As-quenched Martensite Research Articles

Unveiling the roles of initial phase constituents and phase metastability in hydrogen embrittlement of TRIP‐assisted VCrCoFeNi medium‐entropy alloys

Medium-entropy alloys (MEAs) that exhibit transformation-induced plasticity (TRIP) from face-centered cubic (FCC) to body-centered cubic (BCC) are considered promising for liquid hydrogen environments due to their remarkable cryogenic strength. Nonetheless, studies on hydrogen embrittlement (HE) in BCC-TRIP MEAs have not been conducted, although the TRIP effect and consequent BCC martensite usually deteriorate HE susceptibility. In these alloys, initial as-quenched martensite alters hydrogen diffusion and trap behavior, and deformation-induced martensitic transformation (DIMT) provides preferred crack propagation sites, which critically affects HE susceptibility. Therefore, this study aims to investigate the HE behavior of BCC-TRIP MEAs by designing four V10Cr10Co30Fe50–xNix (x = 0, 1, 2, and 3 at%) MEAs, adjusting both the initial phase constituent and phase metastability. A decreased Ni content leads to a reduced fraction and mechanical stability of FCC, which in turn increases HE susceptibility, as determined through electrochemical hydrogen pre-charging and slow-strain rate tests The permeation test and thermal desorption analysis reveal that the hydrogen diffusivity and content are affected by initial BCC fraction, interconnectivity of BCC, and refined FCC. As these initial phase constituents differ between the alloys with FCC- and BCC- dominant initial phase, microstructural factors affecting HE are unveiled discretely among these alloy groups by correlation of hydrogen-induced crack behavior with hydrogen diffusion and trap behavior. In alloys with an FCC-dominant initial phase, the initial BCC fraction and DIMT initiation rate emerge as critical factors, rather than the extent of DIMT. For BCC-dominant alloys, the primary contributor is an increase in the initial BCC fraction, rather than the extent or rate of DIMT. The unraveled roles of microstructural factors provide insights into designing HE-resistant BCC-TRIP MEAs.

Comparison of the Pitting Corrosion Resistance of Bainite and Martensite in Fe-0.4C-1.5Si-2Mn Steel

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.

Rapid tempering of a medium-carbon martensitic steel: In-depth exploration of the microstructure – mechanical property evolution

Rapid tempering is a remarkable sustainable and energy-efficient way to modify properties of as-quenched martensite, particularly its toughness and ductility. In this work, tensile and fracture toughness properties, and microstructural evolution of a medium-carbon (0.4 wt%), low-alloy steel under different rapid tempering circumstances are discussed. The hot-rolled and direct-quenched specimens were subjected to rapid tempering treatments (heating rate of about 90 °C/s) to the different tempering temperatures (320–720 °C). As a reference material, one sample was subjected to conventional tempering at 420 °C for one hour. The results indicated that the mechanical properties and microstructural features are influenced considerably by tempering temperature rather than the holding time. Despite the short tempering duration, the strength of tempered martensite dropped (from a max. tensile strength of ∼ 2100 MPa to a min. of ∼ 1100 MPa) while ductility increased with increasing tempering temperature (from a tensile elongation of 4% to ∼ 15%). However, the rapidly tempered sample at 420 °C experienced a loss of fracture toughness as a result of coarse stick-like cementite precipitation. Microstructural observations revealed that, even with a short time frame, tempering temperature plays a significant role in the resulting cementite morphology. At lower tempering temperatures, the cementite precipitates have a stick-like structure, while at higher temperatures, they become more globular/spheroid-like. This change in cementite morphology led to an enhancement in fracture toughness. Overall, the results indicated that, by a proper design of the rapid tempering process, an excellent combination of tensile properties and fracture toughness can be obtained compared to conventional tempering while significantly saving time and energy.

Substructure and crystallography of lath martensite in as-quenched interstitial-free steel and low-carbon steel

In this study, the substructures and crystallographic features of as-quenched lath martensite in interstitial-free (IF) steel and low-carbon steel (0.2C steel) were investigated using focused ion beam-scanning electron microscopy serial sectioning and advanced transmission electron microscopy. A three-dimensional analysis revealed that the morphology of the so-called lath, the smallest structural unit, exhibited a wedge-shaped plate instead of the ideal lath shape. The habit plane orientations were distributed from (5 7 5)A to (1 1 1)A in the IF steel, whereas they were near to that predicted based on the phenomenological theory of martensite crystallography in the 0.2C steel. In the IF steel, well-developed dislocation cells, tangled dislocations, and curved dislocations could be observed. Conversely, straight dislocations and cementite formation were observed in the 0.2C steel. Moreover, the orientation changed significantly inside a given lath in both steels. The lath boundary comprised regularly spaced steps with (0 1 1)M atomistic terrace planes. Interfacial dislocations corresponding to the discontinuity of the (0 1 1)M lattice planes could be observed at steps, and these dislocations were introduced to compensate for the misorientation between adjacent laths. Additionally, carbon segregations were observed at the lath and block boundaries in the 0.2C steel. Our observation results on the dislocation structures, cementite formation, and segregation of carbon atoms on the lath and block boundaries indicated that the substructure changed significantly during quenching. Based on our experimental results, we discuss the origin of the substructures and crystallographic features of lath martensite.

Yielding behavior of triplex medium Mn steel alternated with cooling strategies altering martensite/ferrite interfacial feature

• Solute segregation at various interfaces characterized atomically in triplex medium Mn steel. • Two cooling strategies produced different martensite/ferrite interfaces for distinct yielding behavior. • Density and mobile ability of dislocations near martensite/ferrite interfaces determine yielding behavior. In this paper, we report the influence of cooling processes on the yielding behavior of a medium Mn steel (MMS) with triplex microstructure, i.e. austenite (γ), ferrite (α) and as-quenched martensite (α'). After the intercritical annealing (IA) at both 725 ℃ and 750 ℃, the steel was subjected to the two cooling processes, i.e. air cooling (AC) and water quenching (WQ). It exhibits the discontinuous yielding after the AC following the IA at 750 ℃ while the continuous yielding after the WQ. Compared with WQ process, both the dilatometry and the microstructural examinations show that the AC process leads to lower M s temperature, larger retained austenite (RA) fraction and less martensite, the latter is always companied with geometry necessary dislocations (GNDs) generated near the α/α' interfaces. Considering the complexity of nanosized tri-phases in this steel, the presence of martensite with key features in the resultant specimens was systematically examined by atom probe tomography (APT) on the samples prepared by the specific target lift-out method. The APT results directly revealed the C/Mn co-segregation at the α'/α interfaces in the AC samples but not in WQ samples. The numerical simulation results further suggest that the segregation of C and Mn at the α'/α interfaces may be due to different mechanisms. We conclude that the yielding of triplex MMS is determined by both the quantity of GNDs generated near the α/α' interfaces, which increases with martensite fraction, and the extent of their immobilization resulting from the interfacial segregation of solute atoms when the presence of martensite is sufficient. WQ tends to suppress the discontinuous yielding of MMS since the rapid cooling may promote more martensite formed with the increasing quantity of GNDs and prevent the interfacial segregation of both C and Mn.

(Corrosion Division Morris Cohen Graduate Student Award) Microscopic Electrochemical Properties of Carbon Steels and Metallurgical Approach for High Corrosion Resistance

Carbon steels are widely used in the production of automobiles, bridges, plates, and so on because of their superior mechanical properties and low cost. Carbon steels contain various types of microstructures such as ferrite, pearlite, and martensite, and desired mechanical properties can be obtained by combining these microstructures. While carbon steels exhibit excellent mechanical properties, their pitting corrosion resistance is relatively low in chloride-containing environments. Although the steels are successfully protected from corrosion by coating and/or painting, localized corrosion is readily initiated at cut edges and in coating defect areas in atmospheric environments. To prolong their service life and improve their reliability, it is necessary to elucidate the initiation mechanism of pitting corrosion on carbon steels. While it is widely known that the microstructures of carbon steels affect their corrosion resistance, the detailed mechanism is still unclear. The objective of this research is to clarify the relationship between the microstructure and pitting corrosion resistance of carbon steels in chloride-containing near-neutral pH environments.The pitting corrosion resistance of the typical microstructures of carbon steels was investigated using a micro-electrochemical polarization technique. Various microstructures (ferrite, pearlite, tempered martensite, decarburized martensite, as-quenched martensite etc.) were obtained by conducting different heat-treatments, and micro-scale electrodes (ca. 100 μm × 100 μm) were prepared with only one microstructure included. Small areas without any non-metallic inclusions were chosen as the micro-scale electrodes to eliminate the effect of pit initiation at the inclusion and evaluate the intrinsic corrosion behavior of the microstructure itself. Then, potentiodynamic anodic polarization measurements were carried out in NaCl-containing boric-borate buffers at pH 8.0. It was determined that the pitting corrosion resistance depends on the type of microstructure, with the as-quenched martensite showing the highest pitting corrosion resistance. The as-quenched martensite includes a high amount of interstitial C in the Fe lattice, and it was also clarified that the anodic dissolution of steels was clearly suppressed with increasing interstitial C content. During pitting corrosion, the local breakdown of the passive film is induced by chloride ions, which results in the initiation of active dissolution. It was considered that the low dissolution rate provided by interstitial C results in the superior pitting corrosion resistance of as-quenched martensite.To clarify the reason for the superior active dissolution resistance obtained by interstitial C, first-principles calculations were conducted. At first, the electronic structure of martensite was analyzed using bulk-model supercells with bct Fe crystal structures which included interstitial C. According to first-principles calculations, the electronic density of states (DOS) at and around the Fermi level decreased due to the presence of interstitial C. In addition, a valence electron transfer occurred from Fe to C atoms, resulting in a decrease in the number of valence electrons of Fe. Because the presence of valence electrons at and around the Fermi level affects the kinetics of electron (charge) transfer in oxidation/reduction reactions, including the active dissolution reaction, it was considered possible that the presence of interstitial C suppressed the electronic reactivity of Fe. Furthermore, first-principles calculations were also carried out for slab-model supercells consisting of a metal/vacuum interface, and the effect of interstitial C on the work function of martensite was analyzed. The result of the calculations indicated the work function of martensite linearly increased with increasing interstitial C content. Since the work function and ionization energy of metals are the same, the electrochemical properties of metals, such as dissolution resistance, are considered to be principally associated with their work function. In other words, it is understood that metals with a high work function are associated with a higher dissolution resistance. According to the above calculation results, the change in the electronic structure of Fe and the increase in the work function provided by interstitial C are key factors contributing to the superior dissolution resistance of martensite.

Correlation between strength and hardness for substructures of lath martensite in low- and medium-carbon steels

In this study, microtensile and nanoindentation tests were systematically performed to evaluate the contribution of the block boundary to yield strength and to correlate the strength and hardness of as-quenched lath martensite steels with low- and medium-carbon contents. A habit-plane-dependent yielding behaviour was observed in the single-block (SB) as well as single-packet (SP) structures, independent of the carbon content. The low-angle boundary is difficult to effectively act as an obstacle for dislocation motion. This suggests that the anisotropic yielding behaviour can be attributed to the dislocation wall structures formed during the early stage of deformation. The contribution of the single-block boundary to the resolved shear stress at the onset of yielding was estimated to be constant with increasing nominal carbon content. This indicates that the yield strength of the lath martensite structure increases with increasing carbon content through block refinement and solid solution strengthening. The relationship between tensile strength and the corresponding hardness was established for the SP and SB structures. The correlation coefficient, including the block boundary strength, was in accordance with the Tabor relationship, whereas the coefficient corresponding to the matrix excluding the block boundary strength was almost half lower than the Tabor's constant.

The Formation Mechanism of Nanocrystals after Martensitic Transformation.

Understanding the ultrafine substructure in freshly formed Fe-C martensite is the key point to reveal the real martensitic transformation mechanism. As-quenched martensite, whose transformation temperature is close to room temperature, has been investigated in detail by means of transmission electron microscopy (TEM) in this study. The observation results revealed that the freshly formed martensite after quenching is actually composed of ultrafine crystallites with a grain size of 1–2 nm. The present observation result matches well with the suggestion based on X-ray studies carried out one hundred years ago. Such nanocrystals are distributed throughout the entire martensite. The whole martensite shows a uniform contrast under both bright and dark field observation modes, irrespective of what observation directions are chosen. No defect contrast can be observed inside each nanocrystal. However, a body-centered cubic {112}<111>-type twinning relationship exists among the ultrafine α-Fe grains. Such ultrafine α-Fe grains or crystallites are the root cause of the fine microstructure formed in martensitic steels and high hardness after martensitic transformation. The formation mechanism of the ultrafine α-Fe grains in the freshly formed martensite will be discussed based on a new γ → α phase transformation mechanism.

Anisotropic cleavage fracture caused by transformation-induced internal stress in an as-quenched martensite

The effect of microscopic internal stresses (Type-II) generated via martensitic transformation from austenite (A) to martensite (M) on {001} M cleavage fracture was investigated in an as-quenched 0.1C–5.0Mn steel (mass%). Crystallographic orientation analysis using the electron backscatter diffraction technique revealed that cryogenic {001} M cleavage fracture was promoted predominantly in mode I fracture by Charpy impact testing, and that the Bain group, where martensite variants belong to the same Bain variants, acted as an effective unit to prevent fracture. It was also discovered that the cleavage crack preferentially propagates on (001) M rather than on (100) M in the Bain group, where (001) M was nearly parallel to (001) A in the Bain lattice correspondence ([−110] A // [100] M , [110] A // [010] M , (001) A // (001) M ). In addition, by using a combination of a micro-scale focused ion beam and high-precision digital image correlation techniques, it was found that microscopic internal stresses were anisotropically developed in each Bain group. The principal axes of the internal stresses corresponded to the <001> M coordinate axes in the Bain lattice correspondence, and the principal stress parallel to [001] M is much higher than that of the other two. After the measurement of the internal stresses, it was proved that the cleavage fracture behavior obeyed the effective normal stress in the normal direction of {001} M consisting of the resolved bending stress in the impact test and the anisotropic internal stress generated via martensitic transformation. This suggests that the transformation-induced internal stress originating from the Bain strain leads to the anisotropy of the cleavage fracture in lath martensite. • The effect of microscopic Type-II internal stress on the macroscopic cleavage fracture was investigated in lath martensite. • FIB-DIC technique revealed that martensitic transformation induces the anisotropic elastic strain in each Bain group while maintaining <001> as the principal axes. • EBSD analysis proved that mode-I cleavage crack preferentially propagates on (001) rather than on (100) in a Bain group regardless of carbon contents. • Transformation-induced internal stress originating from Bain strain leads to the anisotropy of the cleavage fracture in lath martensite.

Hierarchically patterned multiphase steels created by localised laser treatments

The realisation of sophisticated hierarchically patterned multiphase steels has the potential to enable unprecedented properties in engineering components. The present work explores the controlled creation of patterned multiphase steels in which the patterns are defined by two different crystal structures: face centre cubic or fcc (austenite) and body centre cubic or bcc (martensite). These austenite/martensite mesostructures are generated by solid–solid phase transformations during the application of localised laser heat treatments in a Fe-Ni-C alloy. In particular, four patterned configurations are analysed in this work consisting of one or two horizontal austenite line structures imprinted in a base of as-quenched or tempered martensite. Digital image correlation analysis during tensile testing of the developed materials showed that both the strength of the base martensite and the mesostructure at the gauge have a strong effect on the resulting properties. Clear differences were observed among the configurations in strain partitioning, hardening of the different constituents and failure. The uniform elongation and tensile strength are increased with respect to that of the reference martensite and austenite, respectively. Concepts explored in this work can be extended to more complex patterns and other base microstructures, opening novel strategies to engineer properties in steel and other alloys.

Strengthening via Grain Refinement in Lath Martensite on Low Carbon Fe–18Ni Alloys

The lath martensite structure in steel offers high strength with a complex substructure, and its strength increases with carbon content. However, the mechanism of carbon strengthening is yet to be elucidated. In this study, we evaluate the tensile properties of as-quenched lath martensite without retained austenite in Fe–18Ni alloys containing 4–570 ppm carbon. In the 4 mass ppm carbon alloy, whose carbon is almost trapped by Ti(CN) particles, the work hardening behavior during uniform elongation is constant regardless of the size of the effective grain surrounded by a high angle boundary. In contrast, the yield point (YP), 0.2% proof stress (σ0.2%), 0.6% proof stress (σ0.6%), and maximum tensile strength (TS) in 7, 110, and 570 mass ppm carbon alloys increase with the refinement of their effective grain, consistent with the Hall–Petch relationship. The Hall–Petch intercepts for the YP, σ0.2%, σ0.6%, and TS are constant and unaffected by the carbon content. This suggests that the non-occurrence of solution hardening by solute carbon atoms in the lath martensite. The Hall–Petch coefficients for the YP, σ0.2%, σ0.6%, and TS increase with carbon content and are proportional to the square root of the carbon content. This indicates that the increase in carbon content increases the strength of the lath martensite via the refinement of effective grains and the increase in the effectiveness of grain refinement.

Quantitative evaluation of dislocation density in as-quenched martensite with tetragonality by X-ray line profile analysis in a medium-carbon steel

The crystal structure of as-quenched martensite in steel changes from body-centered cubic to body-centered tetragonal because of the presence of interstitial carbon, and the c/a ratio increases with increasing carbon content. Because the X-ray diffraction peak profiles associated with the c-axis overlap those of the a-axis, the diffraction peak observed in martensite gradually broadens as the solute carbon increases. This may cause an overestimation of the dislocation density in the X-ray line profile analysis (for example, modified Williamson-Hall/Warren-Averbach method). In this study, the effect of the tetragonality of martensite in the X-ray line profile analysis was investigated to evaluate the dislocation density in a medium-carbon martensitic steel. The apparent dislocation density estimated without considering peak broadening due to tetragonality was calculated to be 1.7 × 1016 m−2 in the as-quenched 0.55%C steel. This value is approximately six times higher than that estimated using transmission electron microscopy (TEM), indicating that the dislocation density was overestimated. To eliminate the effect of tetragonality, the specimens were tempered at temperatures below 623 K. The estimation based on the TEM method revealed that the dislocation density only slightly decreased by tempering at 573 K, indicating negligible recovery at this temperature. The dislocation density obtained in the tempered specimen was calculated to be approximately 4.5 × 1015 m−2 using the mWH/WA method. This value was regarded as the true dislocation density of the as-quenched martensite in the 0.55%C steel.

Impact of Dislocation Density and Mobility on Yielding Behavior in Quenched Medium-carbon Martensitic Steel Tempered at Low Temperature

Middle-carbon martensite steels are vital materials for mechanical components and their mechanical properties have attracted significant interest. However, the decrease in the elastic limit of the as-quenched materials is one of the remaining puzzles. Herein, we quantitatively characterized the dislocation density and its structure in the as-quenched and tempered martensite steel by neutron diffraction line profile analysis and discussed their impact on the yield stress. The dislocation density in the as-quenched specimen was the highest at 9.7 × 1015 m−2, while it decreased with an increase in the tempering temperature. In addition, the component ratios of edge and screw dislocations decreased and increased, respectively, depending on the increase in the tempering temperature. The dislocation arrangement parameter (M) varied between the tempering temperatures of 220 and 290°C. Although there was a large difference between the yield stress obtained from the tensile test and that estimated from the dislocation density, the experimental results could be explained by correcting them with the inverse of M value as an index showing the effective dislocation density ratio.

Dual-phase hetero-structured strategy to improve ductility of a low carbon martensitic steel

Martensitic transformation significantly increases the strength of low-carbon steels, while it is usually at expense of the formability and ductility. In order to further improve the mechanical properties of low carbon martensitic steel, the strategy of dual-phase heterostructure was proposed. The steel with nano-lamellar structure in size of 83 nm was produced by cyclic annealing & cold rolling (AnnCR) on the martensitic structure. Then, the ultrafine-grained heterostructured dual-phase (UFG-HSDP) steels with outstanding combination of strength and ductility were achieved by subsequent short-time intercritical annealing. A promising heterostructure of soft ferrite grains completely embedded in hard martensite grains was formed in the sample annealed at 820 °C. A high strength of ∼1.1 GPa, close to the as-quenched full martensite steel, was retained in the HSDP steel. While, the uniform elongation was significantly improved to 6% by tailoring the dual-phase distribution. Hetero-deformation induced (HDI) stress, derived from the mechanical incompatibility of the dual-phase, is proposed to provide an extra strain hardening in the HSDP steels. Detailed microstructure analysis indicates that geometrically necessary dislocations piled-up near the zone interfaces produce a long-range back stress in the ferrite zones as well as a corresponding forward stress in the martensite zones, collectively resulting in the hetero-deformation induced (HDI) stress.

Modelling and experimental validation of austenite growth from As-quenched martensite during intercritical annealing of a medium-Mn steel

Modelling and experimental validation of austenite growth from As-quenched martensite during intercritical annealing of a medium-Mn steel

Enhanced ductility of as-quenched martensite by highly stable nano-sized austenite

Austenite plays a key role to improve tensile properties of advanced high strength steel (AHSS). In this study, nano-sized austenite particle was produced in as-quenched martensite by using chemically heterogeneous initial microstructure consisting of Mn-enriched cementite and ferritic matrix. Mn-enriched cementite transforms into nano-sized austenite during austenitzation and considerable amount of them retain after cooling to ambient temperature. The austenite particles have an exceptional stability, which cannot be fully interpreted with the Mn enrichment. Possible contributions from the compressive stress and the C partitioning into the austenite are considered. A persistent TRIP effect by highly stable austenite contributes to remarkable improvement of tensile ductility without compromising tensile strength even in the as-quenched martensite.

Excess Solute Carbon and Retained Tetragonality in Tempered Fe-0.6C-1Mn Martensite and the Effect of Silicon Addition

The tetragonality and carbon distribution in tempered Fe-0.6C-1Mn martensite were investigated by X-ray diffraction and atom probe tomography to elucidate strain relaxation in the tetragonal lattice during tempering and its relationship with the solubility of excess carbon in martensite. Even though tetragonality (c/a) decreased with an increase in the tempering temperature, it persisted at low levels up to 400 °C. Si addition suppressed the decrease in tetragonality at 400 °C by inhibiting recovery in the dislocated matrix. Such persistence implies that dislocation migration is crucial for the complete release of tetragonal lattice strain at such a temperature, in addition to the decrease in the amount of solute carbon in martensite. A low level of tetragonality was observed for martensite containing carbon in the solid solution below the critical value of ~ 0.2 mass pct, at which a bcc structure was predicted. The amount of solute carbon after tempering was linearly correlated with tetragonality in the solute carbon content range of 0.07 to 0.6 mass pct, and the correlation coefficient was similar to those for as-quenched auto-tempered martensite and bainitic ferrite; these results indicate that the amount of excess carbon is simply determined by the amount of tetragonal lattice distortions remaining after carbide precipitation and recovery.

Role of cementite and retained austenite on austenite reversion from martensite and bainite in Fe-2Mn-1.5Si-0.3C alloy

The effects of cementite (θ) and retained austenite (RA) on the kinetics and microstructure evolution in austenite (γ) reversion of an Fe-2Mn-1.5Si-0.3C (mass%) alloy have been studied by investigating the reversion from as-quenched and tempered martensite, and bainite. Pre-tempering of martensite or prolonged austempering, both resulting in θ precipitation, enhanced the formation of globular γ at high reversion temperature, while the presence of RA in carbide-free bainite structure obtained by short time austempering suppressed its formation. High-temperature pre-tempering or prolonged austempering retarded the reversion kinetics, while low-temperature pre-tempering or the presence of RA have negligible influences on the kinetics. Nucleation of γ at θ / ferrite interface results in the formation of globular γ. Theoretical analysis reveals that coarser θ particle has a higher nucleation potency for globular γ, and thus coarsening of θ particles by pre-tempering or austempering promotes its formation. On the other hand, the precipitation of θ particles before γ reversion is suppressed when a large amount of RA is present, thus globular γ formation is suppressed. It was concluded that the retardation on reversion kinetics by high-temperature pre-tempering or austempering is mainly owing to the suppression on the growth of reverted γ because of enhanced Mn enrichment into θ and coarsened θ particle.

Sliding wear of medium-carbon bainitic/martensitic/austenitic steel treated by short-term low-temperature austempering

A medium-carbon Si–Mn–Ni–Cr–Mo alloyed (300M) steel was austempered for various short periods at its martensite-starting temperature of 285 °C to seek improved sliding wear resistance as compared to the traditional martensitic and bainitic steels. Reciprocating sliding wear tests were performed against a WC/Co ball counterpart at a constant load of 49 N. The samples were characterised using field emission SEM, XRD and hardness testing. The associated wear mechanisms were analysed using SEM and cross-sectional TEM. The results revealed that a short austempering time of 6 min produced refined arrays of initial nano-bainitic ferrite laths and inter-lath filmy austenite and the majority martensite and retained austenite, while the majority of the microstructure remained martensitic with retained austenite. The hardness was unchanged to that of the as-quenched martensite of 6.4 GPa. Simultaneously the wear coefficient decreased by 41% from 2.67 to 1.58 × 10-15 m3N-1m-1, which is also superior to both the tempered martensite at 1.65 × 10-15 m3N-1m-1 and the lower bainite at 1.87 × 10-15 m3N-1m-1. Increasing the austempering time to 20 and 60 min resulted in wear coefficients of 1.38 and 2.18 × 10-15 m3N-1m-1, respectively. The improved wear resistance has been explained by the wear induced microstructure evolution, especially the carbon partitioning induced stabilisation of retained austenite. The high-stress sliding wear was found to be dominated by severe shear deformation, which resulted in a nano-laminate structured top layer. Delamination wear was found to take place within the embrittled nano-laminates.

Strain hardening behaviour of as-quenched and tempered martensite

Martensite is a key constituent in advanced high strength steels and plays an important role in providing the high strength. While the strength of martensite has been extensively studied in the past, its low elastic limit and extremely high strain hardening rate remain a puzzle for the steel community. Composite models proposed recently can successfully reproduce these features as result of gradual yielding of microstructural constituents with either variations in intrinsic yield strengths or transformation induced residual stresses. Although these composite models can explain certain observations associated with the deformation of as-quenched martensite, neither can self-consistently describe all the key characteristics in the tension-compression behaviour of as-quenched martensite. Attempts to extend these composite models to tempered martensite have been limited. In this contribution, we conduct a systematic experimental study on the strain hardening of as-quenched and tempered martensite with mechanical testing (e.g. monotonic tension and tension-compression) and interrupted X-ray diffraction. It is shown that the high strain hardening rate, large Bauschinger effect and diffraction line narrowing found in as-quenched martensite during straining can be sustained in tempered martensite tempered up to 400 °C. These phenomena can be understood by considering martensite as a multi-constituent composite having both variations in intrinsic yield strengths and relaxation of transformation induced residual stresses during straining.