Influence of boron content on hot cracking sensitivity of weld heat affected zone in incoloy 800.

Abstract

The influence of B content on the hot cracking sensitivity of the heat affected zone (HAZ) in Incoloy 800 was examined by Varestraint test and by hot ductility test using the thermal cycle simulator. The results indicated that the HAZ cracking sensitivity increased with the increase of B content until the B content exceeded about 0.01%, which is estimated to be the solubility limit, but the cracking sensitivity was decreased above 0.01% B. Similar relationship has been observed in the case of weld metal. However, the mechanism of the HAZ cracking seems to be somewhat different from that of weld metal cracking which depends on the extent of healing phenomenon, as the change of liquid film composition during the weld thermal cycle was observed. On the heating of weld thermal cycle, the composition of the liquid film in the HAZ was assumed to have been mainly that of Cr2B eutectic. However, the diffusion of B atom from grain into the grain boundary, in the weld thermal cycle after the initiation of grain boundary liquation, was so little at low B content below the solubility limit that (Fe, Ni)2B eutectic with a lower melting point than Cr2B eutectic became to dominate the composition of the liquid film, resulting in the extension of the brittle temperature range and the higher cracking sensitivity. On the contrary, at the B content higher than the solubility limit, the diffusion of B atom into the grain boundary was so sufficient that the liquid film preserved the composition of Cr2B eutectic which reduced such the brittle temperature range as described at low B content.The relation between hot cracking sensitivity of HAZ and B content of Incoloy 800 as mentioned above seems to be derived from the change of brittle temperature range due to such a difference of the liquid film composition.