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Effect of Surface Hardness Distribution on SSC Propagation Behavior in Grade X65 Linepipe

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Application of sour linepipes has expanded toward severe sour environment regions including higher H2S partial pressure conditions. In 2013, actual sour gas pipeline failure occurred due to SSC (Newbury et al., 2018). One of the possible root cause of SSC was assumed to be a formation of hard spots in a
surface region of steel. Fairchild et al. investigated and proposed three hard zone formation mechanisms including carbon contamination, dual phase microstructure and heat transfer variation in a recent paper (Fairchild et al., 2019; Newbury et al., 2019).

Product Number: 51323-19238-SG
Author: Junji Shimamura, Daichi Izumi, Mitsuhiko Hakoda, Joe Kondo, Mohammed A. Alanezi, Yasser S. AlSubhi, Satoshi Yamaguchi
Publication Date: 2023
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UOE linepipe has been long used for high-strength and severe sour application, however, recently hard-zone issue has been often discussed regarding sulfide stress cracking (SSC) in severe sour environment containing H2S gas. One of the possible root cause of SSC was assumed by formation of hard-zones in the steel pipe inner surface. Advanced-OLAC-based cooling rate control for surface and mid-thick area can achieve uniform and low hardness at the surface portion while maintaining sufficient tensile properties. By improving cooling homogeneity as well as surface cooling rate control, homogeneous granular bainite microstructure can be obtained, resulting in stable low surface hardness even in the inner surface of Grade X65 pipe. SSC propagation behaviors were investigated using Grade X65 samples with two different artificial hard-zones. One is the hard-zones with shallower depthless than 0.5 mm made by the laser heat treatment. The other is the hard-zones with depth of approx. 1.2 mm made by the accelerated cooling with intentionally high surface cooling rate. Using these samples, four-point bend (4PB) SSC tests were conducted under 1 bar H2S condition. As the result, when the hard-zone depth was less than 0.5 mm, crack arrested regardless of the hard-zone width or length. On the other hand, when the hard-zone depth was approx. 1.2 mm, crack propagated. It was found that the deeper the hard-zone, the easier the crack propagates. In the mill trial tests of Grades X60 and X65 UOE pipes, it was found that lowering the surface hardness to less than 240HV0.5 led to no SSC appearance in 1 bar H2S condition.

UOE linepipe has been long used for high-strength and severe sour application, however, recently hard-zone issue has been often discussed regarding sulfide stress cracking (SSC) in severe sour environment containing H2S gas. One of the possible root cause of SSC was assumed by formation of hard-zones in the steel pipe inner surface. Advanced-OLAC-based cooling rate control for surface and mid-thick area can achieve uniform and low hardness at the surface portion while maintaining sufficient tensile properties. By improving cooling homogeneity as well as surface cooling rate control, homogeneous granular bainite microstructure can be obtained, resulting in stable low surface hardness even in the inner surface of Grade X65 pipe. SSC propagation behaviors were investigated using Grade X65 samples with two different artificial hard-zones. One is the hard-zones with shallower depthless than 0.5 mm made by the laser heat treatment. The other is the hard-zones with depth of approx. 1.2 mm made by the accelerated cooling with intentionally high surface cooling rate. Using these samples, four-point bend (4PB) SSC tests were conducted under 1 bar H2S condition. As the result, when the hard-zone depth was less than 0.5 mm, crack arrested regardless of the hard-zone width or length. On the other hand, when the hard-zone depth was approx. 1.2 mm, crack propagated. It was found that the deeper the hard-zone, the easier the crack propagates. In the mill trial tests of Grades X60 and X65 UOE pipes, it was found that lowering the surface hardness to less than 240HV0.5 led to no SSC appearance in 1 bar H2S condition.

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