Abstract
Stellite type alloys have been utilized as weld hard overlaying alloys for valves used in high temperature and high pressure water. However, overlaying alloys free from cobalt contents have been ardently hoped for because cobalt is one of the main elements to increase radioactivity levels in radiation environment. The present investigation reports development of cobalt-free overlaying alloys based on the composition of high manganese steel with varying contents of nitrogen and molybdenum for these applications. Wear tests are conducted at 573K which will be the use temperature of the alloys, and the most resistant alloy is found to be resistant as high as 2.5 times to the stellite type alloys. Galling tests are also conducted at 573K with the most wear resistant alloy, and the alloy is found to be scuffing resistant as high as 7 times to the currently applied stellite type alloys. Thus, the developed alloy is found to be recommended as a cobalt-free overlaying alloy for valves of used in high temperature and high pressure water. The alloy is a χ-phase strengthened alloy, and utilizes a different strengthening mechanism from currently utilized mechanisms such as carbide strengthening and Laves phase strengthening mechanisms. Actual application procedures of the alloys are under investigation at present as the next step of the development.
