Microstructural analysis of heat-resistant welded pipes

Microstructural analysis of welded joints in pipes of a heat-resistant alloy was carried out. The samples that were studied were cut from pipes that were kept at 800 °C for 10 months in a gas heater used to heat up the gas used for the direct reduction of iron ore. This work is concentrated on the analysis of a series of cracks detected in the base metal and the heat-affected zone (HAZ) of welded joints. It was found that such cracking was of intergranular nature and that the cracks propagated through the primary carbides. Two different types of carbides, which differed in silicon content, were detected. It was also observed that cracks propagated in a preferential way through the rich carbides. This analysis allows for the conclusion that the presence of silicon in the carbides increases the cracking susceptibility of the alloy.