Abstract
Influence of Ni and Mn on toughness of multi-pass weld heat-affected zone (HAZ) in quenched and tempered high strength steels was investigated in order to make clear the metallurgical factors controlling the toughness of the local brittle zone (LBZ) in the HAZ.
Change of simulated HAZ microstructure due to chemical compositions and thermal cycle condition is expressed by a modified hardenability index (SI), which corresponds to ideal critical diameter (DI) taking account of the factors of boron and austenite grain size for the HAZ. Simulated HAZ toughness has good correlation with SI and Ni content. The highest toughness is obtained at optimum SI value (≅ 11) regardless of the chemical compositions and the thermal cycle conditions.
For lower bainite, martensite and their mixtures, cementite which precipitated in the laths of auto-tempered martensite during cooling has a important role for controlling the toughness. But, fracture facet size or retained austenite is not a main controlling factor.
