Evaluation of intergranular corrosion susceptibility in an as welded high alloy austenitic stainless steel casting

Abstract

Intergranular corrosion is a leading cause of failure of pump and valve castings in the chemical process industry. High alloy austenitic stainless steels of the CN7M type. are frequently used because they are highly resistant to sulphuric acid and other aggressive chemicals. Existing specifications normally require a solution anneal after weld repair to eliminate carbide precipitation in the heat affected zone. The introduction of the argonoxygen decarburisation (AOD) secondary refining process in many stainless foundries has allowed the production of CN7M and similar alloys with very low carbon levels. Cast test plates of the CN7MS composition were joined using both gas tungsten arc and shielded metal arc welding. Samples were taken, in the as welded condition, and subjected to aseries of tests based on ASTM A262, for the detection of intergranular carbide precipitation. Particular attention was given to the heat affected zones. The results demonstrate that the heat generated by welding is insufficient to induce sensitisation in the weldheat affected zone. Sensitisation can be induced by longer exposure, in a furnace at 6675°C. These findings indicate that AOD refined high alloy, fully austenitic castings behave similarly to the lower alloyed CF3M types and that a post-weld solution anneal is unnecessary for material with a carbon content below 0·03% after welding with a low carbon filler metal and with careful control of inter pass temperature and heat input.