Heat-affected-zone toughness of welded joints in micro-alloy steels

Abstract

The principal objective of this work was to evaluate the effect of chemical composition and cooling rate on the notch toughness of the heat-affected-zone of welded micro-alloy (HSLA) steel plate. Limited work was carried out on the effect of stress relieving heat treatment on notch toughness. Specimens were prepared by making submerged arc welds over a range of heat inputs on HSLA steel plates containing various amounts of niobium, vanadium and molybdenum. The notch toughness of the heat-affected-zone was measured on precracked specimens as a function of temperature by the instrumented Charpy impact test. Tests recorded the initiation of fracture in terms of load and time over a temperature range that covered the transition from ductile to brittle fracture. Because toughness is related to microstructure, the heat-affected-zone microstructures of specimens were identified. To help in evaluating the microstructures, continuous cooling transformation diagrams were established for all steel compositions. The results indicate that for submerged arc welding, cooling times between 800 and 500 °C of 40 seconds or less do not give rise to toughness-related problems in the heat-affected-zone of single pass welds in typical micro-alloy steels. In this range of cooling rates, toughness increases with increasing rate. The associated microstructural characteristics are fine prior austenite grain size leading to small bainite colony size and an absence of coarse lath boundary carbides or martensite/austenite particles. In practice, a cooling time of 40 seconds corresponds approximately to a heat input of 2.4 kJ/mm in a 12-mm plate or more than 8 kJ/mm in a 50-mm plate. Higher heat inputs result in cooling times of more than 40 seconds. The heat-affected-zone toughness suffers, and at the same time precipitated micro alloy carbides and nitrides are detectable in the microstructure. Loss of toughness in niobium-containing steels stress relieved after welding appears to be particularly serious.