Susceptibility of Carbon Steel Pipe, Fittings and Flanges to Brittle Fracture

Brittle fractures in parent material carbon steel pipe, fittings, and flanges are surfacing in recent ASME B31.3 refinery and gas plant construction and facility start-ups with unexpected low toughness of 3J (2.2 ft-lb) to 7J (5.2 ft-lb) at −10°C (14°F) to −29°C (−20°F). The issue is becoming wide-spread globally, affecting up to 30 percent of materials tested, although many manufacturers are not experiencing this issue. The issue creates a new brittle fracture risk that needs to be addressed as the uncertainty of not knowing suitability for service at temperatures down to −29°C (−20°F) is concerning for reliability and safety. These components are considered by ASME VIII Div I and ASME B31.3 Code as being inherently ductile, and brittle fracture resistant without any Charpy impact testing requirements. Testing showed brittle transgranular cleavage cracks. The components were deemed to be unsuitable and not safe for use at low temperatures even though they complied with the applicable ASME Codes [1, 2] and ASTM material standards. Low toughness can result in brittle fracture of the material during hydrostatic tests, cold start-ups, or upset conditions that result in low temperature operations. Additionally, some ASTM A350 LF2 CL1 [3] forged flanges certified to −46°C (−50°F) exhibited the same 3J (2.2 ft-lb) to 7J (5.2 ft-lb) at −46°C (−50°F). This paper discusses historical literature, metallurgical investigations, findings, and factors that contribute to susceptibility to brittle fracture including chemistry, grain size, heat treatment and forming techniques and also issues of ductile to brittle temperature transition shift, and fracture mechanical assumptions. This paper provides guidance to ensure the components are suitable for service and proposes options in addition to the current minimum Codes requirements to mitigate risks of in-service brittle fracture.