Precipitation Of M23C6 Carbides Research Articles

Microstructure and mechanical-property evolution of the explosive welding joint from the same RAFM steels under explosive welding and post-weld heat treatment

This study investigated the morphological evolution behaviors and the mechanical property of the weld interface during explosive welding for the flyer plate and base plate sourcing from the same reduced activation ferrite/martensitic (RAFM) steel. The relationship between fine-grain formation and the occurrence of dynamic recrystallization was analyzed based on the strain and temperature field distributions at the explosive-weld interface. Although post-weld tempering (PWT) treatment eliminated bent martensite laths, but it did not eliminate fine equiaxed grains and δ ferrite grains near the explosive-weld interface; therefore, the PWT treatment improved the mechanical properties of the explosive-weld RAFM steel joint only minimally. However, implementation of a normalizing process after the explosive welding process (“PWN” treatment) significantly eliminated the gradient structure and the residual δ ferrite grains at the explosive-weld interface of the RAFM steel joint. Based on the PWN state, the inclusion of a tempering treatment (“PWNT” treatment) promoted the uniform precipitation of M23C6 carbides, which significantly improved the tensile strengths of the explosive-weld RAFM steel specimens under both room and high temperatures. To study the influence of different microstructures on the plastic deformation behavior at the explosive-weld interface, the representative volume element (RVE) models basing on crystal plasticity finite element (CPFEM) method was constructed for different post-weld heat treatments (PWHTs). At the explosive-weld interface of the PWT-state RAFM steel joint, the plastic deformation was mainly concentrated in the δ ferrite grains, rather than in the fine equiaxed grains. In addition, only minimal deformation concentration was observed for the PWNT-state RAFM steel joint. In order to reveal the independent influence of the fine equiaxed grains at the weld interface on the mechanical properties of explosive-weld RAFM steel joint, the microstructural and mechanical distinctions between the RAFM steel joints in the N-PWT and PWNT states were investigated detailly. Finally, through optimization of the PWHT method, an explosive-weld RAFM steel (PWNT state) joint with a uniform microstructure and ideal mechanical properties was obtained.

Electric field enhanced diffusion welding of alloy 617: Microstructural characteristics and mechanical properties

This study investigated the microstructural characteristics and mechanical behavior of diffusion welded nickel-based Alloy 617 obtained by electric field-assisted sintering (EFAS) using various parameters. The interfacial microstructure exhibited different characteristics including good grain boundary (GB) migration across the interface in the samples diffusion-welded at 1100 °C and a flat interface in the samples joined at 1000 °C and 1050 °C. The interface consisted of fine Al2O3 oxides, while precipitation of interfacial M23C6 carbides was not observed. Grain boundaries migrated across the Al2O3 oxides, leaving these oxides within the grains. Graded grain size was observed, with grain coarsening being more significant near the sample surface due to the temperature gradient induced by EFAS. Tensile testing revealed that the specimens fractured in the matrix away from the interface, indicting strong diffusion-welded joints. The peak tensile strength of 807 MPa was obtained in the samples welded at 1000 °C due to minimal grain growth. The materials obtained at 1100 °C exhibited reduced tensile strength but improved ductility. Strain maps revealed by digital image correlation showed alternating high and low strain segments in the samples produced at 1000 °C and 1050 °C, indicating that the flat interfaces with no GB migration were less ductile compared to the matrix. A greater strain uniformity was observed along the bond interfaces with improved GB migration. The hardness reduced near the sample surfaces due to enlarged grains induced by temperature gradient. This study demonstrates that GB migration and enhanced mechanical strength can be achieved in diffusion-welded Alloy 617.

Microstructure characterization and mechanical properties of austenitic Super 304H steel after operation

The paper presents the results of testing the microstructure and mechanical properties of austenitic Super 304H steel after approximately 31,000 h of operation at a temperature of 570 °C. The microstructure analysis showed that the utilization of the tested steel contributed to the precipitation of numerous M23C6 carbides and individual sigma phase particles at grain boundaries, while dispersive ε_Cu and MX particles was observed inside the grains. At the grain boundaries, the precipitates sometimes formed a continuous mesh. The presence of numerous secondary phases resulted in higher than standard strength properties while maintaining the required plastic properties.

Phase evolution in Ni-based superalloy K439B during thermal process

Based on the difference in element distribution and diffusion in various regions, the phase transformation of the as-cast, heat-treated and long-term aged K439B superalloys was investigated. Ti, Nb, Ta and Al elements segregated to the interdendritic regions, whereas W, Co and Cr elements segregated to the dendrite core regions in the as-cast alloy. During thermal process, the γ/γ' eutectic and the irregular γ' phase formed in the as-cast alloy completely dissolved into γ matrix. Subsequently, the reprecipitated spherical γ' phase gradually grew up and tended to be cubic. γ' phase was surrounded by the dislocations and a few stacking faults, which reduced the elastic strain energy in the long-term aged alloy. In addition, the degeneration of MC carbide was caused by a discrepancy in element diffusion rate and driving force in the interdendritic and grain boundary regions. The degeneration of MC carbide promoted the precipitation of γ' phase, M23C6 carbide and η phase, M23C6 carbide precipitated from γ matrix and γ' phase transformed into η phase. The preferential precipitation of M23C6 carbide was related to the difference in crystal structure, precipitation temperature range and Gibbs free energy.

Effect of heat treatment on the anisotropic mechanical properties and corrosion resistance of laser powder bed fusion fabricated Inconel 625

The microstructure evolution of laser powder bed fusion processed Inconel 625 after heat treatment at 650 ℃ (aging), 850 ℃ (sensitization) and 1050 ℃ (homogenization) was studied. Relationship of microstructure evolution and the anisotropic mechanical properties and corrosion resistance was analyzed. Decreased corrosion resistance after aging and sensitization was observed, which is related to the different precipitation behavior. After aging, γ″ and MC carbides precipitated along cellular structures further accelerate the corrosion along melting pool boundaries. While after sensitization, δ precipitated along cellular structures aggravated the corrosion along the melting pool boundaries. At the meantime, precipitation of M23C6 carbides and δ along grain boundaries made it vulnerable to be corroded, which further accelerate the corrosion rate. After homogenization, segregation and precipitates are barely observed in the microstructure which contribute to an enhancement of corrosion resistance. Moreover, with the increasing of heat treatment temperature, recovery of anisotropic microstructure contributes to the decrease of the anisotropy of mechanical properties. This study may contribute to the profound comprehension of heat treatment procedure on microstructure evolution as well as mechanical properties and corrosion resistance of laser powder bed fusion processed Inconel 625.

New insights into the stress rupture life enhancement mechanisms of a nickel-based superalloy

In this paper, the effects of hot isostatic pressing (HIP) and heat treatment (HT) on the stress rupture properties of nickel-based superalloy K417G were investigated. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were employed to analyze the microstructure, fracture morphology, determination of crack source and dislocation evolution after stress rupture fracture. The results indicate that, HIP specimens demonstrate a 19.5% reduction in stress rupture life compared to as-cast specimens, whereas HIP + HT specimens exhibit a 41% increase in stress rupture life, reaching a value of 63.04 h. The fracture mechanism of the samples under different heat treatment conditions is different. There are a large number of pores in the as-cast specimen, which leads to the initiation and propagation of cracks and eventually result in fracture. The pores in the HIP specimen are almost eliminated. However, the coarsening and uneven distribution of γ′ phase led to the weakening of the hindrance to dislocation movement. Additionally, the increase in elongated carbides at grain boundaries accelerates crack initiation and propagation. Under HIP and HT, the cubic γ′ phase is uniformly precipitated, which effectively hinders the movement of dislocation lines. Granular M23C6 carbides precipitate at grain boundaries, acting as pinning points to retard grain boundary sliding and crack propagation, thus improving the stress rupture life.

Enhancing the strength-ductility synergy in hot-forged Fe50Mn30Co10Cr10 high entropy alloy (HEA) through carbon additions

In this work, we systematically investigated the effect of carbon additions on the microstructure and room temperature mechanical properties of hot-forged Fe50Mn30Co10Cr10 HEA. The different carbon contents were introduced to Fe50-xMn30Co10Cr10Cx (x = 1, 2.5, 6 at.%, hereinafter, referred as C1, C2.5 and C6, respectively) HEAs to enhance the strength-ductility synergy by triggering the multiple strengthening mechanisms. The results showed that the grain sizes were decreased with increasing of carbon content, while a high fraction of mechanical twins ∼45% were generated in C2.5 HEA, and their fractions were ∼35% in C1 and C6 HEAs. Moreover, the discrete nano-sized M23C6 carbides in C2.5 HEA were transformed into continuous mico-sized M23C6 carbides along the grain boundaries in C6 HEA. Therefore, C2.5 HEA possessed high yield strength of 584 MPa, tensile strength of 1.03 GPa and outstanding ductility of 68%, i-e., breaking the strength-ductility paradox. A highest yield strength of∼662 MPa and tensile strength of more than 1 GPa were observed for C6HEA with refine grains, although the ductility was only ∼10%, which indicated that the massive carbon addition could effectively enhanced the strength of HEA but at the cost of ductility due to the presence of coarse M23C6 carbide precipitation. The enhanced mechanical properties of studied HEAs are mainly ascribed to the contribution of multiple strengthening mechanisms including precipitation, grain boundary, dislocation and interstitial solid solution strengthening.

Effects of Chemical Composition and Solidification Rate on the Solidification Behavior of High-Cr White Irons

The effects of chemical composition and solidification rate on the solidification behavior of high-Cr white irons were investigated through directional solidification. Increasing the solidification rate in hypoeutectic alloys caused finer dendrite-arm spacing, as expected. The eutectic structure, which formed in the interdendritic region, was comprised of M7C3 and austenite; however, secondary dendrite arms of hypoeutectic alloys contained a few M7C3 particles that solidified prior to the eutectic structure. The transition from cellular to dendritic solidification occurred at a solidification rate between 50 µm/s and 100 µm/s in a near-eutectic alloy. In the near-eutectic alloy with cellular solidification, a directionally arrayed in-situ composite of M7C3/austenite formed within the cell. Speckle-like features appeared in the intercellular region due to M23C6 carbide precipitation during subsequent cooling after freezing. Like dendrite-arm spacing in hypoeutectic alloys, the inter-speckle spacing and the inter-fiber spacing became finer with an increasing solidification rate in the cellular solidification range.

Embrittlement Mechanisms of HR3C Pipe Steel at Room Temperature in Ultra-Supercritical Unit

HR3C steel is an austenitic high-temperature-resistant steel. Because of its good strength and high-temperature performance, it has been widely used in ultra-supercritical power plant boilers. With the increasingly frequent start-up and shutdown of thermal power units, leakages of HR3C steel pipes have occasionally occurred due to the embrittlement of HR3C pipe steel after a long service duration. In this study, the embrittlement mechanisms of HR3C pipe steel are investigated systematically. The mechanical properties of the pipe steel after running for 70,000 h in an ultra-supercritical unit were determined. As a comparison, the pipe steel supplied in the same batch was aged at 700 degrees Celsius for 500 h. The mechanical properties and the micro-precipitation of the aged counterparts were also determined for comparison. The results show that the embrittlement of HR3C pipe steel in service for 70,000 h is obvious. The average impact absorption is only 5.5 J, which is a decrease of 96.7%. It is found that embrittlement of HR3C steel also occurs after 500 h of aging at 700 °C, and the average value of impact absorption energy decreases by 70.4%. The comparison experiment between the in-service pipe steel and the aged pipe steel shows that in the rapid decline stage of the impact toughness of HR3C steel, the M23C6 carbide in the microstructure has a continuous chain distribution in the grain boundary. There were no other precipitated phases observed. The rapid precipitation and aggregation of M23C6 carbides leads to the initial embrittlement of HR3C steel at room temperature. The CRFe-type σ phase was found in the transmission electron microscope (TEM) image of the steel pipe after 70 thousand hours of use. The precipitation of the σ phase further induces the embrittlement of HR3C pipe steel after a long service duration.

Development and Application of On‐Line Solution Annealing Process for 304 Austenitic Stainless Steel

After hot rolling, 304 austenitic stainless steel requires a solution annealing treatment to prevent intergranular corrosion and eliminate work hardening effects. Compared to traditional offline processes, on‐line solution annealing offers advantages in terms of cost and time savings. However, both recrystallization behavior and M23C6 carbide precipitation behavior are significantly influenced by the cooling process after rolling, which poses conflicting requirements. This study investigates the precipitation behavior of M23C6 carbides and the recrystallization softening behavior during the continuous cooling process of hot‐rolled samples. The kinetics equations are derived using the Scheil's additivity rule. The temperature profiles in different regions of the plate are studied using finite element analysis. A practical approach for online solution annealing is proposed and applied in industrial testing.

Influence of Boron on Stress-Relief Cracking Susceptibility of T23 Steel

The effect of boron on stress-relief cracking (SRC) sensitivity in the coarse grain heat-affected zone (CGHAZ) of ASME SA213-T23 was studied by thermo-mechanical simulation. Then, the fracture mode, microstructure, carbide evolution, and boron segregation were observed by an optical microscope, a scanning electron microscope, a transmission electron microscope, and an electron probe microanalyzer. Finally, the mechanism of increasing boron content to improve SRC resistance was described. The results showed that when the boron content is lower than 0.0038 wt.-%, T23 steel is sensitive to SRC at 550–750°C (1022–1382°F), and the sensitive temperature range narrows as the boron content increases. When the boron content increases to 0.010 wt.-%, SRC can be eliminated. Moreover, boron addition did not improve grain boundary (GB) strength nor did it fundamentally change the fracture mode at high temperatures, but it significantly improved intergranular ductility. This is because the boron segregation at the GB inhibits the precipitation of M23C6 carbides, which reduces the void nucleation and alloy element depletion near the GB, thus significantly improving intergranular plasticity. The improvement of intergranular plasticity gives the grain sufficient time to deform and greatly improves the overall plasticity of the CGHAZ. As a result, the SRC resistance of the welded joint is significantly improved because the stress can be released through enough plastic deformation during postweld heat treatment or service.

Effect of aging temperature on corrosion resistance of 316L stainless steel in simulated proton exchange membrane fuel cell cathode environment

The effects of aging temperature on the corrosion behavior of 316 L stainless steel in a simulated cathodic environment of a proton exchange membrane fuel cell (PEMFC) were investigated by potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), double-loop electrochemical potential response (DL-EPR). The results show that the corrosion resistance of 316 L is significantly degraded after aging at 750–900 °C for 1 h in the simulated cathodic environment of PEMFC. A strong sensitivity to intergranular corrosion takes place after aging at 850 °C, and chromium rich M23C6 carbides precipitate on grain boundaries. The precipitation of M23C6 leads to discontinuity of the passivation film near the grain boundaries, resulting in Cr-depleted zones at the grain boundaries, increasing intergranular corrosion, and decreasing corrosion resistance. When the aging temperature exceeds 950 °C, grain boundary precipitation of M23C6 does not occur and the corrosion resistance is improved. At an aging temperature of 1050 °C, a dense Cr2O3 film is formed on the surface of the alloy during the polarization test, giving it the best corrosion resistance in the simulated cathodic environment of PEMFC.

Precipitation and Transformation of Carbides during Tempering of 7Cr14 Martensitic Stainless Steel

The precipitation and transformation of carbides in steel are gaining more interest since they are closely related to the properties of steel. Herein, the precipitation and transformation of carbides during tempering of the 7Cr14 martensitic stainless steel subjected to high‐temperature solution treatment (HTST) are investigated. The results indicate that the precipitation of M3C (300–333 °C) and M23C6 (612–645 °C) carbides occurs during tempering of the HTST steel. The effective precipitation activation energies of M3C and M23C6 carbides are 120 ± 7 and 272 ± 13 kJ mol−1, respectively. The precipitation processes of M3C and M23C6 carbides are dominated by C and Cr atomic diffusion, respectively. Moreover, it is found that two transformation processes occur during tempering with increasing temperature, namely, M3C carbides dissolve in the matrix and then M23C6 carbides precipitate along the boundaries of martensitic laths, and the plate‐like M3C carbides decompose into several globular or rod‐like M23C6 carbides that arrange in a chain‐like morphology.

Oxidation behavior and corrosion properties of 2-GPa grade Co-reduced ultrahigh-strength stainless steel

This study investigated the oxidation behavior and corrosion resistance of 2-GPa grade ultrahigh-strength stainless steel. Results revealed that the addition of V to Co-reduced FerriumS53 (7Co1V) accelerated the precipitation of MC carbide during aging, while inhibiting the precipitation of Cr-rich M23C6 carbide. A significant amount of Cr was predicted in the microstructure of the alloys, and this microstructure increased the partitioning of Cr to the alloy surface and provided enhanced resistance against corrosion pitting. These findings suggest that the addition of V into Co-reduced stainless secondary hardening steels can enhance their oxidation and corrosion resistance without compromising their mechanical properties.

Effects of tempering temperature on the microstructure evolution and mechanical properties of 16%Cr–5%Ni super martensitic stainless steel

The effects of tempering temperature on the microstructure evolution and mechanical properties of 16%Cr–5%Ni super martensitic stainless steel (SMSS) were investigated in the present study. Reversed austenite was detected via X-ray diffraction when the as-quenched samples with full martensite were tempered above 500 °C, and its volume fraction attained a maximum of ∼17% at 620 °C. When tempered below 620 °C, all the reversed austenite formed was able to keep stable at room temperature, while partial or even entire reversed austenite formed above 640 °C with Ni content less than 8 wt% transformed back into fresh martensite during cooling, demonstrating that the enrichment of Ni determined the thermal stability of reversed austenite. EDS results revealed that the precipitation of M23C6 carbides caused Ni enrichment in the adjacent regions, promoting the nucleation of reversed austenite, and the growth of reversed austenite was mainly controlled by Ni diffusion. The results of tensile tests and instrumented impact tests indicated that stable reversed austenite effectively improved the ductility and toughness, and offset the deterioration of toughness caused by the M23C6 precipitating at grain boundaries. Tempering at 540 °C achieved an excellent combination of strength and toughness, while the best plasticity of samples was obtained by tempering at 620 °C.

Influence of solution temperature on microstructure and mechanical properties of DZ411 alloys with different Ta compositions

The influence of solution treatment temperature on microstructure and mechanical properties of DZ411 alloys with different Ta compositions (2.72, 3.10, and 4.00 wt.%) is studied in details. With the increase of Ta composition, the average values of the primary dendrite arm spacing (PDAS) in superalloys before heat treatment are found to increase from 145.9 ± 3.7 to 209.8 ± 7.6 μm, the area fractions of carbides increase from 0.59 ± 0.05% to 0.65 ± 0.05%, the area fractions of γ-γ′ eutectics increase from 4.6 ± 0.05% to 8.1 ± 0.05%, and the cubicity of the γ′ phase also increases. After heat treatment, since the γ′ phase shows more superior high temperature performance in alloys with higher Ta composition, the solution temperature of alloys with higher Ta composition should be increased to promote the dissolution of the coarse γ′ phase. The evolution of carbides is mainly divided into two types: the decomposition of MC carbides: MC + γ → M23C6 + γ′ and the precipitation of fine M23C6 carbides. The increase of cubicity of γ′ phase will increase the interfacial energy of two phases, which can lead to the pile-up of dislocation and improve the microhardness of alloys. The fine M23C6 carbides will hinder the initiation and propagation of cracks, and the interaction of fine carbide particles reduces the stress concentration and improve the fracture strength of alloys. Therefore, the addition of Ta increases the mechanical properties including both tensile properties and microhardness of alloys with higher Ta composition increases more obviously after heat treatment at higher temperature.

Effect of Hot-Rolling on the Microstructure and Impact Toughness of an Advanced 9%Cr Steel

A 9%Cr martensitic steel with Ta and B additions was subjected to thermo-mechanical treatment (TMT) including rolling in the range of metastable austenite at 900–700 °C followed by water quenching and tempering at different temperatures. Applied TMT with tempering at T ≥ 700 °C substantially improved the impact toughness. The application of the TMT with subsequent tempering at 780 °C decreased the ductile–brittle transition temperature from 40 to 15 °C and increased the upper shelf energy from 300 to 380 J/cm2 as compared to the normalized and tempered (NT) condition. The microstructural observations with scanning and transmission electron microscopes showed the precipitation of fine Ta-rich MX carbonitride and M23C6 carbide during TMT and subsequent tempering. The analysis of the cleavage facets and the secondary cracks with electron back-scattered diffraction (EBSD) revealed that the brittle fracture occurred via cleavage cracking along {100} planes across the laths, while the high-angle boundaries of martensite blocks and packets were effective barriers to the crack propagation. The increased impact toughness of the tempered TMT steel sample was attributed to enhanced ductile fracture owing to the uniform dispersion of the precipitates and favorable {332}⟨113⟩ crystallographic texture.

The impacts of M/A constituents decomposition and complex precipitation on mechanical properties of high-strength weathering steel subjected to tempering treatment

The impact of microstructure evolution on the mechanical properties of a typical 500 MPa-grade weathering steel produced by an identical thermo-mechanical control process and different tempering treatments at 450–650 °C were thoroughly investigated mainly using scanning electron microscopy (SEM) equip with an electron backscatter diffraction (EBSD), high-resolution transmission electron microscopy (HRTEM), and X-ray diffractometer (XRD). Results indicated that the as-rolled steel consists of granular bainitic ferrite (GBF) and a considerable amount of martensite/austenite (M/A) constituents, exhibiting unsatisfied mechanical properties. As the tempering temperature increased from 450 to 550 and 650 °C, the massive twin-type M/A constituents decomposed preferentially into carbides following the sequence of Fe3C→(Cr, Mn, Fe)3C→(Cr, Mn, Fe)3C + (Cr, Mn)23C6, with a small amount of fine lath-type M/A constituents remained. The matrix recovered with the bainitic ferrite laths merged, the dislocation density decreased and the proportion of high-angle grain boundary (HAGB) increased. Nanoscale (Ti, Nb)C and ε-Cu particles also precipitated simultaneously, leading to an increase of yield strength. The strain hardening capacity and hence the tensile strength reduced resulting from the decomposition of M/A constituents. Moreover, the impact toughness was first enhanced by tempering at 450–600 °C with the decreasing twin-type M/A constituents content and increasing HAGBs proportion, and then deteriorated by tempering at 650 °C due to the precipitation of necklace-like M23C6 carbides at the grain boundaries. An optimum mechanical property combination was achieved via tempering at 550–600 °C.

Microstructure development of modified 9Cr-1Mo steel during laser powder bed fusion and heat treatment

Herein, the effect of laser powder bed fusion (LPBF) parameters on the microstructure development of modified 9Cr-1Mo steel and the relationship between the as-built and heat-treated microstructures were characterized using electron backscattered diffraction (EBSD). The optimal energy density for the steel was determined to be 100–150 J/mm3. The microstructure of the as-built steel was found to be mainly composed of coarse and columnar δ-ferrite grains and fine martensite grains. δ-ferrite was formed through rapid solidification, while martensite was formed in the heat-affected zone during LPBF. A higher energy density and scan strategy with a 67 deg. rotation in each layer were found to be suitable for obtaining a higher area fraction of martensite because they can provide a larger heat-affected zone compared with a lower energy density and a 90 deg. rotation. Tempering reduced the hardness via recovery of martensite and enhanced the precipitation of M23C6 carbide at martensite, while the microstructural morphology in the as-built condition was maintained. The typical as-built microstructure disappeared after the normalizing treatment, while the size of the prior austenite grains was affected by the as-built microstructure. In-situ EBSD observation of the phase transformation behavior during normalization revealed that martensite became fine prior austenite grains, while δ-ferrite became coarse prior austenite grains. From these results, the effects of energy density and scan strategy on microstructure development were identified not only for LPBF but also during the normalizing and tempering heat treatments.

Pitting Performance of Cold- and Hot-Rolled Nickel-Saving High-Strength Metastable Austenitic Stainless Steel

Nowadays, nickel-saving metastable austenitic stainless steel (MASS) has become the right solution to meeting the growing requirement of higher strength, better corrosion resistance and more cost saving for the automobile industry. Better understanding of the pitting mechanism of the MASS after either cold- or hot-rolled can offer guidance for the producing of high-performance automobile steel. In the current work, for uncovering the pitting mechanism of the cold- and hot-rolled MASS, the microstructural evolution and pitting performance of nickel-saving metastable austenitic stainless (MASS) steel after cold- (CR) and hot-rolling (HR) were researched via electron microscopy technique and electrochemical methods. Austenite composites the main phase of the MASS. Small amounts of martensite film were proven to form in the MASS. The precipitation of Cr-rich M23C6 carbides was observed in the CR-MASS, while no carbides existed in the HR-MASS. The pitting resistance of the HR-MASS was better than the CR-MASS, which could be attributed to the fact that the stable pits in CR-MASS were initiated near the carbides, whereas the MnS inclusion would serve as the initiation sites for stable pits in HR-MASS. Findings in this work will provide a guidance for developing new generation MASS for automobile industry.