Chenchong Wang

Austenite layer and precipitation in high Co–Ni maraging steel

In high Co-Ni maraging steel, austenite has a great effect on the fracture toughness of the steel and the precipitated carbides are the main strengthening phase. In this study, both austenite layers and precipitation were observed and their formation theory was analyzed by Thermo-Calc simulation and several reported results. TEM and HRTEM observation results showed that the thickness of the austenite layers was about 5-10 nm and the length of the needle-like precipitated carbides was less than 10nm. The carbides maintained coherent or semi-coherent relation with the matrix.

Effects of Ni on austenite stability and fracture toughness in high Co-Ni secondary hardening steel

Three kinds of high Co-Ni secondary hardening steels with different Ni contents were studied. The nanoscale austenite layers formed at the interface of matensite laths were observed. Both observation and diffusion kinetic simulation results showed that both Ni and Co did not obtain enough time to get the equilibrium content in this system. The Ni content in austenite layers decreased significantly, and Co content increased slightly with the decrease of Ni content in overall composition. The austenite stability was estimated by Olson-Cohen model, in which both chemical and mechanical driving force could be calculated by equilibrium thermodynamic and Mohr’s circle methods, respectively. Simulation and mechanical test results showed that the decrease of Ni content in austenite layers would cause the change of austenite stability and decrease the fracture toughness of the steels. When the Ni content in the overall composition was lower than 7 wt. %, the Ni content in γ phase would be lower than 20 wt. %. And the simulation value of

Variation of microstructure and mechanical properties of medium Mn steels with multiphase microstructure

M_{\rm{S}}^\sigma

Analysis of fracture toughness in high Co–Ni secondary hardening steel using FEM

(stress-induced critical martensite transformation temperature) would be up to 80 °C, which was about 60 °C higher than room temperature. Based on the analysis, the Ni content in the overall composition of high Co-Ni secondary hardening steels should be higher than 8 wt. % in order to obtain a good fracture toughness.

Multi-scale simulation of hydrogen influenced critical stress intensity in high Co–Ni secondary hardening steel

Specimens of medium manganese steels with different manganese contents were austenised at 850°C for 30 min followed by intercritical annealing and water quenching at three different temperatures with different times. After heat treatments, mechanical properties were measured through tensile test. X-ray diffraction, electron back scattered diffraction and scanning electron microscopy examinations were also performed. Results indicate that annealing time, annealing temperature and manganese content all affect the final microstructure, which may consist of ferrite, retained austenite and fresh lath martensite, and variation of microstructure leads to different mechanical properties of steel.