Confusion exists in the literature as to whether the crystal structures are cubic or tetragonal in lath martensites of Fe–C alloys and low-alloy steels. Steels with a range of carbon contents have been quenched and examined by synchrotron x-ray diffraction. The presence of dislocations and residual local strains complicates the analysis since peak splitting of tetragonal lines is obscured by the broadening. Asymmetry of the 200,020/002 lines has been examined and synthesised using model peak functions. A new approach has been to study the 222 peaks which are unique (not split) for both cubic and tetragonal crystals. For low carbon steels (<~0.2%C) the structures are fully or almost completely cubic. Above about 0.7%C the martensite has tetragonal symmetry. Intermediate, medium carbon, steels consist of mixtures of cubic and tetragonal structures.

It is argued that martensite is always tetragonal at the moment when it forms from austenite but it can subsequently decompose into cubic by auto-tempering during the quench. Kinetics of decomposition have been measured using higher carbon steels and these are extrapolated to the situation of auto-tempering. A model is developed whereby the creation of tetragonal martensite according to the Koistinen-Marburger relationship is modified by its rate of decomposition. The final structures are predicted to depend on both the martensite start temperature (Ms) and the cooling rate.

The kinetics of fcc–bcc heterointerface with typical orientation relationships (ORs) during phase transformation of iron and carbon steels is investigated by molecular dynamics simulations. The heterointerface with the Nishiyama–Wasserman (N–W) OR immediately propagates downward keeping parallel to the initial interface. On the other hand, the incubation time appears before the rapid growth of the needle–like bcc phase at the heterointerface with the Kurdjumov–Sachs (K–S) OR. The transformation rate decreases with increasing carbon concentration at both the interfaces with N–W and K–S ORs. Moreover, analyses of mean square displacement revealed that carbon atoms are constrained by surrounding iron atoms and therefore no carbon diffusion occurs during the phase transformation of carbon steels, which can be regarded as a diffusionless phase transformation.

In-situ neutron diffraction measurements were performed on Fe-33Ni-0.004C alloy (33Ni alloy) and Fe-27Ni-0.5C alloy (27Ni-0.5C alloy) during cooling from room temperature to the cryogenic temperature (4 K) to evaluate changes in the lattice constants of austenite and martensite, and changes in the tetragonality of martensite due to thermally induced martensitic transformation. As the martensitic transformation progressed, the lattice constants of austenite in both alloys deviated to smaller values than those predicted considering the thermal shrinkage, accompanied by an increase in the full width at half maximum of austenite. The fresh martensite formed in both alloys had a body-centered tetragonal (BCT) structure, regardless of the carbon content. The tetragonality of martensite decreased with progressive martensitic transformation during cooling in the 33Ni alloy, but was almost constant in the 27Ni-0.5C alloy. This suggests that carbon is necessary to maintain the tetragonality of martensite during cooling. The tetragonality of martensite in the 27Ni-0.5C alloy decreased during room temperature aging because of carbon mobility.

To clarify the geometry of the junction of martensite which forming butterfly-type martensite, electron microscopy, theoretical analysis using the phenomenological theory of martensite crystallography and rank-1 connection were carried out on Fe-18Ni-0.7Cr-0.5C (mass%) alloy. Martensite plates in this alloy exhibit {252}_γ habit plane and K-S OR and are in good agreement with the double shear PTMC by Ross and Crocker. The frequency of formation of V1/V2 and V1/V16 (butterflies) pairs was 20% each, with these two pairs alone accounting for 40% of the total. Theoretical analysis using rank-1 connection of variants revealed that butterfly-pairing is not a geometrically compatible morphology, even for alloys in which butterfly martensite forms frequently. We have also revealed theoretically that the junction plane of the butterfly-pairing keeps (100)_γ independent of the mode of lattice invariant shear (i.e. morphology of martensite plate). Preferential formation mechanism for butterfly-pairing is discussed based on the common lattice invariant deformation between V1 and V16.

The strain distribution due to martensitic transformation in a Fe–Ni alloy was investigated using high-precision markers drawn via electron-beam lithography. This study focused on the strain distribution within the lenticular martensite plate, which developed immediately below the martensite start temperature.

The strain in the central region of lenticular martensite was determined to be greater than that near the interface boundary by measuring the displacement at each intersection of the grid markers. This finding is consistent with the shape strain distribution predicted by the phenomenological theory of martensitic transformation based on the microstructure observations.

In order to characterize martensite microstructure in more detail, the orientation analysis based on Rodrigues-Frank (R-F) space was applied for the composite microstructure consisting of different types of martensite, retained austenite, and undissolved carbide in Cr-containing high-carbon steels. Martensite variants was separated more clearly in the R-F space, and consequently individual martensite blocks characterized by identical variant could be identified accurately in the real space. As a result, fine lath martensite and coarse plate martensite was separated by the critical block length, which is defined with a crystallinity evaluated by electron backscatter diffractometry. Furthermore, in addition to butterfly shaped plate blocks formed within an austenite grain (Type-I) and single block growing along austenite grain boundaries (Type-II), twin blocks pair growing from austenite grain boundaries to inside the grain (Type-III) could be observed as characteristic plate martensite formed in high-carbon steels. The orientation relationship of twin blocks with respect to prior austenite was changed simultaneously from near Nishiyama-Wasserman to Kurdjumov-Sachs in the growth stage. These characteristics of twin pair of Type-III is expected to be effective to reduce the increase of both strain and interfacial energies during martensitic transformation.

Surface microstructures were investigated in pure iron and Fe-1mass%M (M = Mn, Cr, Al, Si) alloys gaseous-nitrided at 1123 K and quenching to reveal the alloying effects on surface hardening by nitrogen (N) martensite. Thicker hardened layers with higher hardness than pure iron were obtained in the Mn-added alloys whereas the additions of Si and Al lead to increase the surface hardness with reduction of the hardened layer thickness. On the other hand, adding Cr decreases both the hardness and thickness of the hardened layer. No precipitation of alloy nitride is observed in austenite nor internal ferrite region in Mn-added alloy. Meanwhile, CrN(B1) and AlN(wurtzite) particles are dispersed in ferrite and austenite regions in the Cr- and Al-added alloys, respectively. Unlike those alloys, (austenite+ α-Si₃N₄) lamellar structure is formed in the Si-added alloy followed by martensite transformation of the high temperature austenite during quenching. Phase diagrams of Fe–M–N systems can consistently describe those alloying effects on the microstructure evolution.

High-strength low-to-medium carbon martensitic steel is increasingly used in the automobile industry. This study investigated the room temperature aging behavior of as-quenched autotempered Fe–C lath martensitic steels (C: 0.07–0.77 mass%) using kinetic analysis of hardness change and interrupted atom probe (AP) analysis to clarify the dominating factor of hardening. Age-hardening at 23°C was confirmed in the autotempered lath martensitic steels, including low-carbon steel with a carbon content of less than 0.25 mass%. The AP and kinetic analyses of hardness evolution indicated that the growth of carbon clusters at dislocations dominates the hardening of martensite. The maximum hardness increment in lath martensite increased with initial excess solute carbon C_sol in the matrix, but the increment in unit C_sol was smaller than that in carbon-supersaturated ferrite. The smaller hardness increase in martensite may indicate the concurrent softening due to the relaxation of the residual lattice strain in martensite by carbon clustering. Interrupted AP analysis of the prolonged aging over two years indicated that the transformation from carbon clusters to iron carbides occurs via an in-situ transformation of the clusters. The microscopic heterogeneity in carbon distribution in the order of martensite blocks and the gradual decrease in excess C_sol during room temperature aging were also confirmed by AP analysis. The persistence of the heterogeneity and excess solute carbon in the martensite matrix after aging and tempering is also discussed.

As-quenched martensite in carbon steels needs to be tempered to restore ductility and toughness for practical applications. During tempering of martensite, microstructural evolutions induced by a series of reactions relevant to carbon diffusion is known to occur. In this study, multi-aspect characterization using advanced techniques such as in-situ neutron diffraction, transmission electron microscopy and three-dimensional atom probe tomography, was performed to investigate the changes in tetragonality, physical properties, microstructure and solute carbon content of high-carbon martensite, with an aim to clarify its low-temperature tempering behaviors. A Fe-0.78 mass%C binary alloy was austenitized and quenched to prepare the as-quenched martensite, followed by tempering via continuous heating at different rates. It was found that various reactions occurred sequentially during tempering, starting from the structure modulation generated by carbon clustering in the 0th stage, then followed by the precipitation of metastable η-carbide preferentially on dislocations in the 1st stage, towards the later decomposition of retained austenite, and precipitation of χ-carbide and cementite in the 2nd and 3rd stages, respectively. After analyzing the experimental results, a compressive residual stress with elastic anisotropy was confirmed in the retained austenite until the temperature range of its decomposition. In addition, the tetragonality and solute carbon content of martensite were found to be continuously decreased especially in the temperature range of the 1st stage. Compared with the tetragonality change of martensite during continuous heating, the lattice volume expansion induced by carbon was found to be more effective to accurately estimate the solute carbon content of tempered martensite.

A thermodynamics analysis based on first-principles calculations was used to establish the behavior of carbon in the matrix during the preliminary stage of the tempering process of steel, as well as the formation mechanism of the η-carbide and cementite phases. Models were constructed in which carbon occupied three octahedral interstitial sites in BCC-Fe, and the cluster expansion and variation method was applied to evaluate the free energy of forming the solid solution. Furthermore, the thermal equilibrium distribution of carbon was obtained using the Monte Carlo simulation method by using the effective cluster interactions. The variable-cell nudged elastic band method was applied to evaluate the energy barrier that exists in the transition process. In addition, transition models considering the influence of interfaces were constructed. The Monte Carlo simulations of BCC-Fe showed well-defined clustering of carbon atoms. Furthermore, the free energies calculated from the cluster expansion and variation method showed two-phase separations between Fe and Fe₂C, suggesting that these clusters are formed by two-phase separation based on atomic ordering. The energies of the transition processes to η-carbide and cementite were calculated from the structural models created based on these local structural findings. By comparing the energy barriers in each transition process, it is shown here that the metastable carbides may occur because of a series of following processes: η-carbide precipitates preferentially in the initial stage of tempering; consequently, cementite precipitates in the third stage using the η-carbides as precipitation sites, and finally, η-carbide transitions to cementite.

Mechanical properties of tempered martensitic steel are controlled by precipitation conditions of carbides. In high strength medium carbon martensitic steel, carbide precipitation at low tempering temperature is strongly affected by silicon addition. Silicon addition prevent carbide transition from ε carbide to cementite, also prevent tempering softening and low temperature temper embrittlement. In this paper, we investigated the carbide transition in medium carbon steel with various Si content, using chemical analysis and XRD analysis of residue carbide after electrolytic extraction, DSC thermal analysis and FE-SEM observation. We found three step carbide transition from ε carbide to para-cementite and ortho-cementite. Especially, para-ortho transition of cementite is not only change in chemical composition of carbide, but also change in the cementite morphology and precipitation site from platelet shape inside of martensite block to granular shape at martensite block boundary. Silicon addition inhibited the formation of para-cementite, and shifted higher the transition temperature of ortho-cementite.

In this paper, the influence of tempering temperatures on microstructures and tensile properties of a Cr–N alloyed medium Mn martensitic steel was studied. The microstructures formed after the tempering below 400°C are composed of recovered martensite as the matrix, ultrafine retained austenite (RA) and carbonitrides. The tempering at 100°C led to the best combination of 2080 MPa ultrahigh ultimate tensile strength (UTS) and 15% total elongation (TE), which is attributed to the prominent strain hardening capacity caused by both the gradually release of internal stress and the pronounced austenite-to-martensite transformation. The tempering at 400°C resulted in the rapid increase of yield strength (YS) by ~500 MPa due to the relief of internal tensile stress and annihilation of dislocations and the best ductility because it produced the most stable RA grains with the highest C concentration for a sustainable austenite-to-martensite transformation over the large plastic straining. The further increase of temperature to 650°C caused ferrite formed, which decrease both YS and strain hardening rate, leading to the lowest UTS. Moreover, it was found that higher N content increased YS but had little influence on both UTS and TE because it mainly contributed to enhanced precipitation of carbonitrides. It is then concluded that the strength and ductility of medium Mn martensitic steel could be increased by increasing the strain hardening capacity through tailoring both the internal stress in martensite and the mechanical stability of RA via a proper tempering treatment.

The precipitation behavior of M₂₃C₆ and Fe₂W was investigated in a boron-doped 9%Cr-3%W-3%Co steel during tempering at temperatures from 500 to 790°C. During tempering at high temperatures, M₂₃C₆ precipitation occurred earlier than Fe₂W, but this precipitation sequence was reversed below 550°C. The Fe₂W precipitates formed on PAGBs during tempering at the lower temperatures showed a continuous film morphology. The film-shaped Fe₂W precipitates were stable after aging for at least 100 h at 650°C.

In order to understand microstructural changes in 9Cr-1Mo-V-Nb weld metal after long term use, microstructure and precipitates distribution before and after aging at 1013 K were investigated. In the weld metal, regions with coarse or fine prior austenite grains were observed due to thermal cycle during welding. In the coarse grain region, precipitate particles inferred to M₂₃C₆ were densely located on grain boundaries, however, in the fine grain regions, they were sparsely observed not only on grain boundaries but also inside grains. Post weld heat treatment (1013 K/7.7 h) followed by aging (1013 K/100 h) led to ferrite grains formation in the fine grain region. EBSD analysis implied that dislocation density in ferrite grains was low. After the aging, mean diameter of particles became coarser and interparticle spacing became sparser in the fine grain region than in the coarse grain region. On the other hand, dislocation density calculated by hardness in martensite structure was almost no deference between these regions before and after the aging. Therefore, it was suggested that ferrite grains were formed because pinning energy by precipitate particles locally reduced in the fine grain region.

This study was set to fundamentally investigate the characteristics of austenite reversion occurring in maraging steels additive-manufactured by laser powder bed fusion (L-PBF). The maraging steel samples manufactured under different L-PBF process conditions (laser power P and scan speed v) were subjected to heat treatments at 550°C for various durations, compared with the results of the austenitized and water-quenched sample with fully martensite structure. The L-PBF manufactured samples exhibited the martensite structure (including localized austenite (γ) phases) containing submicron-sized cellular structures. Enriched alloy elements were detected along the cell boundaries, whereas such cellar structure was not found in the water-quenched sample. The localized alloy elements can be rationalized by the continuous variations in the γ-phase composition in solidification during the L-PBF process. The precipitation of nanoscale intermetallic phases and the following austenitic reversion occurred in all of the experimental samples. The L-PBF manufactured samples exhibited faster kinetics of the precipitation and austenite reversion than the water-quenched sample at elevated temperatures. The kinetics changed depending on the L-PBF process condition. The enriched Ni element (for stabilizing γ phase) localized at cell boundaries would play a role in the nucleation site for the formation of γ phase at 550°C, resulting in enhanced austenite reversion in the L-PBF manufactured samples. The variation in the reaction kinetics depending on the L-PBF condition would be due to the varied thermal profiles of the manufactured samples by consecutive scanning laser irradiation operated under different P and v values.

Ausforming processes are those thermomechanical treatments in which austenite is plastically deformed before either a martensitic or a bainitic transformation takes place. Although the deformation of an austenitized steel at intermediate temperatures has many benefits, it can also induce displacive transformations, sometimes unavoidably at the industrial level. Although the addition of a mechanical driving force, associated to the applied stress, has been shown to accelerate the bainitic transformations and promote transformation plasticity and variant selection, little information is found on the effect of stress on parameters such as bainitic ferrite plate thickness or volume percentage of retained austenite. Some works suggest that constant stresses can coarsen bainitic ferrite plates and increase the amount of transformed bainitic ferrite, although no systematic work has been conducted in this regard. This work aims to better understand the characteristics of bainitic microstructures formed during the application of stress or under constant stress and discuss the mechanisms affecting the bainitic transformations. Among the obtained results, it can be highlighted that, at temperatures below the bainite start temperature, bainitic ferrite plates formed during straining are more refined that isothermally formed plates at the same temperature, whereas constant stresses leads to an increased fraction of coarser bainitic ferrite plates, as compared to those microstructures obtained at the same temperature.

Simulated thermochemical controlled processing (TMCP) was performed on four microalloyed plate steels with Nb and Mo contents varying from 0.03 to 0.045 and 0.03 to 0.15 wt pct, respectively, to investigate influences of both processing and alloying on transformation behavior and microstructural evolution. Dilatometry was performed in situ in a Gleeble^® 3500 during thermomechanical simulation to construct continuous cooling transformation (CCT) diagrams for all alloys. A range of cooling rates between 2°C/s and a target 100°C/s along with two deformation levels, −0.4 total true strain and −0.6 total true strain in the austenite regime, were employed to create a range of microstructures. Increased deformation in the austenite non-recrystallization region promoted both polygonal ferrite and acicular ferrite transformation through an increase in nucleation sites. The increase in nucleation sites also resulted in a finer resultant microstructure with increased deformation. Increased cooling rates reduced transformation start temperatures and favored non-polygonal transformation products. Intermediate cooling rates led to the more desirable microstructures consisting of acicular ferrite and bainite. Both Nb and Mo increased the hardenability of the steel through interactions with the polygonal ferrite transformation. Nb and Mo retarded the polygonal ferrite transformation and favored an acicular morphology. Molybdenum alloying also favored bainite transformation. Desirable microstructures of acicular ferrite and bainite were able to be produced with the combination of higher deformation, intermediate cooling rates, and increased Nb/Mo alloying.

A crystallographic analysis was conducted on the lower bainite formed at the austenite grain boundary in Fe-0.6C-0.8Mn-1.8Si (in mass%) steel at the initial stage of transformation. The variant selection and the effect of the character of the austenite grain boundary on the formation of lower bainite were investigated from several perspectives: the character of the prior austenite grain boundary (PAGB), the crystal orientation relationship between the bainitic ferrite (BF) and the adjacent prior austenite grain (PAG) across the PAGB, and the geometrical relationship between the BF and PAGB plane. BFs do not form at twin boundaries but form at high-angle grain boundaries. The effect of the orientation of the adjacent PAG across the PAGB is not dominant in the case of lower bainite formation, while the PAGB plane strongly affects the formation and the variant selection of BF. BF tends to form when its habit plane or shape deformation direction is nearly parallel to the PAGB plane. It is suggested that the formation of BF, whose HP is parallel to the PAGB, is favored from the perspectives of the interfacial energy, the elastic strain energy, and the plastic accommodation.

A modified continuum composite model was utilized to analyze the stress–strain behavior of as-quenched steels with a fully martensitic microstructure. The model was employed to express the stress–strain curves obtained by a simple tensile test and those obtained by forward and backward loading using a simple shear test machine. The study confirmed that the model can represent the stress–strain behavior under both forward and backward deformation. In addition, the evolution of local strain distributions during plastic deformation was investigated using a digital image correlation method. The strain distributions and their evolution during deformation were qualitatively represented using this model. The discrepancies between the model calculations and experiments are due to the limitations of the iso-work assumption and the impact of slip deformation on the macroscopic work-hardening behavior of martensitic steels. Highly strain-concentrated regions aligned along the longitudinal direction of the lath and block, known as in-lath-plane slips, may not play an important role in the work-hardening behavior of as-quenched martensitic steels. However, the other slips, namely the out-of-lath slip, may play a significant role in the work hardening of as-quenched martensitic steels.

The effects of pre-strain on the mechanical properties of high-strength martensitic steels were investigated using either strain tempering (ST) or quenching and tempering (QT) samples. In the tensile tests at deformation temperatures between 296 and 573 K, the ST sample exhibited an increase in both the tensile strength (TS) and uniform elongation (U.El) at 473 to 523 K, whereas the QT sample showed an increase in U.El with little change in the TS and yield strength (YS). The results of in situ neutron diffraction experiments revealed an increase in the stress partitioning to the bcc phase with an increase in the deformation temperature from 296 to 523 K. The difference in the phase stress between the bcc and cementite phases decreased with an increase in the temperature due to the decrease in the cementite strength. Pre-strain of 0.5% increased the YS at 296 K with a slight work hardening; the initial dislocation density (ρ) decreased at 523 K, but increased significantly after yielding, leading to a better combination of TS and U.El. The combination of pre-strain, tempering, and deformation temperatures caused the change in ρ before deformation and the increase in ρ after yielding of the martensitic steel.

Commonly, plain carbon fresh martensitic steel shows high strength and breaking immediately. However, low C – 2wt.% Si – 5wt.% Mn fresh martensitic steel exhibits excellent strength and ductility. But it’s not clear why 5 wt.%Mn martensitic steel has ductility more than plain carbon martensitic steel. In this study, the effect of C and Mn on strength and ductility is investigated by in-situ X-ray diffraction during tensile tests in SPring-8. The relationship between the work hardening behavior and the dislocation behavior is analyzed. The dislocation density was calculated with modified Williamson-Hall method and modified Warren-Averbach method. XAFS measurement was also performed to investigate the interaction between Mn and C. It was found that the increase rate of dislocation density in plain carbon martensitic steel was higher than in 5%Mn martensitic steel. Adding 5%Mn, the increase rate of dislocation density can be reduced. We found that the tensile strength is determined by the upper limit of dislocation density, and the uniform ductility is determined by the increase rate of the dislocation density. The dislocation arrangement parameter obtained by XRD and TEM observation revealed that adding 5%Mn inhibited the formation of dislocation cell structure, resulting in increasing dislocation density. From XAFS results, it is considered that the attractive distance between Mn atoms and C atoms increases the interparticle distance of C atoms fixed to dislocations. Therefore, it is considered that the shear stress on dislocation for breaking through between the solute atoms decreases, and the work hardening rate decreases.

Linear friction welding (LFW) is a solid-state joining process in which a joint is formed through the relative oscillation of two components under a high contact load. In this method, the joining temperature can be determined from the applied pressure, which is the focus of this study. Quenched and subsequently tempered SCM440 steel was joined using LFW at applied pressures of 150–1200 MPa. The effect of applied pressure on the Vickers hardness and microstructures was investigated. The joining temperature decreased with increasing applied pressure until a pressure of 900 MPa was reached. However, the joining temperature rose again above the A₃ point when the applied pressure increased to 1200 MPa. The deformation during LFW was presumed to be relatively limited to the interface region under extremely high applied pressure, which caused an overshoot in the temperature of the joint interface. In the case of low applied pressure, slightly elongated lath martensitic microstructures with a much smaller size than the usually quenched lath martensitic microstructure were formed; however, the misorientation distribution of the grains was rather similar to that of the as-quenched one. On the other hand, in the case of high applied pressure, an equiaxed extremely fine globular martensitic microstructure as small as 0.2 µm with large misorientation was formed. Martensitic transformation was assumed to have occurred in a single-variant manner from extremely fine, dynamically recrystallized austenite grains. The hardness distributions exhibited good agreement with the microstructural variations with the applied pressure and distance from the joint center.

This study investigated the low-temperature fracture toughness of martensite and bainite with various Bain unit sizes. The three-point bending tests revealed that the apparent fracture toughness increased with decreasing the Bain unit size. We also found that even when the carbide size and distribution were almost the same, the apparent fracture toughness of tempered martensite with Bain unit size of 2.5 µm was much higher than that of bainite with Bain unit size of 16.2 µm. The propagation of micro-crack stopped at the Bain unit boundaries when the Bain unit size was small. The additional load was necessary for further propagation of crack which stopped at the Bain unit boundaries, leading to the improvement of fracture toughness. The critical local fracture toughness corresponding to the propagation of crack across the Bain unit boundaries was estimated at 1.04 MPa m^1/2 by finite element simulations. Based on this value, we proposed that the Bain unit boundary whose interval was less than 9.4 µm could become obstacle for the crack propagation after penetrating matrix/carbide boundary.

In this study, we prepared hypereutectoid SUJ2 and SK3 steels with cementite (θ) particles of varying area fraction, particle size and circularity. The steels were then subjected to a specially-designed heat treatment aimed at eliminating coarse θ particles located on the grain boundaries. The effect of heat treatment on the characteristics of the θ particles and the resultant toughness were evaluated by Charpy impact tests. The designed heat treatment successfully achieved the spheroidization and annihilation of the θ particles on the grain boundaries, especially in the SUJ2 steels. Specifically, the heat-treated SUJ2 samples showed a ductile fracture surface with fine and homogeneously distributed dimples, whereas the SK3 samples showed a fracture surface with a mixture of cleavage and dimple surfaces, and grain boundary fracture was also partly observed. The impact values tended to be high when the size and area fraction of the θ particles on the grain boundaries were small. Furthermore, the circularity of the θ particles at the grain boundaries significantly influenced the impact value. It has been demonstrated that if the circularity of the θ particles is high, the impact values are less likely to deteriorate, even if there are large θ particles on the grain boundaries.

Effect of carbon (C) content (0.05 mass% to 0.3 mass%) on critical intergranular fracture stress of tempered martensite steel was investigated using 3 mass% manganese (Mn) steel. The critical intergranular fracture stress was obtained by calculating the maximum principal stress at fracture in a tensile test of circumferentially notched round bar specimen using elastoplastic finite element analysis. As a result, critical intergranular fracture stress of tempered martensite steel increased with increasing the C content. Therefore, the dominant factors of critical intergranular fracture stress were examined from the viewpoints of the amount of segregation of each element on prior-austenite grain boundaries and the grain size of the martensite substructure. The first result was found to be the effect of reducing the amount of Mn segregation by increasing the C content. This was thought to be because cementite acts as a solid solution site for Mn, and the amount of Mn in solid solution in the matrix phase is reduced by increasing the C content. The second result was found to be the effect of refining the substructure of martensite surrounded by high angle grain boundaries by increasing the C content. This indicates that grain size also affects crack initiation resistance in intergranular fracture by determining the stress concentration at the grain boundaries as the distance of dislocation accumulation.

The microstructure and corresponding tensile properties were examined in quenching and partitioning (Q&P) processed low carbon steels, depending on the silicon content ranging from 0.1–2.0 wt.%. The silicon content and process temperature generated a highly interactive influence on the evolution of final microstructure, including the fraction of constituent phases and their characteristics such as solute carbon content in each phase. The yield strength was nearly unchanged or slightly decreased even with the silicon addition for a given Q&P condition. The change of yield strength showed a reasonable correlation with the loss of solute carbon in martensite or bainite caused by the carbide precipitation and the carbon partitioning into austenite, which depended on the silicon content. High partitioning temperature enhanced the yield strength for a given silicon content and quenching condition, because of the tempering effect on the martensite matrix. Although the fraction and stability of retained austenite were still critical for improving ductility, the intrinsic properties of the martensite matrix, such as the occurrence of tempered martensite embrittlement, governed the ductility of Q&P steels in situations where the role of retained austenite was limited due to low fraction or poor mechanical stability.

To investigate the stand-alone effect of austenite morphology on transformation-induced plasticity (TRIP), martensitic steels with second-phase austenite, whose morphology was controlled by the dissolution of alloy cementite, were fabricated with 0.6%C–3%Mn steel and their mechanical properties were evaluated in terms of the mechanical stability of austenite. The morphology of alloy cementite in the initial microstructure was controlled to lamellar and spherical through Mn partitioning pearlitic transformation and subsequent spheroidization. Alloy cementite was replaced with retained austenite after austenitization due to local austenite stabilization by Mn enrichment. As a result, two types of martensitic steels with lamellar- and spherical-shaped austenite grains could be prepared, where composition, fraction, and number density of the second-phase austenite were kept the same, while only the morphology was changed significantly. Compression tests revealed that the steel with lamellar austenite maintains higher strain hardening rate than that with spherical austenite. The higher strain hardenability was attributed to the TRIP effect, indicating that the lamellar austenite has lower mechanical stability. Furthermore, nanoindentation tests was conducted to directory evaluate the elastic-plastic deformation behavior of second-phase austenite. The deformation behavior of the second-phase austenite showed a clear morphology dependence; the lamellar austenite shows larger strain hardening from the early stage of plastic deformation. Detailed analysis suggested a possibility that the interphase boundary between the martensite matrix and second-phase austenite acts as a preferential nucleation site for deformation-induced martensitic transformation.

The strain distribution in the 9mass%Ni steel introduced by tensile deformation at cryogenic temperature was visualized using a digital image correlation method, and the relationship between the strain distribution and the microstructure of the steel was systematically investigated. Based on the obtained results, the factors that influence the strain distribution were discussed. In the present 9mass%Ni steel, regions consisting of tempered- and fresh-martensite and austenite (TFMA) were embedded in the tempered martensite matrix. The volume fraction of the retained austenite varied along the normal direction of the hot-rolled plates, indicating that Ni segregation occurred during the manufacturing process. As the tensile stress increased, the total elongation remained constant with decreasing temperature. Strain was introduced inhomogeneously via tensile deformation at 77 K. The high- and low-strain regions tended to be distributed in a unit of the block. It indicates that the deformability differed among the blocks. The average strain in the block (ε_block) was strongly correlated with the Schmid factor of the slip system on the habit plane (SF_habit) and area fraction of TFMA in a block (A_MA). The least absolute shrinkage and selection operator regression revealed that the contributions of SF_habit and A_MA to ε_block were nearly equal. Therefore, the deformability of the block in the 9mass%Ni steel is dominated by SF_habit and A_TFMA.

Quenching and Partitioning (Q&P) steel sheets, which utilize the transformation induced plasticity (TRIP) effect of retained austenite to improve the elongation of high strength steel sheets, are expected to become an important material for next-generation automotive structural parts. Although it has been reported that the yield strength (YS) of the Q&P steels consisting of tempered martensitic microstructure with retained austenite (hereafter ”Q&P steels” in this study) is affected by retained austenite, the mechanism has not yet been discussed in detail. The purpose of this study is to clarify the effect of the carbon content in retained austenite on the yielding behavior of the Q&P steels. The chemical composition of the model steel used here was 0.18%C-1.5%Si-3.0%Mn (mass%). The steels were annealed at 1143 K, then cooled to 473 K, followed by holding at the temperatures between 523 K and 673 K for 600 s. The increased carbon content in retained austenite increased the YS of the Q&P steels. It was found that the yielding of the Q&P steels was caused by the stress-induced transformation of retained austenite when the critical stress for the stress-induced transformation was lower than the elastic limit of tempered martensitic matrix. This result revealed that the increased carbon content in retained austenite was able to achieve the higher elastic limit of martensitic steels containing retained austenite.

The effects of retained austenite upon softening during low-temperature tempering at 373 K were investigated using martensitic carbon steels with and without retained austenite. To increase the amount of retained austenite, 10 mass% Ni was added to the base carbon steel (Fe-0.3C alloy). During tempering, the hardness decreased more rapidly in the Ni-added steel containing 6 vol.% retained austenite than in the base steel without retained austenite. Analyses of the microstructure and the carbon content in the solid solution (i.e., the solute carbon concentration) revealed that the retained austenite tended to suppress carbide precipitation and significantly reduced the solute carbon concentration in the martensitic matrix. We demonstrated that retained austenite acts as an effective absorption site for solute carbon in the martensitic matrix; however, the partitioned carbon is unevenly localized near the martensite/austenite interface, owing to the poor diffusivity at 373 K.

Deformation-induced martensitic transformation (DIMT) during tensile or compressive deformations of the bainitic steels with various carbon content (0.15%C, 0.25%C, 0.62%C) was studied. The initial microstructure before the deformation tests was prepared by the austempering at 400°C to obtain bainitic structure consisting of bainitic ferrite and retained austenite. The volume fraction of the retained austenite was larger in the bainitic steel with the larger carbon content. In all of the bainitic steels, the tensile deformation exhibited larger work hardening than the compression. This difference indicates the suppression of the DIMT at the compression, and actually the measurements of electron back scattering diffraction (EBSD) confirmed the less reduction of retained austenite at the compression of all the bainitic steels. Additionally, the steel with the highest carbon content was examined by in situ neutron diffraction and clarified the difference similar to that obtained by the EBSD measurement. The regression of the relation between the fraction of austenite and applied strain with the conventional empirical equation revealed that the kinetic of DIMT is strongly dependent with the stress polarity, but not significantly changed by the carbon content. The mechanism of the DIMT dependence of the stress polarity was discussed with the deformation texture and the crystallographic orientation dependence of DIMT.

The high-speed deformation behaviors of SUS316LN stainless steel, mainly comprised of deformation twin domains, ultrafine lamellae, and shear bands to form a heterogeneous-nano (HN) structure, were systematically investigated by means of Charpy impact tests and dynamic tensile tests. At the lowest applied impact velocity of v = 10⁻² ms⁻¹, the Charpy impact value increased with decreasing temperature from room temperature (RT) down to 173 K. Nevertheless, at the highest impact velocity of v = 10⁰ ms⁻¹, the impact values were approximately identical irrespective of temperature and were lower than those at 10⁻² ms⁻¹. Deformation-induced γ → α’ martensitic transformation was found to take place by the impact tests. The change in the volume fraction f of the α’ phase exhibited the same tendency as the impact value. That is, the value of f increased with decreasing temperature at v = 10⁻² ms⁻¹ and, however, it was almost identical regardless of the temperature at v = 10⁰ ms⁻¹. It was revealed by dynamic tensile tests that the temperature at the gauge section of the specimens increased abruptly with strain rate and reached approximately 720 K at = 10³ s⁻¹. The above experimental results can be reasonably understood from the suppression of the γ → α’ martensitic transformation by the increase in temperature during high-speed deformation. The above analyses strongly suggest that deformation-induced martensitic transformation plays an important role in the Charpy impact value via the transformation-induced plasticity effect.

The hydrogen desorption behaviors of the SUS304 and SUS316L austenitic stainless steels during deformation at ambient temperature were investigated using a tensile test machine in a vacuum chamber equipped with a mass spectrometer. Obvious hydrogen desorption was detected only in the SUS304 steel, which exhibited a distinct martensitic transformation. Because the hydrogen desorption rate in SUS304 decreased when deformation stopped, a significant factor causing transformation-induced hydrogen desorption was an increase in martensite fraction during plastic deformation. Furthermore, hydrogen promoted both martensitic transformations to ε and to α′, which assists the hydrogen desorption. These results indicate the presence of synergistic interactions between the hydrogen uptake/diffusion and martensitic transformation. In contrast, SUS316L steel showed no martensitic transformation and exhibited hydrogen-assisted deformation twinning. No significant increase in hydrogen desorption was observed during plastic deformation. This result indicates that deformation twinning has no effect on hydrogen diffusion/desorption.

It is well known that ferritic steels yield discontinuously with a clear yield point, while martensitic steels yield continuously with a low elastic limit followed by significantly large strain hardening. To evaluate the unique elastic-plastic deformation behavior of martensitic steels easily and accurately, nanoindentation tests were conducted using martensitic steels with lath martensitic structure, and the obtained results were then compared with that of ferritic steel while taking into account the pop-in phenomenon. In the normal load-displacement curves, ferritic steel had pop-in clearly, but no pop-in was observed for martensitic steels, regardless of carbon content. However, the analysis based on Hertz’s contact theory made it possible to quantitatively evaluate the elastic limit of martensitic steel as well as ferritic steel. As a result, it was found that the elastic limit of martensitic steel is much lower than that of ferritic steel, and the plastic strain at yielding is also quite small. The plastic deformation behavior based on dislocation theory suggests that the yielding of ferritic steels is governed by dislocation nucleation and subsequent dislocation avalanche. In contrast, the yielding phenomenon of martensitic steels might be greatly influenced by the motion of pre-existing mobile dislocations introduced through martensitic transformation.

The reverse transformation behavior during heating in Fe-10%Mn-0.1%C (mass%) martensitic alloy consisting of α’-martensite, ε-martensite and retained austenite was investigated using the in-situ neutron diffraction. When the temperature was elevated with a heating rate of 10 K/s, the ε→γ reverse transformation occurred first at the temperature range of 535–712 K, where Fe and Mn hardly diffused. In the temperature range where the ε→γ reverse transformation occurred, the full width at half maximum of the 200γ peak increased, indicating that the austenite reversed from ε-martensite contains high-density dislocations. In addition, the transformation temperature hardly depends on the heating rate and the crystal orientation of the reversed austenite was identical to that of the prior austenite (austenite memory), which suggests that the ε→γ reverse transformation would proceed through the displacive mechanism. After completion of the ε→γ transformation, the α’→γ reverse transformation occurred at the temperature range of 842–950 K. When the heating rate is low (<10 K/s), the reverse transformation start temperature significantly depends on the heating rate. It could be because the diffusional reverse transformation accompanying the repartitioning of Mn occurs. On the other hand, a higher heating rate (≥10 K/s) resulted in the disappearance of the heating rate dependence. This was probably due to the change in the transformation mechanism to the massive-type transformation, which is diffusional transformation without repartitioning of Mn.

The strain distribution and microstructure at the crack nucleation sites in martensitic steel with fine- and coarse-prior austenite grains subjected to tensile deformation were characterized using the digital image correlation method on replica films. Although the tensile properties of the fine- and coarse-prior austenite grain specimens were approximately identical, the total strain was certainly improved in the coarse-prior austenite grain specimen. The crack size increased with the coarsening prior austenite grains, whereas the number of cracks decreased. An inhomogeneous strain was introduced in both the specimens by tensile deformation. The accumulated strain when crack nucleates was approximately the same in both specimens, independent of the prior austenite grain size. In low-strain regions, there were no cracks even though the accumulated strain was comparable to that when crack nucleates in high-strain regions. The strain at the crack nucleation sites was high even before crack nucleation occurred. Cracks primarily nucleated on packet and prior austenite grain boundaries, even in the coarse-prior austenite grain specimen, which confirmed that the prior austenite grain boundary should be a preferential crack nucleation site. It can be concluded that the high local strain and the presence of packet or prior austenite grain boundaries are responsible for crack nucleation in martensitic steel subjected to tensile deformation.

Specimens with different microstructures (bainite, as-quenched martensite, and tempered martensite) were fabricated using a Fe-0.4C-1.5Si-2Mn steel sheet, and the pitting corrosion resistances of these microstructures were compared. Retained austenite was barely detected in the X-ray diffraction analysis. The Vickers hardness values of the microstructures were ordered as (high) as-quenched martensite > tempered martensite ≈ bainite in the 325°C-austempered specimen > bainite in the 425°C-austempered specimen (low). The pitting corrosion resistance of each microstructure was evaluated by potentiodynamic polarization in boric-borate buffer solutions containing NaCl (pH 8.0) under naturally aerated conditions. The pitting corrosion resistances of the microstructures were ordered as (high) as-quenched martensite > bainite in the 325°C-austempered specimen > tempered martensite > bainite in the 425°C-austempered specimen (low). The lower active dissolution rates of the microstructures were determined to provide superior pitting corrosion resistance.