The chemical processing industry (CPI) acquires new equipment in order to increase efficiency in the manufacture of chemical products. In addition to the economical goal of continually increasing plant performance it is also necessary to implement essential advancement in the areas of environmental protection and safety. Developments of new materials in recent years have centered on a significant increase in the content of chromium and molybdenum. However, optimizing a material’s composition regarding the resistance against corrosion by means of approaching the solubility limits of an alloy can cause changes in the microstructure during the manufacture of CPI equipment, especially when heat treatments are applied during the fabrication process.

This paper highlights sensitive areas by demonstrating the impact of unwanted fabrication-related changes, especially during heat treatments and provides information on methods of monitoring. The materials considered are mainly Alloy 31 (UNS No. N08031) and Alloy 654 SMO (UNS No. S32654);

in the case histories a 6% Mo stainless steel (SS) is also considered.

The paper provides time-temperature-sensitization data describing changes in the corrosion behaviour (pitting and intergranular corrosion) and ductility. Based on these data, manufacturers and end-users can avoid wrong heat treatments.

The materials considered in this paper have the tendency to form precipitates in the temperature range between 800 and 1050 C. Alloy 31 is less sensitive than Alloy 654 SMO. Alloy 31 can be held in the non-welded condition for a maximum of 10 minutes within the critical temperature range without detrimental effects on the alloys behavior.

Altogether, a tight quality management is essential to receive CPI equipment made from modern alloys in proper condition.