MAIN FACTORS OF CRACKING BY ELEVATED-TEMPERATURE EMBRITTLEMENT IN THE WELD HEAT-AFFECTED ZONE OF AUSTENITIC HEAT-RESISTANT ALLOY PIPE

Cracking may occur in the weld Heat Affected Zone (HAZ) of Alloy 800 H, one of the austenitic heat-resistant alloys during long-term service in the temperature range of 600 approximately 700-degrees-C. The authors have conducted a series of restraint cracking tests to consider the mechanism of this crack formation and method of ameliorating the cracking. The following results were obtained. (1) This HAZ cracking can be reproduced by a cylindrical type restraint cracking test. (2) Intergranular oxidation and cracking were detected at the bottom of notched HAZ area of as-welded specimens in the temperature range of 600 approximately 650-degrees-C. The area of cracking coincided with the area of gamma' phase precipitation. (3) The intergranular cracking occurs when weld tensile residual stress acts upon grain boundaries whose cohesive forces are severely reduced by the intergranular oxidation and precipitation of gamma' phases in matrix. (4) A solution treatment which reduces initial weld residual stress, plating of Ni-1%B for preventing oxidation, and the reduction of Ti and/or Al contents which prevents the precipitation of gamma' phases are all effective to reduce this type of cracking.