Study On Microstructure And Properties Of9Ni Steel Under Different Treatment Process

With the changes in the international energy situation, LNG (Liquefied Natural Gas)become more and more important in the word energy structure.The demand for liquefied natural gas is increasing, and the related research has gradually been concerned.Due to long-term used in the low temperature, the research to the safety and reliability of LNG storage tanks on service is particularly important. As the key material for the LNG storage tanks,9Ni steel is carried on the related technology research which is of great significance.In this paper, the microstructure and mechanical properties during service and after strain aging of domestic9Ni steel was investigated,and the heat treatment process of copper containing9Ni steel was explored.The microstructure observation and analyses were carried out by using OM and SEM, and the content of organization was measured by XRD.And low-temperature impact toughness and tensile properties were tested on impact testing machine and the universal electronic tensile testing machine. For the practical problems during serving of9Ni steel, mainly the effect on its use due to the change of performance, advisory opinion was given.At the same time,heat treatment process of copper containing9Ni steel was explored and elaborated.Results show that:(1) The effect of strain aging on mechanical properties and microstructure of9Ni steel was studied via various experimental conditions, such as pre-strain, temperature and time.Drawed the conclusion that:(a) Compared to the low temperature (196℃),9Ni steel was more sensitive to the strain ageing under the high temperature (100℃,25℃),and the effect on mechanical properties was larger;(b)9Ni steel was more sensitive to the strain ageing within the low strain range (<2.5%).However there was no obviously change in microstructure,moreover the strength and toughness could meet the demand of service;(c) The main reasons were that the formation of Cottrell atmosphere became more difficult because of the low concentration of interstitial atom, the low temperature of service and the difficult of the atomic diffusion at the short range;(d) Aged at-196℃, the strength and low-temperature toughness of9Ni steel were improved, that was ascribed to some reversed austenite could transform to twin martensite and enhance the uniformity and stability of reversed austenite.(2) The strength and low temperature toughness of9Ni steel after insulation cycle treatment are improved, which is in a higher level than the untreated material. During the insulation cycle treatment, the transformation of reversed austenite into twinned martensite,which become the strengthing phase. There was carbide precipitated in martensitic matrix, forming dispersion strengthening.At the same time, martensitic matrix was refined, so it also can cause strong.All the factors led to the improvement of9Ni steel in strength and toughness. (3)For the copper containing9Ni steel used in the paper, the optimal quenching temperature range and tempering range of QT heat treatment are800～820℃and540～560℃.The mechanical properties of copper containing9Ni steel was more remarkably affected by tempering temperature than quenching temperature, especially the low temperature toughness. In the best heat treatment temperature,there was a lot of dimple in the low-temperature impact fracture of the copper containing9Ni steel, which showed its good low-temperature toughness.