The use of model alloys to develop corrosion-resistant stainless steels

The goal of this program is to develop a cost-competitive replacement for type 347 stainless steel with better high temperature corrosion and creep resistance. Several series of model alloys have been fabricated in order to better understand the role of alloy composition in resisting accelerated corrosion attack due to the presence of water vapor. Alloys were tested at 650°-800°C for 500-5000h in air plus 10 vol.% water vapor. The first series examined Fe-Cr-Ni ternary alloys and found that increased Ni contents improved corrosion resistance, as did decreased grain size. A second series of Fe-16Cr alloys with minor alloy additions was produced to determine if any minor alloy additions might provide improved resistance similar to reactive element additions at higher temperatures. Beneficial effects were observed for additions of Mn, Si, TiB₂ and NbC, but individual additions of Al, Ti, La and Y showed little benefit. A third series with austenitic alloys focused on Mn and Si additions to alloys with 16-20%Cr and 15-20%Ni. Alloys based on Fe-20Cr-20Ni showed the most promise from this testing. The final stage of alloy development will explore alloy additions and processing variations to improve the creep strength for this application.