Martensite Transformation In Steel Research Articles

Phase Transformations in Layered Composites Based on Zr, Nb, Ta or Ti Sheet Explosively Welded to Stainless Steel Plate

The composites based on reactive metals (Zr, Ta, Nb, Ti) sheets explosively welded to stainless steel plates were investigated using X-ray synchrotron radiation, TEM and SEM to characterize phase transformations in near-the-interface layers. SEM and TEM investigations of the solidified melt regions unveiled amorphous and nanocrystalline non-equilibrium phases of variable chemical compositions, incorporating elements from the joined components. Phase analysis in layers near the interface carried out using high-resolution synchrotron radiation show predominantly reflections coming from the main elements of parent sheets/plates. Nevertheless, a closer look at the diffraction patterns shows the presence of reflections coming from the phases based on the two-component equilibrium phase diagrams. The measurements performed at the interface, but including only the steel plate, revealed significant amounts of α-Fe, γ-Fe and ε-Fe phases. Their appearance was attributed to the high pressure and fast cooling rates, which promoted a martensitic transformation in steel.

ON α- MARTENSITE NUCLEATION IN STEELS AND IRON ALLOYS

Purpose. To investigate the possibility of martensite nucleation on magnetic (concentration) inhomogeneities of chrome-nickel steels austenite.
 Research methods. Generalization of experimental and theoretical studies of the strong magnetic field effect on the martensitic transformation in steels and iron alloys, taking into account the magnetic state of austenite
 Results. The analysis of the magnetic state of austenite and the abnormally large strong magnetic field effect on the martensite point displacement in 40X2H20 steel indicates that the transformation of austenite into martensite in it is a “magnetic first order phase transition”.
 Scientific novelty. Some features of the martensitic transformation in steels and iron alloys are explained. It is assumed that martensite during the “magnetic γ→α- transition” originates on magnetic inhomogeneities with a disoriented magnetic structure of the spin glass type. Forced magnetostriction during paraprocess and spontaneous magnetostriction below the Curie point lead to the emergence of collinear ferromagnetism with a change in the coordination number from 12 to 8. High all-round pressure leads to the occurrence of collinear antiferromagnetism and the γ-phase stabilization. In paramagnetic austenite the nucleation of α-martensite occurs on dislocation nuclei, the sizes of which have the same order of magnitude (by volume) as the magnetic inhomogeneities in the Fe-Ni system.
 Practical value. The results obtained in the work expand the concept of martensitic transformation in steels and provide grounds for explaining some kinetic features in them.

Effect of Magnetic State of Austenite on Thermodynamics and Kinetics of Martensitic Transformation in Steel

Effect of Magnetic State of Austenite on Thermodynamics and Kinetics of Martensitic Transformation in Steel

The model of austenite structure state taking into account fluctuations of magnetic nature

The results of magnetic state simulation of carbon steel austenite are presented in this work. Quantitative parameters of the areas of short-range magnetic order, which are presented in paramagnetic austenite matrix and are observed as potential places of α-phase nucleation, are examined. This austenite heterogeneity in steels and clean iron is confirmed by the experimental data; however, detailed quantitative study meets technical complication of such experiments. It is suggested to use the Ising model for realization of the calculation experiment. The obtained spin configurations were analyzed for determination of geometric parameters of magnetic inhomogeneities and period of their sustainable existence. Calculations were conducted within the temperature range of austenite phase existence in steel, as well as without and with taking into account presence of external magnetic field, which strengthens existing austenite heterogeneity and has the effect on conduction of phase transformations in steel. The results of simulation confirm presence of the areas of short-range magnetic order in austenite, their size is close to the existing theoretical assessments and experimental data. It is shown that temperature rise leads to decrease of dimensions, number and life period of clusters, but these parameters are still important at the temperature of martensite transformation in steel. Overlapping of external magnetic field creates the conditions for existence of large-size ferromagnetic clusters also at more high temperatures, as well as increases time of their sustainable existence. The external magnetic field provides the conditions for more intensive and multiplicative nucleation of ferromagnetic phase, similar to short-range magnetic order fluctuations in location of atoms during liquid metal cooling. 

Влияние степени деформации в условиях низких температур на превращения и свойства метастабильных аустенитных сталей

Introduction. For reliable operation of low-temperature equipment, it is necessary to use materials capable of ensuring operability in a wide temperature range under conditions of alternating loads, exposure to corrosive media, etc. Most often, in such cases, metastable austenitic steels (MAS) of various alloying systems are used. Despite sufficient experience in the use of such materials, not enough information is collected on the behavior of such materials at low temperatures, including phase-structural transformations, the features of such transformations in different temperature zones, including when a load is applied, both static and dynamic. The subject of the study in this work is selected MAS 10Cr14NMn20 and 10Cr14Mn14Ni4Ti grades. The purpose of the study is to evaluate the performance of industrially used metastable austenitic steels for its possible use instead of steel 12Cr18Ni10Ti. Research methodology. The phase composition of the samples was studied on a DRON-3.0 X-ray diffractometer. Mechanical tests were carried out in the temperature range from +20 to -196 °C. Static uniaxial tensile tests were carried out on a R-20 tensile testing machine; cylindrical specimens with threaded heads were prepared according to GOST 11150–75, as well as samples with a circumferential notches. Dynamic bending tests were carried out on a pendulum impact tester, using samples according to GOST 9454–78. Results and Discussion. Based on the data obtained, it is found that an increase in the strain rate at low temperatures contributes to a decrease in the number of martensitic phases in the steels under study. It is found that the hardenability during elastic-plastic deformation decreases and completely disappears at the temperature of the material transition to a brittle state. It is shown that an increase in the rate of low-temperature deformation of samples prevents the development of phase martensitic transformations in steels. The results obtained can be recommended for use in the selection of materials for the manufacture of equipment operating at temperatures down to -196 °C. Conclusions. It is shown that the obtained values of the characteristics of mechanical properties make it possible to recommend the studied MAS as a substitute for steel 12Cr18Ni10Ti, down to a temperature of -196 °C.

MODERN STRUCTURAL STEELS WITH TRIP-EFFECT

Purpose. The aim of this work is to analyze the latest literature data on current state of the issue in the development of the chemical composition and heat treatment modes of steels that implement the TRIP effect under loading. Due to the deformation martensitic transformation of retained austenite, these steels have increased mechanical and operational properties with cost-saving alloying. Methodology. The information published over a long period of time in scientific literature, including domestic sources and high-rating foreign publications are used in this work. The information is systematized by the main TRIP-steels’ types and relates to their chemical composition and heat treatment technology. Findings. The article presents a retrospective of the research progress on the development of structural TRIP effect added steels namely: high-alloy single-phase metastable austenitic steels, as well as low-alloy multiphase TRIP-assisted steels, in which the TRIP effect plays an supporting role, as an additional strengthening and providing increased plasticity mechanism. The latter group of steels includes low alloy TRIP-assisted steels, δ-TRIP steels and maraging TRIP steels. The typical alloying schemes and applied heat treatment modes that make it possible to achieve the optimal phase-structural components ratio, the volume fraction and stability of retained austenite to deformation martensitic transformation in steels and, as a result, to provide an increased mechanical and operational properties were described. The key role of the carbide-free structure formation is noted, which is achieved by alloying with silicon and/or aluminum in providing high properties of steel. The prospects of using modern structural TRIP steels in the automotive and in the machine-building industry are shown. Recommendations for further research directions in this area are made. Originality. The article provides an analysis and systematization of relevant literature data on the development of technologies for the production of multiphase structural steels with retained metastable austenite, realizing the TRIP effect. Practical value. The results of the research can be used as reference materials in solving material design problems, as well as for educational purposes in the preparation of specialized professionals in engineering specialties.

X-RAY STUDIES OF COMPOSITE MATERIAL STEEL 20 - STAINLESS STEEL 50CR15MO2V AFTER EXPLOSION WELDING AND NORMALIZATION

The results of studies of the characteristics of the fine structure of a bimetal made of stainless steel 50Kh15M2F and carbon steel 20 after explosion welding and subsequent heat treatment in the joint zone are presented. It was found that normalization at temperatures above 900 ° C leads to a martensitic transformation in steel 50Cr15Мo2V, which is accompanied by an increase in hardness and the development of a fine structure: an increase in the level of elastic deformation of the crystal lattice and fragmentation of coherent scattering regions.

Martensitic transformation in the systems based on Fe-Ni compositions in strong pulsed magnetic fields

Purpose of work. To ascertain the causes of the abnormally large displacement of the martensitic point in steels and iron alloys in strong pulsed magnetic fields at low temperatures. Research methods. Generalization of experimental and theoretical investigations of the strong magnetic field influence on the martensitic transformation in steels and iron alloys, taking into account the magnetic state of austenite. The obtained results. The distributions of the martensitic point displacement ΔMS from the content of the main component - iron and the temperature of the martensitic γ → α- transformation beginning (martensitic point MS) in different experiments are obtained. It is shown that the obtained temperature dependence ΔMS(MS) in a strong magnetic field at low temperatures decomposes into two components, one of which correlates with the generalized Clapeyron-Clausius equations, and the other is opposite to it. In addition, it was found that steels and alloys with intense γ → α- transformation in a magnetic field contain at least 72.5% iron (wt), which at low temperatures in the fcc structure is antiferromagnetic. Scientific novelty. The anomalous temperature dependence of the distribution ΔMS(MS) in a strong magnetic field is explained on the basis of quantum representations of the magnetic interaction of atoms in the Fe-Ni system. This effect is associated with a number of other invar effects, in particular, with an abnormally large spontaneous and forced magnetostriction, a strong dependence of the resulting exchange integral on the interatomic distance. The point of view according to which in these alloys in a magnetic field γ → α- transformation occurs by the type of “magnetic first kind phase transformation” is substantiated. It is assumed that the nucleation of the martensitic phase in a magnetic field occurs in (at) local regions of γ- phase with disoriented atomic magnetic moments (with high compression and increased forced magnetostriction). Practical value. The information obtained in this work provides grounds for explaining the kinetic features of the transformation of austenite into martensite in steels and iron alloys.

Dynamic Theory of the Effect of a Strong Magnetic Field on the Martensitic Transformation in Steels with Austenite Grain Sizes Close to a Critical Value

In the dynamic theory of martensitic transformations, temperature Ms of the start of transformation corresponds to conditions that are optimal for generating waves by nonequilibrium d electrons that control the growth of a martensitic crystal. The recognition of relative damping $${{\Gamma}}_{{\text{e}}}^{'}$$ of s electrons plays a substantial role in this case. A general analysis that has made it possible to propose, for the first time, an analytical formula for critical size Dc $$\left( {{{\Gamma}}_{{\text{e}}}^{'}} \right)$$ of austenite grains is used to interpret the results for chromium–nickel steels, in which the transformation is initiated by strong magnetic fields. Under conditions of positive bulk magnetostriction, a magnetic field with intensity H lowers the chemical potential of electrons. As a result, the Dc( $${{\Gamma}}_{{\text{e}}}^{'},$$ H) value decreases and austenite with a grain diameter of D, which was stabilized by condition D < Dc( $${{\Gamma}}_{{\text{e}}}^{'},$$ 0) in the absence of a field, becomes destabilized, since inequality D > Dc( $${{\Gamma}}_{{\text{e}}}^{'},$$ H) is fulfilled. The dynamic theory predicts a sharp increase in the Dc( $${{\Gamma}}_{{\text{e}}}^{'},$$ 0) value at $${{\Gamma}}_{{\text{e}}}^{'}$$ → 1. This is evidenced by value Dc $$\left( {{{\Gamma}}_{{\text{e}}}^{'}} \right)$$ ≥ 1 mm for steel 67Kh2N22, which is three orders of magnitude larger than Dc( $${{\Gamma}}_{{\text{e}}}^{'},$$ 0) ≈ 1 μm for the Fe–31Ni alloy. Other peculiarities of the effect of magnetic field on the martensitic transformation are also discussed.

Phase field simulation of martensitic-transformation-induced plasticity in steel

The influence of martensitic-transformation-induced plastic deformation in steel is studied using phase field simulations. A phase field framework that incorporates elastic as well as plastic effects is used. A high-carbon steel is considered as an example to illustrate the coupling of martensitic transformations and plasticity. In this work, all 24 transformation variants associated with the Kurdjumov-Sachs orientational relationship are considered to realistically describe the martensitic transformation in steel. Temperature-induced as well as stress-induced transformations are studied. The role of plasticity is investigated by performing simulations with and without the plastic flow. It is found that transformation plasticity plays a dual role: (1) in the case of temperature-induced transformations it helps in the initial nucleation of the martensite by stabilizing the initial embryo, and (2) once the martensite starts to grow, transformation-induced plasticity resists the further growth and results in stabilization of retained austenite. In contrast, the simulations in the absence of transformation-induced plasticity show that the entire system transforms into martensite. For stress-induced transformations, it is found that transformation induces plastic deformations, even though the applied (macroscopic) stress is lower than the yield stress. Although the dominant contribution to the stress-induced strain arises due to the formation of favored martensite variants, transformation-induced plasticity generates significant additional deformation that can influence the mechanical properties and the resulting martensite domain pattern.

On the Role of Isothermal Martensite Formation during Cryogenic Treatment of Steels

Abstract This contribution addresses the effect of sub-zero Celsius treatments at cryogenic temperatures on the steel microstructure. It is shown that the formation of martensite, including the socalled isothermal, i. e. time dependent or thermally activated, martensitic product, can provide an explanation to the observations in the literature that both the temperature of cryogenic treatment and the holding time at cryogenic temperatures may have an influence on the performance of steel products in service. This review traces the most important stages in the development of cryogenic treatments along the last 95 years and describes it in parallel with the description of the current state of understanding of the kinetics of martensitic transformation in steel. In the last part of the contribution, the new insight is put into context with a practical example where various types of subzero Celsius treatments are applied.

On the Role of Isothermal Martensite Formation during Cryogenic Treatment of Steels

Abstract This contribution addresses the effect of sub-zero Celsius treatments at cryogenic temperatures on the steel microstructure. It is shown that the formation of martensite, including the socalled isothermal, i. e. time dependent or thermally activated, martensitic product, can provide an explanation to the observations in the literature that both the temperature of cryogenic treatment and the holding time at cryogenic temperatures may have an influence on the performance of steel products in service. This review traces the most important stages in the development of cryogenic treatments along the last 95 years and describes it in parallel with the description of the current state of understanding of the kinetics of martensitic transformation in steel. In the last part of the contribution, the new insight is put into context with a practical example where various types of subzero Celsius treatments are applied.

Novel -75°C SEM cooling stage: application for martensitic transformation in steel.

Microstructural changes during the martensitic transformation from face-centred cubic (FCC) to body-centred cubic (BCC) in an Fe-31Ni alloy were observed by scanning electron microscopy (SEM) with a newly developed Peltier stage available at temperatures to −75°C. Electron channelling contrast imaging (ECCI) was utilized for the in situ observation during cooling. Electron backscatter diffraction analysis at ambient temperature (20°C) after the transformation was performed for the crystallographic characterization. A uniform dislocation slip in the FCC matrix associated with the transformation was detected at −57°C. Gradual growth of a BCC martensite was recognized upon cooling from −57°C to −63°C.

EBSD investigation of the crystallographic features of deformation-induced martensite in stainless steel

Variant selection during the martensitic transformation in steels may play an important role in determining the transformation kinetics and the resulting mechanical properties. In this study, the variant selection and crystallographic features of deformation-induced martensite were investigated by quasi in situ electron backscatter diffraction (EBSD) in grade SUS321 during tensile deformation. Significant differences in variant selection between austenite (γ)→hcp-martensite (ε)→bcc-martensite (α’) and γ→α’ transformation routes were observed and reported in detail, which demonstrated that ε-martensite plays an important role in the variant selection of α’. Variant selection at different deformation stages was also analysed and revealed that α’ variants with the highest priority and variant pairs were preferred at the initial and last deformation stages in the γ→ε→α’ sequence, respectively. Meanwhile, the single α’ variant nucleated at the thin slip band keeps its crystallography feature upon further deformation in the γ→α’ sequence. In addition, the strain work of the martensitic transformation for applied loads was quantitatively estimated to explain the variant selection and associated mechanism. When these calculations are compared to the experimental results it is found that they are not able to predict which α’ variant is forming preferentially during either during the γ→α’ or the ε→α’ sequences, while only accurate predictions are obtained for the γ→ε transformation which indicates that the γ→α’ variant selection is more complex.

Internal residual stress originated from Bain strain and its effect on hardness in Fe–Ni martensite

To further understand the internal residual stress that is microscopically generated via martensitic transformation in steels, the origin of internal strain attributed to Bain correspondence between face-centered cubic (fcc) and body-centered cubic (bcc) was evaluated from macro- and micro-viewpoints and its effect on hardness was investigated in an interstitial free Fe-16%Ni martensite. Neutron diffractometry and electron backscatter diffraction analysis showed that body-centered cubic (bcc) crystal structure of as-quenched martensite contained elastic distortions leading to small tetragonality, even in martensitic steels without solute carbon, and that the extended [001]bcc of martensite tended to be parallel to <001>fcc of prior fcc austenite. In addition, the combination of micro-scale focused ion beam (FIB) and high-precision digital image correlation techniques revealed that a micropillar fabricated by FIB processing within a martensite block was anisotropically deformed by the release of the residual strain distributed in as-quenched martensite in correspondence with the tetragonal distortions of the bcc crystal structure. These results prove that a small part of the Bain strain remained as an internal elastic residual strain and was microscopically distributed among Bain groups in lath martensite after martensitic transformation. Furthermore, the residual strain generated a hydrostatic internal stress, and therefore, the nanohardness decreased considerably by the micropillar fabrication accompanied by the release of the internal stress. This means that the internal residual stress in martensite among Bain groups influences the mechanical properties of martensitic steel.

Internal Residual Stress Originated from Bain Strain and its Effect on Hardness in Fe-Ni Martensite

To further understand the internal residual stress that is microscopically generated via martensitic transformation in steels, the origin of internal strain attributed to Bain correspondence between face-centered cubic (fcc) and body-centered cubic (bcc) was evaluated from macro- and micro-viewpoints and its effect on hardness was investigated in an interstitial free Fe-16%Ni martensite. Neutron diffractometry and electron backscatter diffraction analysis showed that the as-quenched martensite had a body-centered tetragonal (bct) crystal structure with small tetragonality, even in martensitic steels without solute carbon, and that the [001]bct of bct martensite tended to be parallel to fcc of prior fcc austenite. In addition, the combination of micro-scale focused ion beam (FIB) and high-precision digital image correlation techniques revealed that a micropillar fabricated by FIB processing within a martensite block was anisotropically deformed by the release of the residual strain distributed in as-quenched martensite in correspondence with the orientation of the bct crystal structure. These results prove that a small part of the Bain strain remained as an internal elastic residual strain and was microscopically distributed among Bain groups in lath martensite after martensitic transformation. Furthermore, the residual strain generated a hydrostatic internal stress, and therefore, the nanohardness decreased considerably by the micropillar fabrication accompanied by the release of the internal stress. This means that the internal residual stress in martensite among Bain groups influences the mechanical properties of martensitic steel.

Crystallography of Martensitic Transformation in Steels: Advances in Experimental and Theoretical Research

Crystallography of Martensitic Transformation in Steels: Advances in Experimental and Theoretical Research

Features of Martensitic Transformation in Steel during Quenching in a Constant Magnetic Field

The data on the occurrence of martensitic transformation in steel under the action of a magnetic field were obtained by the electric resistivity method. The obtained data indicate the possibility of stress-assisted martensite formation in the temperature range of Ms-Md (in which superplasticity of austenite is observed). This possibility is due to the magnetic heterogeneity of austenite. Nanosized regions with a ferromagnetic order are present in the paramagnetic matrix. They can perceive the energy of the external magnetic field through the magnetostrictive stresses and change the fields of the elastic forces in the crystal lattice. All this leads to a decrease in the energy of formation of the nucleation center.

Revisiting Temperature and Magnetic Effects on the Fe-30 Wt Pct Ni Martensite Transformation Curve

This work revisits the relationship between the volume fraction of martensite and the transformed microstructure. This relationship is analyzed by considering that although thermodynamic principles determine the possibility of transformation, the size and arrangement of the martensite units over the austenite grains is determined by the local surroundings. The proposed equation for the transformation curve incorporates the probabilistic aspect of the initial transformation in a limited number of scattered austenite grains and autocatalysis. The validation of the model, already verified with data typical of FeC steels, FeNiC and FeNiMn alloys, is extended in this study to a transformation that exhibits microstructure diversity. Finally, we show that the model fits the transformation curves typical of “18Cr-8Ni” stainless steel; this finding demonstrates that the model is applicable to transformation curves characteristic of other systems due to a conceptualization based on the intrinsic aspects of martensite transformation in steels.

Hidden pathway during fcc to bcc/bct transformations: Crystallographic origin of slip martensite in steels

As a classical structural phase transformation with a long history in physical metallurgy, the face-centered cubic (fcc) to body-centered cubic/tetragonal (bcc/bct) martensitic transformation in steels is one of the most studied crystal structure changes induced by either temperature or stress. However, it has yet to be realized that besides the well-known Bain path that leads to {112} twinning in the bct lattice, a hidden pathway could also be activated during the transformation, which leads to the formation of slip martensite and {110} twinning in the bct lattice. We show by symmetry and crystallographic analysis that activation of the hidden pathway is driven by internal or external stress coupled with short-range migration of interstitial carbon atoms. By considering both the hidden pathway and the Bain path, we construct a transformation pathway network for the fcc to bcc/bct transformation, which allows a systematic analysis of defects generation during transformation cycling. A new mechanism for the formation of slip martensite in steels is demonstrated through the analysis of symmetry breaking along both the hidden pathway and the Bain path, by which two distinctive types of martensites, slip and twinning martensites, are placed on an equal footing from a crystallographic point of view. The pathway network provides new insights into structural transformation mechanisms and the nature of transformation induced defects in steels.