optimum technological parameters. After welding, the specimens were subjected to high temperature tempering (710 °C/120 min/AC (air cooling), abbreviation HTT) and incomplete annealing (840 °C/120 min/AC, abbreviation IA) heat treatment, and the microstructure and mechanical properties of the as-welded state and two different heat treatment states were compared. The results showed that the as-welded weld metal (WM) was composed of coarse lath martensite, and the heat affected zone (HAZ) was composed of a large amount of massive martensite and dispersed fine MX and M23C6. Compared with the welded state, the martensite structure of HTT state WM and HAZ were refined, nano-scale M23C6 was precipitated along the boundary of tempered martensite in WM and nano-scale MX was precipitated in tempered martensite lath structure, both of which played a role of dispersion strengthening. Compared with the HTT state, the martensite structure of the WM and HAZ in the IA state became coarser, and both M23C6 and MX tend to grow together. The fracture position of the tensile specimens of as-welded state, HTT state and IA state welded joints were all located on the base metal (BM), and the tensile strength decreased successively, while the yield strength and elongation increased successively. The average impact absorbed energy of the BM, as-welded state, HTT state and IA state welds were 236 J, 42 J, 241 J and 51 J, respectively. The impact performance of HTT state welds surpassed the BM.
The microhardness distribution in the different zones of a friction stir welded reduced activation ferritic/martensitic steel has been investigated and correlated to the hierarchical martensitic microstructure in the respective zones, characterized by electron backscatter diffraction orientation analysis. It is found that the variation of prior austenite grain size, packet size, and block width in different subzones is influenced by the peak temperature and effective strain rate during the friction stir welding process. The distribution of the microhardness correlates directly with the geometrically necessary dislocation density observed in the different zones.
One of the most significant requirements for joining dissimilar materials is to produce a minimum number of deeper-penetration welds and thereby avoid flaws. The Activated Flux TIG welding process improves penetration depth in a single pass, reducing the number of weldments. The dissimilar weld between LAFM – Stainless Steel 316LN also possesses the same requirement for a particular application. The present study aims to investigate the structure-property relationship and mechanical properties of dissimilar weldments of LAFM – SS 316LN using conventional TIG and Activated TIG welding using TiO2 and Co3O4 oxide fluxes. The microstructure study of conventional TIG weld reveals the fine lath martensitic structure as compared to weld prepared with A-TIG using TiO2 flux due to difference in the cooling rate. Also, the carbide formation in conventional TIG and A-TIG using TiO2 confirms the presence of delta ferrite But, the same observation was not confirmed with A-TIG weldments prepared using Co3O4. Similarly, the hardness results also support the microstructure study. The higher transverse tensile strength of weld indicates that it is stronger than base metal. The lower impact energy in case of Activated Flux TIG welding process was reported due to presence of oxide flux.
The present work is concerned with the application of linear friction welding (LFW) process accessible for the weld-jointing of 9Cr reduced activation ferrite/martensite (RAFM) steels. Optical and electron microscopic characterization of microstructures were performed in various regions of the weld joint. Hardness, tensile and Charpy impact tests on the joined samples were processed to examine the mechanical property and reliability of the weld joints. It indicates that, hot plastic deformation induced by linear friction triggers continual dynamic recrystallization in the weld zone, along with high-density dislocation substructures formed by such severe deformation, which leads to a good combination of mechanical performances in the weld joint. Such linear friction welding comes beyond the rotational process restriction in conventional friction stir welding, and avoid significant oxide inclusions, porosities and coarsened grains brought by heat input as well. The work proves that the present LFW technique works well in the welding of 9Cr RAFM steels and inspires us of a future study on optimizing process parameters of the welding process for a better performance.
Point defects induced by irradiation affect the phase decomposition and morphology evolution of Fe‒Cr alloys, which results in the increase of hardness and embrittlement. The coevolution of point defects and the Cr-enriched nanoscale α′ phase was studied by using the phase-field simulation, and the interactive influences between point defects and Cr-enriched nanoscale particles on the kinetics evolution were clarified. It is found that the clustering of vacancy and interstitial atoms is earlier than the separation of α′ phase, the point defects migrate from the initial cluster position to the interface of α/α′ phases, and accumulate into a defect concentration loop around the α′ phase with the growth of α′ phase. In addition, with the increase of initial defects concentration, the phase separation is accelerated via the vacancy diffusion mechanism, and also the separation of Cr-enriched particles promotes the clustering of point defects. The point defects show a significant influence on the phase separation with elevated temperature.
This paper reports on the precipitate evolution and its influence on the high temperature creep resistance of 9% Cr martensite ferritic steel FSW joint. The creep rupture time of the FSW joints welded by 400 rpm and 200 rpm are 859.29 h and 1173.8 h respectively, while that of BM is 3398 h. The creep fracture occurs in ICHAZ where a pronounced increase of coarse Laves phase is detected. Most of the Laves phase particles are large in size and mainly nucleate and grow rapidly at the micrograin boundaries adjoining the M23C6 particles where W, and other elements segregate. The large Laves phase particles will deteriorate the pinning effect of M23C6, weaken the solid solution strengthening effect and attribute to the increased cavity nucleation. In addition the size of Laves phase in ICHAZ is bigger than that in stir zone (SZ). These reasons explain why creep fracture occurs in the ICHAZ and the time to rupture will be shorter than base metal.
Dissimilar welded joints of 9Cr-1W reduced activation ferritic/martensitic (RAFM) steel and 316 L austenitic stainless steel were made by friction stir welding (FSW) with optimized welding parameters. In stir zone (SZ), mechanically mixing and metallurgical bonding of the two materials are observed. A α + γ two-phase microstructure is observed nearby the bonding interface characterized by diffusion of Cr and Ni from 316 L steel side to RAFM steel side during FSW process. The grain size and dislocation density of the SZ 316 L in are critical for high temperature tensile behavior of the joint. Reducing the rotational speed or placing 316 L at AS could enhance the grain refining, but lead to the increase of the dislocation density of SZ 316 L. The tensile properties of the joint at 550℃ are 297.5 MPa yield strength, 546.5 MPa ultimate tensile strength and 17.5% elongation.
The Reduced Activation Martensitic/Ferritic steel (RAFM steel)is known as one of the most important materials for future fusion reactor blanket. Welding process is unavoidable during the blanket manufacturing, so it is necessary to research the welding performance of the RAFM steel. Simulations and experiments regarding fatigue behavior of the RAFM steel welding specimens have been done in this paper. The simulation of fatigue behavior of tungsten inert gas (TIG)welding and electron beam (EB)welding of the RAFM steel were carried out by using ANSYS mechanical engineering code. The fatigue behavior for two types RAFM steel specimens of TIG welding and EB welding were also tested by SDS200 electro-hydraulic servo fatigue testing machine. The fatigue performance of the weld joint both for TIG welding and EB welding was tested by applying the same gradient load. The experimental results were studied to analyze the impact of fatigue resistance for the analyzed welding technologies of RAFM steel, and the results indicate that the EB welding is stable under increasing stress. Outcomes of this study can provide some beneficial indications for the blanket design of China Fusion Engineering Testing Reactor (CFETR)in the future.
In this work, aluminum/titanium carbide (Al/TiC) surface composite has been fabricated by friction stir processing using a novel modular Direct Particle Injection Tool (DPI–FSP). The tool has a unique feature wherein the TiC particles have been transferred from the tool itself by spring adjusted plunger movement into the matrix. The microstructural observations from optical and scanning electron microscope (SEM)-EDS results revealed the homogeneous distribution of particles in the stirred zone (SZ) and the thickness of the formed surface composite layer (SCL) is approximately 0.34(Formula presented.)mm. X-ray diffraction results confirmed that the particles are reinforced in the aluminum matrix, and no intermetallics have been formed in the composite. The microhardness of composite was increased from 68 to 135(Formula presented.)Hv, and the impact test results showed that the toughness was almost comparable to that of the base metal.
A quantitative simulation of the separation of the α' phase in Fe–28 at.% Cr alloy under the effects of applied strain is performed by utilizing a three-dimensional phase-field model. The elongation of the Cr-enriched α' phase becomes obvious with the influence of applied uniaxial strain for the phase separation transforms from spinodal decomposition of 700 K to nucleation and growth of 773 K. The applied strain shows a significant influence on the early stage phase separation, and the influence is enlarged with the elevated temperature. The steady-state coarsening with the mechanism of spinodal decomposition is substantially affected by the applied strain for low-temperature aging, while the influence is reduced as the temperature increases and as the phase separation mechanism changes to nucleation and growth. The peak value of particle size distribution decreases, and the PSD for 773 K becomes more widely influenced by the applied strain. The simulation results of separation of the Cr-enriched α' phase with the applied strain provide a further understanding of the strain effect on the phase separation of Fe–Cr alloys from the metastable region to spinodal regions.
Addition of copper in high-carbon bainitic steel has resulted in attractive elongation. Heterogeneous copper precipitation in the course of bainitic transformation, at low homologous temperature, has resulted into attractive strength–ductility combination despite the limited dislocation controlled plasticity of nanoscale bainite. Early yielding of bainite facilitates strain partitioning between the blocky austenite and bainitic sheaves, leading to the continuous work hardening. In contrast to the tensile fracture strain, the compressive fracture strain has been found to be less dependent on the austenite volume fraction. The nanoindentation results indicated limited work hardening of the bainite sheaves.
Bead-on-Plate Friction Stir Welding (FSW) joints were produced on 3-mm thick Ultra-fine Bainitic Steel using polycrystalline cubic boron nitride (PcBN) tool. The rotational speeds employed include 80, 100, 150, and 200 rpm. All the weld joints were prepared at a traverse speed of 35 mm/min. The noncontact online thermography has been used to measure interface temperature between the top surface of plate and bottom of the tool shoulder bottom. This measurement enabled to determine the peak temperature reached in the stir zone (SZ) during FSW. The interface temperatures at all rotational speeds are ~below 673 K. The microstructure in the base metal (BM) is composed of carbide-free bainite and retained austenite. After FSW, the weld joint revealed BM, stir zone (SZ), and thermomechanically affected zone (TMAZ). The retained austenite decreased with increasing rotational speed in SZ and there has been a gradual transition of microstructure in the TMAZ. Stir zone exhibited higher hardness compared to BM. Maximum hardness in SZ occurred at 150 rpm.
The initial phase separation and coarsening of the Cr-enriched α′ phase in Fe–Cr alloys were studied by utilizing phase field model. The phase separations of the α′ phase via non-classical nucleation and growth, the transition state from nucleation and growth to spinodal decomposition, and the spinodal decomposition are investigated at 700 K for the alloys containing 25, 28 and 35 at.% Cr. The time exponent of the average radius of α′ phase decreases from the growth and coarsening stage to the steady-state coarsening stage for the nucleation growth mechanism, the slope of number density of α′ phase increases at the steady-state coarsening stage with the increased Cr concentration. The velocity of phase separation in 25 at.% Cr alloy with a nucleation growth mechanism are slower than that in high Cr alloys with a spinodal decomposition mechanism. The results indicate that a low concentration alloy is more stable at high temperature, and the phase separation is fast at the early stage, which supply a reference for the alloy composition selection, and for the analysis of the relationship of property and morphology.
A quantitative investigation of phase decomposition in Fe-35 at.% Cr alloy aged at 773 K was performed by using atom probe tomography (APT) and phase-field simulation. The kinetics of the α′ phase formation via spinodal decomposition was studied by the variation of volume fraction, average particle radius and particle size distribution of α′ phase at 700, 725 and 750 K. The simulated morphology and composition are in good agreement with the APT and transmission electron micrograph (TEM) results, demonstrating that the thermodynamic parameters used in the simulation were optimized. The growth and coarsening of the α′ phase were promoted by the increasing aging temperature. Also, the coarsening rate increases and induces a larger slope for the number density of particles at the steady-state coarsening stage. The time exponent of the average radius of the α′ phase shows a three-stages variation from about 0.31 to 0.20 and larger than 0.33 for the initial phase decomposition, growth and coarsening, and the steady-state coarsening stages, respectively. The particle size distribution of the α′ phase is similar with the Brailsford-Wynblatt's (BW) predication at early stage of phase decomposition, and becomes lower than BW's value at coarsening stage. The kinetics of nanoscale α′ phase from initial decomposition to coarsening is useful for the composition design and properties prediction of Fe–Cr alloys working at high temperature.
The effects of tool rotational speed (200 and 700 rpm) on evolving microstructure during friction stir welding (FSW) of a reduced activation ferritic-martensitic steel (RAFMS) in the stir zone (SZ), thermo-mechanically affected zone (TMAZ), and heat-affected zone (HAZ) have been explored in detail. The influence of post-weld direct tempering (PWDT: 1033 K (760 °C)/ 90 minutes + air cooling) and post-weld normalizing and tempering (PWNT: 1253 K (980 °C)/30 minutes + air cooling + tempering 1033 K (760 °C)/90 minutes + air cooling) treatments on microstructure and mechanical properties has also been assessed. The base metal (BM) microstructure was tempered martensite comprising Cr-rich M23C6 on prior austenite grain and lath boundaries with intra-lath precipitation of V- and Ta-rich MC precipitates. The tool rotational speed exerted profound influence on evolving microstructure in SZ, TMAZ, and HAZ in the as-welded and post-weld heat-treated states. Very high proportion of prior austenitic grains and martensite lath boundaries in SZ and TMAZ in the as-welded state showed lack of strengthening precipitates, though very high hardness was recorded in SZ irrespective of the tool speed. Very fine-needle-like Fe3C precipitates were found at both the rotational speeds in SZ. The Fe3C was dissolved and fresh precipitation of strengthening precipitates occurred on both prior austenite grain and sub-grain boundaries in SZ during PWNT and PWDT. The post-weld direct tempering caused coarsening and coalescence of strengthening precipitates, in both matrix and grain boundary regions of TMAZ and HAZ, which led to inhomogeneous distribution of hardness across the weld joint. The PWNT heat treatment has shown fresh precipitation of M23C6 on lath and grain boundaries and very fine V-rich MC precipitates in the intragranular regions, which is very much similar to that prevailed in BM prior to FSW. Both the PWDT and PWNT treatments caused considerable reduction in the hardness of SZ. In the as-welded state, the 200 rpm joints have shown room temperature impact toughness close to that of BM, whereas 700 rpm joints exhibited very poor impact toughness. The best combination of microstructure and mechanical properties could be obtained by employing low rotational speed of 200 rpm followed by PWNT cycle. The type and size of various precipitates, grain size, and evolving dislocation substructure have been presented and comprehensively discussed. © 2017 The Minerals, Metals & Materials Society and ASM International
The present paper presents the results of an extensive electron microscopy investigation on the decomposition modes of high temperature austenite in 9Cr-W-V-Ta reduced activation ferritic-martensitic steels. Although the displacive martensitic transformation is predominant on austenitisation, low volume fraction of Fe rich M3C or M23C6 precipitates formed, when the tungsten content exceeded 1 wt-%. The compositional inhomogeneity introduced in the austenite by the nature, chemistry and kinetics of dissolution of the pre-existing carbides is dependent on the steel composition and austenitisation conditions. The extent of repartitioning of tungsten between M23C6 and ferrite largely influences the kinetics of austenite and martensite transformation, for the same austenitisation conditions. Supporting evidence from calorimetry analysis is also presented.
In this study, the effects of tool rotational speed and traverse speed on welding of AISI 430 (X6Cr17, material number 1.4016) ferritic stainless steels by friction stir welding method are examined. Two specimens with dimension of 3 × 100 × 200 mm were joined in butt position. Tool rotational speeds were determined to be 560–1400 min−1 and traverse speeds as 80–200 mm/min. During the studies, tool pressure force 3.5 kN and tool angle of 0° was kept constant. Hard metal carbide (WC-Co hard metal identified as K10) with equilateral triangle tip profile was used as the tool material. Determination of the tool advance speeds related to the tool rotation speeds giving the best-looking weld seals with acceptable values of mechanical properties was aimed.During welding of the specimens joined in butt position, the temperature change due to time and variation of the pressure force applied on welded specimens by the tool shoulder has been recorded. It has been observed that the best mechanical resistance values were obtained at tool rotational speed of 1120 min−1 through five tool rotational speeds (560–1400). Also it has been observed that the best mechanical resistance values were obtained at traverse speed of 125 mm/min through five traverse speeds (80–200) with the constant tool pressure force of 3.5 kN and tool angle of 0°.
Friction stir welding of steel presents an array of advantages across many industrial sectors compared to conventional fusion welding techniques. However, the fundamental knowledge of the friction stir welding process in relation to steel remains relatively limited. A microstructure and property evaluation of friction stir welded low alloy steel grade DH36 plate, commonly used in ship and marine applications has been undertaken. In this comprehensive study, plates of 2000 × 200 × 6 mm were butt welded together at varying rotational and traverse speeds. Samples were examined microscopically and by transverse tensile tests. In addition, the work was complemented by Charpy impact testing and micro-hardness testing in various regions of the weld. The study examined a wide range of process parameters; from this, a preliminary process parameter envelope has been developed and initial process parameter sets established that produce commercially attractive excellent quality welds through a substantial increase in the conventionally recognised weld traverse speed.
Reduced Activation Ferritic-Martensitic (RAFM) steels are actively considered as structural material for blanket module of fusion reactor for its adequate creep rupture strength and better void swelling resistance than austenitic stainless steels. This paper presents the effect of tungsten and tantalum on the creep deformation and rupture properties of RAFM steel. Four heats of RAFM steel containing 1-2 wt. % tungsten and 0.06 - 0.14 wt. % tantalum have been melted. Creep tests were carried out on the steels at 823 K over a stress range of 180 – 260 MPa. The creep deformation of the steels was found to proceed with relatively shorter primary regime followed by an extended tertiary regime with virtually no secondary regime. The minimum creep rate of the steel was found to decrease with the increase in tungsten content and to increase with the increase in tantalum content. Creep rupture life of the steel increased appreciably with the increase in tungsten content from 1 wt. % to 2 wt. %. The increase in tantalum content from 0.06 wt. % to 0.14 wt. % was found to decrease the creep rupture strength of the steel.
Effects of tungsten and tantalum contents on impact, tensile, low cycle fatigue and creep properties of Reduced Activation Ferritic–Martensitic (RAFM) steel were studied to develop India-specific RAFM steel. Four heats of the steel have been melted with tungsten and tantalum contents in the ranges 1–2 wt.% and 0.06–0.14 wt.% respectively. Increase in tungsten content increased the ductile-to-brittle transition temperature (DBTT), low cycle fatigue and creep strength of the steel, whereas the tensile strength was not changed significantly. Increase in tantalum content increased the DBTT and low cycle fatigue strength of the steel whereas the tensile and creep strength decreased. Detailed TEM investigations revealed enhanced microstructural stability of the steel against creep exposure on tungsten addition. The RAFM steel having 1.4 wt.% tungsten with 0.06 wt.% tantalum was found to possess optimum combination of impact, tensile, low cycle fatigue and creep properties and is considered for Indian-specific RAFM steel.
There have been concerted world wide efforts to develop steels suitable for use in efficient fossil fired power plants. Ferritic alloys containing between 9 and 12 wt-% chromium are seen as the most promising materials in this respect, especially for thick walled components such as headers and the main steam pipe in boilers. However, the performance of the improved steels has often not been realised in service, because premature failures occur in the heat affected zone of welded joints in a phenomenon referred to as type IV cracking. This review assesses the relationship between the composition and microstructure of 9–12 Cr steels, the welding and fabrication procedures and how these factors translate into a propensity for type IV failures.
Mock-ups of DEMO breeding blankets, called Test Blanket Modules (TBMs), inserted and tested in ITER in dedicated equatorial ports directly facing the plasma, are expected to provide the first experimental answers on the necessary performance of the corresponding DEMO breeding blankets. Several DEMO breeding blanket designs have been studied and assessed in the last 20 years. At present, after considering various coolant and breeder combinations, all the TBM concepts proposed by the seven ITER Parties use Reduced-Activation Ferritic/Martensitic (RAFM) steel as the structural material. In order to perform valuable tests in ITER, the TBMs are expected to use the same structural material as corresponding DEMO blankets. However, due to the fact that this family of steels is ferromagnetic, their presence in the ITER vacuum vessel will create perturbations of the ITER magnetic fields that could reduce the quality of the plasma confinement during H-mode. As a consequence, a legitimate question has been raised on the necessity of using RAFM steel for TBMs structural material in ITER. By giving a short description of the main TBM testing objectives in ITER and assessing the consequences of not using such a material, this paper gives a comprehensive answer to this question. According to the working group author of the study, the use of RAFM steel as structural material for TBM is judged mandatory.
The precipitation reactions in two ferritic steels, 9Cr-lMo-V-Nb and 12Cr-lMo-V-W, were studied. Analytical electron microscopy,
optical microscopy, electrolytic extractions, and hardness measurements were used to determine the types, amounts, and effects
of precipitates formed as a function of the heat treatment. The effect of variations in the austenitizing treatment was ascertained.
In addition to variations in the austenitizing time and temperature, different cooling rates after austenitization were also
used. Air cooling after austenitization (normalization) resulted in little precipitation in both alloys. Precipitation in
the 12Cr-lMo-V-W alloy after furnace cooling was found in all cases examined. Under certain conditions precipitation was also
found after furnace cooling the 9Cr-lMo-V-Nb alloy. However, when compared to the amount of precipitate in the fully tempered
state, the 9Cr-lMo-V-Nb showed a much greater variation in the degree of precipitation following furnace cooling. In addition,
the matrix microstructure of the 9Cr-lMo-V-Nb alloy was very sensitive to cooling rate. The precipitation reactions during
tempering after a normalizing treatment were followed as a function of tempering treatment. Tempering temperatures were varied
from 400 to 780 °C. The carbide precipitation was essentially complete after one hour at 650 °C for both alloys. Analytical
microscopy was used to identify the precipitates. In the 9Cr-lMo-V-Nb alloy, a combination of chromium-rich M23C6 and vanadium-niobium-rich MC carbides was found. The carbides in the 12Cr-lMo-V-W alloy were identified as chromium-rich
M23C6 and vanadium-rich MC. The results give an indication of the sensitivity of these alloys to heat treatment variations.
Concepts for the next generation of nuclear power reactors designed to meet increasing world-wide demand for energy include water-cooled, gas-cooled, and liquid-metal-cooled reactors. Reactor conditions for several designs offer challenges for engineers and designers concerning which structural and cladding materials to use. Depending on operating conditions, some of the designs favor the use of elevated-temperature ferritic/martensitic steels for in-core and out-of core applications. This class of commercial steels has been investigated in previous work on international fast reactor and fusion reactor research programs. More recently, international fusion reactor research programs have developed and tested elevated-temperature reduced-activation steels. Steels from these fission and fusion programs will provide reference materials for future fission applications. In addition, new elevated-temperature steels have been developed in recent years for conventional power systems that also need to be considered for the next generation of nuclear reactors.
The inherent properties of reduced activation ferritic/martensitic (RAFM) steels include reduced swelling and high recycling potential, which make them likely candidates for application in commercial fusion power plants. The International Energy Agency (IEA) agreement has been an effective framework for international co-operation in developing RAFM steels. The progress and critical issues observed in this co-operation are reported. The production of RAFM steels on an industrial scale has been demonstrated. Various methods of fusion welding and solid hot isostatic pressing (HIP) are feasible for joining the steels. Manufacturing of complex shapes with the powder HIP method works well for RAFM steels. Major critical issues addressed concern the effects of simultaneous introduction of helium and displacement damage. The availability of a 14 MeV neutron source is identified as an essential tool to determine this effect. Finally, the potential of oxide dispersion strengthening to increase the operating temperature of RAFM steels is considered as an issue that has to be resolved to enlarge the application temperature window of RAFM steels.
The precipitation reactions in two ferritic steels, 9Cr-1Mo-V-Nb and 12Cr-1Mo-V-W, were studied. Analytical electron microscopy, optical microscopy, electrolytic extractions, and hardness measurements were used to determine the types, amounts, and effects of precipitates formed as a function of the heat treatment. The effect of variations in the austenitizing treatment was ascertained. In addition to variations in the austenitizing time and temperature, different cooling rates after austenitization were also used. Air cooling after austenitization (normalization) resulted in little precipitation in both alloys. Precipitation in the 12Cr-1Mo-V-W alloy after furnace cooling was found in all cases examined. Under certain conditions precipitation was also found after furnace cooling the 9Cr-1Mo-V-Nb alloy. However, when compared to the amount of precipitate in the fully tempered state, the 9Cr-1Mo-V-Nb showed a much greater variation in the degree of precipitation following furnace cooling. In addition, the matrix microstructure of the 9Cr-1Mo-V-Nb alloy was very sensitive to cooling rate. The precipitation reactions during tempering after a normalizing treatment were followed as a function of tempering treatment. Tempering temperatures were varied from 400 to 780 °C. The carbide precipitation was essentially complete after one hour at 650 °C for both alloys. Analytical microscopy was used to identify the precipitates. In the 9Cr-1Mo-V-Nb alloy, a combination of chromium-rich M23C6 and vanadium-niobium-rich MC carbides was found. The carbides in the 12Cr-1Mo-V-W alloy were identified as chromium-rich M23C6 and vanadium-rich MC. The results give an indication of the sensitivity of these alloys to heat treatment variations.
Reduced-activation ferritic/martensitic (RAFM) steels are an important class of structural materials for fusion reactor internals developed in recent years because of their improved irradiation resistance. However, they can suffer from welding induced property degradations. In this paper, a solid phase joining technology friction stir welding (FSW) was adopted to join a RAFM steel Eurofer'97 and different FSW parameters/heat input were chosen to produce welds. FSW response parameters, joint microstructures and microhardness were investigated to reveal relationships among welding heat input, weld structure characterization and mechanical properties. In general, FSW heat input results in high hardness inside the stir zone mostly due to a martensitic transformation. It is possible to produce friction stir welds similar to but not with exactly the same base metal hardness when using low power input because of other hardening mechanisms. Further, post weld heat treatment (PWHT) is a very effective way to reduce FSW stir zone hardness values.
The microstructures and mechanical properties of heat affected zones (HAZs) by Gas Tungsten Arc Welding (GTAW) were studied for 9Cr2WVTa steels with carbon content varying from 0.07 wt.% to 0.25 wt.%. Enlarged HAZs samples with 8 mm to 10 mm wide uniform temperature zone were prepared by the thermal-mechanical physical simulator Gleeble 1500 based on the Finite Element Method (FEM) numerical simulation and experimental measurement for the welding thermal cycle process and weld profile. The microstructures were observed by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). In addition, the mechanical properties tests including micro-hardness test, tensile test and impact test were carried out to investigate the effects of the carbon content and the welding thermal cycle. The results show that the big blocky delta ferrite in 9Cr2WVTa steel with lower carbon content deteriorates the impact property. On the other hand, the quenched martensite, especially for the twin martensite in 9Cr2WVTa steel with higher carbon content, deteriorates the impact toughness as well. The weldability of 9Cr2WVTa steel can be improved by adjusting the carbon content between 0.14 wt.% and 0.17 wt.%.
Abstract Formation of delta(δ)-ferrite in the weld metal, during autogenous bead-on-plate welding of Reduced Activation Ferritic Martensitic (RAFM) steel using Gas Tungsten Arc Welding (GTAW) process, has been studied. Composition of the alloy is such that delta-ferrite is not expected in the alloy; but examination of the weld metal revealed presence of delta-ferrite in the weld metal. Volume fraction of delta-ferrite is found to be higher in the weld interface than in the rest of the fusion zone. Decrease in the volume fraction of delta-ferrite, with an increase in preheat temperature or with an increase in heat input, is observed. Results indicate that the cooling rate experienced during welding affects the volume fraction of delta-ferrite retained in the weld metal and variation in the delta-ferrite content with cooling rate is explained with variation in the time that the weld metal spends in various temperature regimes in which delta-ferrite is stable for the alloy during its cooling from the liquid metal to the ambient temperature. This manuscript will discuss the effect of welding parameters on formation of delta-ferrite and its retention in the weld metal of RAFM steel.
Creep test at 600 °C under 130 MPa for the China Low Activation Martensitic (CLAM) steel was performed up to 7913 h in this study. According to the stress level, the crept specimen was divided into three regions in order to investigate the influence of stress on Laves-phase formation. In addition to the expected M23C6 carbide and MX carbonitride, the amount and the size of Laves phase in these three regions on the crept specimen were characterized by transmission electron microscopy. Laves phase could be found in all the regions and the creep stress could promote the formation of Laves phase.
In this study, microstructure and mechanical properties of a friction stir welded 18Cr–2Mo ferritic stainless steel thick plate were investigated. The 5.4 mm thick plates with excellent properties were welded at a constant rotational speed and a changeable welding speed using a composite tool featuring a chosen volume fraction of cubic boron nitride (cBN) in a W–Re matrix. The high-quality welds were successfully produced with optimised welding parameters, and studied by means of optical microscopy (OM), scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD) and standard hardness and impact toughness testing. The results show that microstructure and mechanical properties of the joints are affected greatly, which is mainly related to the remarkably fine-grained microstructure of equiaxed ferrite that is observed in the friction stir welded joint. Meanwhile, the ratios of low-angle grain boundary in the stir zone regions significantly increase, and the texture turns strong. Compared with the base material, mechanical properties of the joint are maintained in a comparatively high level.
A series of 4, 6 and 8 mm DH36 steel welds were produced using optimum conditions for friction stir welding (FSW). Comparator welds in the same thickness from the same plates were produced using a single sided single pass process submerged arc welds (SAW). This work was carried out to evaluate the mechanical properties of FSW material with a view to its possible application in a shipbuilding production process route.
Overall, the performance of the FSW material was superior to the SAW comparators. Areas such as distortion and fatigue were particularly positive in the FSW material. An 8 mm thick plate was also produced using two FSW passes, one from either side, and it was found to have superior toughness and fatigue performance when compared to the single sided 8 mm FSW material. Some of these benefits are thought to have originated from the internal overlap zone between the two passes.
In this paper the microstructure and mechanical properties of electron
beam weld (EBW) joints for China low activation martensitic (CLAM)
steel, which underwent a series of different post weld heat treatments
(PWHTs) were studied. The aim of the study was to identify suitable
PWHTs that give a good balance between strength and toughness of the EBW
joints. The microstructural analyses were performed by means of optical
microscope (OM) and scanning electron microscope (SEM). The mechanical
properties were determined via tensile tests and Charpy impact tests.
The results showed that the tensile strength of the as-weld joint (i.e.
without any PWHT) were close to that of the base metal, but the impact
toughness was only 13% of that of the base metal due to the existence of
a delta-ferrite microstructure. To achieve a significant improvement in
toughness a PWHT needs to be performed. If a one-step PWHT is applied
tempering at 760 °C for 2 h gives EBW joints with high strength at a
still acceptable toughness level. If a two-step PWHT is applied, a
process involving quenching at 980 °C for 0.5 h followed by
tempering at 740 °C or 760 °C for 2 h gives EBW joints with high
strength and toughness properties. Whenever possible a two-step PWHT
should be applied in favor of a one-step process, because of higher
resulting strength and toughness properties.
The feasibility of friction stir (FS) welding of maraging steel is demonstrated using W–Mo base tool. The FS weldements were evaluated by performing hardness, impact toughness and metallographic evaluations. The base metal has a microstructure consisting of martensite and austenite islands. Refined grains of the martensite phase in friction stir zone was observed, and attributed to dynamic recrystalization. A significant improvement in toughness was observed in friction stir welds compared to base metal in both as-welded (27%) and aged (114%) condition. Although more development work is needed, particularly to improve tool materials, the present results indicate that FS welding does indeed have potential for joining maraging steels in aerospace applications.
As traditional 9–12% Cr heat-resistant ferritic steels, T91 steels have been considered as candidate reduced-activation materials for nuclear engineering applying due to its excellent creep resistance and high resistance to void formation during neutron irradiations at elevated temperature. Needle-like M3C precipitates are produced during the routine normalizing process before tempering. Differential scanning calorimetry and infrared radiation thermometer have been employed to study the precipitation behavior of the secondary M3C particles upon subsequent cooling process after austenization. Various austenization conditions (holding time, temperature and the subsequent cooling rate) were carried out to clarify effect of normalizing condition on the formation of the M3C phase. In spite of various austenization conditions applied, it is found that the precipitation of M3C phase is depends only on the cooling rate applied. Furthermore, the precipitation of M3C phase occurs before the onset of the martensite transformation, which is contrary to the previous statement that it takes place during the auto-tempering stage after martensitic transformation. The above observation points out that the precipitation of M3C would produce an effect on the subsequent martensitic transformation behavior, leading to the formation of wide martensite laths with a low dislocation density.
For the development of Reduced Activation Ferritic–Martensitic Steel (RAFMS), for the Indian Test Blanket Module for ITER, a 3-phase programme has been adopted. The first phase consists of melting and detailed characterization of a laboratory scale heat conforming to Eurofer 97 composition, to demonstrate the capability of the Indian industry for producing fusion grade steel. In the second phase which is currently in progress, the chemical composition will be optimized with respect to tungsten and tantalum for better combination of mechanical properties. Characterization of the optimized commercial scale India-specific RAFM steel will be carried out in the third phase. The first phase of the programme has been successfully completed and the tensile, impact and creep properties are comparable with Eurofer 97. Laser and electron beam welding parameters have been optimized and welding consumables were developed for Narrow Gap – Gas Tungsten Arc welding and for laser-hybrid welding.
Fusion welded joints of Cr-Mo steels fail prematurely under creep condition at the heat affected zone (HAZ) close to the base metal, termed as type IV cracking. Optical metallography and hardness testing across the joint establish that the type IV cracking occurs in the soft intercritical HAZ. Based on detailed microstructural studies carried out to understand the evolution of the microstructure and its role in determining the tendency for type IV cracking, the factors that lead to deterioration of creep strength in intercritical HAZ in weld joint of Cr-Mo steels are:
a)
fine grained structure
b)
coarse M23C6 carbides at grain and sub-grain boundaries
c)
dissolution of M2X and MX types of intragranular precipitates.
In the case of low Cr steels, the dissolution of intragranular Mo2C is an important factor among others in determining the tendency to type IV cracking in the weld joint. On the other hand, in higher Cr alloys, M23C6, which plays a dominant role in determining substructure strengthening by stabilizing the substructures, is found to be the main cause of type IV cracking in the weld joint. The dissolution of finer M23C6 and the accompanying coarsening of the large particles leads to the modification of lath-like substructure having high dislocation density into fine polygonal ferrite having low dislocation density, which in turn reduces the creep strength profoundly. The preferential Z-phase formation accompanied with the dissolution of intragranular (Nb,V)(C,N) in the intercritical HAZ is also considered as a factor for the type IV cracking on longer creep exposure. The paper would highlight and discuss in detail some of our results on these lines.
Reduced activation ferritic/martensitic steels (RAFMs) are recognized as the primary candidate structural materials for fusion blanket systems. The RAFM F82H was developed in Japan with emphasis on high-temperature properties and weldability. Extensive irradiation studies have conducted on F82H, and it has the most extensive available database of irradiated and unirradiated properties of all RAFMs. The objective of this paper is to review the R&D status of F82H and to identify the key technical issues for the fabrication of an ITER test blanket module (TBM) suggested from the recent research achievements in Japan. This work clarified that the primary issues with F82H involve welding techniques and the mechanical properties of weld joints. This is the result of the distinctive nature of the joint caused by the phase transformation that occurs in the weld joint during cooling, and its impact on the design of a TBM will be discussed.
EUROFER weldability is investigated in support of the European TBM manufacturing. Electron beam, hybrid, laser and NGTIG processes have been carried out on the EUROFER-97 steel (thickness up to 40mm), a reduced activation ferritic–martensitic steel. It is shown that the most promising processes are laser, electron beam and hybrid welding, depending on the section size and accessibility. They produce similar welding results with high joint coefficients and are well adapted for minimizing residual distortions. The FZ are typically composed of martensite laths, with small grain sizes. In the HAZ, martensite grains contain carbide precipitates. High hardness values are measured in all these zones that if not tempered would degrade toughness and creep resistance. A one step PWHT (750°C/3h) is successfully applied to joints restoring good material performance. Distortion levels, with and without PWHT, are controlled through adaptation of manufacturing steps and clamping devices, obtaining levels not exceeding 120μm (+/−60μm) on a full “one cell mock-up”.
The Cr-Mo ferritic (martensitic) steels are candidates for the structural components of fusion reactors. Irradiation of such steels in a fusion environment produces long-lived radioactive isotopes, which lead to difficult radioactive waste disposal problems once the structure is removed from service. Such problems could be reduced by using steels that contain only elements that produce radioactive isotopes that decay to low levels in a reasonable time (tens of years instead of hundreds or thousands of years). The US Department of Energy has a program to develop steels to meet the criteria for shallow land burial as opposed to deep geologic storage. A review of the alloy development programs indicates that ferritic steels that meet these criteria can be developed.
The on-heating phase transformation temperatures up to the melting regime and the specific heat capacity
of a reduced activation ferritic–martensitic steel (RAFM) with a nominal composition (wt%): 9Cr–
0.09C–0.56Mn–0.23V–1W–0.063Ta–0.02N, have been measured using high temperature differential
scanning calorimetry. The a-ferrite + carbides ?c-austenite transformation start and finish temperatures,
namely Ac1, and Ac3, are found to be 1104 and 1144 K, respectively for a typical normalized and
tempered microstructure. It is also observed that the martensite start (MS) and finish (Mf) temperatures
are sensitive to the austenitising conditions. Typical MS and Mf values for the 1273 K normalized and
1033 K tempered samples are of the order 714 and 614 K, respectively. The heat capacity CP of the RAFM
steel has been measured in the temperature range 473–1273 K, for different normalized and tempered
samples. In essence, it is found that the CP of the fully martensitic microstructure is found to be lower
than that of its tempered counterpart, and this difference begins to increase in an appreciable manner
from about 800 K. The heat capacity of the normalized microstructure is found to vary from 480 to
500 J kg�1 K�1 at 500 K, where as that of the tempered steel is found to be higher by about, 150 J kg�1 K�1.
The status of development and commercialization of a modified 9 Cr-1 Mo alloy is presented. The alloy is modified by the addition of 0.06 to 0.10% Nb and 0.18 to 0.25% V. The alloy is recommended for use in the normalized and tempered condition (1040/sup 0/C for 1 h, air cooled to room temperature; 760/sup 0/C for 1 h, air cooled to room temperature). Heat treatment, Charpy impact, tensile, and creep properties of the alloy are described in detail along with a brief description of other properties. The modified alloy has creep strength that exceeds that of standard 9 Cr-1 Mo and 2 1/4 Cr-1 Mo steels for the temperature range from 427 to 704/sup 0/C. The total-elongation and reduction-of-area values for all test temperatures and rupture times up to 22,500 h exceed 15 and 70%, respectively. The estimated design allowable stresses for this alloy are higher than those for standard 9 Cr-1 Mo and 2 1/4 Cr-1 Mo steel. At 550/sup 0/C and above, these values are twice those of the other alloys. Operating experience on this alloy is being obtained by installing tubes in various steam power plants. 13 figures, 5 tables.
The comprehensive body of knowledge that has built up with respect to the friction stir welding (FSW) of aluminium alloys since the technique was invented in 1991 is reviewed. The basic principles of FSW are described, including thermal history and metal flow, before discussing how process parameters affect the weld microstructure and the likelihood of entraining defects. After introducing the characteristic macroscopic features, the microstructural development and related distribution of hardness are reviewed in some detail for the two classes of wrought aluminium alloy (non‐heat‐treatable and heat‐treatable). Finally, the range of mechanical properties that can be achieved is discussed, including consideration of residual stress, fracture, fatigue and corrosion. It is demonstrated that FSW of aluminium is becoming an increasingly mature technology with numerous commercial applications. In spite of this, much remains to be learned about the process and opportunities for further research and development are identified.
The status and key issues of reduced activation ferritic/martensitic (RAFM) steels R&D are reviewed as the primary candidate structural material for fusion energy demonstration reactor blankets. This includes manufacturing technology, the as-fabricated and irradiates material database and joining technologies. The review indicated that the manufacturing technology, joining technology and database accumulation including irradiation data are ready for initial design activity, and also identifies various issues that remain to be solved for engineering design activity and qualification of the material for international fusion material irradiation facility (IFMIF) irradiation experiments that will validate the data base.
Friction stir welding (FSW) is a relatively new solid-state joining process. This joining technique is energy efficient, environment friendly, and versatile. In particular, it can be used to join high-strength aerospace aluminum alloys and other metallic alloys that are hard to weld by conventional fusion welding. FSW is considered to be the most significant development in metal joining in a decade. Recently, friction stir processing (FSP) was developed for microstructural modification of metallic materials. In this review article, the current state of understanding and development of the FSW and FSP are addressed. Particular emphasis has been given to: (a) mechanisms responsible for the formation of welds and microstructural refinement, and (b) effects of FSW/FSP parameters on resultant microstructure and final mechanical properties. While the bulk of the information is related to aluminum alloys, important results are now available for other metals and alloys. At this stage, the technology diffusion has significantly outpaced the fundamental understanding of microstructural evolution and microstructure–property relationships. # 2005 Elsevier B.V. All rights reserved.
Relative type-IV cracking susceptibility in 2.25Cr-1Mo, 9Cr-1Mo, and 9Cr-1MoVNb ferritic steel weld joint has been assessed.
The type-IV cracking was manifested as preferential accumulation of creep deformation and cavitation in the relatively soft
intercritical region of heat affected zone of the weld joint. The type-IV cracking susceptibility has been defined as the
reduction in creep-rupture strength of weld joint compared to its base metal. The 2.25Cr-1Mo steel exhibited more susceptibility
to type-IV cracking at relatively lower temperatures; whereas, at higher temperatures, 9Cr-1MoVNb steel was more susceptible.
The relative susceptibility to type-IV cracking in the weld joint of the Cr-Mo steels has been rationalized on the basis of
creep-strengthening mechanisms operating in the steels and their venerability to change on intercritical heating during weld
thermal cycle, subsequent postweld heat treatment, and creep exposure.
Chinese low activation martensitic steel (CLAM) has been designed with improved composition, and its performance, such as tensile properties, ductile–brittle transition temperature (DBTT41J), creep and thermal physical properties, has been determined. The interaction experiments between CLAM and plasma were carried out in the HT-7 tokamak facility and the activities, afterheat and gamma dose rate for CLAM as a function of cooling time (CT) were calculated to obtain the required control levels of impurities in CLAM. The insulator coatings on CLAM steel prepared by the CVD process at 700 °C and 740 °C have pure Al2O3 and Al2O3 with an oxygen deficiency layer on surface about 1 μm thick. The electrical resistivity of the coating reaches about 104 Ωm2 on the surface.
A series of R&D activities on the structural material China Low Activation Martensitic steel (CLAM) and related blanket technology are being carried out in Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). A summary of these activities is presented, mainly covering the composition design, property tests, techniques for HIP joining and coating, and activation analysis. In addition, a nuclear material database FUMDS is introduced, which is under development based on the requirement for CLAM data management.
This paper presents the results on the physical metallurgy studies in 9Cr Oxide Dispersion Strengthened (ODS) and Reduced Activation Ferritic/Martensitic (RAFM) steels. Yttria strengthened ODS alloy was synthesized through several stages, like mechanical milling of alloy powders and yttria, canning and consolidation by hot extrusion. During characterization of the ODS alloy, it was observed that yttria particles possessed an affinity for Ti, a small amount of which was also helpful in refining the dispersoid particles containing mixed Y and Ti oxides. The particle size and their distribution in the ferrite matrix, were studied using Analytical and High Resolution Electron Microscopy at various stages. The results showed a distribution of Y 2 O 3 particles predominantly in the size range of 5-20 nm. A Reduced Activation Ferritic/Martensitic steel has also been developed with the replacement of Mo and Nb by W and Ta with strict control on the tramp and trace elements (Mo, Nb, B, Cu, Ni, Al, Co, Ti). The transformation temperatures (A c1 , A c3 and M s ) for this steel have been determined and the transformation behavior of the high temperature austenite phase has been studied. The complete phase domain diagram has been generated which is required for optimization of the processing and fabrication schedules for the steel. © 2010 Elsevier B.V. All rights reserved.
Recent research results obtained in Europe, Japan, China and the USA on reduced activation ferritic/martensitic (RAFM) steels are reviewed. The present status of different RAFM steel products (plate, powder HIPped steel, many types of fusion and diffusion welds, unirradiated and irradiated states) is sufficient to present a strong case for the use of the steels in ITER test blanket modules. For application in DEMO, more research is needed, including the use of the International Fusion Materials Irradiation Facility (IFMIF) in order to quantify the effects of large amounts of transmutation products, such as helium and hydrogen. (C) 2007 Elsevier B.V. All rights reserved.
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