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A metallurgical investigation was undertaken to identify the factors that are responsible for sensitivity to sulfide-stress cracking (SSC) in the heat-affected zone (HAZ) of microalloyed high-strength line-pipe steels, when the HAZ does not contain regions of high hardness. Sustained-load tests were conducted in the National Association of Corrosion Engineers (NACE) laboratory sour-gas environment to determine the regions in which SSC initiated, and thereby to identify which regions were most sensitive to SSC. In every steel studied, SSC initiated in a specific region of the HAZ, usually in the region that was heated into the intercritical temperature range during welding. One steel exhibited SSC sensitivity in the region adjacent to the weld metal, which was heated above the upper critical temperature during welding, and another steel exhibited SSC sensitivity in the base metal following a laboratory normalizing heat treatment. In all of the steels that were studied, the region that exhibited sensitivity to SSC contained columbium carbonitride precipitates that were coarser than those that were present in the pipe body, which was immune to SSC as fabricated. The only steel that did not fail in the SSC experiments was not alloyed with columbium. These observations suggest that sensitivityo to SSC is related to the growth of columbium carbonitride precipitates from a size below 50 Å to a size ranging from 100 to 200 Å during welding or during some heat treatments. Thus, it should be possible to reduce sensitivity to SSC by modifying the chemical composition or the cooling rate from welding or both to suppress precipitate growth.
sulfide-stress cracking, pipe steel, weld heat-affected zone, columbium carbonitrides, fatigue (materials), cracking, environmental effects