Transformation Start Temperature Research Articles

Abnormal martensitic transformation and morphology induced by chemical heterogeneity in additively manufactured maraging steel

We report on the effect of chemical heterogeneity on martensitic transformation in a maraging steel produced through laser-based directed energy deposition. Unlike conventional maraging steels with lath martensitic microstructures, the maraging steel exhibited an abnormal morphology, which was refined and elongated in the building direction. The growth direction of the abnormal lath martensite did not occur along the habit plane, while the lath martensite morphology corresponded to the cellular dendritic microstructure of the maraging steel. Among the elements partitioned to the interdendritic region, Ni played a major role in the reduction of the martensitic transformation start temperature (Ms temperature), thereby interfering with the martensitic transformation across the interdendritic region. The interdendritic region acted as a new domain boundary of martensitic transformation, where incomplete martensitic transformation in the region resulted in the retention of austenite.

Interpass temperature strategies for compressive residual stresses in cladding low-transformation-temperature material 16Cr8Ni via wire arc additive manufacturing

Low-transformation-temperature (LTT) materials induce compressive residual stresses within an effective depth in wire arc additive manufacturing (WAAM), which is well suited for cladding applications. However, this role was found to be sensitive to the phase transformation sequence. We developed an LTT material—16Cr8Ni (16Cr8Ni-LTT)—and to explore its potential, it was clad onto a KA36 substrate via WAAM. Three different interpass temperature strategies were designed as follows: (1) no interpass temperature control; (2) interpass temperature controlled at approximately 270 °C above the martensite transformation start temperature (Ms = 200 °C) for simultaneous phase transformation; and (3) interpass temperature controlled at approximately 50 °C below the martensite transformation finish temperature (Mf = 60 °C) for sequential phase transformation. A thermal elastic-plastic numerical model considering the phase transformation-induced plasticity (TRIP) was developed using in-house software, JWRAIN-Hybrid, to reproduce the temperature field and residual stresses of 16Cr8Ni-LTT cladding. X-ray diffraction (XRD) and contour methods were employed for residual stress measurements. The high accuracy of the model was validated in terms of the temperature, deformation, and residual stress. The reproduced maximum historical temperature distributions were combined with hardness tests to identify the depths and temperatures of the heat-affected zone (HAZ). The measured maximum compressive residual stresses induced by the three interpass temperature strategies for cladding 16Cr8Ni-LTT were −503, −420, and −720 MPa. Moreover, irrespective of the adopted interpass temperature strategy, both longitudinal and transverse residual stresses were compressive in the 16Cr8Ni-LTT cladding. This study provides scientific insights into LLT-induced compressive residual stresses and highlights the superiority and flexibility of cladding LTT materials via WAAM for improving the resistance of large metal components to fatigue and corrosion.

Prediction of martensitic transformation start temperature of steel using thermodynamic model, empirical formulas, and machine learning models

Three methods are used to predict the martensitic transformation start temperature (Ms ) of steel. Based on the database containing 832 compositions and corresponding Ms data, prediction models are built, modified, and trained. Firstly, Ms was re-calculated by establishing a thermodynamic model to link the martensitic transformation driving force (Gibbs free energy difference of martensite and austenite) with resistance (elastic strain energy, plastic strain energy, interface energy, and shearing energy). Secondly, the existing Ms data is cleaned and re-predicted using traditional empirical formulas within different composition application ranges. Thirdly, four different algorithms in machine learning including random forest, k nearest neighbor, linear regression, and decision tree are trained to predict 832 new Ms values. By comparing the Ms results re-predicted by the mentioned three methods with the original Ms values, the accuracy is evaluated to identify the optimal prediction model.

Transformation Behaviors and Microstructure Modifications in Hydrogen-Charged Ti–Ni Shape Memory Alloy

AbstractThe effect of hydrogen charging duration on the transformation behavior, microstructural evolution, and dynamic microstructural changes associated with thermoelastic martensitic transformation in Ti–Ni shape memory alloy was investigated. Compared with the uncharged specimen, the martensitic transformation start (Ms) and reverse transformation finish (Af) temperatures increased with charging time, whereas the martensitic transformation finish and reverse transformation start temperatures remained almost unchanged. In situ SEM results were consistent with these behaviors. Upon cooling, the transformation progressed from the center to the surface in charged specimens, indicating a higher transformation temperature in the center than the surface. The latent heat of transformation decreased with increasing charging time, quantitatively attributed to an untransformed region consisting of hydrogen-induced martensite and a hydrogen-affected layer. The hydrostatic effect from those layers on the interior B2 phase was proposed as the origin of the increased Ms and Af temperatures.

Effect of quenching temperature on the austenite stability and mechanical properties of high-strength air-cooled TRIP steel prepared with hot-rolled C–Si–Mn sheets

This paper systematically studies the effect of quenching temperature on the microstructure evolution and the relationship between the microstructure and mechanical properties of air-cooled TRIP steel prepared with C–Si–Mn hot-rolled sheet. The results indicate that when the quenching temperature is lower than (200 °C) or close to (250 °C) the Ms temperature, the martensite transformation mainly occurs during quenching or at early stage of air cooling. When the quenching temperature reaches 300 °C, lathy bainite begins to form. As the quenching temperature increases, the morphology of bainite is transformed from lathy to granular. Bainite transformation during air cooling can promote the C enrichment in austenite, thus improving the thermal stability of the austenite. However, martensite transformation also occurs during subsequent air cooling, and the start temperature of martensite transformation first decreases and then increases with the increase of quenching temperature from 300 °C to 450 °C. The yield and tensile strength of TRIP specimens first decrease and then increase, while the retained austenite content, elongation and product of strength and elongation (PSE) first increase and then decrease. When the quenching temperature is 400 °C, the best elongation and PSE are obtained. The best elongation of 400TRIP specimen is mainly attributed to its highest content and suitable mechanical stability of retained austenite, which is favorable for the expansion of the TRIP effect range. The TRIP specimens prepared with hot-rolled sheet all exhibit ultra-high tensile strength (over 1050 MPa), which is related to the ultrafine ferrite grains, martensite transformation during air cooling and TRIP effect of retained austenite.

Basics of Heat Treating: Martempering

Abstract Martempering involves cooling steel from the austenitizing temperature and rapidly cooling into either specially formulated petroleum oil or a molten salt bath with a specific composition to a temperature slightly above the martensite start (Ms) transformation temperature of the steel. Martempering is performed to reduce distortion problems that may be encountered with conventional quenching methods. This article, adapted from the latest ASM Handbook on quenching and quenchants, describes suitable steels for martempering and variables that influence the process.

Microstructures, transformation temperatures and superelastic properties of the rapidly solidified (TiZrHf)50Ni25Co10Cu15 HESMAs

In this study, the effects of the rapid solidification process on microstructures, transformation behaviors and superelastic properties of the multi-component (TiZrHf)50Ni25Co10Cu15 (at%) high-entropy shape memory alloy (HESMA) were investigated. The as-spun (TiZrHf)50Ni25Co10Cu15 fibers were prepared by a rapid solidification process. The solution-treated (TiZrHf)50Ni25Co10Cu15 alloy bulk specimen consisted of a (NiCoCu)-rich matrix, (TiZrHf)2(NiCoCu)-type phase and carbide, while the as-spun fiber specimen consisted of (TiZrHf)-rich matrix and carbide. The (TiZrHf)2(NiCoCu)-type phase is dissolved in the matrix of as-spun fibers due to the rapid solidification process. The martensitic transformation start temperature of the (TiZrHf)50Ni25Co10Cu15 alloy increased from 53.5 °C to 91.5 °C after the rapid solidification process. Both the (TiZrHf)50Ni25Co10Cu15 alloy bulk and fiber specimens showed clear superelasticity and the total superelastic recovery strain increased from 4.6 % to 5.7% after the rapid solidification process.

Phase Transformation Temperature Prediction in Steels via Machine Learning.

The phase transformation temperature plays an important role in the design, production and heat treatment process of steels. In the present work, an improved version of the gradient-boosting method LightGBM has been utilized to study the influencing factors of the four phase transformation temperatures, namely Ac1, Ac3, the martensite transformation start (MS) temperature and the bainitic transformation start (BS) temperature. The effects of the alloying element were discussed in detail by comparing their influencing mechanisms on different phase transformation temperatures. The training accuracy was significantly improved by further introducing appropriate features related to atomic parameters. The melting temperature and coefficient of linear thermal expansion of the pure metals corresponding to the alloying elements, atomic Waber-Cromer pseudopotential radii and valence electron number were the top four among the eighteen atomic parameters used to improve the trained model performance. The training and prediction processes were analyzed using a partial dependence plot (PDP) and Shapley additive explanation (SHAP) methods to reveal the relationships between the features and phase transformation temperature.

The Mechanical and Functional Behaviors of Mn–15 wt%Cu Alloy Produced by Selective Laser Melting

Mn–Cu‐based alloys possess excellent mechanical and functional properties (damping capacity and shape memory effect). This work utilizes a typical additive manufacturing technique, selective laser melting (SLM), to fabricate Mn–15 wt%Cu alloys to explore the mechanical and functional behaviors. The results indicate that the γ‐(Mn, Cu) and a small amount of γ′‐(Mn, Cu) phase are the main phase compositions in the as‐SLMed Mn–15 wt%Cu alloy, as well as twins and second‐phase precipitation particles. The microstructure shows a fine cellular γ‐(Mn, Cu) dendrite with primary dendrite spacing of ≈0.9 μm in the columnar grains with a size of ≈8 μm. The martensitic transformation start temperature (MS) and phase transformation hysteresis are ≈132.6 and ≈5.1 °C, respectively. The existing nanoscale chemical segregations of Mn and Cu are attributed to the spinodal decomposition of γ‐(Mn, Cu). A compositional modulation is observed with a wavelength of ≈20 nm and an amplitude fluctuation of Mn (55–85 wt%) and Cu concentrations (15–45 wt%). Finally, the as‐SLMed Mn–15 wt%Cu alloy boasts good tensile properties (359, 268 MPa, and 3.6%), damping (internal friction of 0.032 when the strain amplitude is 9 × 10−4), and shape memory performances (one‐way ηow = 37–48%, and two‐way ηtw = 16–20% under the pre‐deformation strain of 2–3%).

Pre-existing orthorhombic embryos-induced hexagonal–orthorhombic martensitic transformation in MnNiSi1–x (CoNiGe) x alloy

The thermal–elastic martensitic transformation from high-temperature Ni2In-type hexagonal structure to low-temperature TiNiSi-type orthorhombic structure has been widely studied in MnMX (M = Ni or Co, and X = Ge or Si) alloys. However, the answer to how the orthorhombic martensite nucleates and grows within the hexagonal parent is still unclear. In this work, the hexagonal–orthorhombic martensitic transformation in a Co and Ge co-substituted MnNiSi is investigated. One can find some orthorhombic laths embedded in the hexagonal parent at a temperature above the martensitic transformation start temperature (M s). With the the sample cooing to M s, the laths turn broader, indicating that the martensitic transformation starts from these pre-existing orthorhombic laths. Microstructure observation suggests that these pre-existing orthorhombic laths do not originate from the hexagonal–orthorhombic martensitic transformation because of the difference between atomic occupations of doping elements in the hexagonal parent and those in the pre-existing orthorhombic laths. The phenomenological crystallographic theory and experimental investigations prove that the pre-existing orthorhombic lath and generated orthorhombic martensite have the same crystallography relationship to the hexagonal parent. Therefore, the orthorhombic martensite can take these pre-existing laths as embryos and grow up. This work implies that the martensitic transformation in MnNiSi1−x (CoNiGe) x alloy is initiated by orthorhombic embryos.

Uncovering the isothermal transformation dynamics and elemental diffusion behavior of reversed austenite in PH13-8Mo steel

The isothermal transformation kinetics and microstructural evolution of the reversed austenite significantly influence the properties of PH13-8Mo steel. However, the microscopic phase transformation behavior related to elemental diffusion during isothermal processes for PH13-8Mo steel needs further examination. Herein, we systematically investigate the isothermal transformation and elemental diffusion behavior of reversed austenite in PH13-8Mo steel. Phase transformation kinetic measurements unravel that the complete elimination of residual austenite can be achieved. Only hierarchical martensitic microstructure without austenite is detected after the solution-cooling treatment at 927 °C with a cooling rate of 10 °C/s, determining the critical martensite transformation start and finish temperature to be 176.6 °C, 11.6 °C. The isothermal kinetics of reverted austenite satisfy the Johnson-Mehl-Avrami relationship. The fraction of reversed austenite increases from 0.14 % at 510 °C to 8.73 % at 593 °C. Lath martensite boundaries with large kernel average misorientation values promote elemental diffusion at the interface. Further, the underlying behavior of elemental diffusion is elucidated during the isothermal aging process. The interfacial model indicates the growing width of the reversed austenite up to 100 nm and the compositional gradient formed by the elemental diffusion during aging for 0–5 h at 593 °C. The interdiffusion coefficient calculations interpret the diffusion direction of alloying element Ni from martensite to reversed austenite; Cr and Mo may form alloying carbides at the austenite-martensite interface; Al possesses the fastest diffusivity in martensite and austenite phases. The present work unveils the phase transformation kinetics of PH13-8Mo steel, achieving quantitative regulation for maraging stainless steel.

Effect of Mn content on microstructure and transformation behavior of TiZrHfNiCoCu multi-component high-entropy shape memory alloys

Multi-component Ti16.667Zr16.667Hf16.667Ni25Co10Cu15-XMnX at.% (X = 0, 1, 3, 5 and 7) high-entropy shape memory alloys (HESMAs) were prepared by arc-melting and then microstructures, transformation temperatures, phase constituents and superelastic properties were investigated by scanning electron microscope observation, differential scanning calorimetery, X-ray diffraction and dynamic mechanical analysis in tensile mode, respectively. The microstructure of solution-treated TiZrHfNiCoCuMn HESMAs specimens consisted of (NiCoCuMn)-rich matrix, (TiZrHf)2(NiCoCuMn)-type phase and (TiZrHf)(NiCoCuMn)-type phase. The area fraction of (TiZrHf)2(NiCoCuMn)-type phase increased from 1.7 to 5.1% with increasing Mn content from 0 to 7 at.%. The area fraction of the (TiZrHf)(NiCoCuMn)-type phase increased from 1.9 to 17.2% with increasing Mn content from 3 to 7 at.%. -ΔHmix effect was dominant over ΔSmix and δ effects for phase constitution in TiZrHfNiCoCuMn HESMAs. The martensitic transformation start temperature decreased with the addition of Mn content up to 3 at.% and then increased with increasing Mn content from 3 at.% to 7 at.%. TiZrHfNiCoCuMn HESMAs showed clear superelasticity and the total recovered strain decreased with increasing Mn content.

MICROSTRUCTURE EVOLUTION AND PHASE TRANSFORMATIONS IN MICROALLOYED ARMOX 500T STEEL DURING A DILATATION PROCESS

With this work we successfully developed two modified Armox 500T alloys using microalloying with different amounts of niobium (Nb) and boron (B) to obtain a finer grain size, which in return enhances the remnant properties. Furthermore, different heat-treatment cycles were designed and performed using a quenching dilatometer to study the combined effect of thermal cycling and microalloying with Nb and B on the microstructure and transformation temperatures including the start and finish temperature of the austenite transformation (Ac1, Ac3) and the martensite start and finish temperature (Ms, Mf) of the investigated alloys. Dilatometry results show that increasing the content of Nb from 0.07 w/% to 0.13 w/% and B from 0.0035 to 0.0046 w/% increases the temperature range between Ac1 and Ac3 by 55 °C, indicating a broader range for changing heat-treatment temperatures. In addition, the Ms temperature is reduced by 13 °C due to austenite refinement caused by the microalloying of Nb and B. The effect of the annealing temperature at a constant heating rate showed a significant impact on the austenite grain size and hardness. Furthermore, the kinetics of phase transformations were theoretically studied using Thermo-Calc, and the numerical predictions were confirmed experimentally with dilatometry results. Metallography investigations using a scanning electron microscope (SEM) and an optical microscope (OM) were conducted to evaluate the microstructure evolution of the developed alloys. Hardness tests were performed to evaluate the effect of the grain refinement of martensite lathes caused by microalloying with Nb, B, and heat-treatment thermal cycling. It is found that the hardness of the modified Armox alloys in this research was improved by 14 % in comparison with the conventional Armox 500T.

Effects of Thermal Cycling and Porosity on Phase Transformation of Porous Nanocrystalline NiTi Shape Memory Alloy: An Atomistic Simulation

Herein, the effects of thermal cycling and porosity on the martensitic phase transformation behavior of porous nanocrystalline (NC) NiTi shape memory alloys (SMAs) at the atomic scale are investigated by molecular dynamics simulation. The simulation results show that thermal cycling causes the NC NiTi SMAs to repeatedly undergo martensitic phase transformation and inverse phase transformation processes. The martensitic phase transformation capability is suppressed with increasing porosity and number of cycles, but the suppression effect of thermal cycling is weakened with increasing porosity. Moreover, the start temperature of martensitic transformation, residual martensitic phase, and the fraction of interstitial atoms increase with increasing porosity. Thermal cycling causes the disordered structures and shear plastic deformation to accumulate as cycling increases. Recoverable shear deformation is generated within the grain, but shear plastic deformation is mainly concentrated at both grain boundaries and pore surfaces. Residual martensitic phase increases after thermal cycling. These results provide important explanations and references for a deeper understanding of phase transformation behavior and pore effects caused by thermal cycling.

Effect of recrystallization on bainite transformation and mechanical properties of complex phase steel with high formability (CH steel)

Herein, the effect of recrystallization on the bainite transformation and mechanical properties of CH steel were investigated using SEM, TEM, EBSD, dilatometry, and tensile testing machine. First, precipitation was introduced to hinder recrystallization by batch annealing. Subsequently, the competitive relationship between austenite transformation and recrystallization was revealed using different annealing processes. Finally, the effect of recrystallization on bainite transformation was analyzed, mainly including bainite morphology, fraction, transformation rate, bainite nucleation rate of untransformed austenite and austenite-bainite orientation relationship, as well as the effect on the mechanical properties. After batch annealing, a large amount of cementite and nanoscale (Nb, Ti)C precipitated in the hot-rolled sheet, the austenite transformation start temperature (Ac1) and the pearlite to austenite transformation end temperature (Ac1f)of the cold-rolled sheet increased, and the austenite transformation end temperature (Ac3) of the cold-rolled sheet decreased, and the austenite transformation process accelerated. The microstructure of annealed sheets is consisted of ferrite, granular bainite, martensite, and a small fraction of retained austenite. Carbide hindered the recrystallization process during the annealing process, resulting in significant refining of the grains. The lower degree of recrystallization promoted bainite transformation, providing more nucleation sites, higher grain boundary and autocatalytic nucleation activation energy, improving bainite's fraction and transformation rate. The bainitic ferrite was mainly lathy in the recrystallized zone and polygonal in the non-recrystallized zone. Both a K-S and an N-W relationship between retained austenite and bainite, while the N-W relationship in the non-recrystallized zone was relatively higher. Without severe loss of plasticity, hindering recrystallization increased the dislocation, grain refinement, and precipitation strengthening values of the samples, and the strength significantly improved. However, with the increase of annealing temperature, recrystallization fraction increased gradually, and the increase in strength became less noticeable.

INFLUENCE OF THE THERMAL CONDITION OF STEEL ON THE TRANSFORMATION TEMPERATURES OF TWO CHROMIUM HOT-WORK TOOL STEELS

The influence of the thermal condition of the steel on the transformation temperatures of two chromium hot-work tool steels was investigated. The steels studied were in two different thermal states: the soft -annealed state and the hardened-and-tempered state. The soft-annealed condition, i.e., the fully annealed condition, is a thermal state of steels in which the matrix is ferritic, and the carbon is chemically bonded in spherical carbides. The hardened-and-tempered condition, on the other hand, means a fully hardened-and-tempered martensitic matrix with uniformly distributed (primary and secondary) carbides. The samples were analysed in a simultaneous thermal analyser (STA) using the differential scanning calorimetry (DSC) method to determine the transformation temperatures. We also performed calculations based on the CALPHAD method to obtain the equilibrium temperatures of the transformations. The aim of the study was to determine the influence of different thermal conditions of chromium hot-work tool steels on the transformation temperatures such as solidus/liquidus temperatures, eutectoid transformation temperatures (A1 and A3), austenite solidification temperature and martensite transformation start temperatures. Since DSC analysis also measures thermal influence, we were able to determine the energies absorbed during eutectoid transformation and melting (endothermic processes) and the energies released during the solidification of δ-ferrite and γ-austenite (exothermic processes), as well as the energies released during martensite transformation. It was found that hardening and tempering reduce both eutectoid transformation temperatures and that the solidification intervals are closer to those calculated. From an energetic point of view, hardening and tempering reduce the energies absorbed during melting.

Study of ageing and size effects in Nickel–Titanium shape memory alloy using molecular dynamics simulations

ABSTRACT The effect of ageing process at 1073.15 K on the athermal phase transformation in different nanograins (4.5, 6 and 9 nm) of Nickel–Titanium shape memory alloy is studied using molecular dynamics simulations. The martensitic transformation start temperature and transformed martensite volume decrease with decreasing grain size. The two-stage phase transformation with intermediate phase formation is observed only in the smallest grain (4.5 nm) where a single martensite variant is formed. On the contrary, larger grains show a self-accommodated twinned martensite variants with one-stage transformation from austenite to martensite. The ageing process hardly influences the morphology of the martensite phase, however, it rises the martensite start temperature and reduces the austenite start temperature. The nano grain size models demonstrate the constrained transformation due to the transformation barrier and the accumulation of internal stress, while the ageing process shows the tendency for stress relaxation.

SAG usage wear-resistant steel with rare retained austenite achieve trade-off between impact toughness and strength

The impact toughness is the key to determine the service life of high strength wear-resistant steel for the lining boards of large-scale semi-autogenous grinding mill (SAG). It is difficult to predict the impact fracture failure of lining boards when SAG is running, which can cause damage to equipment once it occurs. Therefore, novel high strength wear-resistant steel was designed to improve impact toughness in this study. It is interestingly found that the microstructure contained finer bainite/martensite, and rare retained austenite (around 2 vol.%), which achieved trade-off between impact toughness and strength after the optimal heat treatment scheme (900 °C normalizing → 900 °C quenching → 300 °C tempering). The element segregation of Al and Si was improved and the start temperature of martensitic transformation during quenching was reduced, resulting in the refinement of martensite laths. Thus, the higher density of high-angle grain boundary (HAGB) and dislocation were obtained, which contributed to excellent impact toughness. Based on the instrumented impact curve, the total impact energy can be divided into four parts: elastic deformation energy (Ee), plastic deformation energy (Ed), crack stable propagation energy (Ep1) and crack unstable propagation energy (Ep2). Traditional toughening mechanisms focus on Ep1, which was based on the tortuous crack propagation path. Our experimental results show that Ed was another effective control unit to ensure the toughness. The difference between Ed and Ep1 in improving absorption energy was discussed in detail by combining HAGB density and dislocation density.

The improvement of the shape memory effect of Cu-13.5Al–4Ni high-temperature shape memory alloys through Cr-, Mo-, or V-alloying

This study presents the effect of small additions of Cr, Mo, or V on the microstructure, crystallographic features of martensitic transformation, and shape memory effect of Cu-13.5Al-4Ni master alloy. The results show that the studied alloys consist of dominant β'1 (18R) martensitic structure and fine bcc precipitates which are riched in the alloying elements but poor in the Cu content. The martensitic transformation start temperatures of the alloys are 414K, 385K, and 394K when X = 0.5Cr, 0.5Mo, or 0.5V, respectively. More importantly, a drop of grain size was achieved in the studied alloys, leading to the improvement of the shape memory effect in Cu-13.5Al-4Ni alloys. As the result, the studied alloys exhibit a steady increase of shape memory effect as the pre-strain changed from 6% to 10%, which the maximum shape memory effect reaching 6.2%, 5.8%, and 5.5% for X = 0.5Cr, 0.5Mo, and 0.5V, respectively.

Role of Magnetism in Lattice Instability and Martensitic Transformation of Heusler Alloys

Heusler alloys are subject of considerable interest because they exhibit a martensitic transformation (MT), a shape-memory effect and a giant magnetocaloric effect. As it is commonly believed, the pronounced magnetoelastic coupling plays a crucial role; however, the effect of alloy composition on MT is still under discussion. To describe the features of MT in Ni0.75−xMnxGa0.25 Heusler alloys, the phenomenological model that consistently considers the magnetic and lattice degrees of freedom and their mutual interplay has been developed. The magnetic entropy contribution was estimated within the framework of the microscopic approach. The proposed model allows us to describe the dependence of the martensitic transformation start temperature Ms(x) on the Mn concentration x in reasonable agreement with the experiment.