Analysis of properties and microstructure of ASTM 5% Ni steel used for cryogenic pressure vessel

The influence of the QLT (quenching + lamellarizing + tempering) heat treatment process on the structure and properties of ASTM 5% Ni steel is analyzed. Using QLT heat treatment is to improve the cryogenic toughness of steel, which is generally just related to the reversed austenite. The present study deals with the analysis of the role of QLT heat treatment on structure transformation by researching the relationship among QLT, reversed austenite, and cryogenic toughness. After QLT heat treatment, the structure of ASTM 5% Ni steel transforms from coarse F + B to uniform and fine-tempered sorbite. Grain refinement and high dislocation density bring excellent comprehensive mechanical properties. Alloying elements influence the process and results of the phase transformation of austenite and martensite by getting into them to form new segregation structural units. The oscillographic impact results show that unstable crack growth does not occur during the fracture process. The fracture is upper platform type, which indicates that 5% Ni steel has good cryogenic crack arrest performance. Austenite is not found in the structure under X-ray and transmission electron microscope. The results of the electronic probe indicate the condition of creating conversed austenite is not met. So, the good low-temperature toughness is not from austenite but grain refinement and the increase of the ratio of the large angular grain boundary. This study not only provides proof that the QLT process is not the only factor in the forming of reversed austenite, but also gives inspiration that cryogenic toughness should not rely too much on the reversed austenite.