The Influences of Heat Treatment on the Microstructure and Mechanical Properties of 3.5Ni Steel

Abstract

The influence of heat treatment on microstructure and mechanical properties of 3.5Ni steel has been studied in this paper. The experiment results showed that the microstructure and grain of the normalized and tempered steel exhibited a low uniformity, which deteriorated toughness. The grain could be refined after quenching and tempering, and the carbides were recognized dispersing at grain boundaries, the low temperature toughness was improved. After secondary quenched and tempered, the size of grains became smaller, and the low temperature toughness was better. Keywords-component; low temperature pressure vessel; microstructure; low temperature toughness; 3.5Ni; quenching; tempering. I. FOREWORD The pressure vessels designed temperature below -20°C (such as liquefied ethylene, liquefied natural gas, liquid nitrogen, liquid hydrogen and other containers for producing and storing) were called the low temperature pressure vessels, steels for manufacturing these containers were called low temperature pressure vessel steel. The Ni series low temperature vessel steel was an very important one, its temperature range was -40°C~-196°C, and it can satisfy the most manufacturing requirements of the low temperature pressure vessels. Its low temperature toughness was outstanding, and its cost was lower than that of the Cr-Ni stainless steel, so it had been widely used in the United States, Japan and other developed countries. 3.5Ni steel was typical one, and it was mainly used for manufacturing equipment of oil and air separation, and methanol washing tower, H2S concentration tower, CO2 tower and other equipment for synthetic ammonia. It was generally used at -101°C, but it can also work between -110°C and 120°C . There were some standards for the Ni series low temperature steels in china, and had manufactured related products, but the industrial production scale were small, and most of them depend on importing. At present, domestic steel enterprises have conditions to produce 3.5Ni low temperature steels. But their performance was not always excellent, for example, bad low cryogenic toughness. Though there were some research work on low temperature toughness of the 3.5Ni steel, its related heat treatment process was not comprehensive.So, this paper introduced the effect of different heat treatment process on the mechanical property of 3.5Ni steel. II. EXPERIMENTAL PROCEDURES A. Chemical composition 3.5Ni steel was smelt by converter and secondary refining, and section size of its casting billet is 250 mm x 1550 mm, then it was rolled into the steel plates with 36 mm, 60 mm thickness in 2800 mm rolling mill. Its chemical composition is given in Table 1. TABLE 1 CHEMICAL COMPOSITION OF 3.5NI (WT%)