Multiscale mechanics of TRIP-assisted multiphase steels: II. Micromechanical modelling

The stress and strain partitioning between the different phases of transformation-induced plasticity (TRIP)-aided multiphase steels is evaluated using a mean field homogenization approach. The change of the austenite volume fraction under straining is predicted using a micromechanics-based criterion for the martensitic transformation adapted to the case of small, isolated, transforming austenite grains. The parameters of the model are identified from the mechanical response and transformation kinetics measured under uniaxial tension for two steels differing essentially by the austenite stability. The model is validated by comparing the predictions with tests performed under different loading conditions: pure shear, intermediate biaxial and equibiaxial. An analysis of the effect of the austenite stability on strength and ductility provides guidelines for optimizing properties according to the stress state.     

Multiscale mechanics of TRIP-assisted multiphase steels: I. Characterization and mechanical testing

The mechanical behaviour of transformation-induced plasticity (TRIP)-assisted multiphase steels is addressed based on three different microstructures generated from the same steel grade. The mechanisms responsible for the work-hardening capacity and the resulting balance between strength and resistance to plastic localization are investigated at different length scales. The macroscopic mechanical response is determined by simple shear, uniaxial tension, Marciniak and equibiaxial tension supplemented by earlier tensile tests on notched and cracked specimens. It is shown that the transformation rate reaches a maximum for stress states intermediate between uniaxial tension and equibiaxial tension. At an intermediate length scale, the true in situ flow properties of the individual ferrite–bainite and retained austenite phases are determined by combining neutron diffraction and digital image correlation. This combined analysis elucidates the partitioning of stress and strain between the different constitutive phases. Based on these results, supplemented by transmission electron microscopy and electron backscattered diffraction observations, a general overview of the hardening behaviour of TRIP-assisted multiphase steels is depicted.     

Effect of the kinetics of the martensitic transformation on the cavitation resistance of unstable austenitic steels

Metal Science and Heat Treatment -     

Effect of warm rolling on the martensitic transformation and properties of austenitic steels

Conclusion The optimal preliminary rolling conditions are 300°C with =45%, which produces fairly high strength and ductile characteristics.S. M. Kirov Ural Polytechnical Institute. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 12, pp. 42–44, December, 1974.     

On the role of crystallographic texture in mitigating hydrogen-induced cracking in pipeline steels

Cathodic hydrogen charging was applied to low-carbon pipeline-steel samples produced using different thermomechanical paths. The samples developed similar microstructures but different crystallographic textures and grain–boundary distributions. This made it possible to investigate the resistance to hydrogen-induced cracking (HIC) of steels with strong {1 1 1}ND and {1 1 2}ND texture fibres, steels with a dominating {0 0 1}ND texture fibre, and steels with close-to-random textures; {h k l}ND representing grain orientations with {h k l} planes parallel to the steel rolling plane. The results show that strong {1 1 1}ND fibre textures produced by warm-rolling schedules significantly increase HIC-resistance of pipeline steels, whereas {0 0 1}ND and close-to-random textures make steels HIC-prone.     

A note on the effect of testing temperature on the stress-corrosion cracking of martensitic stainless steels

Abstract

It has been proposed that stress-corrosion cracking of strong (> 70 tons/in.²) martensitic stainless steels is a form of brittle fracture caused by the absorption of hydrogen which is a cathodic reaction product. There is an apparent anomaly in that the tendency to stress-corrosion cracking becomes greater with increasing temperature whereas steel becomes tougher. The work described here was carried out to determine whether brittle fracture due to dissolved hydrogen can occur at elevated temperatures (up to 340°) and two test techniques were used to this end. It is concluded that the results are consistent with a hydrogen-embrittlement stress-corrosion cracking mechanism.     

Deformation-induced martensitic transformation behavior in cold-rolled and cold-drawn type 316 stainless steels

We investigate deformation-induced martensitic transformation behavior in cold-rolled and cold-drawn specimens of type 316 stainless steel. Deformation-induced martensite preferentially nucleates at the twin boundary between the austenite matrix and a deformation twin. In the cold-rolled specimen, martensite formed at the twin boundary has a Kurdjumov–Sachs (K–S) relationship with both the austenite matrix and the deformation twin (“double K–S relationship”). In the cold-drawn specimen, two kinds of deformation twins with different twin planes are typically formed, and therefore deformation-induced martensites are formed where the deformation twin boundaries intersect: martensite thus has an imperfect “triple K–S relationship” with the austenite matrix and the two deformation twins. The complicated crystallographic orientation relationship between austenite and martensite grains strongly restricts the formation of some variants of deformation-induced martensites. Because of the difference in number of nucleation sites in the cold-drawn and cold-rolled specimens, martensitic transformation is more enhanced in the former than in the latter.     

Strain-induced martensitic transformation in stainless steels: A three-dimensional phase-field study

A three-dimensional elastoplastic phase-field model is developed to study the microstructure evolution during strain-induced martensitic transformation in stainless steels under different stress states. The model also incorporates linear isotropic strain hardening. The input simulation data is acquired from different sources, such as CALPHAD, ab initio calculations and experimental measurements. The results indicate that certain stress states, namely uniaxial tensile, biaxial compressive and shear strain loadings, lead to single variant formation in the entire grain, whereas others, such as uniaxial compressive, biaxial tensile and triaxial strain loadings, lead to multivariant microstructure formation. The effects of stress states, strain rate as well as temperature on the mechanical behavior of steels are also studied. The material exhibits different yield stresses and hardening behavior under different stress states. The equivalent stress is higher at low strain rate, whereas a higher elongation is obtained at high strain rate. The deformation temperature mainly affects the hardening behavior of the material as well as the transformation, i.e. martensite volume fraction decreases with increasing temperature. Some of the typical characteristics of strain-induced martensite, such as the formation of thin elongated martensite laths, shear band formation and nucleation of martensite in highly plasticized areas, as well as at shear band intersections, are also observed.     

In situ neutron diffraction investigation of the collaborative deformation–transformation mechanism in TRIP-assisted steels at room and elevated temperatures

The deformation behaviour of two transformation induced plasticity (TRIP)-assisted steels with slightly different microstructures due to different thermo-mechanically controlled processing (TMCP) was investigated by the in situ neutron diffraction technique during tensile straining at room temperature and two elevated (50 and 100 °C) temperatures. The essential feature of the TRIP deformation mechanism was found to be significant stress redistribution at the yield point. The applied tensile load is redistributed within the complex TRIP-steel microstructure in such a way that the retained austenite bears a significantly larger load than the ferrite–bainite α-matrix. The macroscopic yielding of the steel then takes place through the simultaneous cooperative activity of the austenite-to-martensite transformation in the austenite phase and plastic deformation in the α-matrix. It is concluded that, although its volume fraction is small, the martensitically transforming retained austenite phase dispersed within the α-matrix governs the plastic deformation of TRIP-assisted steels.     

Influence of different heat treatments on corrosion resistance of martensitic stainless steels

The changes in microstructure, caused by different heat treatments, have considerable influence on the corrosion resistance of stainless steels. The heat treatment causes an alteration of carbide contained in steels. Changes in the constant B for these steels, (B = Rp · i_COR) have been observed in quenched, tempered and annealed conditions. Pitting corrosion resistance of martensitic stainless steels in quenched and tempered conditions in 0.1M H₂SO₄ by adding Cl^? ions has been investigated.