X7Ni9 (9Ni) steel heat treatment

X7Ni9 (9Ni) steel is a kind of low carbon quenched and tempered steel, the microstructure is martensite plus bainite. 9%Ni steel has good toughness and high strength at extremely low temperature, and has a small thermal expansion coefficient compared with austenitic stainless steel and aluminum alloy, and is economical. It is one of the main materials for the manufacture of large LNG storage tanks. The main features of X7Ni9 (9Ni) steel are high nickel content, high purity, high strength, high low temperature impact toughness, and good weldability.

X7Ni9 steel delivery status: normalized, normalized + tempered, quenched and tempered

Heat treatment and changes of X7Ni9 steel:
As the tempering temperature of X7Ni9 steel increases, the content of reverse-transformed austenite in the steel increases, and the low-temperature impact toughness increases. At the same time, the matrix structure is purified, and the strength first increases and then decreases.

When tempering at 580℃, with the decrease of holding time, the strength of the steel increases, the plasticity and toughness decrease, and the plasticity is lower than the standard value; the tempering process of QLT heat treatment is 580℃×60min.