Stress Relaxation Cracking Characteristics in Stainless Steel and Nickel Base Superalloys for High Temperature Service

Stress relaxation cracking (SRC), known as reheat or strain age cracking (SAC), remains a significant challenge for industries such as molten salt storage and petrochemicals. This phenomenon is particularly prevalent in precipitation-strengthened stainless steel and nickel-based superalloy welds, occurring in pressurized or highly constrained components operating above 500°C. Extensive literature highlights the influential factors of stress, strain, temperature, and microstructural variations on time to failure. The formation of susceptible microstructures conducive to cracking typically involves the presence of strong grains at the expense of localized weaker regions along grain boundaries, often manifested through precipitate free zones (PFZs) facilitating intergranular fracture.

This study delves into a combination of experimental investigations and case studies on SRC. Experimental stress relaxation tests were conducted to establish the intricate relationship between applied stress, strain, temperature, and time to failure across various materials including precipitate strengthened stainless steels and Ni-base superalloy welds. Furthermore, comparisons between matching filler and alternative fillers for stainless steel alloys, such as E1682, were explored. Microstructural analyses employing scanning electron microscopy (SEM) and transmission electron microscopy (TEM) along with fractography, were employed to investigate both intragranular and intergranular phases, as well as fracture characteristics, respectively.