Martensite Lath Boundaries Research Articles

In situ tempering of martensite during laser powder bed fusion of Fe-0.45C steel

During laser powder bed fusion (L-PBF), materials experience cyclic re-heating as new layers are deposited, inducing an in situ tempering effect. In this study, the effect of this phenomenon on the tempering of martensite during L-PBF was examined for Fe-0.45C steel. Detailed scanning electron microscopy, transmission electron microscopy, atom probe tomography, and hardness measurements indicated that martensite was initially in a quenched-like state after layer solidification, with carbon atoms segregating to dislocations and to martensite lath boundaries. Subsequent tempering of this quenched-like martensite was the result of two in situ phenomena: (i) micro-tempering within the heat affected zone and (ii) macro-tempering due to heat conduction and subsequent heat accumulation. Hardness measurements showed that although both influenced martensite tempering, micro-tempering had the most significant effect, as it reduced martensite hardness by up to ∼380 HV. This reduction was due to the precipitation of nano-sized Fe3C carbides at the previously carbon-enriched boundaries. Lastly, the magnitude of in situ tempering was found to be related to the energy input, where increasing the volumetric energy density from 60 to 190 J/mm3 reduced martensite hardness by ∼100 HV. These findings outline the stages of martensite tempering during L-PBF and indicate that the level of tempering can be adjusted by tailoring the processing parameters.

The effect of reversed austenite on mechanical properties of 13Cr4NiMo steel: a CPFEM study

In order to investigate the effect of reversed austenite on the mechanical properties of 13Cr4NiMo steel, a meso scale crystal plasticity finite element model (CPFEM) was established to simulate the deformation induced martensitic transformation (DIMT) and micromechanical behavior during tensile deformation. The results demonstrate that the mechanical stability of the film-like austenite embedded in the martensite lath boundary is greater than blocky austenite within the martensite lath. The film-like austenite will preferentially undergo DIMT during initial plastic deformation prior to blocky austenite. After deformation, the tempered martensite yields and deforms plastically, while adjacent new martensite is elastically deformed. The incompatible deformation leads to the introduction of dislocations in the tempered martensite and increases the work hardening rate. The new martensite, that is transformed from film-like austenite, significantly increases the true strain of the adjacent martensite matrix, which is beneficial to the plasticity and toughness, but has little contribution to the increase of matrix stress. New martensite, transformed from blocky austenite, increases the von Mises stress of the adjacent martensite matrix, which improves the tensile strength and strain hardening capacity, but makes little contribution to the increase of matrix strain.

Measurement of degree of sensitization in post-weld heat treated 13Cr-4Ni martensitic stainless steel clad using double loop electrochemical potentiokinetic technique

In this paper, the susceptibility to intergranular corrosion (IGC) of 13Cr-4Ni martensitic stainless steel clad deposited by submerged arc welding was studied in the as-welded condition and after post-weld heat treats (PWHT) at 420, 500, 540, and 580 °C. Microstructural studies showed a martensitic matrix, about 20% δ-ferrite and carbide precipitates in the microstructure under the as-welded condition. The results of the double loop electrochemical potentiokinetic technique revealed that the PWHT process significantly increased the degree of sensitization (DOS). The highest DOS corresponded was 87.9% in the sample treated at 500 °C. PWHT at temperatures higher than 500 °C returned the DOS to lower values. PWHT at 500 °C and below promoted the decomposition of δ-ferrite, and hence some new chromium carbides precipitated at δ-ferrite and martensite interfaces, whereas tempering at temperatures higher than 500 °C accounted for carbides precipitation at martensite lath boundaries owing to the formation of tempered martensite. The DOS value decreased with increasing PWHT time due to chromium diffusion to chromium-depleted regions. Also, the potentiodynamic studies showed an inverse relationship between the DOS and pitting potential; the higher DOS, the lower the pitting potential.

Liquid metal embrittlement of 12Cr ferritic/martensitic steel thin-walled tubes exposed to liquid lead-bismuth eutectic

In this work, liquid metal embrittlement (LME) susceptibility of 12Cr ferritic/martensitic (F/M) thin-walled tubes has been tested in liquid lead-bismuth eutectic (LBE) under different temperatures (i.e., 350, 400 and 450 °C), oxygen concentrations (i.e., oxygen-poor and oxygen-saturated) and strain rates (i.e., 5 ×10−5, 1 ×10−6 and 5 ×10−7 s−1). The results show that LME occurs under nearly all the testing conditions, and the most influential factors upon the LME susceptibility are oxygen concentration and strain rate. Transmission electron microscopy (TEM) characterization at crack tips shows that LME cracks propagate preferentially along microstructural boundaries, such as martensite lath boundaries, subgrain boundaries and carbides/lath interphase boundaries.

Influence of the Evolution of 9CrMoCoB Steel Precipitates on the Microstructure and Mechanical Properties during High-Temperature Aging

In this study, the short-term aging was carried out to reveal the evolution of precipitates and mechanical properties of heat resistant 9CrMoCoB steel during the early creep, replacing the conventional creeping. The tempered martensite lath structure (TMLS) and precipitates were observed in the as-aged 9CrMoCoB steel. TMLS in the matrix underwent a transition to the polygonal ferrite after aging only for 300 h. In comparison, the mean diameter of the precipitates increased from 183 to 267 nm after aging at 650 °C for 300 h. Also, the mean diameter of the precipitates increased from 183 to 302 nm at 700 °C. The room-temperature and high-temperature strength of 9CrMoCoB steel decreased after high-temperature aging, which may be mainly due to precipitates coarsening. Many M23C6 phases precipitate in the prior austenite grain boundary (PAGB) and lath boundary. After aging 100 h, TMLS transformed into polygonal ferrite, and the size of the precipitate at the subgrain boundary was about 100 nm, while after 300 h of high-temperature aging, large precipitates appear (400 nm) in the matrix. After 200 h of high-temperature aging, the obvious growth of precipitates on the PAGB and lath boundary weakens the pinning effect on the PAGB and martensite lath boundary and accelerates the transformation of microstructure and mechanical properties.

Superior low temperature toughness in a newly designed low Mn and low Ni high strength steel

In this study, a low cost 2.5Mn-1.5Ni low carbon steel with a superior low temperature toughness (Akv at −140 °C: 210 J) is developed, due to the formation of highly stable, nano-sized, and film-like retained austenite along the martensitic lath boundaries by using Quenching-Lamellarization-Tempering (QLT) process.

Tuning process parameters to optimize microstructure and mechanical properties of novel maraging steel fabricated by selective laser melting

Maraging steel is a promising material for additive manufacturing due to its ultrahigh yield strength, reasonable ductility and good weldability. However, the ductility of the fabricated part will be degraded after aging treatment. In this regard, we firstly designed a new composition of maraging steel Fe-18.3Ni-9Co-4.84Mo-0.92Ti-0.27Al-0.13Cr-0.01C (wt.%), whose strength and ductility can be simultaneously improved by selective laser melting. The relationship between laser process parameters and forming defects has been studied. Using single-track and single-layer experiments, fully dense parts were fabricated with a certain range of process parameters, corresponding to the 30–50% lap rate in the X–Y plane and 70% remelted rate along Z-axis. Besides, we found film-like reverted austenite along the martensite lath boundaries in the as-fabricated part. The effects of heat treatment processes on the reverted austenite and mechanical properties of the fabricated parts were also studied; the strength-ductility trade-off of maraging steel after heat treatment can be alleviated. The tensile strength and elongation of printed samples after direct aging treatment can respectively reach 2037 MPa and 6.4%, and 2182 MPa, 4.8% after solution and aging treatment.

Microstructure evolution of T91 steel after heavy ion irradiation at 550 °C* *Project supported by Guangdong Major Project of Basic and Applied Basic Research (Grant No. 2019B030302011), the National Natural Science Foundation of China (Grant Nos. U2032143, 11902370, and 52005523), the International Science and Technology Cooperation Program of Guangdong Province, China (Grant No. 2019A050510022), the China Postdoctoral Science Foundation (Grant

Fe-Cr ferritic/martensitic (F/M) steels have been proposed as one of the candidate materials for the Generation IV nuclear technologies. In this study, a widely-used ferritic/martensitic steel, T91 steel, was irradiated by 196-MeV Kr+ ions at 550 °C. To reveal the irradiation mechanism, the microstructure evolution of irradiated T91 steel was studied in details by transmission electron microscope (TEM). With increasing dose, the defects gradually changed from black dots to dislocation loops, and further to form dislocation walls near grain boundaries due to the production of a large number of dislocations. When many dislocation loops of primary a 0/2〈 111 〉 type with high migration interacted with other defects or carbon atoms, it led to the production of dislocation segments and other dislocation loops of a 0 〈 100 〉 type. Lots of defects accumulated near grain boundaries in the irradiated area, especially in the high-dose area. The grain boundaries of martensite laths acted as important sinks of irradiation defects in T91. Elevated temperature facilitated the migration of defects, leading to the accumulation of defects near the grain boundaries of martensite laths.

Cu precipitation-mediated formation of reverted austenite during ageing of a 15–5 PH stainless steel

A Cu precipitation-mediated austenitic transformation during ageing treatment of a 15–5 PH stainless steel is revealed through atom probe tomography, in situ synchrotron X-ray diffraction and computational thermodynamics and kinetics. The austenitic transformation is proposed to occur through the pathway: Cu precipitation at the martensite/retained austenite interfaces or at martensite lath boundaries → partitioning of austenite stabilizing elements towards interfaces of the Cu precipitates → reverted austenite formation.

Cracks Forming and Propagation Mechanism for the Q345 Steel during Cooling

Cracks always occur for the Q345 steel during cooling, which usually lead to Welding failure. Through investigation of the cracks forming position and propagation path, its forming and propagation mechanism for the melted Q345 steel during cooling was studied in detail by using of SEM, EBSD and Nano Indenter equipment in this paper. SEM observation results imply that cracks mainly initiated at martensite phase region close to basal material. Moreover, it also can be observed that the cracks always propagated along the martensite lath or packets boundaries, but not the original prior austenite grain boundaries. The hardness differences between cracks adjacent positions and matrix is about 2.48 GPa, which is larger than that of melted region and matensite (0.70GPa). In addition, EBSD results show that the average prior austenite grains boundaries angles (30-50º) are always smaller than that of martensite lath and packet boundaries (>50º). Combining investigation of the cracks forming position, propagation path and microstructure misorientation, it proves that the occurrence of the cracks at junction of martensite phase region close to basal material is related with larger hardness difference or larger residual stress between martensite and ferrite, while cracks propagation path is controlled by the boundaries angles or local misorientation. The cracks incline to propagate along or deflect to large martensite lath and packets boundaries.

Modeling of Reversed Austenite Formation and Its Effect on Performance of Stainless Steel Components

Abstract The kinetics of reversed austenite formation in 301 stainless steel and its effect on the deformation of an automobile front bumper beam are studied by using modeling approaches at different length scales. The diffusion-controlled reversed austenite formation is studied by using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model, based on the experimental data. The model can be used to predict the volume fraction of reversed austenite in a temperature range of 650–750 °C. A three-dimensional elastoplastic phase-field model is used to study the diffusionless shear-type reversed austenite formation in 301 steel at 760 °C. The phase-field simulations show that reversion initiates at martensite lath boundaries and proceeds inwards of laths due to the high driving force at such high temperature. The effect of reversed austenite (RA) and martensite on the deformation of a bumper beam subjected to front and side impacts is studied by using finite element (FE) analysis. The FE simulations show that the presence of reversed austenite and martensite increased the critical speed at which the beam yielded and failed. RA fraction also affects the performance of the bumper beam.

Microstructural evolution and martensitic transformation in FeCrV alloy fabricated via additive manufacturing

FeCrV alloy (high-wear-resistance steel or HWS) has been extensively used in additive manufacturing for hardfacing and repairing owing to its excellent hardness, superior weldability, and wear resistance. This alloy must possess superior mechanical property under harsh environments, such as friction and wear, to facilitate its application in repairing gears and strengthening the surface of a die locally. HWS deposited by directed energy deposition was subjected to heat treatment by quenching and tempering (QT) to improve the mechanical property and investigate the influence of martensitic transformation. During the solidification of the molten pool, the as-deposited HWS exhibited compressive residual stress (−102 (±72) MPa) on the surface, resulting from martensitic transformation. In contrast, the surface of the heat-treated HWS (HT-HWS) had tensile residual stress (150 (±34) MPa), which was induced by released carbon from the matrix and decarburization at the surface. QT heat treatment precipitated spherical chromium carbide (W0·07Mo0·07Cr1·31V0·31Fe1·38C) and rod-like vanadium carbide (W0·07Mo0·08Cr0·48V0·56Fe1·08C) on the grain boundaries and nanosized-vanadium carbide on the martensite lath boundaries. During heat treatment, the austenite of the as-deposited HWS transformed into martensite, and only 4.80% of the austenite remained. Despite heat treatment, the dislocation density did not decrease because the martensitic transformation caused plastic deformation of the adjacent grains. In addition, the precipitation of fine carbide inhibited grain growth during the heat treatment. For these reasons, the hardness of the as-deposited HWS increased by 17% (i.e., from 659 (±18.7) HV to 773 (±3.33) HV), corresponding to the hardness of carburized SCM420 (774 (±4.31) HV). The tensile strength of the as-deposited HWS increased from 570.6 (±21.5) MPa to 848.1 (±47.7) MPa by removing the melt pool boundary and needle-shaped austenite which are the crack propagation paths of the QT-heat-treated alloy. The carbide that precipitated during heat treatment changed the fracture mechanism from intergranular with dimples to cleavage.

Enhanced Cross-Tension Property of the Resistance Spot Welded Medium-Mn Steel by In Situ Microstructure Tailoring

Medium Mn steels, one of the most promising 3rd generation advanced high strength steels (AHSS), achieve an encouraging trade-off between the outstanding mechanical property and the production cost. As a typical medium Mn steel, 7Mn steels have superior mechanical property but their poor cross-tension property becomes the Achilles' heel, hindering the application in the automotive industry. The current study focuses on the cross-tension property of the resistance spot weld of 7Mn steel. Generally, martensite is produced in the nugget during the resistance spot welding (RSW). However, the microstructure in the weld nugget can be correspondingly tuned by directly optimizing the welding parameters. With post-weld pulses, in situ tempering occurs, which can decrease the segregation of Mn existing along martensite lath boundaries and facilitate the microstructure transition from martensite to tempered martensite. The tuning on the nugget microstructure facilitates the increase of cross-tension strength (CTS) from 1.5 to 3.7 kN. Although both cases fail in an interfacial fracture mode, a partial ductile fracture is demonstrated in the specimen with post-weld treatment, which is attributed to the occurrence of low-carbon α phase and second phase particles. This study elucidates that the decrease of segregation and the microstructure transition in the nugget are the dominant factor determining the CTS. It is therefore demonstrated that the reduction of Mn segregation and the formation of tempered martensite can increase the weldability of RSW joints of the medium Mn steels.

A refined oxidation mechanism proposed for ferritic-martensitic steels exposed to oxygen-saturated liquid lead-bismuth eutectic at 400°C for 500 h

The structure of the oxide scales formed on three ferritic/martensitic (F/M) steels, including HT9, T91 and CLA16 exposed to oxygen-saturated lead-bismuth eutectic (LBE) at 400°C for 500 h has been studied. The results show that the oxide scales in the three steels have a similar triple-layered structure. The outer layer is a nanosized/ultrafine magnetite containing massive Pb solid solution nanoparticles. An Fe-Cr spinel constitutes the intermediate layer without the Pb particles. The innermost layer is a Cr oxide (i.e., Cr2O3) that forms preferentially at the martensite lath boundaries. A refined oxidation mechanism is proposed with consideration of the Cr oxide formed at the oxidation front, as well as of an improved “Pb nano-channels” theory.

The formation of a mixed martensitic/bainitic microstructure and the retainment of austenite in a medium-carbon steel during ultra-fast heating

The current work focuses on the microstructure evolution of a medium-carbon steel during an ultra-fast heating cycle. Electron microscopy methods such as Scanning Electron Microscopy, Electron BackScatter Diffraction and Transmission Electron Microscopy are utilized for the characterization of microstructural constituents of the studied steel samples. The applied heating rate during this treatment was 100 °C/s and the heating temperature was 930 °C, slightly above the Ac3 temperature, and subsequent quenching in water. It is found that the microstructure consists of martensite (60 %), lower bainite (37 %) and retained austenite (3%). The formation of bainite during this treatment is attributed to the heterogeneity of the chemical composition in the microstructure due to the lack of time for carbon to diffuse from cementite in long distances. Regions that are located away from the former pearlitic colonies are not enriched in carbon and manganese along the austenitization process and, according to the Controlled Cooling Transformation diagram, will form bainite upon cooling. The retainment of austenite in films is observed between the bainitic plates and on the martensitic lath boundaries, where carbon and manganese are segregated. Such microstructures are utilized by the current 3rd Generation Advanced High Strength Steels for the optimization of the mechanical properties.

Research on the correlation between impact toughness and corrosion performance of Cr13 Super martensitic stainless steel under deferent tempering condition

Super martensitic stainless steel (SMSS) is characterized by improved mechanical properties and corrosion resistance. However, during production the impact toughness will be significantly reduced under certain heat treatment regimes (tempering at 873 K followed by quenching at 1213 K and tempering at 913 K). Double loop electrochemical potentiokinetic reactivation (DL-EPR) were used to investigate the mechanism for the reduction of impact toughness in this experiment. It was found that the primary cause for the decrease was precipitation of continuously distributed Cr-rich carbides (M23C6) along martensite lath boundaries. The Ra value obtained by DL-EPR is positively correlated with the amount of Cr-rich carbide precipitation. As Ra increases, impact toughness increases first and then decreases rapidly, which indicates that it is reasonable to use Ra to qualitatively predict the impact toughness of the steel.

Influence of Tempering in Different Melting Routes on Toughness Behavior of AISI 4340 Steel

Martensitic microstructure of AISI 4340 steel can be heat treated to achieve the desired mechanical properties. However, mechanical properties degrade owing to the impurities and cleanliness during the steel production. In this work, AISI 4340 steel was produced through three different routes, such as vacuum degassing (VD), electro slag remelting (ESR), and vacuum arc remelting (VAR), followed by austenizing, hardening, and tempering. Further, mechanical characterization such as tensile, hardness, and toughness were carried out in a wide range of tempering temperatures (171-649 °C). A variation in mechanical properties was observed due to the evolution of precipitated carbide with the tempering temperature in all three routes. A thicker carbide layer along the martensitic lath boundary led to higher embrittlement in VD and VAR for tempering regime 171-427 °C. The absence or lesser embrittlement in ESR attributes to the homogeneously distributed fragmented carbides. Martensitic lath coarsening, ferritic phase formation along with the precipitated carbide distribution, significantly enhances the fracture toughness over the impact toughness at higher tempering temperature (> 316 °C). The difference in the mechanical properties in all the three routes is found to be sensitive toward the chemical composition causing a marked difference in the carbide precipitation and its distribution along the martensitic lath boundaries.

Nucleation of W-rich carbides and Laves phase in a Re-containing 10% Cr steel during creep at 650 °C

The nucleation of the Laves phase particles in a Re-containing 10% wt. Cr-3% Co-3% W steel with a low nitrogen and a high boron contents during creep is characterized by distinctive features. The precipitation process can be written as M23C6 carbide → M6C carbide → Laves phase. The nucleation of all phases in this precipitation sequence is heterogeneous. The M23C6 carbides precipitate on the boundaries of martensitic structure during tempering at temperature of 770 °C. The M6C (Fe3W3C) carbides precipitate during both tempering at 770 °C and creep at 650 °C on the M23C6/ferrite surfaces and on the martensitic lath boundaries. The M6C carbides have the mutual relationship of crystal orientations with both the M23C6 carbides and ferrite. The chemical composition of the M6C carbides depends on their nucleation sites. After 83 h of creep, the M6C carbides starts to dissolve, whereas the Laves phase particles are nucleated on the M6C/ferrite surface and separately on the martensitic lath boundaries. The Laves phase particles nucleate on the M6C/ferrite surface are smaller than those nucleated on the martensitic lath boundaries. The Laves phase particles exhibit the unique orientation relationships with a high misfit. The transformation of M23C6 → M6C → Laves phase is an in-situ transformation accompanied by chemical compositional changes.

Investigation on Temper Embrittlement of TS1100 MPa Grade Ultra-High Strength Steel

Temper embrittlement is one of the major challenges encountered during the heat treatment of ultra-high strength steels. In this work, the microstructural evolution and mechanical properties during the austenite region quenching and tempering (Q-T) and the intercritical quenching and tempering (IQ-T) processes in a tensile strength (TS) 1100 MPa grade steel targeted for the manufacturing of a mooring chain were investigated. The temper embrittlement presented in the Q-T sample was attributed to the carbide network decorating the prior austenite grain and martensitic lath boundaries. In contrast, the IQ-T sample, where intercritical holding before quenching at the temperature of 12 °C lower than Ac3 was applied, exhibited a reasonable CVN impact energy of 188 J at − 20 °C with a slightly reduced strength. The ferrite formed during intercritical holding not only added extra interphase boundaries, but also refined the substructure of the quenched martensite. These increased boundary areas provided more nucleation sites for carbide precipitation during tempering. Meanwhile, the refined martensitic substructure promoted the diffusion of Cr, encouraging the fast coarsening of Cr-rich carbides. The formation of a carbide network was eventually prevented in the IQ-T sample. Thus, the IQ-T process was an effective approach to eliminate temper embrittlement caused by the carbide network.

Kinetics of Reverted Austenite in 18 wt.% Ni Grade 300 Maraging Steel: An In-Situ Synchrotron X-Ray Diffraction and Texture Study

In this study, we investigated the transformation kinetics of martensite → reverted austenite in 18 wt.% grade 300 Ni maraging steel. The kinetics was evaluated based on the in situ synchrotron x-ray diffraction data collected during isothermal heat treatment at 570°C. The onset of transformation martensite → reverted austenite was detected after ~ 5 min of aging. The austenite fraction increased as a function of annealing time and reached approximately 30 vol.% after 3 h of heat treatment. The electron backscatter diffraction technique revealed that reverted austenite is formed preferentially on both the martensitic lath boundaries and sub-grain boundaries inside the laths, in particular in those with high Taylor factor values. The reverted austenite maintains an orientation relationship with the prior austenite; however, variant selection can take place.