STUDY ON THE CRITICAL CTOD VALUES IN THE HEAT AFFECTED ZONE OF C-Mn MICROALLOYED STEEL.

Crack Tip Opening Displacement (CTOD) tests were carried out to evaluate the Heat Affected Zone (HAZ) toughness. Crack initiation properties were strongly affected by the microstructures of the HAZ. The LBZ deteriorated the critical CTOD values in the HAZ because of both the coarsened grain size and existing martensite-austenite (M-A) constituent. The fine ferritic microstructures improve the critical CTOD values in the HAZ, through reducing the fraction of M-A constituent. A good correlation was found between the critical CTOD values in the HAZ and fracture facet size, although the effect of fracture facet size on the critical CTOD values in the HAZ was different for the fraction of M-A constituent. The close correlation was found between the critical CTOD values in the HAZ and the fraction of M-A constituent. Griffith crack was observed at M-A constituent and propagated along M-A constituent/matrix interfaces, resulting in low critical CTOD values.