Xueping Ren

Effect of Temperature on Microstructure and Deformation Mechanism of Fe-30Mn-3Si-4Al TWIP Steel at Strain Rate of 700 s−1

As twinning-induced plasticity (TWIP) steel is one potential material for shaped charge liner due to the combination of high strength and high plasticity, deformation mechanism at high strain rate and high temperature is required to study. Compression experiments of Fc-30Mn-3Si-4 Al TWIP steel were conducted using a Gleeble-1500 thermal simulation machine and a split-Hopkinson pressure bar (SHPB) between 298 and 1073 K at strain rates of 10–3 and 700 s–1, respectively. Microstructures were observed using optical microscopy COM) and transmission electron microscopy (TEM). Results show that flow stress and densities of deformation twins and dislocations decrease with increasing deformation temperature at strain rates of 10–3 and 700 s–1. The stack fault energy (SFE) values (Γ) of Fe-30Mn-3Si-4 Al TWIP steel at different temperatures were calculated using thermodynamic data. Based on corresponding microstructures, it can be inferred that at 700 s–1, twinning is the main deformation mechanism at 298 — 573 K for 30 mJ/m2≤Γ≤63 mJ/m2, while dislocation gliding is the main deformation mechanism above 1073 K for Γ≥145 mJ/m2. In addition, with increasing strain rate from 10–3 to 700 s–1, the SFE range of twinning is enlarged and the SEF value of twinning becomes higher.

Effect of superplastic deformation on the bonding property of 00Cr25Ni7Mo3N duplex stainless steel

The superplastic deformation diffusion bonding of 00Cr25Ni7Mo3N duplex stainless steel was performed on a hot simulator. The microstructure of the bonding interface was characterized by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The mechanical properties of the specimen were investigated by a shear strength test. The results indicated that the shear strength was improved with the increase of superplastic deformation reduction. When the deformation reduction was up to 50%, the shear strength of the specimen achieved 417 Mpa, approaching to that of the base metal. In addtion, the superplastic diffusion bonding technique was not very sensitive to surface roughness levels. When the surface roughness of the bonding specimen surpassed 0.416 μm (level G2), the shear strength achieved at least 381 MPa.

Design of a low-alloy high-strength and high-toughness martensitic steel

To develop a high strength low alloy (HSLA) steel with high strength and high toughness, a series of martensitic steels were studied through alloying with various elements and thermodynamic simulation. The microstructure and mechanical properties of the designed steel were investigated by optical microscopy, scanning electron microscopy, tensile testing and Charpy impact test. The results show that cementite exists between 500°C and 700°C, M7C3 exits below 720°C, and they are much lower than the austenitizing temperature of the designed steel. Furthermore, the Ti(C,N) precipitate exists until 1280°C, which refines the microstructure and increases the strength and toughness. The optimal alloying components are 0.19% C, 1.19% Si, 2.83% Mn, 1.24% Ni, and 0.049% Ti; the tensile strength and the V notch impact toughness of the designed steel are more than 1500 MPa and 100 J, respectively.

Effect of high temperature and high strain rate on the dynamic mechanical properties of Fe-30Mn-3Si-4Al TWIP steel

The dynamic mechanical properties of Fe-30Mn-3Si-4Al twinning induced plasticity (TWIP) steel were studied by the split-Hopkinson pressure bar (SHPB) at temperatures of 298–1073 K and strain rates of 700, 2500, and 5000 s−1. The TWIP steel indicates strain rate hardening effect between 700 and 2500 s−1, but it shows strain rate softening effect between 2500 and 5000 s−1. In addition, the strain rate softening effect enhances with an increase in deformation temperature. After deformation, the microstructures were studied by optical microscopy (OM). It is shown that the deformation bands become more convergence, a part of which become interwoven with an increase in strain rate, and the dynamic recovery and recrystallization are enhanced with an increase in both temperature and strain rate.