Stability Of Retained Austenite Research Articles

The effect of retained austenite stability on high speed deformation behavior of TRIP steels

The safety of passengers is important during an automobile collision. Because a collision event involves high speed deformation, it is necessary to develop property data and understand the applicable deformation mechanisms to aid in the selection of proper materials for crash-related automotive components. Therefore, dynamic mechanical properties of low carbon TRIP steels with varying retained austenite stabilities were evaluated over a wide range of strain rates using a high-velocity hydraulic tensile testing machine. Tensile tests were performed at strain rates ranging from 102 to 6×102 s1 using standard ASTM E-8 specimens with an elastic strain gage attached to the sample grip end to measure load, and a plastic strain gage mounted onto the gage section to measure strain. Ultimate tensile strengths (UTS), strain rate sensitivities, and strain hardening behaviors are reported. TRIP steel with high stability retained austenite exhibited higher yield stress, lower UTS and lower strain hardening than TRIP steel with low stability retained austenite.

In Situ Neutron Diffraction Analysis of Phase Transformation Kinetics in TRIP Steel

The precise characterization of the multiphase microstructure of low alloyed TRIP steels is of great importance for the interpretation and optimisation of their mechanical properties. In-situ neutron diffraction experiment was employed for monitoring of conditioned austenite transformation to ferrite, and also for retained austenite stability evaluation during subsequent mechanical loading. The progress in austenite decomposition to ferrite is monitored at different transformation temperatures. The relevant information on the course of transformation is extracted from neutron diffraction spectra. The integrated intensities of austenite and ferrite neutron diffraction profiles over the time of transformation are then assumed as a measure of the volume fractions of both phases in dependence on transformation temperature. Useful information was also obtained on retained austenite stability in TRIP steel during mechanical testing. The in-situ neutron diffraction experiments were conducted at two different diffractometers to assess the reliability of neutron diffraction technique in monitoring the transformation of retained austenite during room temperature tensile test. In both experiments the neutron investigation was focused on the volume fraction quantification of retained austenite as well as on internal stresses rising in structure phases due to retained austenite transformation.

Evaluation of retained austenite stability in heat treated cold work tool steel

In the paper interdependence between retained austenite volume fraction and external tensile stresses as well as temperature and tempering time have been observed. On the basis of the obtained research a parameter characterising mechanical stability of retained austenite has been introduced. Low mechanical stability of retained austenite has been observed in the structure of the researched 70MnCrMoV9-2-4-2 steel which was not tempered after hardening.

Microstructure and tensile behaviour of cold-rolled TRIP-aided steels

The transformation of austenite to martensite is fundamental to the hardening of carbon steels. This transformation plays an important role for the mechanical behaviour of low-carbon ferrous alloys containing about 10 vol.% retained austenite. The effect, known as transformation-induced plasticity (TRIP), is manifested by unusual high work hardening and high uniform elongation—properties very desirable for thin sheets applied for automotive parts. Tensile tests of cold-rolled sheets at room temperature allowed to study the retained austenite stability against strain-induced martensitic transformation. The influence of the processing texture (specimen orientation) and the strain rate (2 × 10 −2 and 2 × 10 −3 s −1) on the uniform elongation was observed experimentally. Results show that a homogeneous microstructure and the absence of initial blocky martensite ensure long deformation paths. At the same time, tensile data reveal only a small influence of deformation parameters on the ultimate strength.

A physically based model for TRIP-aided carbon steels behaviour

A physically based model for TRIP carbon steels is developed suitable to predict the macroscopic behaviour of multi-constituent aggregates. It includes the effects of phase composition and morphology on flow stress and strain hardening. In a first part, a detailed description of the stress-assisted and strain-induced martensitic transformation kinetics is given based on a generalised form of the Olson–Cohen model. The appearance of the much harder martensitic phase during plastic straining gives rise to a strong hardening of the retained austenite islands. The matrix behaviour is described using a model previously developed for ferritic–martensitic steels. A quite simple but accurate homogenisation approach is used to determine the TRIP steel behaviour. The predicted evolution of strain-induced martensite volume fraction, flow stress and incremental work hardening is in good agreement with experimental data and illustrates the critical importance of the retained austenite stability on the formability of TRIP steels.

Effect of heat treatment on formability in 0.15C−1.5Si−1.5Mn multiphase cold-rolled steel sheet

The effects of volume fraction and the stability of retained austenite on the formability of a 0.15C−1.5Si−1.5Mn (hereafter all in wt.%) TRIP-aided multiphase cold-rolled steel sheet were investigated after various heat treatments. The steel sheets were intercritically annealed at 800°C, and isothermally treated at 400°C and 430°C. Microstructural observation, tensile tests and limiting dome height (LDH) tests were conducted on the heat-treated sheet specimens, and the changes in retained austenite volume fraction as a function of tensile strain were measured using an X-ray diffractometer. The results showed a plausible relationship between formability and retained austenite stability. Although the same amount of retained austenite was obtained after isothermal holding at different temperatures, better formability was obtained in the specimens with the higher stability of retained austenite. If the stability of the retained austenite is high, the strain-induced transformation of retained austenite to martensite can be stably progressed, resulting in a delay of necking to the high strain region and improvement in formability.

Low‐alloy TRIP steels: a correlation between mechanical properties and the retained austenite stability

In recent years the technology of low‐alloy TRIP steels has considerably advanced. The mechanical properties are characterised by a combination of high yield strength and high uniform elongation as well as enhanced formability. In the present work an effort to correlate mechanical properties with the retained austenite stability was made. Two low‐alloy TRIP steels were investigated. The first of them represents a typical composition of the low‐alloy TRIP steels, while the other one contains aluminum as alloying element. The influence of the heat treatment on the mechanical properties and especially on the amount and stability of the retained austenite was determined. The retained austenite stability was measured with a single specimen technique, in which a tensile specimen was used to determine the MσS temperature with a loading‐unloading procedure. The results showed that there is a strong influence of the stability of the retained austenite on the mechanical properties. Increased stability combined with a high amount of retained austenite, exhibited an increase in both, yield strength and uniform elongation while increased amount of retained austenite with low stability did not show the same good combination of mechanical properties. The results clearly indicate that in order to get the maximum TRIP effect, a good combination of austenite stability and amount is required.

Development of high strength low alloy TRIP‐aided steels with annealed martensite matrix

Formable high‐strength low‐alloy TRIP‐aided sheet steels with annealed martensite matrix or TRIP‐aided annealed martensitic steel were developed for automotive applications. The steels possessed a large amount of plate‐like retained austenite along annealed martensite lath boundary, the stability of which against the strain‐induced transformation was higher than that of the conventional TRIP‐aided dual‐phase steel with polygonal ferrite matrix. In a tensile strength range between 600 and 1000 MPa, the TRIP‐aided annealed martensitic steels exhibited superior large elongation and reduction of area. In addition, the steels possessed the same excellent stretch‐flangeability and bendability as TRIP‐aided bainitic steel with bainitic ferrite matrix. These properties were discussed by matrix structure, a strength ratio of second phase to matrix, retained austenite stability, internal stress in matrix and so on.

Ductility and Formability of Newly Developed High Strength Low Alloy TRIP-aided Sheet Steels with Annealed Martensite Matrix.

Formable high-strength low-alloy sheet steels with annealed martensite matrix or TRIP-aided annealed martensitic steel were developed for automotive applications. The steels possessed a large amount of plate-like retained austenite along annealed martensite lath boundary, whose stability against the strain-induced transformation was higher than that of the conventional TRIP-aided dual-phase steel with polygonal ferrite matrix. In a tensile strength range between 600 and 1000 MPa, the annealed martensite steels exhibited a superior large elongation and reduction of area. In addition, they possessed the same excellent stretch-flangeability and bendability as TRIP-aided bainitic steel with bainitic ferrite matrix. These properties were discussed by matrix structure, a strength ratio of second phase to matrix, retained austenite stability, internal stress and so on.

Influence of rolling of TRIP steel in the intercritical region on the stability of retained austenite

Abstract Deformation in the intercritical region—or two-phase ( α + γ ) area—has a substantial effect on the microstructures and mechanical properties of TRIP-aided steel. It indeed increases dislocation density, arranging it in a subgrain substructure which in its turn affects the mechanisms occurring during the ageing treatment that is given on the run-out table. The first purpose of the present work is to investigate the effect of increasing amounts of prior rolling deformation at a given temperature in the intercritical area upon the resulting microstructure. Rolling in the intercritical region stabilizes retained austenite against strain induced transformation on room temperature straining. Increased dislocation density, grain refinement and carbon enrichment are the main factors that govern the retained austenite stability, and consequently the strength and work-hardening behaviour of the material. The second purpose of the study, hence, is the understanding of these stabilising effects. It was found that the morphology of the retained austenite and of the surrounding phase should also be involved as factors that dictate the retained austenite transformation to martensite. These two factors are related to the internal stress state.

Effect of Niobium on the Mechanical Properties of TRIP Steels

Three TRIP steels have been laboratory melted with 0.17% C, 1.4% Mn, and 1.5% Si as the base composition and additions of 0.02 or 0.04% niobium. After hot and cold rolling the effect of annealing treatment on microstructure development and resulting mechanical properties was evaluated. Niobium retards the isothermal bainite transformation and thus leads to higher retained austenite values. The mechanical properties are dependent on the grain size, the volume fraction of retained austenite, the retained austenite stability, local transformation stresses after bainite formation, and on carbide precipitates. Compared to conventional high strength steels TRIP steels offer an improved strength-ductility relationship and expand the range of cold formable steels to higher strength values.

Cyclic deformation behavior of a transformation-induced plasticity-aided dual-phase steel

Cyclic hardening-softening behavior of a TRIP-aided dual-phase (TDP) steel composed of a ferrite matrix and retained austenite plus bainite second phase was examined at temperatures ranging from 20 °C to 200 °C. An increment of the cyclic hardening was related to (1) a long-range internal stress due to the second phase and (2) the strain-induced transformation (SIT) behavior of the retained austenite, as follows. Large cyclic hardening, similar to a conventional ferrite-martensite dual-phase steel, appeared in the TDP steel deformed at 20 °C, where the SIT of the retained austenite occurred at an early stage. This was mainly caused by a large increase in strain-induced martensite content or strain-induced martensite hardening, with a small contribution of the internal stress. In this case, shear and expansion strains on the SIT considerably decreased the internal stress in the matrix. With increasing deformation temperature or retained austenite stability, the amount of cyclic hardening decreased with a significant decrease in plastic strain amplitude. This interesting cyclic behavior was principally ascribed to the internal stress, which was enhanced by stable and strain-hardened retained austenite particles.

On the thermodynamic stability of retained austenite in 4340 steel

The effect of retained austenite on the mechanical behavior of quenched and tempered steels depends on its thermodynamic stability against mechanically-induced martensitic transformation during loading. In this work, the stability of retained austenite is characterized by the Md, temperature and experimentally determined values of the Md, temperature are compared with model predictions. The comparison suggests that significant carbon stabilization of the retained austenite occurs during tempering for both, high and low temperature, austenitizing treatments employed. In addition, it appears that the assumption of a fully-biased rather than a fully-random distribution of nucleation-site potencies is in better agreement with the observed experimental behavior. Finally, the stress state dependence of the retained austenite stability could account for the mechanical behavior variation reported in literature.

Retained austenite and mechanical properties in bainite transformed low alloy steels

Uniform ductility and formability of low alloy steels can be improved by the transformation plasticity effect of metastable retained austenite. In this work, intercritical annealing followed by bainite transformation resulted in the retention of austenite with sufficient stability for transformation plasticity interactions. The effect of retained austenite on mechanical properties was studied in two low‐alloy steels. Bainite transformation was carried out in the range of 400 to 500°C. The strength properties (yield strength and ultimate tensile strength) were more sensitive to bainite isothermal transformation temperature than holding time. Maximum strength properties were obtained for the lower transformation temperatures. On the other hand, high uniform and total elongation values were obtained at lower transformation temperatures but were sensitive to bainite isothermal transformation time. Variations in uniform elongation with holding time were linked to variations in retained austenite stability. Maximum values of uniform elongation occurred at the same holding times as the maximum amount of retained austenite. The same was true for total elongation and ultimate tensile strength. The above results indicate a strong correlation between retained austenite stability and uniform ductility and suggest that further optimisation regarding chemical composition and processing with respect to austenite stabilisation may lead to a new class of triple‐phase high‐strength high‐formability low‐alloy steels.

Effect of bainitic transformation on microstructure of Si-Mn steel

Several Si-Mn steels with similar Si and Mn levels and carbon contents, ranging from 0.25 to 0.75 wt %, were studied to determine the effect of bainitic transformation on the microstructure of Si-Mn steel. The microstructure was categorized by optical metallography, scanning and transmission electron microscopy, and X-ray diffraction. The results showed the existence of an optimum transformation time to produce the maximum content of retained austenite, though the retention of a large amount of retained austenite was encouraged as a result of bainitic transformation. The microstructure consisted of carbon-free upper bainite whose individual ferrite was separated by the ‘thin-film’ type of retained austenite, while the ‘blocky’ type of austenite was also found. The results also showed that carbide precipitation occurred in the residual austenite after the optimum time, which decreased the retained austenite content. The retained austenite stability is discussed in relation to the carbon content and morphology of the retained austenite.

Ductility and strain-induced transformation in a high-strength transformation-induced plasticity-aided dual-phase steel

The influence of forming temperature and strain rate on the ductility and strain-induced transformation behavior of retained austenite in a ferritic 0.4C-1.5Si-1.5Mn (wt pct) dual-phase steel containing fine retained austenite islands of about 15 vol pct has been investigated. Ex- cellent combinations of total elongations (TELs), about 48 pct, and tensile strength (TS), about 1000 MPa, were obtained at temperatures between 100 °C and 200 °C and at a strain rate of 2.8 X 10-4/s. Under these optimum forming conditions, the flow curves were characterized by intensive serrations and increased strain-hardening rate over a large strain range. The retained austenite islands were mechanically the most stable at temperatures between 100 °C and 200 °C, and the retained austenite stability appeared to be mainly controlled by strain-induced martensite and bainite transformations (SIMT and SIBT, respectively), with deformation twinning occur- ring in the retained austenite. The enhanced TEL and forming temperature dependence of TEL were primarily connected with both the strain-induced transformation behavior and retained aus- tenite stability.

Effect of retained austenite on the yielding and deformation behavior of a dual phase steel

Tensile tests were conducted on a vanadium containing dual phase steel at temperatures between −53 and + 187°C to determine the effect of retained austenite stability on tensile properties. The transformation of retained austenite to martensite with stress/strain was shown to be a contributing factor in the yielding and strain hardening behavior of the dual phase steel. Increasing the stability of the austenite, by increasing the test temperature, caused the expected shift in the austenite to martensite strain transformation to higher strains. This led to a lower initial strain hardening exponent which increased with strain, compared to the ambient and sub-zero temperature deformation where the initial strain hardening exponent was higher but decreased with strain. The former behavior, which accentuated the strain hardening ability at higher strains, led to an increase in the uniform and total elongations, suggesting that the ductility of dual phase steels can be further improved by optimizing the stability of the retained austenite.

Medium carbon steel alloy design for wear applications

The fracture characteristics of steels are strongly influenced by martensite substructure, retained austenite stability, and morphology. Attractive strength-toughness properties have been attained with Fe-Cr-C-Mn alloys. These alloys, when tested under sliding wear conditions, also exhibit good wear resistance which compares favorably with that of commercial wear-resistant alloys. The most significant finding is an apparently strong correlation between sliding wear resistance and retained austenite, which in turn appears to correlate with Charpy impact properties. Little correlation was observed between hardness and wear resistance for the experimental steels.