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To answer a longstanding question towards how the austenitizing temperature affects the sulfide stress cracking (SSC) resistance in high strength low alloyed (HSLA) steels for oil country tubular goods (OCTG), it is necessary to study effects of the austenitizing temperature on martensite microstructure, diffusion and distribution behaviors of hydrogen atoms and SSC resistance. The current work showed that prior austenite grain size (PAGS) greatly increased as the austenitizing temperature was increased from 820 °C to 910 °C, but slightly increased with a further increase to 1000 °C. Similar trend arose in the size and volume fraction of Cr-rich carbides and effective grain size of martensite. The dislocation density was similar after quenching from various austenitizing temperatures, but after tempering it was found to be increased with the increase in austenitizing temperature. Moreover, although dislocations remarkably decreased the effective hydrogen diffusion coefficient (D0), these trapped hydrogen atoms could readily detrap, increasing the apparent hydrogen solubility (Capp). High angle grain boundaries (HAGBs) decreased Capp, indicating their role as obstacles other than as short-circuit diffusion paths for hydrogen diffusion. Compared with coarse Cr-rich carbides, their small counterparts could stably trap more hydrogen atoms. Without sacrificing the strength, the SSC resistance was greatly increased through decreasing the austenitizing temperature, reasons came down to less dislocations, higher HAGBs density, more recrystallized regions and finer Cr-rich carbides.

This study investigates the effect of adjusting the carbon nitrogen ratio on the microstructure and mechanical properties of martensitic stainless bearing steels, keeping the equivalent total carbon and nitrogen content. The results demonstrate that the sample with nearly equal weight percentages of C and N exhibits optimal comprehensive performance, achieving impressive yield strength, ultimate tensile strength, and uniform elongation of up to 1806.5 MPa, 2279.3 MPa, and 4.5%, respectively. Distinct phase ratios and morphologies observed across the three C/N ratios after identical heat treatment process suggest varying intrinsic mechanisms due to differences in C and N solid solubility in the matrix. The specimen with almost balanced C and N shows the highest interstitial atom solubility in the austenite phase, stabilizing austenite and lowering stacking fault energy. Abundant fine twins in martensite and stacking faults in austenite contribute to increased elongation without compromising strength. In contrast, specimen with higher N/C ratio exhibits numerous nitrides, limiting N atom solid solution in austenite during austenization, resulting in martensite with twins and dislocations. Specimen with higher C/N ratio shows extensive carbide precipitation and mainly dislocation-type martensite. The study deepens the understanding of C and N interactions in advanced bearing steels and provides a foundation for improving mechanical properties of such steels.

Martensite is the result of a diffusionless displacive phase transition. In Fe-based alloys it transforms the FCC to the BCC, BCT, or HCP structures and occurs at high strain rates. It has two typical morphologies, known as lath and plate. A quantitative constitutive description of the slip-twinning transition in the martensitic transformation is presented. It is based on the temperature and strain-rate sensitivities of slip, which are much higher than those for twinning. Thus, twinning becomes a favored deformation mechanism at low temperatures and high strain rates. The Hall-Petch coefficient, for the inclusion of grain size effects, is two times larger for twinning than slip. Constitutive relationships for slip and twinning are presented and applied to the martensitic transformation in steels; the lath-to-plate morphology change observed with increasing carbon content is successfully predicted as a function of grain size by calculations incorporating the two modes of deformation. A simple calculation of the strain rates during martensitic transformation is also provided. This methodology is applied to the Fe–C system and can be extended to the Fe–Ni–C system and to thermoelastic martensites, where twinning is favored over slip to enhance reversibility.

This study investigates the tempering behavior of bainite and martensite in a medium carbon, high silicon steel, with a focus on the microstructural evolution and the attainment of equilibrium, over a temperature range of 200–650 °C. The dissimilarities between the characteristics of the two initial microstructures, both comprising a C-saturated tetragonal ferrite matrix and retained austenite, are reflected in the differences observed in their evolution towards equilibrium as the tempering temperature increases. Therefore, while retained austenite plays a pivotal role in the bainitic microstructure, in the martensitic microstructure it is the ferritic matrix, which is highly dislocated and enriched in carbon, that plays a determinant role. The findings demonstrate that while both bainite and martensite can converge towards the same equilibrium state upon high-temperature tempering (600–650 °C), the pathway to this convergence is markedly different, with bainite exhibiting a slower transformation rate. This path to equilibrium has been characterised by means of high-resolution dilatometry, scanning electron microscopy and detailed X-ray diffraction analysis. This has provided a dynamic and detailed picture of the most relevant microstructural changes and tempering mechanisms in high silicon steels. It has also provided a foundation for tailoring heat treatment processes to optimise the mechanical properties of advanced bainitic and martensitic steels.

The Laser Shock Peening (LSP) is commonly used to strengthen the surface of materials. However, the microstructures after LSP impact are complex due to the intense forces involved. In this study, we explore how martensitic microstructures evolve in a half cylinder of NiTi under LSP impact using the Electron Backscattered Diffraction (EBSD) technique. A gradient of orientation of martensite is observed after LSP impact: the upper layers show highly oriented martensite, characterized by blocky shapes; while the lower layers display more uniform and needle-like martensitic structures. To better understand these microstructural changes, we used Finite Element (FE) simulations to analyze the stress distribution caused by the LSP. From the simulated stress conditions, the reorientation and changes of microstructures of the martensite was satisfactorily predicted by using the Interaction Work (IW) model. Overall, this comprehensive analysis provides valuable insights into the understanding of microstructural changes induced by LSP impact in martensitic NiTi alloys.

Rapid heating is a promising technique for achieving excellent mechanical properties. This study innovatively applies rapid heating to a novel Ti–Mo martensite steel through in-situ observation experiments. By comparing the microstructure evolutions at heating rates of 1 and 30 °C/s, the relationship between microstructure and property evolution was elucidated. The results indicate that the refined austenite grains resulting from rapid heating stem from enhanced grain nucleation, accumulated dislocations, and the increased formation of new carbides under the large superheat. Furthermore, carbide maturation predominates during the slow heating process, while rapid heating accelerates new carbide precipitation with limited carbide maturation. Moreover, during rapid heating, both coherent and non-coherent relationships were observed between the (Ti, Mo, Fe)C particles and the martensite matrix, demonstrating a remarkable pinning effect on grain boundary migration. Nevertheless, during slow heating, large-sized (Ti, Mo)C carbides were formed, presenting a non-coherent relationship with the martensite matrix. Additionally, the primary differences in strength caused by rapid heating compared to slow heating include grain refinement, dislocation, and precipitation strengthenings, with dislocation strengthening playing the dominant role.

Tensile residual stress (TRS) is a well-known factor that deteriorate the integrity of welded joints. Fatigue failure is accelerated by the existence of TRS introduced during the welding process. There have been efforts in the last two decades to develop filler alloys that can reduce TRS by introducing compressive residual stress (CRS) to oppose the TRS in high strength steel welded joints. These works are based on the theory of austenite (γ) to martensite (α’) transformation and the filler is often called a low transformation-temperature (LTT) alloy. Many studies have reported that the fatigue strength (FS) of weld joint made with LTT alloy is many times better than that of the conventional fillers. It is reported to be particularly useful in the repair of high strength steel structures. However, studies on the fatigue crack growth (FCG) behaviour of these LTT alloys is scarce. In this work, we developed Fe-CrNiMo based LTT weld metal composition, assessed its FCG behaviour and compared the results with that of a conventional welding wire (ER70S-6). It is found that ER70S-6 weld metal obtained under relatively fast cooling is extremely tough, but the associated heat affected zone (HAZ) has poor resistance to FCG which obscured the benefit of the tough weld metal. High heat input or condition that results to slow cooling of the ER70S-6 weldment deteriorates its resistance to FCG. Unfortunately, despite its low martensite start temperature of 231±7 and the anticipated beneficial effect of induced CRS, the LTT alloy studied had the lowest FCG resistance. The LTT alloy appears to have an intrinsic microstructural feature or a ‘fault line’ that reduced its resistance to FCG. While the LTT alloy weld metal has poor resistance to FCG, the associated HAZ resisted FCG more than the HAZ associated with ER70S-6 weld metal. It is observed that aligning the ER70S-6 weld metal perpendicular to the crack front produced the highest resistance to fatigue crack initiation and propagation. In the case of ER70S-6, it is believed that the weld metal induced a CRS at the notch tip which resulted to the high fatigue resistance. In the case of the LTT alloy, perpendicular alignment of the weld metal produced slight improvement.

The paper discusses the effect of prior martensite on the bainitic transformation of a high Si hypereutectoid steel. The research uses a combination of experimental techniques, high-resolution dilatometry and X-ray diffraction analysis, and theoretical calculations to investigate the impact of martensite on the transformation kinetics and microstructure evolution. The study shows that the presence of martensite, regardless of the amount, accelerates the bainitic transformation, particularly in the early and fastest stages, and at higher isothermal transformation temperatures. At lower temperatures, the preconditioning of the austenite prior to the bainitic transformation is such that its mechanical stability increases to the point where the whole transformation is inhibited. It is also suggested that the formation of cementite due to the tempering of martensite at the isothermal stage controls the final fraction of bainitic ferrite. The findings provide valuable insights into the role of prior martensite in bainitic transformation kinetics and microstructure development.

This study investigates the effects of 0.12 wt% boron addition on the microstructure and mechanical properties of a Fe-0.08C-4.5Mn-2.5Cr–1Si medium-Mn steel in as-rolled conditions and after the intercritical annealing process. In the as-rolled conditions, boron addition increased the strength and reduced the alloy ductility. In as-hot rolled conditions, an ultimate tensile strength (UTS)/elongation of 1195 MPa/5.5% and 1008 MPa/7% was achieved in the boron-added and boron-free alloys, respectively. However, the effect of boron on the intercritically annealed specimens was diverse. The intercritical annealing had no tangible effect on the mechanical properties of the hot-rolled specimen. However, the intercritical annealing at 700 °C for 600 s led to a UTS/elongation of 1029 MPa/19.5% in the boron-added alloy, the highest UTS/elongation combination among all specimens. The formation of carbo-boride precipitates on lath boundaries of the boron-added alloy possibly locked the boundaries thereby a temper-resistant martensitic structure was attained. The lath structure of the annealed alloy caused an enhanced strength in the boron-added specimen. Nonetheless, in the boron-free alloy, martensite was normally tempered and a considerable reduction in the strength was observed.

ABSTRACT: Objective: The aim of this study was to investigate and compare the effect of temperature on the cyclic fatigue resistance of conventional (ProTaper Universal [PTU]), Gold-Wire (ProTaper Gold [PTG]), and Fire-Wire (EdgeTaper Platinum [ETP]) nickel-titanium alloy files. Method: Twenty files from each system were tested for cyclic fatigue resistance in an artificial canal model. The experiments were performed at room temperature and body temperature in controlled temperature water. Magnified videos were recorded using a dental operating microscope integrated camera during testing to detect file fracture. The number of cycles to failure (NCF) was calculated. The type of failure was investigated macroscopically and microscopically with a dental operating microscope and scanning electron microscope, respectively. Result: The NCF at room temperature was significantly higher compared with body temperature in each system (P < .001). Compared at the same temperature, the ETP group demonstrated the highest NCF, followed by the PTG and PTU groups (P < .001). All files demonstrated cyclic fatigue failure macroscopically and microscopically. Conclusions: The 3 alloy files were affected by temperature. The cyclic fatigue resistance was reduced at the higher temperature and increased at the lower temperature. If the files are geometrically identical, files made of Fire-Wire are preferred compared with Gold-Wire and conventional nickel-titanium alloys based on cyclic fatigue resistance.

Identifying the fracture resistance capabilities and gaining a deeper understanding of the controlling damage mechanisms are important to the development of press-hardened steels (PHS) for automotive applications. In this study, the fracture properties of a novel PHS alloyed with Cr and Si (CrSi-PHS) were assessed by using uniaxial tensile and double-edge notched tensile (DENT) tests. Under uniaxial tension, the fracture resistance is characterized by the fracture strain and work of fracture, and the CrSi-PHS presents a desirable compromise with strength when comparing with other grades of advance high-strength steels (AHSS). The large fracture strain is attributed to the high resistance to damage nucleation even with a high density of grain boundaries. Fracture toughness is characterized by the DENT tests. The fracture toughness parameters of CrSi-PHS are lower than that of the commercial PHS 22MnB5 but comparable to the multiphase AHSSs with lower strength levels. The stress triaxiality near the fatigue pre-crack in DENT tests leads to a change of damage mechanism in CrSi-PHS. The crack propagates discontinuously by joining the micro-cracks and subsequently by the ductile fracture of micro-ligaments. The local plasticity at the crack tip region is suggested to contribute significantly to the energy dissipation. The observations demonstrate a strong dependence of the fracture mode and fracture resistance on the loading condition in the ultrahigh-strength martensitic steels.

In this study, a full view of the shear location was presented and ultrafine grain structure with excellent thermal stability during 700–850 °C was obtained through cold shearing and heat treatment of martensitic Cr4Mo4Ni4V steel. The results reveal that initial martensite blocks were refined through local rotating the elongated blocks into small grains in the shear region with effective strain 2–2.5. Driven by the shear deformation and heat treatment, ferrite recrystallization (about 1 μm) bands formed in the shear region mainly through continuous recrystallization at 700–850 °C, and mixed-grain structure (6.2 μm in average) formed at 1000 °C. Interestingly, the size and distribution of carbides in the shear region and undeformed region exhibited a dramatic difference due to the ultrafine recrystallization. This study provided information regarding achieving stable ultrafine grain structure through cold shearing and heat treatment of Cr4Mo4Ni4V martensitic steel.

A phase field model is presented to investigate the influence of cleavage planes in martensite and austenite phases and the loading rate on the crack propagation process with considering the Kurdjumov-Sachs (K-S) orientation relationship in martensitic steels by the finite element method. The transgranular (through the martensite and austenite phases) and intergranular (along the martensite/austenite interface) fracture processes in martensitic steels can be simulated by this model. This model can simulate the crack propagation in martensitic steels with displaying the crack propagation path in austenite and martensite phases and stress distribution directly. Moreover, this model reveals the energy difference required by transgranular and intergranular fracture in martensitic steels and the hindering effect of martensite on the crack propagation process. The simulation results further indicate that the preferential cleavage planes of crack propagation in martensite and austenite phases are (100)α’ and (111)γ planes, respectively, which is consistent with the experimental results in martensitic steels. An interesting finding is that the influence of the loading rate on the maximum loading for material failure only acts on the crack propagation stage rather than the crack initiation stage. Therefore, this model can be a powerful tool to investigate the cleavage mechanism of the crack propagation process during the water quenching process and to avoid water quenching cracking in martensitic steels.

ABSTRACT: Introduction: The aim of this study was to assess the dynamic cyclic fatigue resistance of an R-Motion file at simulated body temperature and document corresponding phase transformations compared to OneCurve and HyFlex EDM (HFEDM). Methods: R-Motion (25/.06), OneCurve (25/.06), and HFEDM (25/.06) files were selected and divided into 3 groups (n = 9) according to the file type. Dynamic cyclic fatigue testing was done with a custom-made artificial stainless-steel canal that had a 90° angle of curvature and a 5-mm radius of curvature. Files were operated continuously at body temperature until fracture in the artificial canal. The time to fracture was calculated. Statistical analysis was performed, and significance was set at 5%. Phase transformation temperatures for 2 instruments of each group were analysed by differential scanning calorimetry (DSC) analysis. Results: The highest mean time to fracture value was measured in the HFEDM group (277.84 ± 2.51), followed by the R-Motion group (115.09 ± 0.01), whilst the lowest value was found in the OneCurve group (44.28 ± 3.63). Post hoc pairwise comparisons were all statistically significant (P < .001). DSC heating curves show austinite start temperatures to be 33.94 °C and 43.32 °C and austinite finish temperatures to be 35.09 °C and 50 °C for R-Motion and HFEDM, respectively. DSC cooling curves show martensite start temperatures to be 27.54 °C and 44.52 °C and martensite finish temperatures to be 29.13 °C and 37.68 °C for R-Motion and HFEDM, respectively. DSC curves of OneCurve failed to demonstrate transformation temperatures within the tested heat range. Conclusions: Crystalline arrangement of Ni and Ti atoms within the NiTi alloys greatly affects the dynamic cyclic fatigue resistance of the file.

In present work, tensile behaviors of cold-rolled 304L stainless steel were investigated at both room temperature (RT) and liquid nitrogen temperature (LNT). After cold-rolling with a thickness reduction of ∼92%, the 304L stainless steel exhibits an ultrahigh yield strength of 1787 MPa at the expense of ductility (1.06% of uniform elongation) at RT. At LNT, the yield strength (upper yield point of σyu = 2308.4 MPa and lower yield point of σyl = 1894.2 MPa) and uniform elongation (23%) increase greatly. Detailed microstructural investigation reveals that the cold-rolled 304L stainless steel comprises mainly of deformation induced martensite and high density of dislocations (9.06 ×1014 m−2), leading to a lack of dislocation storage capacity and thus brittle fracture at RT. While interrupt tests at LNT reveal a first increase and then decline in dislocation density with tensile strain, accompanied with phase transformation from austenite to martensite. The delamination events also occur, characterized by the multiple separated laminated ligaments observed at fracture surface. Therefore, the high dense dislocations and transformation-induced plasticity (TRIP) effect as well as delamination toughening mechanism contributes to the excellent combination of strength and ductility at LNT. Our work provides experimental and microstructural insights into the deformation mechanisms of the ∼92%-cold-rolled 304L stainless steel during tensile deformation at RT and LNT.

In this study, we investigated the hydrogen embrittlement (HE) of martensitic medium-Mn steels with varying levels of S and Ti contents using thermal desorption analysis, silver decoration, slow strain rate tensile testing, and fractography. The S or S–Ti added specimens demonstrated higher HE resistance compared to the S-free specimens. This is because the S or S–Ti added specimens had a large volume fraction of fine MnS or Ti(C,N)–MnS complex inclusions with a size less than ∼1 μm, which provide strong irreversible H trap sites. However, coarse elongated inclusions were found to be detrimental to HE resistance. Therefore, we concluded that to improve the HE resistance of martensitic medium-Mn steel, it is important to distribute a large volume fraction of fine MnS or Ti(C,N)–MnS complex inclusions without coarse elongated ones.

The fusion zone (FZ) hardness of resistance spot welds is a crucial factor affecting the performance and durability of the welds. Failure mode transition, tendency to fail in interfacial mode, interfacial failure load, and the impact of liquid metal embrittlement cracks on the weld strength are influenced by the FZ hardness. Therefore, accurately predicting the FZ hardness in resistance spot welds made on automotive steels is essential. A simple thermal model is used to calculate the time required for the temperature to drop from 800 °C to 500 °C (Δt85). With the aid of continuous cooling transformation (CCT) diagrams and experimental confirmation, it is shown that in most automotive steels, the FZ exhibits an almost full martensitic microstructure. Generally, the FZ hardness tends to increase in the order of interstitial-free (IF), drawing quality specially killed (DQSK), high strength low alloy (HSLA), ferrite-martensite dual-phase steels, transformation induced plasticity (TRIP), and quench & partitioning (Q&P) steels. To predict the FZ hardness, data-driven regression-based models have been developed based on carbon content, carbon equivalent concept, and the strengthening mechanisms of the martensite. Among these models, the model based on martensite strengthening mechanisms showed the best performance and robustness in estimating the FZ hardness. The assessment of the strengthening mechanism of the FZ showed that most important factors determining the FZ hardness are dislocation hardening due to carbon atoms, interface hardening due to block boundaries, and solid solution hardening due to substitutional alloying elements, respectively. A simple relation is proposed to estimate the load-bearing capacity of automotive steel spot welds during interfacial failure based on the FZ hardness.

The main goal of present paper is to highlight the influence of type of hard phase at different volume fraction onto the cutting force of dual matrix structure ductile iron (DMS-DI) in order to achieve the DMS-DI articles having the suitable machining performance for the DMS-DI market development. DMS-DI specimens were prepared by casting, followed by partially austenitising at temperatures of 870 °C for various times and then quenching into water or molten salt with the temperature 350 °C. The machinability of the workpieces was investigated by adopting, cutting index (relative cutting force with respect to AISI 1110 steel) as a general criterion. Fitting the data to the Johnson-Avrami model revealed the ferrite-to-austenite phase transformation kinetics with two consecutive distinguishable stages with Avrami exponents of 3.5 and 2. In both series of DMS-MDI (martensitic DMS) and DMS-ADI (ausferritic DMS) samples, with increasing percentage of hard phase (Vf), the hardness value was increased, while, surprisingly the cutting index was improved by enhancing of Vf to ≈37%, and then worsened. The dependence of the cutting force on the cutting depth was linear for steel AISI 1110, DMS-MDI and DMS-ADI. The slope of the line and its y-intercept depend on the base materials, type of hard phase, the feeding rate and the spindle turning speed. The hardness value of the DMS-MDI was higher than the DMS-ADI, despite this, for Vf≈37%, the cutting index of the former was up to 20% and 60% better than the DMS-ADI and regular austempered ductile iron, respectively.

Effect of tempering time on microstructure and tensile properties of a low carbon martensitic stainless bearing steel was studied. Results showed that martensitic blocks became coarser and dislocation annihilation continuously took place during tempering process. It was also found that the number of M2C and M6C was increased firstly and then decreased, while their sizes were increased constantly. Moreover, the austenite content was increased due to the formation of reverted austenite. For tensile properties characterization, there was a transition from a ductile fracture to a quasi-cleavage fracture with increasing tempering time. Yield strength peaked when the steel was tempered for 24 h, followed by a constant decline. The abnormal decrease in elongation after long-term tempering was due to the formation of coarsening M23C6 at grain boundaries.

White Etching Layer (WEL) and Brown Etching Layer (BEL) always form on the surface of rails and bearings subjected to rolling cyclic contact stress. Because of the different properties between WEL/BEL and matrix, the cracks initiate and propagate easily, leading to failure ultimately. The WEL and BEL found on pearlite rail, have been widely investigated, but WEL and BEL on bainite rail and martensite bearing have not been further studied. Especially the BEL on martensite bearing raceways has rarely been reported. This study investigates the features of WEL and BEL found on martensite bearing raceway. The BEL locates in the middle of WEL and matrix, but its hardness values are the highest, which is different from the general results reported: WEL > BEL > matrix. The WEL is mainly consisted of austenite with coarse grains, ferrite and retained tempered martensite. The mainly composition of BEL is quenched martensite. Thermal effect is the main formation mechanism of the WEL and BEL in this study. BEL forms because the surface temperature exceeds Acm point, and then rapidly decreases. The formation of WEL may originate from two processes: (1) The austenitizing and slowly cooling process again of BEL; (2) The austenitizing and slowly cooling process of the matrix. WEL and BEL forming processes are closely related to the diffusion and redistribution of carbon. The contributory heat conduction and diffusion mechanism in the confined spaces of grease lubricated bearings have been also discussed in this study.

Steels with outstanding mechanical properties are desired in industrial applications. Martensitic structure significantly increases the strength in low-carbon steels, but usually sacrifices their ductility. In order to improve the combination of strength and ductility, a full martensitic steel was further processed by cyclic intercritical rolling to produce a novel heterogeneous lamellar structured dual-phase (HLSDP) structure. This processing route constructs a small amount of lamellar ferrite surrounded by high volume fraction of refined martensite, which is a promising heterostructure for hetero-deformation induced (HDI) hardening. The HLSDP steel shows a simultaneously improved strength and ductility (ultimate tensile strength, UTS: 1.85 GPa; uniform elongation, UE: 5.6%), comparing to the lath martensite steel (UTS: 1.68 GPa and UE: 4.1%). The superior tensile properties of HLSDP steel are attributed to the higher strain hardening ability, which is related to the significant HDI hardening. Due to the deformation incompatibility between the soft zone (ferrite) and hard zone (martensite), strain partitioning is occurred during plastic deformation. In addition, micro digital image correlation (μ-DIC) analysis reveals the evolution of strain field during deformation. A significant strain gradient is produced near the interface of ferrite and martensite in the HLSDP steel.

A novel 2000HS hot stamping steel was fabricated for automotive applications. Compared to the traditional 1500HS hot stamping steel, it showed an excellent combination of strength and plasticity, and the thickness of the oxide layer was dramatically reduced during the hot stamping process. The tensile experiment, scanning electron microscope (SEM), electron backscattered diffraction (EBSD), transmission electron microscope (TEM) and x-ray diffraction (XRD) characterization methods were employed, to clarify the best comprehensive mechanical properties of 2000HS steel. Experimental results indicated that the microstructure of Z740-A950-T170 consisted of lath martensite, large M23C6 precipitates, small (Nb, V) C precipitates and retained austenite (4∼6vol.%). The obtained best comprehensive mechanical properties of Z740-A950-T170 sample of the tensile strength was 2160 MPa, the yield strength was up to 1318 MPa, and the total elongation reached at 12%. The addition of Mo element inhibited the coarsening and lengthening of the precipitation. Large M23C6 precipitates (101.2 μm) and small (Nb, V) C precipitates (17.5 μm) played the role of pinning the dislocation and increased the yield strength. Regardless of any process, retained austenite was kept at room temperature (4–6 vol.%), which improving the strength and maintaining the total elongation by transformation induced plasticity (TRIP) effect. In addition, the oxidization layer thickness of the invented steel (12.4 μm) was thinner than that of the 1500HS (23.6 μm) after hot forming at 950 °C for 5 min. The significant reduction of oxidation was due to the high percent of Cr and Si contents within the designed steel, the formation of dense Cr and Si at the bottom of the oxide layer, which improving the antioxidant properties for studied steel.

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.

In quenched Fe-C (C: 0.0∼2.0 wt.%) binary alloys, the body-centered cubic (BCC) {112}-type twin structure (density, size and morphology) in martensite was investigated by means of transmission electron microscopy (TEM). In the samples quenched to room temperature, the twin density increased as the carbon content increased. In the carbon-free or pure iron sample, no twin structure was observed. In high carbon martensite, a high density of twins could be seen with twin thickness of 1 nm–2 nm, which is of the scale of the smallest α-Fe grain. The twin density variance is discussed based on a detwinning process, which occurs upon cooling. The twin, as an initial product of martensitic transformation, would experience a higher temperature auto-tempering process in low carbon alloys than in high carbon samples. A noticeable detwinning process takes place in low carbon alloys and results in a low density of twins observed at room temperature. Martensite starting (Ms) temperature plays a crucial role in the detwinning or auto-tempering effect on the twins.

Laser engineered net shaping (LENS) is an advanced additive manufacturing technology combining rapid prototyping and synchronization technology. Its multi-powder feeder delivery system enables multi-materials building in single deposition, which is appropriate for additive manufacturing of functionally graded materials (FGMs). In this study, martensitic-stainless steel (MSS)/austenitic stainless steel (ASS) FGMs with composition transitioning from 100% MSS incrementally graded to 100% ASS by 25% composition gradients are fabricated by LENS. The Vickers hardness of MSS/ASS FGMs ranges from 358 to 170 HV. The decrease of hardness is found to relate to the grain-growth region with the increase of austenite. The obtained specimens of MSS/ASS FGMs show a tensile strength of 669 MPa and an elongation of 19%. In addition, the fracture location of MSS/ASS FGMs in tensile test is in the region of 100% ASS, which is dominated by austenite structure. Finally, the Scheil–Gulliver model is introduced to validate the phase formation of MSS/ASS FGMs’ failure region. Experimental and modeling results indicate that precipitation of α-ferrite in the austenite structure leads to reduced ductility of MSS/ASS FGMs.

Deep cryogenic treatment applied in high-speed steel yielded promising results, and it was found that tempering played a vital role. This paper systematically studied the effect of tempering temperatures on the microstructural composition (residual austenite, martensite, and carbides) and mechanical properties (Vickers hardness and impact toughness) of AISI M35 high-speed steel subjected to deep cryogenic treatment. An increase in the tempering temperatures facilitated the transformation of residual austenite and the formation of martensite blocks corresponding to a decrease in the number of dislocations. Moreover, the carbide precipitation (secondary carbides and nanoscale carbides) began at a tempering temperature of 350 °C, increased at 450 °C, and reached its maximum at 550 °C. The fracture mechanism on the micro-level could be interpreted as follows: cracks occurred in the carbides with larger sizes (primary carbides and large secondary carbides) and at the carbide/matrix interface, and small secondary carbides decohered at the interface, forming microvoids and facilitating plastic deformation. In addition, the specimen tempered at 150 °C exhibited the highest hardness of 880.4 HV1 due to the highest number of dislocations. The impact toughness of the sample tempered at 550 °C was the best, namely 2.50 MJ m−2, due to an increase in the number of martensite block boundaries and the more homogeneous carbide precipitation.

In this study, the additive manufacturing of a functionally graded material (FGM) via directed energy deposition was investigated as an alternative to joining dissimilar metals. The metal powder composition of the FGM was gradually changed from fully low-carbon steel to austenite steel along the building direction. A convolutional neural network model was employed to classify the austenite, martensite, and ferrite phases in the FGM. The volume fraction of the phases was calculated using X-ray diffraction Rietveld refinement and compared with that predicted by the thermodynamic model and that determined from electron-backscattered-diffraction maps. The volume fraction of the bcc phase gradually increased, and the grain size decreased from top to bottom. Nanostructural investigations confirmed the absence of carbide and twin structures due to the relatively low carbon concentration in the upper layers and the presence of a hexagonal ω-Fe phase with twin structures in the interlayers. Furthermore, electron channeling contrast images and kernel average misorientation maps revealed the activation of the deformation twinning and strain-induced transformation of the retained austenite to martensite, which increased the strain-hardening rate. This study can guide the selection of a tailored manufacturing strategy and process parameters to obtain the required material distribution.

Localised laser treatments enable the creation of sophisticated austenite/martensite mesostructures in Fe–Ni–C steel with the potential of achieving enhanced mechanical performance. The control of phase topology is essential to modify the properties of these structures on demand and requires a profound understanding of the effect of the processing parameters on the development of the different phases upon the application of laser treatment. In this work, the microstructure evolution under exceptional gradients in temperature and heating rates is thoroughly investigated. The extent of the laser-affected zone and the heat input were tailored by varying laser parameters and specimen thickness, based on a model that considers transient material properties and the coupling between temperature and microstructure. The predicted temperature fields resulted in a complex interplay between martensite to austenite phase transformation and martensite tempering. Considering the high heating rates of up to 25000 K/s and the observed microstructures, it is suggested that austenite was formed by a pseudo-displacive mechanism and subsequently fully recrystallised in the zones most directly affected by the laser heat source. A smooth strength transition from austenite to martensite, affected by the laser parameters, could be exploited for more effective deformation mechanisms and improved material mechanical properties.

The microstructure of steel greatly influences the mechanical properties. For 9 wt% Cr steels, which are widely used in the power generation industry, the steels have a ferritic and martensitic microstructure which can be altered by heat treating and chemical composition variations. Fully martensitic steels typically having high yield strengths but low ductility. Tempering can reduce the amount of martensite in the steel lowering the yield strength but increasing the ductility of the alloy. Alloying can alter the time required for a martensitic transformation. In authors’ previously published research, the authors used machine learning methodology to predict room temperature tensile properties from scanning electron microscopy (SEM) images of the initial steel microstructures from a wide range of steel compositions. This data-in-brief supplies the raw image files and the associated tensile properties for the authors’ previously published research utilized to predict tensile properties of steels [1].

This paper aims at the phase identification and quantification in transformation induced plasticity duplex stainless steel (TDSS) base and weld metal containing ferrite, austenite, and martensite. Light optical microscopy (LOM) and electron backscatter diffraction (EBSD) analysis were employed to analyze phases. Samples were either mechanically or electrolytically polished to study the effect of the preparation technique. Mechanical polishing produced up to 26% strain-induced martensite. Electrolytic polishing with 150 g citric acid, 300 g distilled water, 600 mL H3PO4, and 450 mL H2SO4 resulted in martensite free surfaces, providing high-quality samples for EBSD analysis. Martensite identification was challenging both with LOM, due to the similar etching response of ferrite and martensite, and with EBSD, due to the similar lattice structures of ferrite and martensite. An optimized Beraha color etching procedure was developed that etched martensite distinctively. A novel step-by-step EBSD methodology was also introduced considering grain size and orientation, which successfully identified and quantified martensite as well as ferrite and austenite in the studied TDSS. Although here applied to a TDSS, the presented EBSD methodology is general and can, in combination with knowledge of the metallurgy of the specific material and with suitable adaption, be applied to a multitude of multiphase materials. It is also general in the sense that it can be used for base material and weld metals as well as additive manufactured materials.

In this study, the effect of cryogenic treatment on the microstructure, especially the retained austenite (RA) evolution behavior, and mechanical properties of 55Cr17Mo1VN plastic die steel was investigated. The quenching microstructure consisted of martensite, RA, and undissolved carbides. After the cryogenic treatment, the RA volume fraction decreased from 44.7 to 35.4%, while the hardness increased from 39.0 to 54.6 HRC because of the increase in the dislocation density. Besides, the “new martensite” phase broke the blocky RA phase during the cryogenic treatment, and the two phases keep a strict K–S or N–W orientation relationship, which improved the stability of the steel. Interestingly, in the cryogenically treated sample, the RA decomposed completely. On the other hand, the untreated sample retained 20.7% of its RA phase after the tempering process. The cryogenic treatment decreased the thermal stability of the RA phase during the tempering process, which can be attributed to the release of the hydrostatic pressure and the generation of a large number of precipitates from the RA phase. As a result of precipitation strengthening, fine grain strengthening, and dislocation strengthening, the cryogenically treated sample showed excellent tensile strength (∼2241 MPa) and high hardness (∼56.2 HRC).

Interstitial-free steel is very soft and ductile, with a fully ferritic microstructure at room temperature. In order to address this issue in the present work, the cold-rolling increases tensile strength vividly by strain hardening. The heat-treatment thereafter optimizes strength-ductility, with an excellent combination, through fine recrystallized grains. The warm-rolled sample provides relatively an inferior property by partial recrystallization of the sample. Irrespective of unannihilated dislocations, both the samples after the recrystallization treatment in the austenitic domain at 925 °C, were water quenched in a laboratory scale to explore phase transformations additionally in the system. The microstructural characterization reveals that the nucleation of massive ferrite under rapid cooling requires austenite conditioning, similar to martensite in the alloy, constrained by strained lattice. The outcome fundamentally argues against the notion that the accumulation of crystal defects/dislocations promotes an extra driving force for a reconstructive phase transformation leading to massive ferrite.

In this study, the effect of the initial microstructure (i.e., ferrite + pearlite or martensite) on the graphitization behavior and microstructural evolution during heat treatment of a hot-rolled medium-carbon, high-silicon steel (Fe–0.55C–2.3Si) is investigated. When a sample consists of ferrite and pearlite, its microstructure gradually changes to ferrite and graphite during heat treatment via the decomposition of pearlite and the formation of graphite particles. In the sample with a martensite microstructure, the phase transformation occurs in the following step: martensite → ferrite + cementite → ferrite + graphite. Although the graphitization process of the martensite sample is more complex than that of the ferrite + pearlite sample, the graphitization rate is substantially faster in the former owing to more abundant grain boundaries and triple junctions that act as nucleation sites for graphite. When the initial microstructure changes from ferrite + pearlite to martensite, the graphite completion time significantly reduces from 16 to 2 h at 650 °C and from 6 to 0.5 h at 700 °C. After complete graphitization, the average size and number density of graphites in the martensite sample are respectively smaller and higher than those in the ferrite + pearlite sample. The rate of hardness decrease (i.e., material softening) during heat treatment is also faster in the martensite sample. These results demonstrate that changing the initial microstructure from ferrite + pearlite to martensite is an effective means to accelerate the graphitization behavior and acquire a graphitized steel with uniformly distributed fine graphite particles.

The initial corrosion process of dual phase (DP) steel produced by the continuous annealing process (CAP) in NaCl solution was systematically studied in this paper. The microstructure, nobility of the phases, and micro-galvanic corrosion behavior were characterized by scanning electron microscope (SEM), electron backscattered diffraction (EBSD) technique, scanning Kelvin probe force (SKPFM), and electrochemical impedance spectroscopy (EIS). Results indicate that there are two types of martensite, namely fresh martensite (FM) and tempered martensite (TM), embedded in the ferrite matrix. The two types of martensite are surrounded by dislocations and connect to each other as in a chain-like network at the ferrite grain boundary. Both FM and TM can act as a micro-cathode to promote the anodic dissolution of adjacent ferrite matrix since FM is up to 15 mV noble to the ferrite matrix and TM is to 30 mV noble. The initial corrosion process of DP steel can be divided into two stages. The first stage involves an increasing corrosion rate of the steel due to the successive emergence of cathodic TM and FM, while the corrosion rate decreases in the second stage because of the separation of martensite.

Cu–Al–Mn–Ti–xTa alloys (x = 0, 1, 2, 3, wt.%) prepared by spark plasma sintering were investigated for the effect of Ta content on the microstructure, mechanical and damping properties. The microstructure and phase composition indicate that the alloy is mainly composed of β′1 martensite, γ′1 martensite, Ti-rich and Ta-rich phase. As the Ta content increases, the grain size of the alloy first decreases and then increases. The reverse trend was observed for hardness, tensile and compressive strength. The hardness, tensile strength and compressive strength increased by 18.2%, 44.9% and 28%, respectively, when the Ta content was 1 wt.% compared to the alloy without Ta element. The presence of martensite provides the alloy with promising damping properties. Meanwhile, the formation of the second phase has a two-sided effect on the damping characteristics. That is, the increase in grain boundaries provides more interfaces for energy dissipation, but the increased compressive stress between the interfaces also hinders the movement of the interfaces. Excellent damping performance is demonstrated with the addition of 1 wt.% of Ta element. The peak values of damping capacity at room temperature and at about 580 °C reached 0.026 and 0.19, respectively. The results confirm that the addition of Ta elements is achievable to obtain Cu–Al–Mn–Ti alloys with both high mechanical and damping properties.

Beta-type titanium alloy surfaces (Ti–35Nb–10Ta (TNT) and Ti–30Nb–4Sn (TNS)) were modified by laser using a power of 1000 W and speed of 6.7 mm/s (Condition A) and a power of 1500 W with speed of 10 mm/s (Condition B). Increasing laser conditions, the thickness of the molten layer was also increased. The initial equiaxed grains changes to quite elongated grains shape. The surfaces were structured under α″ martensite, α and matrix β. The β phase content decreased slightly and the α″ phase increased for both alloys increasing the laser conditions. The condition A increased the elastic modulus (E) and decreased the hardness while condition B did not affect the mechanical properties surface for the TNT system compared to base metal (BM). In TNS system the laser condition A decreased the E and increased the hardness while increasing the laser parameters (Condition B) both E and hardness decreased compared to BM. The laser surface treatment was influenced by the levels of alloying elements present promoting most significant changes in the microstructure and mechanical properties in the TNS system.

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.

In the present work, the laser-induction hybrid cladding technology with induction post-heating (post-LIHC) was utilized to deposit coatings on the full-scale rail surface, and the effects of different induction post-heating temperature on the morphology, microstructure and mechanical performance of the heat affected zone (HAZ) were first in-depth analyzed. Results indicate that with the increase of the average induction post-heating temperature (Ta) in post-LIHC, the microstructure of the HAZ first transforms from the fine acicular martensite structure to the composite structure of martensite and pearlite, then changes to the pure pearlite structure, and finally retransforms to the coarse martensite structure. Especially, the morphology of the “Martensite Zone” (MZ) in HAZ undergoes three remarkable change with Ta increasing: “flat-U-shaped” → “flat-W-shaped” → “vanished” → “flat-V-shaped”. Under the induction post-heating window of Ta = 508 °C ~ 565 °C, not only the MZ in HAZ can be inhibited absolutely, but also fine pearlite with much lower interlamellar spacing, smaller grain size and higher grain misorientation forms instead, making the strength and toughness are both enhanced significantly than the other three type HAZs and the rail substrate. In addition, increasing the induction post-heating temperature within the window of Ta = 508 °C ~ 565 °C can increase the interlamellar spacing and reduce the ferrite grain misorientation of the pearlite structure in HAZ, but has no obvious effect on the grain size therein.

Ti-6Al-4V alloy has limitations in terms of globularization when using additive manufacturing (AM), which requires a final forging step. To optimize this approach, this study investigated the effect of processing variables on the dynamic/static globularization of AM-processed Ti-6Al-4V alloy. Eight double-cone specimens were prepared with different processing variables such as effective strain, solution treatment, hot-forging temperature, and subsequent annealing. Microstructural evolutions were quantitatively characterized to interpret the kinetics of globularization based on the aforementioned variables. In particular, the combination of solution treatment, low-temperature (1073 K) forging, and subsequent annealing significantly accelerated the overall globularization. Such an accelerating effect stemmed from the reduced path for boundary splitting in fine α′ martensitic laths, which was induced via solution treatment. This accelerating effect disappeared at a high temperature (1223 K), which implies the necessity of optimizing the thermomechanical route to exploit martensitic laths.

Quenching and partitioning (Q&P) treated steels are traditionally alloyed with silicon (Si), but its precise role on microstructural mechanisms occurring during partitioning is not thoroughly understood. In this study, dilatometric analysis has been combined with detailed microstructural characterization to unravel the competing mechanisms occurring during partitioning either in parallel or in succession. Three 0.4 wt.% carbon steels with varying Si contents were quenched to 150 °C for ~20% untransformed austenite, and partitioned for 10–1000 s in the temperature range 200–300 °C. The steel with low Si content (0.25 wt.%) exhibited substantial bainitic transformation during partitioning at 300 °C and only 4% retained austenite (RA) at room temperature (RT) even after 1000 s hold. In contrast, a high Si fraction (1.5 wt.%) enabled retention of ~18% austenite under similar conditions. While η-carbides precipitated within the martensite laths in the high-Si steel, cementite precipitated in the low-Si variant. Furthermore, carbide precipitation and growth were strongly suppressed by high Si content. Secondary martensite formation occurred from carbon-enriched austenite during final cooling, irrespective of Si-content. Results illustrate that Si retards austenite decomposition at higher partitioning temperatures but does not improve carbon partitioning at lower temperatures.

Laser-powder bed fusion (L-PBF) has gained significant research interest, not only for its profound advantage of producing near-net shape complex geometries of metallic parts, but also for the possibility of producing tailored microstructures. Here we exploit the capability of manipulating the crystallographic texture by L-PBF to tailor the deformation behavior of austenitic stainless steels. In specific, by adjusting the laser power and the laser scanning speed, tailored crystallographic textures can be obtained, along the uniaxial loading direction in 304L stainless steel samples produced by L-PBF. In situ neutron diffraction and uniaxial tension and compression tests are undertaken to investigate the extent of the transformation induced plasticity effect and to correlate it with the tailored macrostructures. The influence of the initial and the evolving crystallographic texture on the deformation behavior is demonstrated and elaborated accordingly. The observed asymmetry in the deformation behavior between tension and compression is also discussed in detail.

This present work is the first investigation on the laser welding of the novel hot-stamped steel with 2000MPa (HSS2000). Softening was present in mixed-grain HAZ and tempered zone, due to martensite transformation, precipitated carbons and partial recovery of martensite lath. Tensile strength of welded joint was ∼1500 MPa, 80% of base metal, due to the presence of soften zone. However, the softening cannot be eliminated with decreasing laser heat input but it did not influence the tensile strength of welded joints. Thus, the laser welding of HSS2000 is acceptable in the heat input range of 67∼86 J/mm because the tensile strength can reach 1500 MPa, which is superior to other ultra-high strength steels at present. HSS2000 has a potential application in the automotive manufactures in the future.

The effect of Ti addition (0.15 and 0.20 wt%) on mechanical properties and wear resistance of steel excavator bucket teeth was investigated. Teeth were produced by commercial sand casting technique. After production normalizing, hardening, and tempering heat treatment procedures were applied to the teeth. Lath martensitic microstructure was observed and small amount of carbide particles were present in the microstructures of Ti containing ones. In addition, small amount of hard and brittle TiN particles were observed in the microstructure of 0.20 wt% Ti containing teeth. The best mechanical properties were obtained for 0.15 wt% Ti containing teeth with considerably high abrasive wear resistance. However, 0.20 wt% Ti containing teeth exhibited worst abrasive wear resistance and limited ductility due to the formation of TiN particles.

Shape memory alloys (SMA) are reputable for their capability of regaining shape at certain temperatures even after large deformations. In the martensite phase, when a load is applied on SMA wire, it accommodates strain rather than breakage by unfolding its lattice that results in improving damping and stiffness. Also, SMA generates stresses due to phase transformation from martensite to austenite at higher temperatures. This coupling effect of SMA in response to load and temperature makes its use for improving properties of composites by embedding in composite structures. The major factor considered during embedding SMA wires into composite structures is the compatibility between SMA and matrix. SMA wires should have excellent adhesion with matrix otherwise, cracking and delamination in the composite will occur. Therefore, different techniques are carried out for surface modification of SMA wire to improve the bonding between matrix and SMA wire. These techniques are expensive, time-consuming and sometimes give undesirable results. An alternative approach used in this research is to weave SMA wires in 3D structures for providing better grip to these wires before composite fabrication. 3D structures have higher through-the-thickness properties and delamination resistance, but their in-plane properties are lower than 2D laminated composites. Due to the weaving of SMA wires into 3D structures, binder yarns provide a better grip to SMA wire that ultimately increases the interfacial strength of composite while SMA wire improves the tensile properties of 3D structures. Results show that a single SMA wire embedded in three different 3D configurations contributes in improving the tensile properties of each 3D structure depending on the interlocking behavior of fibers with SMA wire. The layer-to-layer 3D configuration loosely grips SMA wire, hence SMA wire faces less resistance upon activation and improves Young’s Modulus to 34.9%. The modified 3D structure provides a strong grip to the SMA wire, hence limitise the increment of Young's Modulus to 16.06%. Tensile behavior along with structural failures of SMA embedded 3D woven composites are discussed in detail.

In this work, the influence of thermomechanical treatments, consisting of a combination of ausforming followed by isothermal holding and cooling to room temperature, on the stability of retained austenite of a carbide-free medium carbon-high silicon steel was investigated. The amount of retained austenite and its transformation to martensite for a range of ausforming treatments was determined by means of X-ray diffraction, dilatometry signal analysis, and metallographic investigation. Different amounts of deformations were applied at 600 °C in the austenite region prior to fast cooling into the bainitic transformation region. Four isothermal temperatures (325, 350, 375, and 400 °C) with a holding time of 1800 s were selected to estimate the extent of the stability of the retained austenite after the completion of the bainitic transformation. The results show that the deformation-free austenite was more stable for the samples isothermally treated at temperatures between 325 and 350 °C. However, the decomposition of retained austenite to martensite during cooling to room temperature increased for isothermal holding temperatures above 350 °C. It was found that the stability of retained austenite was a function of the temperature and percent deformation, and the critical temperature and percent deformations were determined. It was also found that ausforming enhanced the formation of blocky-shaped retained austenite for isothermal holdings above 350 °C, resulting in a decrease in retained austenite stability. The results were analyzed in terms of the influence of thermomechanical conditions on bainitic transformation and its impact on the transformation of retained austenite to martensite.

The evolution of microstructure and mechanical properties during tempering of M50 steel with bainite/martensite (B/M) duplex structure have been investigated. The microstructural observation shows that the redistribution of carbon atoms and precipitation of transition carbides in the B/M duplex specimens are slightly decreased during the initial stage of tempering, as compared with the fully martensitic specimens. After tempering of 550 °C, not only finer ferrite packets are obtained, but also the precipitation of nano-scale carbides is enhanced for the B/M duplex specimens. The mechanical characterization indicates that the hardness is decreased while the impact toughness is significantly improved when tempering temperature increases to 300 °C. When the tempering temperature increases to 550 °C, due to the decomposition of retained austenite (RA) and precipitation of alloy carbides, the hardness exhibits an obvious improvement while the toughness first decreases and then increases. The impact toughness of the B/M duplex specimens is higher than that of the fully martensitic specimens during the whole tempering process, although they have the same tendency varying with tempering temperature. Furthermore, the final wear resistance is significantly improved in the austempered specimens due to the enhanced precipitation during tempering.

The present study emphasizes on the mechanical characterization of medium carbon AISI 1040 and 4140 duplex steels. AISI 1040 and 4140 steels are generally used as structural steels for varying applications, ranging from construction field material to heat resistant parts for moderate temperatures applications. The objective of the present work is to corroborate microstructure and mechanical properties by varying the intercritical temperature in duplex steels. The normalized specimens of as bought steel are heated to a different intercritical temperatures (750, 770, and 790 °C), followed by isothermal holding for 2 h and quenching to room temperature develops duplex phase structure. This treatment grows microstructure with a variable quantity of martensite embedded in a fine ferrite matrix. For the same carbon content in the duplex alloy steels, the addition of Cr, Mo, Si, and Mn, increases the tensile strength and hardness of the material but decreases the toughness and ductility. As the intercritical temperature varies, the amount of martensite and ferrite phases present in the duplex steel also varies, which intern alters the strength and hardness of the material. Duplex steels have higher strength and hardness than as bought, even though the impact strength and ductility are poor. Microhardness is used to identify the micro phases present in the material viz., ferrite and martensite. Microstructure shows the evidence of the formation of a duplex structure with a variation in the amount of aforesaid phases. The result obtained validates the influence of microstructure on mechanical properties of AISI 1040 and 4140 steels.

High temperature shape memory alloys of the Ni-Ti-Hf system are potential candidates for aerospace applications where powder bed additive manufacturing technologies are being increasingly used. In this work, a Ti-rich NiTiHf high temperature shape memory alloy powder was processed by laser powder bed fusion. The standard heat treatment of 550 ⁰C for 3 hours was imposed to promote H-phase precipitation. At room temperature, the material has a dual-phase microstructure composed of martensite, the matrix, and H-phase, as a strengthening precipitate. High energy synchrotron X-ray diffraction is used to evaluate, in-situ, the elastic behaviour of the fabricated part. The deformation anisotropy of several (h k l) families of planes of both phases is evidenced. No major texture changes were observed upon macroscopic elastic loading. We illustrate the potential of using high energy synchrotron X-ray diffraction for detailed analyses of minority phases in additively manufactured components.

The age hardenability of Al-alloyed lightweight stainless steels with the base chemical composition Fe–10.5Cr–3Al (wt.%) and Ni concentrations in the range 3–15 wt.% was studied. Alloys containing 3% and 6% Ni exhibited almost fully ferritic matrix microstructures and a weak age hardening response. Alloys containing 9%, 12%, and 15% Ni, on the other hand, developed primarily martensitic microstructures. Differential scanning calorimetry measurements indicated the occurrence of an exothermic reaction in the approximate temperature range 375–625 °C. Dilatometry measurements indicated that the exothermic reaction was accompanied by a net contraction. Hardness measurements after aging for 5 min indicated significant hardening of alloys already at 350 °C due to the formation of B2-(Ni,Fe)Al intermetallic precipitates. The age hardening response was significantly superior to that of conventional precipitation-hardenable martensitic stainless steels. Tensile elongation in the aged condition was negatively influenced by the presence of soft ferrite regions. Processing conditions associated with a fully martensitic microstructure prior to aging are required to render a uniform age hardening response. Guidelines for the development of a new family of lightweight precipitation-hardenable steels with lower raw material costs and a higher corrosion resistance compared to the standard 18Ni maraging steels are provided.

The study focuses on the superelastic effect in single-crystalline boron-doped Fe-based shape memory alloys. The homogenized and quenched single crystals were subjected to a heat treatment at 973 K for variable aging times. As a result, small and coherent nanometer-sized γ′ (Ni3Al-type) precipitates were formed. It was established that Fe-28Ni-17Co-11.5Al-2.5Ta-0.05B single crystals oriented along [001] direction exhibit the fully reversible superelastic behavior up to 14.3% compression strain at 77 K reaching the maximum theoretical value. The boron addition suppressed completely the formation of the brittle β phase and reduced the average precipitate size of the γ′ precipitates. Using high-energy synchrotron radiation and high-resolution transmission electron microscopy analysis the volume fraction and precipitate size of γ′ were determined indicating that both factors are critical in obtaining the largest superelastic reversibility. Boron addition counters the initial effect of mechanical stabilization which was detected in single crystals without boron. Unlike the thermally induced martensitic transformation, applied stresses produce a different austenite/martensite interface composed of interchanging austenite and martensite variants. It is also demonstrated that upon loading/unloading cycles the moving transformation front divides the material into three district regions i.e. single variant of austenite, austenite intermixed with martensite and single variant of martensite.