Effect of Hot forming on Mechanical properties of 9% Ni Steel for Low temperature service

Abstract

Effect of hot forming process such as hot bending, line heating and spot heating on the mechanical properties of double normalized and tempered 9% Ni steel was studied by examining both tensile properties and impact behaviours.
The results are summarized as follows:
1. Most suitable heating temperature in hot bending was 820°C to 950°C, and normalizing at 900°C, normalizing at 790°C or normalizing at 790°C, and tempering at 570°C followed by air cooling were needed to obtain good notch toughness after forming.
2. The examinations of microstructures showed that good notch toughness was correlated with solution increase, spherical degree of carbides.
3. Line heating tended to harden especially the heating surface, and greatly reduce the notcc toughness. However the notch toughness reduction at the mid-wall and the bottom surface (plate thickness was 13.5 mm) was slight in the case when water spray cooling from maximum heaating temperature of 550°C to 900°C was applied.
4. Spot heating must be avoided, for local shrinkage makes the notch toughness scatter and reduce the toughness value itself.