AP-FIM STUDY ON THE EFFECT OF MO ADDITION ON MICROSTRUCTURE IN TI-NB STEEL

AP-FIM technique was applied successfully to analyze the strengthening mechanism of a newly developed Ti-Nb-Mo-bearing fire-resistant steel, which maintained high yield strength at elevated temperatures. FIM ob servations of Ti-Nb-Mo and Ti-Nb steels before and after aging at 873 K for 0.9 ks made it clear that Nb(C, N) precipitated in ferrite matri x of Ti-Nb-Mo steel during aging was finer in size (2.3 nm) and greate r in number than that of Ti-Nb steel. In the as-hot-rolled condition, corresponding to the situation before aging of those steels, a similar tendency was recognized with Nb(C, N), being coarse in size and small in number. These facts suggest that precipitation hardening of Nb(C, N) at 873 K and room temperature in Ti-Nb-Mo steel is higher than that in Ti-Nb steel. AP analyses of Mo-bearing steels, Ti-Mo steel and Ti- Nb-Mo steel, before and after aging, clearly yielded us two important facts. One was that almost all Mo atoms were in solution in ferrite ma trix of both of the Mo-bearing steels, which suggested Mo solid-soluti on hardening was equal for both of the Mo-bearing steels. The other wa s that Mo atoms strongly segregated at the Nb(C, N)/ferrite matrix int erface in Ti-Nb-Mo steel, which probably suppressed the diffusion of N b atoms into Nb(C, N) from ferrite matrix. This directly results in fi ne precipitates in Ti-Nb-Mo steel.