Influence of chemical composition on microstructure and phase evolution of two HP heat resistant stainless steels after long term plant-service aging

Abstract

The present work is aimed on the microstructure characterisation of two different HP steels after over 80 000 h of service in petrochemical plants of steam reforming. By means of optical and electron microscopy, as well as energy dispersive X-ray elements mapping and X-ray diffraction techniques, the alloys have been deeply investigated in order to make clear the influence of chemical composition, in particular titanium and tungsten additions, on the microstructural evolution after long service aging. The most evident features of aged materials can be summarised in terms of coalescence and coarsening of interdendritic precipitates, precipitation of secondary carbides in the austenite matrix and transformation of niobium-rich carbides in the G-phase silicide. We have observed that titanium and tungsten additions have played a synergistic role in preventing and reducing chemical evolution of the secondary phases present in the matrix.