Hydrogen-Induced Delayed Cracking in TRIP-Aided Lean-Alloyed Ferritic-Austenitic Stainless Steels
Susceptibility of three lean-alloyed ferritic-austenitic stainless steels to hydrogen-induced delayed cracking was examined, concentrating on internal hydrogen contained in the materials after production operations. The aim was to study the role of strain-induced austenite to martensite transformation in the delayed cracking susceptibility. According to the conducted deep drawing tests and constant load tensile testing, the studied materials seem not to be particularly susceptible to delayed cracking. Delayed cracks were only occasionally initiated in two of the materials at high local stress levels. However, if a delayed crack initiated in a highly stressed location, strain-induced martensite transformation decreased the crack arrest tendency of the austenite phase in a duplex microstructure. According to electron microscopy examination and electron backscattering diffraction analysis, the fracture mode was predominantly cleavage, and cracks propagated along the body-centered cubic (BCC) phases ferrite and α’-martensite. The BCC crystal structure enables fast diffusion of hydrogen to the crack tip area. No delayed cracking was observed in the stainless steel that had high austenite stability. Thus, it can be concluded that the presence of α’-martensite increases the hydrogen-induced cracking susceptibility.
Synchrotron x-ray diffraction and crystal plasticity modeling study of martensitic transformation, texture development, and stress partitioning in deep-drawn TRIP steels
Abstract The micromechanics of the formability of a lean duplex TRIP steel was investigated using Synchrotron X-Ray Diffraction (S-XRD) measurements and Crystal Plasticity Finite Element Modeling (CPFEM). Specifically, the effect of the ferrite phase on the reduction of stress concentrations in the martensite phase and the influence of the austenite texture on the distribution of the martensite phase in the deep-drawn duplex TRIP steel were studied in comparison to a TRIP steel case. A series of deep-drawing processes were carried out to examine the sheet formability at ambient temperature, followed by S-XRD evaluations of the phase fraction, texture, and the residual-stress distributions in the deep-drawn cups. The macroscopic residual stress and its partitioning among constituent phases were studied using both S-XRD and CPFEM. In the deep-drawn TRIP steel, large tensile hoop residual stresses concentrated in the strain-induced α’ martensite phase, correlating well with the cracking phenomenon observed. Furthermore, the initial austenite texture influenced the martensite transformation kinetics during the deep-drawing process, resulting in a heterogeneous distribution of the martensite phase fractions around the circumference of the deep-drawn cups, which, in turn, caused an orientation-dependent cracking behavior. In the deep-drawn duplex TRIP steel, the tensile hoop residual stresses in the α’ martensite phase were significantly reduced due to a favorable load partitioning to the ferrite phase, resulting in a better formability.
Hydrogen solubility, diffusivity and trapping in a tempered Fe–C–Cr martensitic steel under various mechanical stress states
Abstract Electrochemical permeation test under stress conditions was carried out to determine the consequences of lattice distortion and defects on hydrogen solubility, diffusivity and trapping in a quenched and tempered martensitic steel. We focused our attention within the “engineering” elastic domain which can be divided into three domains: elasticity, micro-plasticity and generalized plasticity. The local elastic distortion associated with hydrogen atoms in lattice sites and residual vacancies seems to affect hydrogen lattice concentration. The hydrogen trapped in elastic fields shows a complex behaviour as a function of stress related to a possible internal relaxation of stresses around precipitates with the occurrence of plasticity. The plastic deformation caused a substantial increase of irreversible trapping sites in relation with the dislocation multiplication. Apparent diffusion coefficient decreased in this deformation domain in agreement with classical trapping models.