The precipitates in the commercial LCN-155 alloy aged at a temperature between 500°C and 950°C for a time up to 3, 000 hours were identified by the electrolytic extraction method. Further, the effect of additional elements such as N, Mo, W, and Cb on these phases was examined.
The results obtained were as follows:
1. The phases which were identified in the commercial LCN-155 alloy were the carbides, Cb (C, N), M23C6, X phase and M6C, and the intermetallic compound, Laves phase. These phases changed as follows by aging:
Cr23C6→(Cr, Mo, W) 23C6→ X phase→ M6C→ Laves phase
2. The stable Cb compound, Cb (C, N), was detected in the almost examined alloys containing Cb, but, as an exceptional case, it was unstable in a narrow range between about 750°C and 850°C in the alloys containing Cb, Mo, and W.
3. The principal precipitates which contributed to age-hardening were the carbides M23C6. and X phase. Though the carbide M23C6 was stable in the alloys without N, it was less stable than X phase in the alloys with N, Mo, and W.(The X phase was a new type secondary phase which was detected by authors.)
4. The carbide M6C and the Laves phase were identified in alloys of this type containing Mo, W, and Cb aged at a higher aging temperature above about 750°C for a longer time, and scarcely made a contribution to hardening.
5. At about 500°C, no precipitate was found by aging in spite of producing a remarkable volume-change so called as the 1st stage in the tempering process.
6. It was considered that the two steps of age-hardening in the LCN-155 alloy were caused by carbide reaction changing from M23C6 to X phase and the excellent age-hardenability of this type alloy containing N, Mo, and W would be due to a nature of the X phase.