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Effect of Ni addition on the fracture behaviour of a cast ferritic stainless steel
Abstract
The effect of 3 wt% Ni addition on microstructure and fracture behaviour of a cast ferritic stainless steel with 0.3 wt% C and 29 wt% Cr after various heating times at a temperature of 660 °C has been investigated. The addition of Ni changed the as-cast microstructure from ferrite dendrites surrounded by an interdendritic network of eutectic constituent (ferrite + M7C3 carbides) to a largely ferritic matrix containing small austenitic islands and isolated areas of eutectic constituent (ferrite + M23C6 carbides). The Charpy impact energy was markedly higher when Ni was present. The fracture mode changed from cleavage/interdendritic brittle fracture to partially ductile fracture due to the presence of austenite. The fracture mechanism was found to be governed by three competing phenomena: the decrease of eutectic constituent amount, the precipitation of M23C6 carbides at the α/γ interface and the increase of austenite amount.
... The element of Ni has been demonstrated to influence the stacking fault energy of austenite [22,23], and stacking faults can have an impact on dislocation behavior which makes dislocation slip easier at low temperatures, thereby preventing stress concentration at low temperatures and increasing the plasticity of the steel. Adding appropriate Ni has become an important way to improve the low temperature toughness of steel in engineering [24][25][26]. Norstrom and Vingsbo [27] found that the DBTT would decrease by 20 • C for every 1% increase of Ni. It not only reduces the temperature sensitivity of flow stress, but also increases the fracture stress [26,[28][29][30][31]. JFE and Nippon Steel and other companies added Ni content to 3.35% or even higher when developing 800 MPa grade heavy gauge steel plates more than 20 years ago, and they were able to develop 210 mm heavy industrial steel plates with tensile strength of 780 MPa grade [32,33]. ...
... Norstrom and Vingsbo [27] found that the DBTT would decrease by 20 • C for every 1% increase of Ni. It not only reduces the temperature sensitivity of flow stress, but also increases the fracture stress [26,[28][29][30][31]. JFE and Nippon Steel and other companies added Ni content to 3.35% or even higher when developing 800 MPa grade heavy gauge steel plates more than 20 years ago, and they were able to develop 210 mm heavy industrial steel plates with tensile strength of 780 MPa grade [32,33]. ...
The significant effect of Ni content (0.92, 1.94 and 2.94 wt.%) on ductile-brittle transition temperature (DBTT) and microstructure in a 1000 MPa grade high-strength low-alloy (HSLA) steel was studied. Using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), Charpy impact test and low-temperature tensile test to study the fundamental reasons for the effect of Ni content on toughness. The results indicated that increasing the Ni content can reduce the DBTT of HSLA steel and improve the impact toughness at low temperatures. EBSD data post-processing analysis revealed that the key reason for the increase in low-temperature toughness is the refinement of the microstructural crystallographic structure, specifically the significant increase in the boundary density of the block and packet. With the increase of Ni content, the density of grain boundary with an orientation difference greater than 5° between two adjacent {110} crystal planes was higher, which can form a higher density of dislocation pile-up group,thus better reducing local stress concentration. Meanwhile, the stacking fault energy (SFE) increases with the increase of Ni content, which made the screw dislocation more prone to cross slip at low temperature,resulting in an increase in plasticity at low temperatures. These observed phenomena and reasons provided a theoretical explanation for the role of Ni content in reducing DBTT and enhancing the toughness of the core in heavy gauge plates.
... The production of liquid and volatile MoO 3 will be significantly inhibited at temperatures below 600 • C [17], and thus Mo may play a positive impact on the oxidation resistance of the tubing in the ISC environment. In addition, alloying element Ni is also commonly used to improve the electrochemical corrosion resistance and mechanical properties of the tubing in oil fields [18,19]. Although Ni element has little effect on the oxidation resistance in air [19][20][21][22], the NiMoO 4 formation has a beneficial impact on Mo-containing alloys [15]. ...
Fe-9Cr, Fe-9Cr-1Mo and Fe-9Cr-0.5Mo-0.5Ni (wt%) alloys have been prepared for studying the effect of (Mo, Mo/Ni) on carbide precipitation, oxidation behavior and high-temperature tensile property of Fe-9Cr alloy. Rod-shaped Cr23C6 precipitates in Fe-9Cr alloy were refined and spheroidized by Mo addition, while Mo/Ni addition had no impact on Cr23C6 precipitates. Adding (Mo, Mo/Ni) to Fe-9Cr alloy reduced inward oxygen diffusion and the growth rate of an inner Cr-rich layer, thereby accelerating chromia formation and improving the oxidation resistance. High-temperature strength of Fe-9Cr alloy was also enhanced by alloying elements (Mo, Mo/Ni).
... Table 2 shows the energy dispersive spectroscopy (EDS) analysis of the matrix microstructure in zone C. Some kinds of phases exist in this zone. The white carbide network is rich in W, the black carbide network is rich in Cr, the carbide particle in the black matrix between the carbide networks is eutectic carbide, and the matrix consists of pearlite, martensite and retained austenite [10,11] . Fig. 5 presents the micrographs of the matrix microstructure in zone B. Ni concentration of the matrix in zone B is 2.8 wt%. ...
Microstructure of the matrix directly influences the performance and the application of metal matrix composites. By using vacuum casting-infiltration method to manufacture casting tungsten carbide particle reinforced composite, the addition of Ni can alter the microstructure of the matrix of composite. High carbon chromium steel was chosen as the substrate. The casting process was achieved at 1580 degrees C with vacuum degree of 0.072-0.078 MPa. Padding of the molten steel in each part of the preform was different, and the solidification of each part of the composite was different, too. Microstructure of the matrix was various in different parts of the composite. The Ni addition had enlarged the austenite zone in matrix, which would improve the corrosion resistance of the composite. The phase identification of the composite was performed by X-ray diffraction technique. The result showed that Fe3W3C was the primary precipitated carbide and its composition had a direct link with the decomposition of the casting tungsten carbide particles. The hardness of the matrix mainly depended on the reinforced carbide, i.e. Fe3W3C.
Through the idea of replacing Ni with Mn, a shortened process of ultra-fast cooling and two-step intercritical annealing for medium-Mn and low-Ni steel after rolling was designed. Microstructures were characterized through optical microscopy, electron probe micro analyzer, scanning electron microscopy, electron backscattered diffraction, and X-ray diffractometer. Furthermore, the effects of different intercritical annealing temperatures on austenite including content, stability, and the impact toughness of experimental steel were investigated. The microstructures of the experimental steel comprised retained austenite (RA), fine-grained ferrite, and part of martensite. The results show that fine grain structure and austenite with high stability enriched by Mn and Ni provided excellent mechanical properties for experimental steel with yield strength of 645 MPa, tensile strength of 745 MPa, total elongation of 32%, and high impact energy of 133 J at -150°C. As the intercritical annealing temperature increased, the microstructures of the matrix coarsened, and the volume fraction and size of RA increased, while the stability decreased significantly. RA with high stability provided long-lasting work hardening ability through continuous deformation-induced transformation (TRIP effect) during the deformation of steel, which played an important role in toughening the multiphase system.
The aim of the present study was to identify the effect of solute Ni element on the plastic deformation and its thermal activation process of 18Cr ferritic stainless steels. Single-crystal micropillars with different diameters of approximately 2 and 5 μm were fabricated on the surface of Fe–18%Cr–Ni alloy sheets containing different contents of Ni (0–2 mass%). Compression tests were performed on the micropillars at different initial strain rates to investigate the strain rate sensitivity (m) of stress for the slip initiation and the specimen-size dependence of the m value. In micropillars with a small diameter of 2 μm, the Fe–18%Cr binary alloy specimen exhibited a relatively higher strain-rate sensitivity of initial slip stress (m = 0.12), whereas m was reduced to 0.01 by adding 2% Ni. The same trend was observed in micropillars with a larger size of approximately 5 μm. The experimental results were utilized to evaluate the activation volume of plastic deformation. The activation volumes of the Fe–18%Cr alloy and alloy containing 1% Ni followed the specimen-size dependence trend of activation volume in bcc metals; however, the alloy containing 2% Ni exhibited larger activation volumes, suggesting a different thermal activation process presumably due to the significant interaction that occurred between dislocations and solute Ni atoms in bcc solid-solutions.
The effects of Ni content and heat-treatment process on the toughness of a super ferritic stainless steel with 26 wt% Cr and 3.5 wt% Mo were investigated. It was found that with the increase of Ni content, the Charpy impact toughness improved remarkably, and transformed from cleavage brittle fracture to the most ductile fracture. There were no obvious differences between the high- and low-Ni contents on the microstructure and mechanical properties since the addition of Ni did not influence crystal structure, phase composition, and precipitation of ultra pure ferritic stainless steels. Meanwhile, the heat-treatment process was a key point to maintain a high level of toughness by optimizing structure and removing detrimental precipitation, i.e., chi phase.
In this work, the high temperature oxidation mechanism of a high chromium (28 wt-%) cast stainless steel containing 3%Ni, and with an austenitic-ferritic microstructure, has been studied and compared to that of a ferritic stainless steel with similar chromium and carbon contents but without any nickel additions. Two different oxidation temperatures (800 and 970degreesC) were selected for both the as cast and annealed (600degreesC for 2200 h) conditions. The oxidation mechanism of the Ni-containing duplex alloy has been found to involve the formation of four successive layers: (i) the external scale exhibiting the crystalline structure of Cr2O3 (ii) a single phase metal layer in contact with the oxide and consisting of austenite (iii) a decarburised zone exhibiting a two phase structure of ferrite and austenite (iv) the initial alloy microstructure consisting of ferrite, austenite and alloy carbides. The oxidation resistance of the austenitic-ferritic alloy is in all cases 50-100% higher than that of the ferritic alloy, while that of the annealed samples (all other experimental conditions remaining constant) is ten times higher than that of the as cast samples.
In the present study, mechanisms of sticking that occurs during hot rolling of modified STS430J1L ferritic stainless steels
were investigated by using a pilot-plant-scale rolling machine, and the effects of alloying elements on sticking were analyzed
by the high-temperature oxidation behavior. The hot-rolling test results indicated that the Cr oxide layer formed in a heating
furnace was broken off and infiltrated the steel, thereby forming Cr oxides on the rolled steel surface. Because the surface
region without oxides underwent a reduction in hardness rather than the surface region with oxides, the thickness of the surface
oxide layer favorably affected the resistance to sticking. The addition of Zr, Cu, and Ni to the ferritic stainless steels
worked in favor of the decreased sticking, but the Si addition negatively affected the resistance to sticking. In the Si-rich
steel, Si oxides were continuously formed along the interfacial area between the Cr oxide layer and the base steel, and interrupted
the formation and growth of the Cr oxide layer. Because the Si addition played a role in increasing sticking, the reduction
in Si content was desirable for preventing sticking.
The joining of duplex stainless steel (DSS) to carbon steel (CS) was attempted by shielded metal arc welding, with E2209 and E309 electrodes. The hardness and impact strength of the weld metal produced with E2209 electrodes were found to be better than that obtained with E309. Though the general corrosion resistance of the weld metal produced with E309 was superior in 1 M NaCl solution, they exhibited a higher pitting susceptibility in this test environment. The passivation behaviour of the weld metal with E2209 was observed to be on par with that of the duplex stainless steel base material (DSSBM) in 1 M H2SO4 solution; however, in terms of pitting resistance in 1 M NaCl solution both weld metals were inferior to the DSSBM. Though E309 electrodes are widely employed for producing dissimilar weld joints, based on the observations in the current work, it is concluded that the E2209 electrode is the most suitable consumable for joining DSS to CS.
The mechanical properties of a new ferritic stainless steel consisting essentially of 29 pct Cr, 4 pct Mo, 2 pct Ni (29-4-2)
have been evaluated. The mechanical properties of the alloy are dependent on the thermomechanical processing and the final
heat treatment conditionsi.e., both annealing temperature and cooling rate from the anneal. The alloy has excellent toughness, ductility and strength at
room temperature when fast cooled from elevated temperatures. Slow cooling from elevated temperatures results in a degradation
of impact resistance and an increase in strength. The alloy is subject to two major forms of embrittlement. One form results
from the precipitation of intermetallic compounds in the temperature range 704C (1300F) to 954C (1750F) while the other
results from the classical phenomenon called 475C (885F) embrittlement in the temperature range 399C (750F) to 510C (950F).
Degradation of room temperature impact resistance occurs faster after the high temperature type of embrittlement and failure
is characterized by an intergranular fracture mode. Embrittlement after exposure to 475C (885F) results in a slower degradation
in toughness and results in failure by a transgranular cleavage mode. Impact resistance and tensile ductility are also decreased
by exposure to 593C (1100F); however, to a lesser degree than 475C (885F) or 760C (1400F) exposure. The alloy deforms
by slip or twinning depending on the metallurgical condition of the material. Deformation by twinning rather than slip is
not manifested by a reduction in either toughness or ductility. Exposure to 482C (900F) promotes deformation by twinning
whereas exposure to 760C (1400F) does not.
A study of the role of C and N in the brittle fracture of Fe-26 wt pct Cr has been under-taken on alloys containing combined
C and N levels of 67 and 570 ppm. Mechanical and microstructural characterization has been performed on structures involving
C and N in the states of solute, grain boundary precipitate, and intragranular precipitate (with emphasis on plate-like intragranular
nitrides). Fracture mechanisms have been elucidated through microscopic evaluation of electropolished strips pulled in the
ductile to brittle transition temperature (DBTT) range. DBTT variations larger than 200°C were observed. The alloys are embrittled
by grain boundary carbonitrides and intragranular nitrides. Quenching to suppress precipitation was beneficial to the low
C and N alloy but led to severe embrittlement at the higher C and N level. Roles of carbides, nitrides, and twins in microcrack
formation were revealed. A relationship between precipitation annealing temperature, embrittlement mode, and DBTT was established.
A study has been carried out on the effects of isothermal heat treatment at 475 and 550‡C and of continuous cooling at different
rates from 850°C on the brittleness (as assessed by the ductile-brittle impact transition temperature) of a vacuum melted
Fe-25 Cr alloy. The ductile-brittle transition temperature was found to be the lowest for the water quenched condition and
highest for the furnace cooled condition and for material aged at 475‡C for long times (~500 h). An increase of brittleness
with decreased cooling rate in the continuously cooled samples is attributed to the formation of more continuous and larger
amounts of chromium nitrides and carbonitrides at the grain boundaries. Very little or no body centered cubic chromium-rich
phase (alpha prime), associated with 475°C embrittlement, was observed. On aging at 550°C, the increased brittleness with
time is also accounted for by the formation of grain boundary nitrides and carbonitrides. Although a similar effect takes
place in the alloy heat treated at 475°C, the precipitation of alpha prime after long aging times enhances the brittleness.
The tendency towards a more brittle condition with aging treatment and slower cooling rate is explained in terms of the Cottrell
theory for brittle fracture.
It is shown that the fracture toughness in type 304 austenitic stainless steel can be obtained from convenient measurements of the dimple size at the bottom of the cup portion in the fracture surface of round tension samples, employing SEM techniques and quantitative fractography. KIC measurements were performed for sensitized samples after heating for 1, 12, 24, and 48 h at 973 K. Fracture toughness is shown to decrease monotonically with increasing time of heat treatment. Two models based on axial stress conditions, Krafft's model and a model proposed by Schwalbe and Backfisch, were tested and compared with Barsom and Pellegrino's model, which reflects closely the necessary plane strain conditions for KIC measurements. The crack growth increment was considered equal to the dimple size, which was measured by means of the mean intercept length of three dimensional bodies or mean chord in space, L3.
Type 316 stainless steel is aged at 550, 650, 750 and 900 °C for various times to induce grain boundary carbides of different sizes and densities. The impact energy at − 196 °C is shown to be seriously reduced by the grain boundary carbides and the fracture surfaces become intergranular. Fine dense grain boundary carbides are most detrimental to the low temperature impact energy. It is also shown that a thin layer of grain boundary martensite can cause a significant reduction in the room temperature impact energy of this steel.
This research is a study of the effects of size and distribution of grain boundary carbides on the tensile and impact properties of type 316 stainless steel. After solution treatment, this steel was heat treated at 550, 650, 750 and 900 °C to develop stable grain boundary carbides. Treatments were terminated before 7the formation of intragranular precipitates. It is shown that grain boundary carbides have little effect on the yield and tensile strengths. Except for specimens treated at 900 °C, in which the boundary carbides are large and sparse, the moderate reduction in tensile ductility and the severe reduction in impact energy were found to be caused by densely distributed grain boundary carbides. Fracture mechanisms were studied with a scanning electron microscope and are associated with the size and distribution of the grain boundary carbides.
The effects of deformation temperature and additions of impurity elements on the ductility of solution–treated 25Cr–20Ni steels have been examined at the relatively high strain rate of 0·11 s−1 by means of hot tensile tests. The ductility v. temperature curve can be divided into three regions: region I, 1000–1200 K, where there is a ductility minimum due to M23C6 precipitation on grain boundaries; region II, 1225–1400 K, which exhibits a plateau or a slight trough of ductility with corrugated boundaries as a result of dynamic recrystallization localized near the boundaries; and region III, 1450–1600 K, where recrystallization through the whole specimen leads to ductile fracture with complete necking. It was also confirmed that the grain boundary segregation of Sb and S and the sulphide precipitation on the grain boundaries accelerate intergranular cracking and reduce ductility in the range 1075–1300 K, and that, because sulphide particles melt on boundaries, the zero ductility temperature is markedly lowered by the addition of impurities such as S, P, or Sb.MST/364
Type 308 stainless steel is often used as weld filler metal in light water reactor piping systems. In similar cast CF8 type steels, the ferrite phase has been shown to be responsible for the reported degradation in properties after long term aging at temperatures between 300 and 400°C. Despite the relatively low volume fraction of ferrite in the type 308 materials, long term aging at the maximum operating temperature (343°C) results in a significant loss of impact toughness. Using TEM, G phase precipitation was detected in the ferrite matrix of the aged welds both in the matrix and on dislocations. Composition modulations in the aged ferrite consistent with spinodal decomposition were detected in atom probe composition profiles. A variety of statistical techniques was used to analyse the atom probe composition profiles. The spinodal decomposition of the ferrite into iron rich and chromium enriched regions is the most likely cause of the mechanical property degradation of these materials during aging.MST/1188
The fracture behaviour of three 29 wt-%Cr ferritic steels, two containing zirconium and titanium respectively, has been investigated in the as cast condition and after annealing at 660°C for different times up to 2210 h. The fracture energy and the mode of fracture depend on both the morphology and the nature of the eutectic, which consists of carbides and ferrite. In the as cast condition, fracture is predominantly transgranular cleavage and it can be associated with the discontinuous morphology of the M7C3 carbides present in the eutectic as coarse particles surrounded by the eutectic ferrite. After prolonged heating, the ambient fracture energy decreases and the interdendritic mode of fracture is enhanced. This change in fracture mechanism is associated with transformation of the M7C3 to M23 C6 carbides. The M23 C6 carbides, unlike the coarse M7C3 carbides, form a continuous network within the eutectic mixture and constitute an easy path for crack propagation. The zirconium and titanium additions result in a more massive morphology of the carbides in the eutectic mixture and accelerate the M7C3 to M23C6 transformation during the heat treatments, enhancing the interdendritic mode of fracture both in the as cast and in the annealed condition.MST/1734
Although Type 316 austenitic stainless steel is widely used in steam generating plants and nuclear reactors the knowledge
about aging reactions, nature of precipitates, and precipitation kinetics during high temperature exposure is limited. Time-temperature-precipitation
(TTP) diagrams were determined between 400° and 900°C for up to 3000 hr as a function of carbon content, solution treatment
temperature, and cold work. The nucleation and growth phenomena, morphology, and composition of the various carbide (M23C6, M6C) and intermetallic phases (σ, χ, η were determined. The complex sequence of phase instabilities can be explained on the
basis of the carbon content, effect of molybdenum and chromium on the carbon solubility, thermodynamic stability of the phases,
and the kinetics of the various precipitation reactions.
The 885odgF (475°C) embrittlement of seven heats of chromium steels was investigated: four vacuum-melted heats with C + N
< 0.008 pct and 14 pct Cr, 14 pet Cr-2 pet Mo, 18 pct Cr, or 18 pet Cr-2 pet Mo, and three air-melted heats with C + N > 0.09
pet and 18 pet Cr, 18 pct Cr-2 pet Mo, or 18 pet Cr-2 pet Mo-0.5 pct Ti. The steels were heated at 600° (316°), 700° (371°),
800° (427°), 900° (482°), and 1000°F (538°C) for various times up to 4800 h and the influence of this aging was investigated
by hardness measurements, impact tests, and electron metallography. It was demonstrated that the embrittlement due to 885°F
(475°C) exposure was caused by precipitation of a chromium-rich α’ phase on dislocations. The nucleation rate of α’ was calculated with the aid of Becker’s theory and the results were used to extrapolate experimental data obtained in this
study. After an exposure of about 1000 h at 1000°F (538°C), a decrease in room temperature toughness was observed for all
steels investigated. The decrease in toughness was not caused by immobilization of dislocations by α’, but by precipitation of carbonitrides.
Reference is made to a previous paper in which the microstructural stability of type 316 stainless steel to aging in the range 550 to 710 degree C was reported for a wide range of thermal and mechanical pretreatments. The present communication reports the tensile properties for samples given a similar broad range of pretreatments at both room and elevated temperatures and correlates these results with the microstructural data of the previous paper.
The mechanical properties and microstructures of commercial 11 to 29 pct Cr ferritic steels were examined as functions of
aging times to 1000 h at 371, 482, and 593°C. Of the properties evaluated, changes in impact transition temperatures were
the best measure of embrittlement. Embrittlement at 482°C occurs most rapidly in the 29 pct Cr alloy and somewhat more slowly
in the stabilized 26 pct Cr alloy. The stabilized 18 pct Cr alloy embrittles much more slowly while little, if any, embrittlement
was detected in a stabilizedll pct Cr alloy. Embrittlement at 482°C was characterized by a rapid change in properties followed
by a plateau region and then further property changes. The early property change is attributed to precipitation of interstitial
compounds and the later change to classic 475°C embrittlement. The onset of 475°C embrittlement in the two highest Cr alloys
was accompanied by clustering of Cr atoms along {100} planes indicative of spinodal decomposition. Concurrent with clustering
there was also a change from turbulent slip to a more planar slip along {110} planes. Some embrittlement was observed after
longer exposures at 371°C which was attributed to a combination of 475°C embrittlement and the precipitation of interstitial
compounds. Two of the alloys also embrittled at 593°C, accompanied by optically observable precipitates. The precipitate in
the stabilized 18 pct Cr alloy was identified as Laves (Fe2Ti) phase. One of the precipitates in the 29 pct Cr alloy was identified as sigma phase.
The effects of creep on the mechanical properties of type 316 stainless steel were studied. Tensile and Charpy specimens were
machined from the oversize specimens crept at 750 °C and 103 MPa. The ambient fracture energy was found to deteriorate rapidly
after creep. The ambient yield stress was increased moderately, but the tensile ductility was severely reduced. The effects
of intergranular carbides alone on mechanical properties were studied with specimens thermal aged without load. These carbides
were shown to cause a moderate reduction in fracture energy and tensile ductility but had little effect on yield stress. Extensive
grain boundary separations were observed on the fracture surfaces. SEM studies showed that these grain boundaries were covered
with micro voids initiated by the dense intergranular carbides. Frequently, large dimples on grain boundary joined up and
initiated shear fracture into the grain. In the crept specimens additional microstructural changes in the form of intragranular
carbides and subgrain boundaries were observed. Both are responsible for the increase in yield stress and the further reduction
in tensile ductility and fracture energy. The intragranular carbides also modified the size and density of the dimples on
the fracture surfaces.
- Auger
- Alexander
- Ohmori
- Chung