Service conditions in methanol production plants create a driving force for nitrogen uptake in austenitic stainless steel piping, resulting in formation of internal nitride precipitates. Nitrides have been associated with loss of ductility in steels. However, their effects on the performance of specific commercial alloys such as the widely used Alloy 800H have not been studied. Hence any resulting degradation of properties is unaccounted for in component lifetime calculations, potentially leading to unexpected and costly failures. Characterisation of the nitrided microstructure is a critical first step towards understanding the evolution of mechanical properties during service. Limited existing information on microstructural development of Alloy 800H during nitridation restricts the current ability to correlate microstructural features and mechanical performance.
In this work, a multi-scale approach was taken to comprehensively characterise the microstructure of 800H following nitridation at service-relevant temperatures. Samples were nitrided at 800-1000 °C for 50-750 hrs in a 95%N2/5%H2 atmosphere. Optical, scanning electron, and transmission electron microscopy techniques were used for analysis, which focussed on AlN, Cr2N and CrN nitride phases. Features such as precipitate morphology, crystallography, orientation relationships, location, and penetration were documented. Effects on mechanical performance were examined through tensile and hardness testing of nitrided and un-nitrided material.
This work demonstrates the ongoing need for characterisation of the fundamental relationships between microstructure and properties of commercial alloys, in order to optimise service lifetime. This is becoming more critical as industries strive for better efficiency in a world where resource availability and waste are growing issues. It is also shown that microscopic analysis over length scales spanning several orders of magnitude is vital to build a comprehensive picture of microstructural changes occurring in service. Advances in microscopy mean that the capability to do so is now more accessible than ever.