Fe-12Ni-4Co-2Mo-.05Ti alloy for use at 77°K and below

A variant of the maraging class of steels is proposed for application at 77$sup 0$K and below where a combination of very high strength and good toughness is required. The alloy has a composition of Fe-12Ni-4Co-2Mo-0.05Ti where low interstitial content. The as quenched and quenched and aged structures were completely martensitic with a prior austenitic grain size of 10 to 12 $mu$m. This structure had a Y.S. of 138.5 ksi and 154 ksi before and after aging respectively. All aging was done at 444$sup 0$C for 4 hours. The DBTT was shown to lie above 77$sup 0$K as measured by C/sub v/ testing. Based on dilatometric studies of the $alpha$ $Yields$ $gamma$ and $gamma$ $Yields$ $alpha$ transformation temperatures a cycling treatment consisting of reportedly heating to above the A/sub f/ temperature followed by a water quench was utilized to further reduce the prior $gamma$ grain size to approximately 4 to 6 $mu$m. The structure was completely martensitic and possessed a Y.S. of 151 ksi at 77$sup 0$K in the unaged condition with a Y.S./K/sub IC/ ratio of 1.9 while the aged structure showed a Y.S. of 162 ksi with a Y.S./K/sub IC/ ratio of 1.3. C/sub v/ testing showed the DBTT to lie between 77$sup 0$K and 4.2$sup 0$K. Further grain refinement was accomplished by a 2 phase decomposition procedure which resulted in a grain size of 1 to 2 $mu$m. The structure which contained decreasing amounts of austenite with temperature (3.0 percent at R.T. to 1.0 percent at 4.2$sup 0$K) showed the best combination of strength and ductility at 4.2$sup 0$K. A Y.S. of 205 ksi with a Y.S./K/sub IC/ ratio of 0.84 was achieved before aging. The aged structure was brittle at 4.2$sup 0$K with a Y.S. of 218 ksi and a Y.S./K/ sub IC/ ratio of 0.425. (auth