A new processing route to develop nano-grained structure of a TRIP-aided austenitic stainless-steel using double reversion fast-heating annealing

Abstract A novel processing route comprising double reversion annealing (DRA) was designed for developing bulk nano-grained (NG) structure of an austenitic stainless steel (Type 301LN). The new processing concept of DRA comprised two subsequent intrinsic type processes i.e., two times cold reductions (∼53 % and 63 %) followed by fast induction heating (∼200 °C/s) and short duration annealing at different temperatures (first at 690 °C/60s and second at 750–900 °C/0.1–1s). The NG structure revealed a remarkable improvement of the mechanical properties compared to the counterparts processed by single reversion annealing. Furthermore, outstanding combination of strength and formability is achieved for the DRA structures, significantly higher than those of high-Mn TWIP steels, low-alloy TRIP steels and 304 stainless steel. For instance, a superior combination of yield strength (∼950–1030 MPa) and formability index (11.8–12.5 mm) obtained after DRA at 750 °C/0.1s and 800°C/1 s, respectively. However, the corresponding values are 300 MPa and 12 mm for TWIP steels, 500 MPa and 10 mm for TRIP steels, and 270 MPa and 12 mm for 304 stainless steel. In order to reveal the effect of DRA on the stretch formability, Erichsen cup testing was conducted of both the initial and DRA steel specimens. Moreover, Erichsen cup testing also simulated by the finite element method (FEM) to survey further details of their deformation

High-speed Erichsen testing of grain-refined 301LN austenitic stainless steel processed by double-reversion annealing

Abstract Austenitic Cr–Ni stainless-type 301LN steel was subjected to a double-reversion annealing (DRA) treatment to develop bulk grain-refined microstructures. The tensile properties and formability of the DRA structures were determined by high-speed tensile and Erichsen cupping tests at a strain rate of 1.5 s⁻¹ (50 mm s⁻¹) and compared with those of coarse-grained steel. Detailed microstructural features of the DRA structures were characterized using the electron backscatter diffraction technique and X-ray diffraction analysis. The DRA structures achieved by annealing for 1 second at 800 °C and 900 °C exhibited a superior combination of yield (~ 950 and 770 MPa, respectively) and tensile (~ 1050 and 950 MPa, respectively) strengths and ductility (~ 35 and 40 pct, respectively, as well as reasonable Erichsen index values under high-speed biaxial strain. Due to adiabatic heating, the DRA structures had higher austenite stability during high-speed stretch forming, i.e., were less prone to strain-induced martensitic transformation. The finite-element method (FEM) was used to conduct coupled field thermomechanical analyses of the high-speed deformation processes for the coarse-grained and DRA structures. Comparison of the FEM analyses with the experimental results revealed a considerable influence (~ 20 pct) of martensitic transformation on the adiabatic temperature rise. The balance of the yield strength and Erichsen index value of the developed nanograined microstructure is comparable to that of coarse-grained commercial steel

Micromechanical analysis and finite element modelling of laser-welded 5-mm-thick dissimilar joints between 316L stainless steel and low-alloyed ultra-high-strength steel

Abstract As base metals (BMs), plates of 5-mm-thick low-alloyed ultra-high-strength carbon steel (LA-UHSS) with a tensile strength of 1.3 GPa and 5-mm-thick 316L austenitic stainless steel were laser-welded at two different energy inputs (EIs; 60 and 100 J/mm). The microstructural characteristics of the fusion zones (FZs) in the welded joints were examined using electron backscattering diffraction (EBSD) and transmission electron microscopy. The fine microstructural components, such as the prior austenite grain size (PAGS) and effective grain size of the fresh martensite promoted during welding, were analysed by processing the EBSD maps using MATLAB software. The micromechanical performance of the weldments was investigated using microindentation hardness (HIT) to display the mechanical responses of different zones. Uniaxial tensile testing was conducted to explore the joint strength and plasticity failure. The dominant phase structures promoted in the FZs at low and high EIs were similar, that is, martensite with a small fraction of austenite. The HIT values displayed a distinct variation in strength between different zones. The HIT values of 316L, LA-UHSS, and FZ were 1.95, 5.55, and 4.63 GPa, respectively. The PAGS increased from 45 to 70 μm with an increasing EI, and a finer martensitic grain structure with an average size of 2.62 μm was observed at high EIs. The mechanical tensile properties of the dissimilar joints at the studied EIs closely matched those of the BM 316L, demonstrating comparable yield and tensile strengths of 225 MPa and 650 MPa, respectively. This similarity can be attributed to the localized plastic tensile deformation occurring primarily within the relatively softer BM 316L, ultimately resulting in joint failure. The flow behaviour of the dissimilar joints under uniaxial tensile testing was analysed using finite element modelling to determine the stress and strain distributions. The plastic strain was mainly localised within the soft metal 316L owing to enhanced dislocation-mediated plasticity

Enhancement of strength in laser-joined Al-TRIP and Si-TRIP steels:microstructural insights and deformation analysis

Abstract This study highlights the strengthening mechanisms observed during the metal joining of high-strength grade steels (Al-TRIP and Si-TRIP) by providing a concise investigation of microstructural features, mechanical strength evaluation, and employing Finite Element Method (FEM) analysis to understand the deformation behaviour in the joint. The base metals (BMs), Al-TRIP and Si-TRIP are cold-rolled sheets with thicknesses of 0.9 mm and 1.3 mm, respectively. Al-TRIP contains 2.4 wt% Al, while Si-TRIP contains 1.5 wt% Si. The Al-/Si-TRIP joint was processed by laser welding at low energy input 24 J/mm. Electron backscattering diffraction and transmission electron microscopy extensively characterized the microstructural features in the fusion zone (FZ) and heat-affected zone (HAZ) to study strengthening mechanisms induced by welding. Uniaxial tensile tests examined joint mechanical strength, while microindentation hardness (HIT) measurements evaluated mechanical response in the weld zones. The FZ showed a fully martensitic structure, while the HAZs displayed refined grains. Ultrafine-grained structures with an average size of 1 μm were observed in the HAZs, resulting in higher HIT hardness values (∼6.7 GPa) compared to the FZ (∼6.3 GPa). Interestingly, the mechanical tensile properties of the joint were unaffected as failure occurred in the thinner Al-TRIP steel. Finite Element Method (FEM) analysis simulated the tensile testing, revealing localized plasticity in the thinner Al-TRIP and explaining the observed fracture