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Effect of M7C3→M23C6 transformation on fracture behaviour of cast ferritic stainless steels
Abstract
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
... Cui et al. [35] carried out mathematical modeling of the synthesis and destruction of core-shell carbides. Early studies show that during the transformation of M 7 C 3 → M 23 C 6 carbides, their morphology does not change [36,37]. ...
... Inoue et al. [36] showed that each stage of the transformation M 3 C → M 7 C 3 → M 23 C 6 → M 6 C in situ occurs within a narrow time and temperature interval in the region of 500-600 °C from 1 h or more. Vardavoulias et al. [37] prove that the M 7 C 3 → M 23 C 6 transformation of carbides occurs without changing the morphology, which was later confirmed by a number of researchers. Wang et al. [17] showed that the transformation of the morphology of carbides occurs if the holding temperature is increased to 1000 °C and the holding time is increased to 4 h. ...
Direct energy deposition was used to obtain the carbides MxCy in carbon-fibers-reinforced Fe–Ni matrix composites. The direct energy deposition method has been proposed to deposit 316L stainless steel on carbon fibers and Inconel 718 substrate. Carbon fiber is the source of block-like Cr7C3 carbide, which is the basis for growing Cr23C6 carbide with a developed morphology. The carbides of the Cr23C6 type at the microstructure of the carbon-fibers-reinforced stainless steel 316L specimen, obtained by direct energy deposition on Inconel 718 substrate, (Inconel 718/CFs/316L) were investigated. The formation of globular carbides can be achieved by heating at 1000 °C for 4 h, cooling in a furnace for 10 h, and quenching. The polycrystalline structure of the Cr23C6 carbides obtained by direct energy deposition is present. A numerical simulation was used to describe the process of modifying the morphology, assumed strain, thermal strain, thermal history and microstructure of Cr23C6 carbides obtained by DED during heat treatment of Inconel 718/CFs/316L.
... This observation seems contrary to the Cr 7 C 3 -> Cr 23 C 6 transformation, which has been discovered in both austenitic and ferritic stainless steel systems [116]. Early studies indicated that the reaction between Cr and Cr 7 C 3 to form Cr 23 C 6 could be facilitated by a long time annealing (t > 100hours) [51,113]. Nevertheless, the transformation from Cr 23 C 6 to Cr 7 C 3 could happen at a critical point when Cr to C ratio is low enough, as sketched in Fig. 7.1 [11]. ...
... alloy. In this alloy system, the Cr to C ratio is relatively low compared to previously reported systems [51,113,116], the Cr 23 C 6 firstly appeared when the alloy was aged at 823-923 K (550-650 • C). ...
This research explores intrinsic properties of the carbon-rich subsurface zone (“case”) that low-temperature carburization generates in AISI-316 austenitic stainless steel. Foils of this steel were carburized to obtain concentrated interstitially dissolved carbon distributed uniformly throughout their thickness. Compared to the as-received AISI-316 foils, such “full” carburization increases the ultimate tensile strength to 3 times, the yield strength to 4 times, and Young’s modulus to 1.5 times, respectively. On the other hand, the strain to failure decreases to (9 ± 1) 10^(-3). For comparison, foils with larger thickness were carburized as well. Decreasing the ratio of “case” to foil thickness was found to decrease the ultimate tensile strength, yield strength, and Young’s modulus, while increasing the strain to failure. This research also investigates the impact of concentrated interstitial carbon on electrical conductivity, thermal conductivity, and conduction electron density and mobility. Foils with uniform carbon levels exhibit room-temperature electrical and thermal conductivity corresponding to only 0.8 and 0.7 times those measured in the as-received state, respectively. Hall-effect measurements revealed that concentrated interstitial carbon does not significantly reduce conduction electron mobility, but decreases the electron density to 0.7 of what we measured for as-received material. These observations suggest that the interstitial carbon atoms form covalent bonds with the metal atoms. With their unique combination of properties, free-standing uniform concentrated solid solutions of interstitial carbon in austenite can be regarded as a new material. Besides, this research also explores the thermal stability of the “case” at elevated temperature. Between 1300-1400 K, carbide transformation from M_{23}C_6 to M_7C_3 was observed. Finally, this research introduces a potential biomedical application of “case” on the Co–Cr–Mo alloy for surface wear improvement. A low-temperature encapsulation nitro-carburization method was developed. After heat-treating for 7.2 ks at 800 K, the resulting “case” features nitrogen and carbon fractions up to 0.075 and 0.035 respectively. This increases the surface hardness of the alloy to 16 GPa, twice as high as in the as-received state. Moreover, the treatment significantly improves the wear resistance of Co–Cr–Mo alloy in air as evaluated according to ASTM G133-05.
... However, they found that M 23 C 6 carbide formed in the interior of M 7 C 3 carbide. This appearance was also observed by Inoue et al. [20] in the Cr-W steel as well as by Vardavoulias et al. [21] in ferritic stainless steel. These authors stated that the primary M 7 C 3 carbide acted as an effective nucleation site to promote M 23 C 6 carbide formation during heat treatment. ...
... Based on the coherent relationship between M 23 C 6 carbide and the matrix, the matrix can provide the favorable conditions for the formation of M 23 C 6 . Evidentially, it is contrast with the results reported in the steels [20,21]. The main reason may be attributed to the different crystal structures of M 7 C 3 carbide in different alloys. ...
The carbides evolution characteristics and mechanisms in a cobalt-base superalloy at elevated temperatures between 1140 and 1265 °C have been investigated to provide the basis for potential service and heat treatment of the alloy. The in-situ transformation of M7C3 → M23C6 is observed directly, which has not been reported in previous studies of cobalt-base superalloys. M23C6 carbide nucleates at the M7C3/matrix interface due to the coherent relationship between M23C6 carbide and the matrix, and grows towards M7C3 carbide. On the other hand, it is found that primary MC carbide degenerates and releases a large number of Ti and W. Thus, residual MC carbide exhibits increased concentration ratios for Ta and Zr and shows enhanced thermal stability. The results indicate that the degeneration behavior of MC carbide is temperature dependent: MC decomposes into M6C carbide at lower temperature and dissolves partially into the matrix at higher temperature.
... Moreover, the inset clearly shows that some transformation had taken place (indicated by the dashed-green enclosure) as visualized by the contrast difference. This was also observed in other studies [37][38][39] and could be attributed to the destabilization process. The EDS concentration map (Fig. 2c.2) clearly shows the distribution of Ni and Cr within the sample. ...
In the present work, the sub-surface microstructure of a heat treated and worn 26 wt% Cr white cast iron was investigated to gain better insight into the tribological behaviour of the material. The samples were destabilized at 980 °C for 0 (Q_0), 30 (Q_30) and 90 (Q_90) minutes followed by air cooling, and later subjected to dry-sliding linear reciprocating wear tests. The microstructural characterization of the area under the wear track was carried out using a combination of SEM, EDS and EBSD. Additionally, nanoindentation (NI) measurements were used to corroborate the mechanical behaviour with the microstructural observations. EBSD and NI measurements indicated that the matrix area underneath the wear track in Q_0 had undergone significant plastic deformation resulting in a drastic increase in hardness, whereas no such phenomena was observed in the Q_90. This was attributable to the relatively high amount of retained austenite in the former and a predominately martensitic matrix in the latter. Moreover, the large M 7 C 3 eutectic carbides were less cracked in the destabilized samples compared to the as-cast sample owing to the presence of martensite and dispersed secondary carbides, leading to an increased matrix load-bearing capacity. These factors led to the destabilized samples showing a lower wear rate compared to the as-cast sample, and the Q_0 showing the best wear resistance amongst all the samples.
... The formation of M 7 C 3 is reported to involve uphill diffusion of alloying elements, most importantly chromium [1]. The M 7 C 3 carbide has been reported to transform to M 23 C 6 after thermal treatment [3,4]. M 7 C 3 is capable of in-situ transformation into M 23 C 6 [1,3,5]. ...
Crystallographic orientation relationship between M23C6 carbides and austenite in the lightweight high carbon stainless steel Fe−17Cr−9Ni−6Mn−4Al−0.42C (wt.%) was investigated by diffraction analysis in scanning and transmission electron microscopes. Electron backscatter diffraction analysis in SEM indicated the existence of an orientation relationship approximated by the Kurdjumov-Sachs (K-S) orientation relationship between the M23C6 carbides induced by aging and the austenitic matrix. The observation of an orientation relationship deviating from the commonly reported cube-on-cube orientation relationship was explained by the formation of M23C6 carbides via M7C3 carbides as an intermediate phase.
... This mechanism, previously observed by Vardavoulias et al. [30], is especially dangerous in this case as it is observed in regions with depths between 1 and 2 mm and is where most gas porosities were found. These depth values coincide with those of the long cracks observed in Fig. 14 c and the thickness values of the samples that were detached especially in zones beyond a circling diameter of 320 mm (Fig. 20-a) where wear rates are at a maximum. ...
The aim of this study is to identify the failure mechanism behind the damage found on the impeller of a slurry pump operating in a phosphoric acid production plant. The failure analysis was carried out first by identifying the solid particles found in the slurry, then by analysing the damaged impeller itself. The observed damage was non-uniform throughout the different regions of the impeller, especially in the back shroud. This is why a series of samples was extracted from different regions to perform hardness tests, microhardness profiles along its cross-section as well as cross-sectional microstructure examinations. Slurry erosion and cavitation erosion were the main damage sources and were particularly effective due to the uncharacteristically low surface hardness value. This was due to decarburization and the continuous network of carbides that facilitated crack propagation in the grain-carbide interface in near surface areas.
... If we consider the Griffith criteria for the crack propagation, the cementite has an intra-precipitate propagation and the Cr 23 C 6 carbide has an interfacial propagation in agreement with experimental observation. Le carbure M 23 C 6 est aussi souvent présent dans les aciers ferritiques et martensitiques et plus précisement dans les aciers contenant un pourcentage significatif de chrome [119,99,120]. Dans ces aciers, ces carbures sont présents aux joints de grains comme dans les aciers austénitiques [121,122]. Avant d'étudier les propriétés mécaniques, nous avons besoin de représenter les interfaces carbure-métal de notre cellule de simulation. ...
Les aciers ferritiques et austénitiques ainsi que les alliages à base nickel contiennent généralement des carbures. Ces carbures présentent des ruptures interfaciales ou intra-précipité. L'objectif de cette thèse est de prédire les ruptures des interfaces et des carbures à partir de simulations et de modèles atomistiques. Les paramètres de fracture calculés permettront d'alimenter des simulations par éléments finis avec des zones cohésives. Les courbes de décohésion sont obtenues en utilisant le modèle UBER (Universal Bonding Energy Relation) qui utilise des paramètres caractéristiques de l'interface, du carbure ou du métal comme l'épaisseur du volume affectant la rupture, son module d'Young ou l'énergie de Griffith. Pour valider ce modèle, des simulations d'essais de traction menées jusqu'à rupture sont effectuées par DFT (Density Functional Theory). Différents carbures (Fe3C, Cr23C6 et TiC) sont étudiés durant cette thèse en faisant varier les atomes, la structure cristallographique et l'ordre magnétique de la matrice métallique. Nous prédisons qu'en fonction de la nature du carbure, la fracture se localise soit à l'interface (Cr23C6) soit à l'intérieur du carbure (Fe3C). Ces prédictions de rupture sont en accord avec les observations expérimentales. Finalement des calculs par éléments finis sont effectués. Dans le cas du carbure Cr23C6, les calculs par éléments finis cristallins et l'utilisation des paramètres calculés par DFT prédisent une rupture de l’interface. Dans le cas du carbure de titane, la rupture prédite en couplant les deux approches peut être intra-précipité ou interfaciale.
... This process is promoted under cyclic loading leading to a high volume fraction of M 23 C 6 carbides [56]. M 23 C 6 carbides are more voluminous than M 7 C 3 carbides and promote cracking at interdendritic sites [57]. The crack morphology in Fig. 11 suggests that such local fractures indeed occurred and get connected to the growing crack. ...
The performance of Stellite 6 coating deposited on X32CrMoV33 hot work tool steel via Plasma Transfer Arc (PTA) process was investigated under steel thixoforming conditions. The Stellite 6 coating made a very favorable impact on the thermal fatigue performance of the hot work tool steel. The coated hot work tool steel survived much longer, for nearly 5000 cycles before the first thermal fatigue crack was detected on the coating. This marked improvement is attributed to the higher oxidation resistance of Stellite 6 alloy and its ability to retain its mechanical strength at elevated temperatures. The Cr content of the Stellite 6 alloy facilitated the formation of stable Cr-rich oxides which sustained the thermal stresses generated at the surface without spalling and thereby retarded crack initiation. The peak compressive and tensile stresses acting on the coating were estimated to be 500 MPa and 170 MPa, respectively, and eventually led to thermal fatigue cracking. Once the crack initiated, the impact of microstructural features was only minor.
... [68] M 23 C 6 carbides are more voluminous than M 7 C 3 carbides and lead to fracture along the interdendritic sites where they are located. [69] IV. CONCLUSIONS Stellite 12 coating deposited on X32CrMoV33 hot work tool steel by the PTA process has made a favorable impact on the thermal fatigue performance of the X32CrMoV3 hot work tool steel. ...
The thermal fatigue performance of Stellite 12 coating deposited on X32CrMoV33 hot work tool steel via the plasma transfer arc (PTA) process was investigated under steel thixoforming conditions. Stellite 12 coating has made
a favorable impact on the thermal fatigue performance of the X32CrMoV33 hot work tool steel. The latter survived steel thixoforming
conditions lasting much longer, for a total of 5000 cycles, when coated with a PTA Stellite 12 layer. This marked improvement
is attributed to the higher resistance to oxidation and to temper softening of the Stellite 12 alloy. The Cr-rich oxides,
which form during thermal cycling, provide adequate protection to high-temperature oxidation. In contrast to hot work tool
steel, Stellite 12 alloy enjoys hardening upon thermal exposure under steel thixoforming conditions. This increase in the
strength of the coating is produced by the formation of carbides and contributes to the superior thermal fatigue resistance
of the Stellite 12 alloy. When the crack finally initiates, it propagates via the fracture of hard interdendritic carbides. The transformation of M7C3 to M23C6, which is more voluminous than M7C3, promotes crack propagation.
Volume 12 covers all aspects of fractography relevant to its use in materials evaluation, failure analysis, and quality control. In addition to an extensive collection of fractographs, it includes information on the types and appearances of fracture, visual examination and imaging methods, quantitative fractography, surface profiling, and fractal analysis. It describes the morphology of fracture surfaces at different length scales and identifies markers and patterns that shed light on how and why fractures occur. It explains how loading conditions, operating environments, and material composition and structure influence the mode and mechanism of fracture in cast irons and steels, nonferrous alloys, plastics, ceramics, polymer matrix composites, and welded and soldered joints. Through the presentation of fractographs and spectroscopic data, it assists readers in recognizing the characteristic features of different types of fracture, the potential role of material dislocations and impurities, and the contributing effects of corrosion, embrittlement, and fatigue. An overview of the history of fractography and its use in archaeometallurgy is also included. For information on the print version of Volume 12, ISBN: 978-1-62708-385-0, follow this link.
Heat-resistant alloys with excellent mechanical properties are widely used in various fields, and further improvement in their properties is essential to meet the requirements in new-generation advanced supercritical boilers, nuclear reactors, superheaters, and other new materials applications. To effectively enhance the comprehensive performance of heat-resistant alloys, second-phase particle strengthening has been widely studied, and in the face of different service environments of advanced heat-resistant steels, the selection of suitable second-phase particles is essential to maximize the performance of these alloys. To this end, three major types of reinforcing phases in heat-resistant alloys such as carbides, rare earth oxides, and intermetallic compounds are summarized. A comparative analysis of the precipitation behavior of the reinforcing phases with different types as well as the risks and means of controlling their use in service, is presented. Key parameters for the application of various types of second-phase particles in heat-resistant alloys are provided to support the design and preparation of new ultrahigh-performance heat-resistant alloys.
In this paper, we present a case study that illustrates the detrimental effects decarburization has on the resistance of high chromium cast iron to erosion and corrosion. Two samples are analysed: the first was extracted from a heat-treated impeller used for phosphoric acid slurry pumping. This resulted in decarburization and a gradual change in microstructure from the surface to the core of the material where zones closer to the surface not only had a smaller CVF (Carbide Volume Fraction) but formed a continuous carbide network which facilitated cracking propagation. This led to a significant drop in surface hardness. The second sample was extracted from an unused, non-treated pump made using a similar chemical composition to the failed impeller. This sample had a microstructure that is uniform across its regions. Samples were subjected to corrosion testing in a phosphoric acid solution at 80 ℃, and then cleaned and eroded using a sand jet impingement erosion tester, and corrosion-tested again in the same conditions. A comparison is made each time between the mass losses and polarization curves of the samples as well as a microstructural analysis of their cross-sections to evaluate the erosion and corrosion resistance of the high chromium cast irons at their two different states.KeywordsErosionCorrosionDecarburizationChromium carbidesHigh chromium cast ironMicrostructure
To predict the solidification and product properties of tool steels with complex chemical compositions, an understanding of the transformation behaviour during and after solidification, depending on the chemical composition, is crucial. Therefore, the quaternary Fe‐C system with 10 wt.% Cr and 3 wt.% W (subsystem of cold work steels, with M7C3 and M23C6 carbides) and the Fe‐C system with 6 wt.% W and 5 wt.% Mo (simplified high speed steel without vanadium, with M6C and MC carbides) were selected. The motivation for this work was to develop a methodology for the safe and fast production of high‐quality model alloys and the safe, fast and close to equilibrium performance of DSC (differential scanning calorimetry) measurements. As common DSC measurements of as‐cast materials may not allow the equilibrium transformation temperatures of carbides to be detected, a promising time‐temperature DSC setup was developed. Regular diffusion annealing of as‐cast carbidic steels is time‐consuming, but with an additional heat treatment during the DSC measurement in the semi‐solid zone (30‐50 % liquid phase fraction), a status close to equilibrium can be achieved within minutes due to the high diffusion. To prove the potential of the high‐temperature equilibration by partial premelting in the DSC, additional equilibration and quenching experiments were performed at selected temperatures in a Tammann furnace and investigated using SEM‐BSE and XRD analysis. By combining these methods, carbide types and the transformation temperatures can be verified to evaluate and construct complete phase diagrams in the high temperature range. This article is protected by copyright. All rights reserved.
The M7C3/M23C6 in situ transformation in high‐carbon martensitic stainless steel was investigated. Eutectic Cr‐rich M7C3 carbide was generated during solidification; it was metastable and could almost completely transform into other phases when the temperature was <1000 °C. The eutectic M7C3 carbide transformed into an M7C3‐M23C6 core‐shell structure after heat treatment at 1000 °C for 1 h, and then completely transformed into M23C6 carbides after heat treatment at 1000 °C for 7 h. The eutectic M7C3 carbide with the same orientation in the as‐cast state transformed into dozens of M23C6 carbide grains with varying orientations during the in situ transformation, and the transformed M23C6 carbides had no constant orientation relationship with the adjacent matrix and eutectic M7C3 carbide core. The formation of the M23C6 shell was the result of not only the in situ transformation but also the coarsening of M23C6 carbides. The diffusion of carbon atoms in the M23C6 shell was the restrictive factor of the M7C3/M23C6 in situ transformation and maintained the transformation. The diffusion of the chromium atoms in the matrix was the restrictive factor in the coarsening of the M23C6 shell, which continued coarsening and resulted in a significant increase in the final area fraction of carbides. This article is protected by copyright. All rights reserved.
In this study, Fe-based composite coatings were produced on a medium carbon steel surface by the Tungsten inert gas method. The microstructure and phase structure of the coatings were analyzed by an optical microscope with clemex image analysis, scanning electron microscope equipped with an energy-dispersive spectroscopy and X-ray diffractometer. The results show that the coating is metallurgically bonded to the substrate when it is produced with sufficient energy input. In the microstructure of Fe-Cr-C coatings, hexagonal-shaped M 7 C 3 carbides solidified in γ-(Fe,Cr) eutectic matrix, while in Fe-Cr-C/NbC composite coatings, quadrangular-shaped NbC carbides solidified. As the amount of FeNb melted increased, the amount of NbC in the structure increased.
This study investigates the mechanism driving the formation of core-shell carbides during laser metal deposition (LMD). The interaction between CFs and 316L metal powder under laser irradiation results in the formation of new composite materials (316L/CFs) because of two-phase (solid-liquid) melting. 316L/CFs comprise a metal matrix with M7C3 and M23C6 carbides. The metal matrix also contains raw 316L powder particles. The composites thus obtained (316L/CFs) were characterized using X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron backscatter diffraction (EBSD), and microhardness studies. The characterization revealed an in-situ transformation of M7C3 carbides to M23C6, which either occurs completely or ceases during transition stages resulting in the formation of core-shell carbides. The morphological analysis illustrates that primary carbides are of core-shell types with a single-crystal lattice. Moreover, the heat treatment can be used for the coalescence, coagulation, and partial dissolution of eutectic M23C6 carbides. It was observed that the morphology of eutectic and prime carbides are altered during the heat treatment at 1050 °C (4 h) and smooth cooling to 350 °C (11 h). Thus, the transformation of Cr7C3 (skeletal) → Cr23C6 (skeletal) → Cr23C6 (globular) occurs. Furthermore, the characterization of heat-treated samples confirms the core-shell nature of carbides. On the other hand, the core microhardness decreased by 1.5 times compared to that of the shell.
The transformation from M7C3 → M23C6 in a high performance micro alloy (HP-MA) was studied using different techniques. Following the cooling during the centrifugal casting, the as-cast HP-MA alloy consists of an austenitic matrix with the primary carbide network consisting of a combination of M7C3 (M being mainly chromium) and MC (M being mainly niobium). During the heat treatment at 1000 °C, the primary chromium carbides transform from M7C3 to M23C6. The incompletely transformed primary carbide consists of an outer shell of M23C6 carbide (coherent with the austenitic matrix) and a core of M7C3 type carbide. The experimentally determined M23C6 shell thickness agrees reasonably well with the estimated diffusion distance of carbon in M23C6 based on Zener's relation implying that the M7C3 → M23C6 transformation is mainly controlled by the diffusion of the carbon from M7C3 to the matrix through the M23C6 shell. This mechanism is discussed vis-à-vis the literature reported mechanism on the transformation.
Different amounts of niobium carbide (NbC) powder (5, 10 and 15 wt%) are mixed and added to 440C stainless steel powder. The composite powders are sintered at 1260, 1270, 1280 and 1290 °C for 1 h, respectively. The experimental results show that the optimal sintering temperature for the 440C-NbC composites is 1270 °C. Meanwhile, the 440C specimens with 5% NbC addition possess the optimal transverse rupture strength (TRS) value of 1985.2 MPa, as well as the highest polarization resistance of 1.01 × 10² Ω cm². While the 440C specimens contain or with 15 wt% NbC show the highest hardness value of HRA 80.9 after sintering at 1280 °C. Furthermore, the microstructural evaluation reveals that the rod-shaped M7C3 carbides located on the grain boundaries are gradually reduced after NbC particles are added, and the spherical-shaped M7C3 carbides are precipitated and dispersed in the matrix. After heat treatment, the primary M7C3 carbides are converted to M23C6 carbides, which results in a secondary hardening. The results clearly show that heat treatment effectively improves the particle size of the carbides and strengthens the matrix of the 440C-NbC composites.
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.
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.
Improving the performance of the extrusion tools and predicting their life postulates knowledge of their failure mechanisms. In this work, actual failure cases of brass extrusion dies made from stellite 4B are presented and their mechanisms are studied. It is found that exposed parts of the die suffer from cyclic loading which, in combination with manufacturing defects and design imperfections, promote the initiation of fatigue cracks. Aging during service of stellite 4B inserts reduces die life due to the formation of a network of brittle carbides surrounding the ductile cobalt dendrites. During extrusion, the M7C3 carbide dissolves in the cobalt matrix leading to precipitation of new M23C6 and η-carbide types. This occurs at relatively low working temperatures, in the range of 600–650 °C, instead of 850–1100 °C as referred in the resent literature, and is probably assisted by high stresses prevailing during the extrusion process.
Ti-additions together with boron had the most significant influence on the length and thickness of columnar crystals and on the size of the equiaxed zone. Microprobe analysis has shown segregation of titanium, zirconium and sulphur. Microscopical examination of alloys containing titanium has shown the presence of sulphur-titanium eutectic in the interdendritic space. Precipitation conditions are discussed in relation to calculated activities of titanium and sulphur in liquid steel. Electron microscopical investigation has shown existence of boron-containing eutectic in interdritic space and boron-rich phase along grain boundaries. Examination of mechanical properties has pointed out deleterious effect of yttrium on toughness. Yttrium addition increased both ductility and tensile strength of alloys containing up to 1 wt. % Ti.
Centrifugally cast 0. 4C-25Cr-20Ni (wt%) steel (HK 40) has been subjected to detailed microscopic examination both in the as-cast condition and after long-term creep. The as-cast alloy contained a eutectic of M//7C//3 and austenite, which was subjected to energy-dispersive X-ray analysis. Observations by transmission electron microscopy revealed high dislocation densities near cell boundaries. After creep in the range 750-1000 degree C, the eutectic carbides coarsened and transformed from M//7C//3 to M//2//3C//6; moreover, the M//2//3C//6 carbide precipitated predominantly on dislocations as a finer dispersion within the grains. During coarsening, the M//2//3C//6 particles generated further dislocations in the austenite. The higher creep strength of the cast HK 40 compared to wrought alloys is discussed in terms of the detailed microstructural observations, in particular the M//2//3C//6 dispersion.
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.
A study has been made of the effect of systematic variations in σ phase content on the ductility of a wrought type 310 stainless steel. The steel initially contained 1.3% σ-ferrite in the form of platelets lying in the rolling plane. Heat treatment at 700°C caused individual platelets of σ-ferrite to transform to σ phase with little change in size or morphology. The number of fully transformed platelets increased progressively with time at temperature until, after about 1 h, virtually every ferrite platelet had transformed. The variation in mechanical properties with overall degree of transformation has been studied using tensile tests backed up by post-test metallography on deformed specimens. The presence of σ phase results in large specimen-orientation-dependent reductions in ductility. The results are discussed in terms of the orientation dependence of the fracture characteristics of the σ phase platelets and the associated void nucleation and growth processes which control the ductility of the material.
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 C, Zr, Ti, and Nb on the oxidation resistance of 17% Cr steels have been investigated by means of isothermal
heating and cyclic heating at temperatures up to 1000°C. It has been found that C has a detrimental effect on the oxidation
resistance of 17% Cr steels. The oxidation behavior of steels containing Zr is different from that of steels containing Ti
and Nb. Zr improves the oxidation resistance of 17% Cr steels to a great extent. Particularly, alloys containing Zr at a concentration
which exceeds the stoichiometric equivalent of the total amount of C and N show excellent oxidation resistance up to 1000°C.
Electron probe microanalysis shows the presence of a protective oxide layer consisting of Si‐rich oxide at the metal‐oxide
interface, and no Zr is detected in the scales. With steels containing Ti and Nb, on the other hand, there is no protective
Si‐rich oxide, but Ti and Nb are detected in the scales. These scales are not protective. As the effective factors of excellent
oxidation resistance of Cr steels containing Zr, the behavior of carbide, grain growth, and phase transformation have been
investigated.
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 a series of austenitic high chromium cast irons (15%Cr and 2.7% C) containing a constant volume percentage of M7C3 eutectic carbides in distributions that, varies from continuous network to isolated globules was studied.The modification of the eutectic carbide morphology was achieved by the addition of molybdenum followed by high temperature heat treatments. These microstructural changes in the eutectic carbide morphology gave up to 50%improvement in fracture toughness without significant loss in hardness. In these alloys, fracture toughness was found to be controlled by attainment of a critical stress-state ahead of the crack tip despite the dimpled fracture surface between the failed eutectic carbides by cleavage mode. This stress-state is strongly influenced by the eutectic carbide distribution. The data were applied to a model correlating microstructural parameters with the K
IC values.
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 niobium additions to a “straight chromium” alloy were investigated by optical metallography and creep testing
at 1033 K. The results indicate that by increasing the amount of interdendritic constituent, niobium adds to creep strength
and fracture ductility. Optimum alloy design requires balancing niobium, chromium, and carbon content.
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.
La rupture des metaux
- D Francois
- L Mix
- K B Alexander
- M K Miller
- D J Alexander
- R K Nanstad
- H J Goldschmidt
- S B Biner
- H M Chung
- P Auger
- F Danoix
- A Menand
- S Bonnet
- J Bourgoin
- M Guttmann
- Hou Wen-Tai
- Y Ohmori
- Y Maehara