Influence of Martensite-Austenite Constituent on Toughness of Heat-Affected Zone of High-Strength Weldable Structural Steels

Abstract

A study has been made of the cause of heat-affected zone (HAZ) embrittlement from the metallurgical point of view. The special emphasis has been placed on the clarification of the role of martensite-austenite (M-A) constituent. A synthetic weld-thermal-cycle technique has been applied to various high-strength steels in order to simulate the coarse-grained HAZ over a wide range of welding heat input/cooling time.
In each steel, an initial increase in cooling time results in a microstructure change from martensite to a mixed structure of martensite and lower bainite, which lead to an increase in HAZ toughness. Further increase in cooling time causes microstructure to change from the mixed structure to upper bainite, which is accompanied with a marked degradation in HAZ toughness.
A close correlation is found between HAZ toughness and the amount of M-A constituent, whereas the effect of austenite grain size or fracture facet size is only capable of explaining less than 30 percent of loss in HAZ toughness. The deteriorating effect of the constituent is evaluated to be 8°C/volume percent of M-A constituent in terms of V-notch Charpy FATT.
Characteristics of an extremely low carbon steel have also been examined with satisfactory results due to its minimized tendency to the formation of M-A constituent.