Stabilization mechanisms of retained austenite in transformation-induced plasticity steel

Three stabilization mechanisms—the shortage of nuclei, the partitioning of alloying elements, and the fine grain size—of the remaining metastable austenite in transformation-induced plasticity (TRIP) steels have been studied by choosing a model alloy Fe-0.2C-1.5Mn-1.5Si. An examination of the nucleus density required for an athermal nucleation mechanism indicates that such a mechanism needs a nucleus density as large as 2.5 · 10¹⁷ m⁻³ when the dispersed austenite grain size is down to 1 μm. Whether the random nucleation on various heterogeneities is likely to dominate the reaction kinetics depends on the heterogeneous embryo density. Chemical stabilization due to the enrichment of carbon in the retained austenite is the most important operational mechanism for the austenite retention. Based on the analysis of 57 engineering steels and some systematic experimental results, an exponential equation describing the influence of carbon concentration on the martensite start (M _s) temperature has been determined to be M _s (K)=273+545.8 · e ^−1.362w _c(mass pct). A function describing the M _s temperature and the energy change of the system has been found, which has been used to study the influence of the grain size on the M _s temperature. The decrease in the grain size of the dispersed residual austenite gives rise to a significant decrease in the M _s temperature when the grain size is as small as 0.1 μm. It is concluded that the influence of the grain size of the retained austenite can become an important factor in decreasing the M _s temperature with respect to the TRIP steels.     

Retained austenite characteristics in thermomechanically processed Si-Mn transformation-induced plasticity steels

It is well known that a significant amount of retained austenite can be obtained in steels containing high additions (>1 pct) of Si, where bainite is the predominant microconstituent. Furthermore, retained austenite with optimum characteristics (volume fraction, composition, morphology, size, and distribution), when present in ferrite plus bainite microstructures, can potentially increase strength and ductility, such that formability and final properties are greatly improved. These beneficial properties can be obtained largely by transformation-induced plasticity (TRIP). In this work, the effect of a microalloy addition (0.035 pct Nb) in a 0.22 pct C-1.55 pct Si-1.55 pct Mn TRIP steel was investigated. Niobium was added to enable the steel to be processed by a variety of thermomechanical processing (TMP) routes, thus allowing the effects of prior austenite grain size, austenite recrystallization temperature, Nb in austenite solid solution, and Nb as a precipitate to be studied. The results, which were compared with those of the same steel without Nb, indicate that the retained austenite volume fraction is strongly influenced by both prior austenite grain size and the state of Nb in austenite. Promoting Nb(CN) precipitation by the change in TMP conditions resulted in a decrease in the V _RA . These findings are rationalized by considering the effects of changes in the TMP conditions on the subsequent transformation characteristics of the parent austenite.     

High-strain-rate behavior of low-alloy multiphase aluminum- and silicon-based transformation-induced plasticity steels

High-strength, low-alloy transformation-induced plasticity (TRIP) steels are advanced multiphase steel grades that combine high-strength levels with an excellent ductility, making them ideally suited for application in crash-relevant parts of automotive car bodies. The enhanced plastic hardening and deformability are due to a complex interaction between the microstructural phases and to the transformation of metastable austenite to martensite during plastic deformation. During high-strain-rate loading, not only the material but also the transformation will be influenced by adiabatic heating. The impact-dynamic properties of CMnAl- and CMnSi-TRIP steels were determined in the range of 500 to 2000 s⁻¹ using a split Hopkinson tensile bar (SHTB) setup. Bake-hardening treatments were applied to study the effect of strain aging. The experiments show that strain-rate hardening is superior to thermal softening: yield stresses, deformation, and energy dissipation increase with the strain rate. Phenomenological material models were investigated to describe the strain-rate and temperature-dependent behavior of TRIP steels. Both the Johnson-Cook model and an extended version of the Ludwig model were found to give good agreement with the experimental data.     

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 M^σ_S 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.     

Effect of microstructure on the stability of retained austenite in transformation-induced-plasticity steels

Two Fe-0.2C-1.55Mn-1.5Si (in wt pct) steels, with and without the addition of 0.039Nb (in wt pct), were studied using laboratory rolling-mill simulations of controlled thermomechanical processing. The microstructures of all samples were characterized by optical metallography, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The microstructural behavior of phases under applied strain was studied using a heat-tinting technique. Despite the similarity in the microstructures of the two steels (equal amounts of polygonal ferrite, carbide-free bainite, and retained austenite), the mechanical properties were different. The mechanical properties of these transformation-induced-plasticity (TRIP) steels depended not only on the individual behavior of all these phases, but also on the interaction between the phases during deformation. The polygonal ferrite and bainite of the C-Mn-Si steel contributed to the elongation more than these phases in the C-Mn-Si-Nb-steel. The stability of retained austenite depends on its location within the microstructure, the morphology of the bainite, and its interaction with other phases during straining. Granular bainite was the bainite morphology that provided the optimum stability of the retained austenite.     

The mechanical stability of austenite in maraging steels

Two different types of retained austenite were encountered in maraging steels: one obtained by the usual overaging/reversion process, and was found to be mechanically unstable at room temperature; the other, obtained in a sequence of isothermal heat treatments leading to the formation of microsegregational content zones of molybdenum and cobalt in the lath-martensite, and was found to be mechanically stable at room temperature (RT). In the unstable case the austenite transformed to martensite upon cold working at RT. In the stable case, as was shown by careful Mössbauer-effect spectroscopy and X-ray diffraction studies, the amount of retained austenite was not affected by the cold-working at RT, whereas some amount of the martensite was transformed into a ferromagnetic-type of austenite. Complementary studies by electron diffraction have shown that both Kordjumov-Sachs and Nishiyama crystallographic orientation relationships may exist between austenite and martensite, depending on the local molybdenum and cobalt segregational contents in the lath martensite. The appearance of ferromagnetic austenite, as well as other segregational effects observed by the Mössbauer-effect spectroscopy are discussed.     

Effects of strain states on stability of retained austenite in medium Mn steels

Based on uniaxial tensile and plane strain deformation tests,the effects of strain states on the stability of RA(retained austenite)in medium Mn steels,which were subjected to IA(intercritical annealing)and QP(quenching and partitioning)processing,were investigated.The volume fractions of RA before and after deformation were measured at different equivalent strains.The transformation behaviors of RA were also investigated.The stability of RA differed across two different transformation stages at the plane strain state:the stability was much lower in the first stage than in the second stage.For the uniaxial tension strain state,the stability of RA corresponded only to a single transformation stage.The main reason was that there were two types of transformations from RA in the medium Mn steel for the plane strain state.One type was that the martensite originated in the strain-induced stacking faults(SISF).The other type was the strain-induced directly twin martensite at a certain equivalent strain.However,for the uniaxial tension state,only the strain-induced twin martensite was observed.Dislocation lines and dislocation tangles were also observed in specimens deformed at different strain states.In addition,complex microstructures of stacking faults and lath-like phases were observed within a grain at the plane strain state.     

Experimental determination of the stability of retained austenite in low alloy TRIP steels

The stability of retained austenite is the most important parameter controlling the transformation plasticity effects in multiphase low alloy TRIP steels. In this work the thermodynamic stability of the retained austenite has been determined experimentally by measuring the M^σ_s temperature as a function of bainite isothermal transformation (BIT) temperature and time in two low alloy TRIP steels. A single-specimen temperature-variable tension test technique (SS-TV-TT) has been employed, which allowed to link the appearance of yield points in the stress-strain curve with the mechanically-induced martensitic transformation of the retained austenite. The results indicated that the M^σ_S temperature varies with BIT temperature and time. Higher austenite stability is associated with a BIT temperature of 400°C rather than 375°C. In addition, the chemical stabilization of the retained austenite associated with carbon enrichment from the growing bainite is lowered at short BIT times. This stability drop is due to carbide precipitation and comes earlier in the Nb-containing steel. At longer BIT times the retained austenite dispersion becomes finer and its stability rises due to size stabilization. The experimental results are in good agreement with model predictions within the range of anticipated carbon enrichment of the retained austenite and measured austenite particle size.     

残留奥氏体对中碳双相钢冲击性能的影响

采用不同的热处理工艺研究了残留奥氏体对中碳双相钢冲击韧性的影响。利用金相显微镜、扫描电镜、透射电镜和摆锤式冲击试验机,对不同试样的显微组织与冲击韧性进行观察、检测和分析。试验结果表明：中碳贝氏体钢的冲击性能显著高于Q/P马氏体钢（室温冲击功是57J对应15J,-40℃冲击功是33J对应9J）,可能的原因是贝氏体钢中薄膜状残留奥氏体,对裂纹扩展的阻止效应更显著。     

The effect of retained austenite on the fracture toughness of high-speed steels

The aim of this work was to find the quantitative dependences between fracture toughness K_lc and the volume fraction of retained austenite in the matrix of quenched high-speed steels. The tests were carried out on three model alloys of a different content quotient of Mo: W which, after quenching, were gradually supercooled up to ? 196°C and then tempered at 450°C. Also the measurements of the content of retained austenite in the vicinity of the surface of a sample fracture were carried out. It was determined that after tempering at 450°C the fracture toughness of the matrix of high-speed steels is directly proportional to the content of retained austenite in it. Every 1 % by volume of retained austenite increases the fracture toughness K_lc of the matrix by about 5%, despite the fact that most probably it is completely transformed into fresh martensite in front of a propagating crack. Higher fracture toughness of the matrix of high-speed steels rich in molybdenum should be explained exlusively by a larger content of retained austenite. Transformations in the martensitic part of the matrix of the alloys richer in molybdenum clearly reduce the advantageous effect of retained austenite on this steel feature.     

The role of retained austenite in stress corrosion cracking of steels

The ability of retained austenite to affect stress corrosion cracking susceptibility has been examined in two steels, containing mechanically stable and unstable retained austenite of varying amounts and distributions. While limited improvements due to this constituent were observed in both threshold (where it may act as a compcting energy trap) and in growth rates (where it can act as a mechanical barrier and hydrogen sink), these were neither consistent nor more effective than other heterogeneities. Contrary to popular belief, retained austenite does not appear to be a consistently effective microstructural constituent, at least if the aim is to improve SCC performance at strength levels up to about 1000 MPa.     

Thermal stability of retained austenite and mechanical properties of medium-Mn steel during tempering treatment

The thermal stability of retained austenite(RA)and the mechanical properties of the quenched and intercritical annealed 0.1C-5Mn steel with the starting ultrafine lamellar duplex structure of ferrite and retained austenite during tempering within the range from 200 to 500°C were studied by X-ray diffraction(XRD),transmission electron microscopy(TEM)and tensile testing.The results showed that there was a slight decrease in the RA volume fraction with increasing tempering temperature up to 400°C.This caused a slight increase in the ultimate tensile strength(UTS)and a slight decrease in the total elongation(TE);thus,the product of UTS to TE(UTS×TE)as high as 31GPa·% was obtained and remained nearly unchanged.However,aportion of the RA began to decompose when tempered at 500°C and thus caused a~35% decrease of the RA fraction and a~16%decrease of the value of UTS×TE.It is concluded that the ultrafine lamellar duplex structure is rather stable and the excellent combination of strength and ductility could be retained with tempering temperature up to 400°C.Thus,thermal processes such as galvanization are feasible for the tested steel provided that their temperatures are not higher than 400°C.     

The role of retained austenite in stress corrosion cracking of steels

The ability of retained austenite to affect stress corrosion cracking susceptibility has been examined in two steels, containing mechanically stable and unstable retained austenite of varying amounts and distributions. While limited improvements due to this constituent were observed in both threshold (where it may act as a compcting energy trap) and in growth rates (where it can act as a mechanical barrier and hydrogen sink), these were neither consistent nor more effective than other heterogeneities. Contrary to popular belief, retained austenite does not appear to be a consistently effective microstructural constituent, at least if the aim is to improve SCC performance at strength levels up to about 1000 MPa.     

The role of retained austenite in stress corrosion cracking of steels

Metallurgical and Materials Transactions A - The ability of retained austenite to affect stress corrosion cracking susceptibility has been examined in two steels, containing mechanically stable and...     

Modelling of austenite stability in low-alloy triple-phase steels

A model for the stability of dispersed austenite in low alloy triple-phase steels has been developed. The model was based on the dislocation dissociation model for classical heterogeneous martensitic nucleation by considering stress effects on the nucleation site potency distribution. The driving force for martensitic transformation has been calculated with the aid of computational thermodynamics. The model allows for the effects of chemical composition of austenite, mean austenite particle size, yield strength of the steel and stress state on austenite stability. Chemical enrichment in C and Mn, as well as size refinement of the austenite particles lead to stabilization. On the contrary, the increase in the yield strength of the steel and triaxiality of the stress state lead to destabilization. The model can be used to determine the microstructural characteristics of the austenite dispersion, i.e. chemical composition and size, for optimum transformation plasticity interactions at the particular stress state of interest and can then be useful in the design of low-alloy triple-phase steels.     

Mechanical stability of retained austenite in tempered 9Ni steel

The behavior of the thermally stable austenite in the ductile fracture surface layer of a grain-refined and tempered 9Ni steel broken at 77 K was studied through use of Möss-bauer spectroscopy and transmission electron microscopy. Thin foils revealing the mi-crostructural profile of the fracture surface layer were prepared by electroplating a thick pure iron layer on the fresh fracture surface, then thinning a profile sample through a combination of conventional twin-jet electropolishing and ion milling techniques. The re-sults of both Mössbauer spectroscopy and TEM studies showed that the thermally stable austenite transforms to a dislocated martensite in the deformed zone adjacent to the duc-tile fracture surface. This result suggests that transformation of the retained austenite present in tempered 9Ni steel is compatible with low temperature toughness, at least when the transformation product is a ductile martensite.     

Effects of silicon additions and retained austenite on stress corrosion cracking in ultrahigh strength steels

A study has been made of the effects of silicon additions and of retained austenite on the stress-corrosion cracking (SCC) behavior of commercial ultrahigh strength steels (AISI 4340 and 300-M) tested in aqueous solutions. By comparing quenched and tempered structures of 4340 and 300-M i) at equivalent strength and ii) at their respective optimum and commercially-used heat-treated conditions, the beneficial role of silicon addition on SCC re-sistance is seen in decreased Region II growth rates, with no change in K’_ISCC. The beneficial role of retained austenite is demonstrated by comparing isothermally transformed 300-M, containing 12 pct austenite, with conventionally quenched and tempered structures of 300-M and 4340, containing less than 2 pct austenite, at identical yield strength levels. Here, the isothermally transformed structure shows an order of magnitude lower Region II SCC growth rates than quenched and tempered 300-M and nearly two orders of magnitude lower Region II growth rates than 4340, K ISCC values remaining largely unchanged. The results are discussed in terms of hydrogen embrittlement mechanisms for SCC in martensitic high strength steels in the light of the individual roles of hydrogen diffusivity and carbide type.     

残留奥氏体对微纳贝氏体钢塑韧性的影响

采用高碳和中碳低温贝氏体转变工艺（095C钢为200℃等温10d,030C钢为320℃等温1d）研究了残留奥氏体对微纳结构钢塑韧性的影响,对不同试样的显微组织、各相体积分数、伸长率和冲击韧性进行观察、检测和分析。试验结果表明,中碳钢贝氏体转变的塑韧性明显高于高碳钢贝氏体转变,主要原因是中碳钢贝氏体转变中存在一定的亚微米级薄膜状残留奥氏体,在拉伸或冲击过程中引起的残留奥氏体的塑性变形,使断裂的能量增加,可以显著提高样品的塑韧性。