Effect Of Intercritical Quenching On The Element Segregation And Cryogenic Toughness Of 9% Ni Steel

Liquefied natural gas(LNG)has been attracting more attention as a clean and efficient energy source in recent years.At present,many countries have constructed large numbers of LNG facilities around the world.9% Ni steel is widely used in the construction of LNG projects because of its excellent cryogenic properties.As one of the major structural materials for LNG storage and transportation,the cryogenic toughness of 9% Ni steel has always been an important direction of steel research.This paper mainly uses 9% Ni steel as the research object.A series of systematic studies on the influence of intercritical zone holding time,QT,QLT and LT heat treatment processes on the microstructure and mechanical properties of 9% Ni steel.Scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffractometer(XRD)and multi-function internal friction test equipment were used to study the influence of elemental partitioning behavior on cryogenic toughness during heat treatment of 9% Ni steel.The results of this paper are as follows:The characteristics of microstructure and properties under different heat treatment processes were studied experimentally.The final microstructure of 9% Ni steel treated by QT process is mainly tempered martensite,and the reversed austenite is annular oblate.The microstructure of the QLT process is mainly tempered sorbite,and the reversed austenite is mostly island-shaped,block and film-like.The final structure after the LT process is mainly tempered sorbite,and the reversed austenite is mostly blocky and partially ring-shaped.The reversed austenite content under different heat treatment processes of QT,QLT and LT is different.The reversed austenite content under QLT process is the most,and the degree of elemental segregation in reversed austenite is different under different heat treatment processes.The experimental results show that the more reversed austenite,the better the cryogenic impact toughness.At the same time,the effects of the elemental content of reverse transformation austenite on the thermal stability of 9% Ni steel and the microstructure and mechanical properties of the experimental steel under different holding time in intercritical zone were studied.The final microstructure obtained by 9% Ni treatment by QLT process is mainly tempered sorbite,and the reversed austenite has the characteristics of forming along the martensite placket boundary and the original austenite grain boundary.And as the holding time of the intercritical zone increases,the length of the martensite lath gradually increases.The reversed austenite in the samples with intercritical temperature insulation for 30 min and 100 min is mainly island or block,while the reversed austenite in the sample kept for 50 min,the reversed austenite was finely film-like and separated from the block-shaped reversed austenite.It is proved by experiments that these film-like reversed austenite can absorb more energy,which is beneficial to inhibit crack growth and improve cryogenic toughness.The cryogenic impact test results show that the cryogenic toughness of 9% Ni steel depends on the stability of the reversed austenite.The content of Ni and Mn in the reversed austenite affects the stability of the reversed austenite.