2002 Volume 20 Issue 2 Pages 266-275
The hot cracking sensitivity was examined on a series of weld metals of which the carbon content was varied systematically from 0.07 to 0.95%. The cracking sensitivity of weld metal was compared by the cracking test with a self-restraint type specimen. A weld metal was deposited in a circular groove of 80mm in diameter by MAG welding. Weld metals of various carbon contents were obtained by combining each of twelve electrode wires (0.004 to 1.0%C) and three base metals (0.23 to 0.77%C). The cracking sensitivity of each weld metal was expressed by the total length of cracking. The cracking sensitivity increases abruptly when the carbon content of weld metal exceeds 0.20%. It increases to the maximum at 0.40%C and then reduces to the minimum at 0.65%C. This result quite differs from the generally accepted idea that the more is the carbon content of weld metal, the greater becomes its cracking sensitivity. This result was referred to the phase diagram of Fe-C-0.5%Si system in which the peritectic reaction; delta-ferrite+liquid→austenite takes place. For the alloys of 0.20 to 0.65%C, the liquid remains after the reaction. Phosphorus once dissolved in delta-ferrite will be rejected into the remaining liquid. It lowers the final freezing temperature of remaining liquid locating between the dendrites and increases the cracking sensitivity of weld metal. This action is likely to arise most intensively in the alloy of 0.4%C and least in that of 0.65%. Sulfur seems to assist the harmful effect of phosphorus.
