Abstract
An investigation was made to clarify the influence of Local Brittle Zone (LBZ) on critical CTOD (δC) of multi-pass weld Heat-Affected Zone (HAZ) and metallurgical factors controlling the toughness of the LBZ by using four HT50 structural steels of various chemical compositions. The multi-pass welded joint exhibited low δCin case the fatigue-crack front of a COD specimen reached LBZ. The LBZ was identified to be a grain-coarsened zone reheated between AC1 and AC3 by the subsequent welding pass. Simulated HAZ with double thermal cycle reproduced the main microstructure of the LBZ. In the simulated HAZ COD test, cleavage fracture was initiated from a high carbon-martensite-island formed by the reheat-thermal cycle. By temper-thermal cycle due to further welding pass, the martensite-islands were decomposed into cementite-ferrite aggregates to various degree, depending on the chemical compositions of the steels. The higher degree of the decomposition resulted in the greater increase in δC. Close correlation was found between the minimum value of the measured δC in the welded joints and δC of the simulated HAZ with triple thermal cycle.
