Anisotropy of Retained Austenite Stability during Transformation to Martensite in a TRIP‐Assisted Steel

Retained austenite as a key constituent in final microstructure plays an important role in TRansformation Induced Plasticity (TRIP) steels. The volume fraction, carbon concentration, size, and morphology of this phase are the well‐known parameters which effects on the rate of transformation of retained austenite to martensite and the properties of steel, are studied by many researchers. Of the transformation of retained austenite to martensite under strain in a TRIP steel is studied in this paper. The experimental results show that the transformation rate of retained, austenite with similar characteristics, to martensite in differently processed TRIP steel samples, exhibits an anisotropic behavior. This phenomenon implies a kind of variant selection of martensitic reaction of retained austenite under strain and is explained by ferrite texture developed in steel.     

Role of retained austenite on the deformation of an Fe-0.07 C-1.8 Mn-1.4 Si dual-phase steel

The influence of retained austenite on the work hardening behavior of dual-phase steel has been investigated with an Fe-0.07 C-1.8 Mn-1.4 Si steel. With a constant cooling rate of 5 °C per second after intercritical annealing at 780 °C, a significant quantity (about 8 vol pct) of retained austenite was obtained in the dual-phase microstructure. The retained austenite was classified morphologically into either ‘isolated’ or ‘capsulated’ types by TEM observation. The ‘capsulated’ type, which was found without a particular shape inside the microtwinned martensite particle, withstood much deformation by being protected by the surrounding martensite. While the ‘isolated’ type, which was found with an equiaxed shape and was isolated from martensite particles, was easily deformed by the first several percent plastic strain. The increase in work hardening rate, caused by the strain induced transformation of retained austenite to martensite, was ascribed to the contribution of the ‘isolated’ type, the major volume fraction of retained austenite. The effect of the retained austenite on the yielding of dual-phase steel was not indicated since the reduction in the volume fraction of retained austenite was negligible at the initial deformation stage.     

Effect of Free Surface on the Stability of Individual Retained Austenite Grains in a Duplex Stainless Steel

The present work explored the effect of free surface on the stability of individual austenite grains in a duplex stainless steel. It was found that martensitic transformation took place automatically in the retained austenite grain when a free surface was introduced. This is due to the fact that the martensite nucleation energy barrier can be lowered to a thermally surmountable value as the strain energy induced by martensitic transformation is largely lowered when the matrix constraints were removed.     

形变过程中TRIP效应的相变热动态研究

采用拉伸与测温试验同时进行的方法,将应力应变曲线与热能曲线相结合,动态研究热轧TRIP钢拉伸过程中的相变热.研究表明:热轧TRIP钢在拉伸过程中材料增加的热能由部分转变的塑性功和马氏体相变热组成,因此,拉伸过程中实际测得的试样热能高于由塑性功转变的热能.利用平均综合热能损失系数对低速拉伸的TRIP钢的热能进行补充,通过计算与推导,证实了试样在刚进入塑性变形时,一定数量的较不稳定残余奥氏体首先集中发生马氏体相变,随着应变的进一步加大,剩余的较稳定的残余奥氏体根据其稳定情况发生马氏体相变的数量逐渐减少,在试样均匀延伸结束前绝大部分残余奥氏体已转变为马氏体.结合相变热变化可动态描述热轧TRIP钢形变过程中马氏体相变的情况.      

Strengthening of an Fe−Ni−V−C alloy by cyclic phase transformation

The martensitic strength of an Fe-22 Ni-1 V-0.3 C alloy exposed to direct and reverse martensitic transformation between ?196° and 610°C was increased by as much as 35,000 psi from the strength in the uncycled condition. This increase was nearly double the strength increase produced in a martensitic Fe-25 Ni-0.3 C alloy after cycling. Substantial ductility increases accompanied the strengthening. A calibration curve of yield strength vs pct uncycled martensite allowed martensitic strength comparisons to be made in spite of variations in the amount of retained austenite. Transmission electron microscopy showed that fine carbide particles on the order of 100 Å in diam formed during heating in the martensite to austenite portion of a transformation cycle on the Fe?Ni?V?C alloy. The particles were identified as VC from extraction replicas. The strengthening produced by cycling was analyzed by summing calculated strength increases due to substructural changes and fine particle dispersion and the calculated decreases due to the reduced solid solution strengthening that accompanies the formation of carbides.     

Analysis of the Strain‐Induced Martensitic Transformation of Retained Austenite in Cold Rolled Micro‐Alloyed TRIP Steel

The stability of retained austenite and the kinetics of the strain‐induced martensitic transformation in micro‐alloyed TRIP‐aided steel were obtained from interrupted tensile tests and saturation magnetization measurements. Tensile tests with single specimens and at variable temperature were carried out to determine the influence of the micro‐alloying on the M_s^σ temperature of the retained austenite. Although model calculations show that the addition of the micro‐alloying elements influences a number of stabilizing factors, the results indicate that the stability of retained austenite in the micro‐alloyed TRIP‐aided steels is not significantly influenced by the micro‐alloying. The kinetics of the strain‐induced martensitic transformation was also not significantly influenced by addition of the micro‐alloying elements. The addition of micro‐alloying elements slows down the autocatalytic propagation of the strain‐induced martensite due to the increase of the yield strength of retained austenite. The lower uniform elongation of micro‐alloyed TRIP‐aided steel is very likely due to the presence of numerous precipitates in the microstructure and the pronounced ferrite grain size refinement.     

Structure and properties of a microduplex maraging steel

A simple two-step thermal processing technique was devised to impart a microduplex structure in a high strength 250 grade commercial maraging steel. A martensite grain size of approximately 1 μm was obtained with interspersed islands of retained austenite whose volume fraction and mechanical stability could be controlled by varying the thermal processing conditions. The microstructure and mechanical properties of the microduplex structure were compared to those of the alloy in the maraged, martensitic condition. Due to the presence of the austenite phase, the microduplex structure showed a much smaller temperature and strain rate dependence of deformation than the martensitic structure. A remarkable increase in uniform elongation was observed below theM _d temperature of retained austenite. The microduplex structure did not show any significant advantage in fracture toughness over the martensitic structure when compared at similar strength levels. By suitably adjusting austenitic stability a deformation-induced phase transformation (TRIP) of the retained austenite in the microduplex structure could be made to occur; however, the transformation did not lead to any evident increase in toughness. The micro-duplex structure exhibited a slight improvement in fracture toughness at high strain rate in contrast to the martensitic structure in which the rate effect significantly reduced the toughness.     

常化后冷却工艺对1600MPa级超高强钢组织性能的影响

为改善高强度钢的塑性和韧性,对中碳低合金马氏体高强度钢分别采用常化后空冷＋回火和常化后控冷＋回火工艺,研究常化后冷却工艺对钢中残余奥氏体及力学性能的影响.采用扫描电镜获得钢的组织形态,利用X射线衍射和电子背散射衍射技术分析钢中残余奥氏体的体积分数、形貌和分布.发现两种工艺下均得到板条马氏体＋残余奥氏体组织,残余奥氏体均匀分布在板条之间,随工艺参数不同,其体积分数在3%~10%变化.常化后加速冷却能显著细化马氏体板条,提高钢的屈服强度和抗拉强度100 MPa以上,冲击功下降4 J.残余奥氏体的体积分数随常化控冷终冷温度的升高呈现先升高后降低的变化,常化后的控制冷却也可以作为进一步改善马氏体类型钢组织和性能的方法      

On the influence of interactions between phases on the mechanical stability of retained austenite in transformation-induced plasticity multiphase steels

The mechanical stability of dispersed retained austenite, i.e., the resistance of this austenite to mechanically induced martensitic transformation, was characterized at room temperature on two steels which differed by their silicon content. The steels had been heat treated in such a way that each specimen presented the same initial volume fraction of austenite and the same austenite grain size. Nevertheless, depending on the specimen, the retained austenite contained different amounts of carbon and was surrounded by different phases. Measurements of the variation of the volume fraction of untransformed austenite as a function of uniaxial plastic strain revealed that, besides the carbon content of retained austenite, the strength of the other phases surrounding austenite grains also influences the austenite resistance to martensitic transformation. The presence of thermal martensite together with the silicon solid-solution strengthening of the intercritical ferrite matrix can “shield” austenite from the externally applied load. As a consequence, the increase of the mechanical stability of retained austenite is not solely related to the decrease of the M _s temperature induced by carbon enrichment.     

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.     

Structure and properties of a microduplex maraging steel

A simple two-step thermal processing technique was devised to impart a microduplex structure in a high strength 250 grade commercial maraging steel. A martensite grain size of approximately 1μm was obtained with interspersed islands of retained austenite whose volume fraction and mechanical stability could be controlled by varying the thermal processing conditions. The microstructure and mechanical properties of the microduplex structure were compared to those of the alloy in the maraged, martensitic condition. Due to the presence of the austenite phase, the microduplex structure showed a much smaller temperature and strain rate dependence of deformation than the martensitic structure. A remarkable increase in uniform elongation was observed below theM _d temperature of retained austenite. The microduplex structure did not show any significant advantage in fracture toughness over the martensitic structure when compared at similar strength levels. By suitably adjusting austenitic stability a deformation-induced phase transformation (TRIP) of the retained austenite in the microduplex structure could be made to occur; however, the transformation did not lead to any evident increase in toughness. The microduplex structure exhibited a slight improvement in fracture toughness at high strain rate in contrast to the martensitic structure in which the rate effect significantly reduced the toughness.     

In situ X-Ray Diffraction Analysis of Carbon Partitioning During Quenching of Low Carbon Steel

In situ X-ray diffraction investigations of phase transformations during quenching of low carbon steel were performed at the European Synchrotron Radiation Facility (ESRF, Grenoble, France) at beamline ID11. A dynamic stabilization of the retained austenite during cooling below martensite start was identified, resulting in an amount of retained austenite of approximately 4?vol pct. The reason for this dynamic stabilization is a carbon partitioning occurring directly during quenching from martensite (and a small amount of bainite) into retained austenite. A carbon content above 0.5?mass pct was determined in the retained austenite, while the nominal carbon content of the steel was 0.2?mass pct. The martensitic transformation kinetic was compared with the models of Koistinen-Marburger and a modification proposed by Wildau. The analysis revealed that the Koistinen-Marburger equation does not provide reliable kinetic modeling for the described experiments, while the modification of Wildau well describes the transformation kinetic.     

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.     

Niti and NiTi-TiC composites: Part II. compressive mechanical properties

The deformation behavior under uniaxial compression of NiTi containing 0, 10, and 20 vol pct TiC participates is investigated both below and above the matrix martensitic transformation temperature: (1) at room temperature, where the martensitic matrix deforms plastically by slip and/or twinning; and (2) at elevated temperature, where plastic deformation of the austenitic matrix takes place by slip and/or formation of stress-induced martensite. The effect of TiC particles on the stress-strain curves of the composites depends upon which of these deformation mechanisms is dominant. First, in the low-strain elastic region, the mismatch between the stiff, elastic particles and the elastic-plastic matrix is relaxed in the composites: (1) by twinning of the martensitic matrix, resulting in a macroscopic twinning yield stress and apparent elastic modulus lower than those predicted by the Eshelby elastic load-transfer theory; and (2) by dislocation slip of the austenitic matrix, thus increasing the transformation yield stress, as compared to a simple load-transfer prediction, because the austenite phase is stabilized by dislocations. Second, in the moderate-strain plastic region where nonslip deformation mechanisms are dominant, mismatch dislocations stabilize the matrix for all samples, thus (1) reducing the extent of twinning in the martensitic samples or (2) reducing the formation of stressinduced martensite in the austenitic samples. This leads to a strengthening of the composites, similar to the strain-hardening effect observed in metal matrix composites deforming solely by slip. Third, in the high-strain region controlled by dislocation slip, weakening of the NiTi composites results, because the matrix contains (1) untwinned martensite or (2) retained austenite, which exhibit lower slip yield stress than twinned or stress-induced martensite, respectively. K.L. FUKAMI-USHIRO, formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology D. MARI, formerly Postdoctoral Fellow, Department of Materials Science and Engineering, Massachusetts Institute of Technology     

The stability of precipitated austenite and the toughness of 9Ni steel

A correlation was confirmed between the good low temperature Charpy toughness of 9Ni steel and the stability of its precipitated austenite against the martensitic transformation. Changes in the microstructure during isothermal tempering were studied in detail. The austenite/martensite interface is originally quite coherent over ∼100 A distances. With further tempering, however, the dislocation structure at the austenite/martensite interface changes, and this change may be related to the increased instability of the austenite particles. The reduction in austenite carbon concentration does not seem large enough to account for the large reduction in austenite stability with tempering time. The strains inherent to the transformation of austenite particles create dislocation structures in the tempered martensite. The large deterioration of the Charpy toughness of overtempered material is attributed, in part, to these dislocation structures.     

The size of martensite plates in 01 tool steel

The size of martensite plates formed in a commercial 01 tool steel austenitized at 982°C (1800°F) to an ASTM grain size No. 9.4 has been studied and data are compared with those obtained previously on the same steel austenitized at 1204°C (2200°F) to an ASTM grain size No. 2.5. In the fine-grained austenite the first half of the martensitic transformation involves plates of the same diameter and thickness whereas in the coarse-grain austenite transformation involves plates of gradually decreasing diameter but of constant thickness. The reason for this behavior is unclear. Formerly stu-dent at Purdue University     

A Mössbauer study of the surface martensite in 1095 steel

The extent of the martensitic transformation on the surface was measured with reference to the bulk value in a quenched 1095 steel using the Mössbauer effect scattering technique. The difference between surface and bulk austenite concentration was most evident in oil-quenched specimens which have 27 to 29 pct austenite in the bulk while retaining only 15 to 17 pct on the surface. This effect persisted even when 0.1 cm of material was removed by electropolishing of the as-quenched surface, confirming that the martensitic transformation could proceed spontaneously at the new surface exposed by electropolishing. Analysis of the Mössbauer and X-ray diffraction data indicated that the surface martensite extended to a depth greater than 0.3 Μm and less than 20 to 30 μm. Implications of these observations to the determination of retained austenite are discussed.     

Influence of intercritical annealing temperature on microstructure and mechanical properties of medium Mn TRIP steel

The transformation, microstructure and mechanical properties of the 0. 2C- 5Mn TRIP steel after intercritical annealing were investigated using dilatometer, scanning electronic microscopy (SEM), transmission electron microscopy(TEM), X- ray diffraction (XRD), and tensile testing machine. The phase transformation thermodynamics of the investigated steel after intercritical annealing was calculated by Factsage software and the characteristics of the transformation were discussed. The results show that the reversed austenite content increases with the increasing of the intercritical annealing temperature, the carbon content in reversed austenite firstly increases and then decreases, manganese content in reversed austenite decreases, which results in the decreasing of the thermal stability of reversed austenite. When the intercritical annealing temperature is 700??, an obvious martensitic transformation occurs during the cooling process. With the increasing of intercritical annealing temperature, cementite is gradually dissolved, but it cannot be completely dissolved due to the short transformation time. When the intercritical annealing temperature is 600-675??, the microstructure after intercritical annealing consists of ferrite, cementite and retained austenite. When the intercritical annealing temperature is 700??, the microstructure after intercritical annealing consists of ferrite, retained austenite, martensite and a small amount of undissolved cementite. The engineering stress and strain curves of the investigated steel are significantly changed with increasing intercritical annealing temperature. At the same time, the optimal mechanical properties with tensile strength of 1138MPa and total elongation of 23% can be obtained after annealed at 675?? for 3min.     

A M?ssbauer study of the surface martensite in 1095 steel

The extent of the martensitic transformation on the surface was measured with reference to the bulk value in a quenched 1095 steel using the M?ssbauer effect scattering technique. The difference between surface and bulk austenite concentration was most evident in oil-quenched specimens which have 27 to 29 pct austenite in the bulk while retaining only 15 to 17 pct on the surface. This effect persisted even when 0.1 cm of material was removed by electropolishing of the as-quenched surface, confirming that the martensitic transformation could proceed spontaneously at the new surface exposed by electropolishing. Analysis of the M?ssbauer and X-ray diffraction data indicated that the surface martensite extended to a depth greater than 0.3 Μm and less than 20 to 30 μm. Implications of these observations to the determination of retained austenite are discussed.     

Modeling tensile deformation of dual-phase steel

An analytical model has been developed which can describe the tensile deformation behavior of dualphase steel containing retained austenite which transforms to martensite during deformation. The model takes into account the internal back stresses created in the material as a result of the deformation. The influence of various metallurgical factors, such as the amounts of the secondary phases (martensite and retained austenite), strength ratio of the phases, work hardening coefficients, and the stability of retained austenite with respect to the strain-induced transformation, was analyzed. The strongest influence on both strength and ductility was found to result from a large work hardening coefficient of the martensite. Increasing the stability of retained austenite to strain-induced transformation improved the ductility remarkably. The model developed was used to predict the tensile deformation behavior of a commercial dual phase steel fairly accurately.