Metallurgical Changes in steel

Steels subjected to high temperatures and stress for long periods can undergo metallurgical change. This change results in various conditions, including carburization, decarburization, spheroidization and grain growth. All of these conditions lead to a general reduction in mechanical strength or a change in ductility, which may eventually result in failure of the material.  Some materials, including 5Cr-1/2Mo, may be susceptible to precipitation hardening when concentrations of residual elements such as phosphorus, tin and antimony are above certain threshold levels and exposed to heater operating temperatures for a sufficient time period. The result may be temper embrittlement with a loss of elongation and notch ductility as these elements precipitate to the grain boundaries after about a year at temperatures from 572°F (300°C) to 1112°F (600°C). Accordingly, the tube materials have ductile-to-brittle transition temperatures as high as 300°F (149°C) and brittle cracking has been experienced. See API RP 571 for a detailed description of these forms of deterioration.

Type 410 stainless steel can be susceptible to alpha-prime embrittlement or “885°F embrittlement”, depending on the trace elements present in the composition. Alpha-prime significantly reduces the toughness of the metal when at temperatures below 200°F (93°C). Brittle fracture can result from impact loads during downtime so extra caution during handling is prudent. At operating temperatures, the material has acceptable toughness.