Fracture mechanics evaluation of a 0.5Mo carbon steel subjected to high temperature hydrogen attack

Several 0.5Mo carbon steel tubes, subject to high temperature hydrogen attack (HTHA) are analyzed for fracture toughness using crack tip opening displacement (CTOD). A three condition fit-for-service evaluation of HTHA material is presented. The three conditions include plastic collapse analysis, static averaged fracture toughness analysis, and dynamic “good wall” fracture toughness analysis. Macro- and microhardness, scanning electron microscope, fractography, tensile tests, and light microscopy are used to supplement the overall fracture toughness analysis. Cold end strain hardening of the tubes is noticed to result in a 15% reduction in fracture toughness. The HTHA zone reduction in fracture toughness averages 28%. A unique zone outside but adjacent to the primary HTHA zone contains a relatively high hardness in the decarburized zone. This zone is located at the inside edge of the hot end tubesheet. The average fracture toughness reduction for this zone is 27%, with the most severe reduction being 42%. Due to the design stress condition of tubes and the lack of extensive HTHA, all tubes analyzed are found fit-for-service.