Stress relaxation cracking (SRC) is known to occur in austenitic stainless steels and nickel alloys operating between 550°C (1020°F) and 750°C (1380°F). Commonly, failures occur in heavy wall welded components. This paper, however, will discuss two SRC failures that occurred in thin non-welded components. Both of these failures occurred in electric heaters and, specifically, in tubular heating elements made of alloy 800 and 800H. Failure analysis of the components indicated intergranular cracking due to SRC as the cause of these failures. Both failures initiated in areas of relatively high hardness due to cold work. This paper provides details related to manufacturing, environmental conditions, metallurgical analysis and provides recommendations to avoid such failures in the future.
Stress relaxation cracking (SRC) is a failure mechanism known to occur in austenitic stainless steels and nickel alloys operating at moderate to high temperatures.
Typically, SRC failures tend to occur under the following conditions:1-6
1. Susceptible material: 800H, 347H, 617, etc. (typically materials with low creep ductility)
2. High residual stresses: Hardness > 200 HV (welded thick section)
3. Specific temperature range: usually between 500 °C (932 °F) and 750 °C (1382 °F).
Under these conditions, component stresses are relieved by time dependent inelastic deformation.3 In susceptible materials, this process occurs by intergranular cracking and is essentially a creep mechanism.2-6 In this respect, materials with low creep ductility tend to be prone to this type of damage mechanism. On the other hand, materials that have good creep ductility can tolerate the inelastic strains due to relaxation without cracking.3
SRC is known by several other names such as stress-relief-cracking, reheat cracking, strain-age cracking (SAC), Stress oxidation cracking, stress assisted grain boundary oxidation as well as others.3-4,6
Commonly, failures occur in heavy wall welded components. however, this paper will discuss two SRC failures that occurred in thin unwelded components.