Austenite layer and precipitation in high Co–Ni maraging steel

• Both austenite layers and precipitation were observed by TEM and HRTEM.
• The microstructure of the steel included martensite, austenite layers and precipitated carbides.
• The thickness of the austenite layers was about 5–10 nm and the length of the needle-like precipitated carbides was less than 10 nm.
• The needle-like carbides maintained coherent or semi-coherent relation with the matrix.

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 10 nm. The carbides maintained coherent or semi-coherent relation with the matrix.