CTOD properties and M-A constituent in the HAZ of C-Mn microalloyed steel.

Crack Tip Opening Displacement (CTOD) properties in the HAZ of 50 kgf/mm2 (490 MPa) class steels were investigated from the view point of fracture facet size and fraction of M-A constituent. The C-Mn microalloyed steels with carbon content of 0.025 to 0.19 wt. % were selected in this study. The results obtained were as follows:(1) The CTOD properties in the HAZ were strongly affected by the carbon content which varied both the fracture facet size and fraction of M-A constituent in the local brittle zone near fusion line.(2) The fracture facet size got small and the fraction of M-A constituent was increased by increasing the caron content.(3) The distributions of carbon content in the HAZ were analyzed by the EPMA. The peak points in carbon content corresponding to the M-A constituent were 1.07-1.32 wt.% in spite of that the carbon contents in the bulk of base metal were 0.025 to 0.19 wt.%.(4) The cementite as well as the M-A constituent were observed in the HAZ of the steel plate containing carbon more than 0.06 wt. %.(5) The mechanism of the M-A constituent formation was discussed in condensing the carbon atoms.(6) The CTOD properties, as compared with the Charpy impact properties, were greatly influenced by the M-A constituent.