Improved process procedures for upgrading and repair of high alloy and stainless castings

The program was designed to provide a data base and a greater depth of understanding of the behavior of cast high alloy austenitic stainless steels, CK-3MCuN, CN-3MN, and CN-7M, and the Ni-base alloy, CW-6MC, in regard to corrosion response and weldability characteristics. Wrought counterpart materials, 254 SMO@reg;, AL-6XN®, Alloy 20® and Inconel Alloy 625®, were also included for comparison. The data base developed is significant and the conclusions reflect a higher degree of understanding of the performance of the cast materials.

Extensive pitting corrosion testing per ASTM G 48 - 92 and intergranular corrosion testing per ASTM G 28 - 95 Method A were performed. All intergranularly corrosion tested samples were subjected to a 2t radius bend test. Weldability tests were carried out per ASTM A 494 - 94. Effects of autogenous GTA welding, solution annealing temperature and cooling rate after the solution soak, were also evaluated.

Test results show that cast materials generally have lower critical pitting temperatures than their wrought counterparts, but have comparable intergranular corrosion rates. In general, solution annealed castings have better pitting corrosion resistance than those in the as-cast condition. Intergranular corrosion tested as-cast materials usually reveal intergranular separations upon bending, while adequately solution annealed materials do not. Autogenous welding has little or no effect on the corrosion behavior of cast materials. However, adverse affects are found when wrought materials are autogenously welded. The use of ASTM recommended minimum solution heat treating temperatures was found to improve corrosion resistance of cast materials over the as-cast condition. Solution heat treatment of as-cast materials at temperatures higher than the ASTM minimum temperature may result in inferior corrosion responses. The reason for this behavior should be further investigated. Slow cooling after solution soaking was found to be detrimental to corrosion resistance and water quenching must be considered the optimum cooling method.

In the weldability tests conducted per ASTM A 494, ail but 3 cast lots passed the bend test indicating that cast materials exhibit good weldability.