Preliminary Study Of Process 、microstructure And Properties Of Low Temperature Nickel Steel With Reduced Nickel Content

The low carbon steel containing nickel between 5% and 9% was developed and pushed to the markets in USA in the 1940 s to 1950 s, which is mainly used in low temperature environment. The 9Ni steel still has excellent toughness at liquid nitrogen temperature, it also has the advantage of high strength and easy welding, so it is widely used in the liquefied natural gas tank. With the world energy shortage and environmental pressure, natural gas is being paid more and more attention in the world as a kind of clean energy, so the storage and transportation technology of natural gas are also paid lots of research all over the world. The low carbon steel with high nickel content has great research value because of its promising application prospect. This kind of low temperature steel also has the possibility of building domestic liquefied natural gas ship, so it has to be researched furthermore.This dissertation researches four kinds of high nickel steel with different composition and heat treatment process, their mechanical and impact properties are tested, in which the impact performance is the critical performance to value low temperature steel, because only the steel with high impact toughness has enough safety to satisfy application. On the basis of test results, through the analysis of metallographic and SEM photos, the factors influencing the impact property and the mechanism of low temperature fracture can be revealed.In this research, four different kinds of high nickel steels are tested the mechanical and impact properties, the relation between chemical compositions and impact property can be understood, the result can guide the practical manufacture. Then the four kind of steel are further processed by a three-step heat treatment: quenching, direct lamellarizing at dual-phase zone, tempering(QLT), their impact properties are all greatly improved, the microstructure factor influencing low impact toughness is analyzed. The reverted austenite retained during the QLT process has benefit to the low temperature toughness, it can be used in the practical manufacture. Overall, this study can give us a comprehensive understanding of high nickel steel for low temperature application, the low temperature impact toughness can be greatly improved, the QLT heat treatment can offer a way to reduce the nickel content with no toughness lose.