This work began with a study of the effects of cobalt, silicon, nickel and aluminum additions on the tempering response of hot work die steels. While it was expected that cobalt and silicon would both enhance secondary hardening, the significant effects of nickel and aluminum in enhancing secondary hardening were not expected. The base secondary hardening steel used in this work had a hardness of Rockwell-C 51 after tempering for one hour at 550C. An addition of 4 wt. % nickel increased the hardness, after tempering at 550oC. by 2 Rockwell-C. The addition of 2 wt. % aluminum increased the hardness after tempering at 550oC by 2.5 Rockwell-C. What was done next was to add 2 wt. aluminum and 4 wt. % nickel in combination to the base steel. It was anticipated that the steel would not only be strengthened by the effects of nickel and aluminum on alloy carbide formation but that it would also be strengthened by the precipitation of particles of NiAl. The steel modified by the aluminum and nickel in combination had a hardness after tempering at 550oC of Rockwell-C 62 and a Charpy impact energy of 11 ft-lbs. When asked by General Electric Aircraft Engines to propose a new steel for their engine shafts it was quite natural, based on previous experience to suggest combining strengthening by alloy carbides and by particles of NiAl by making additions of aluminum to the high nickel secondary hardening steel, AF1410.