Method of making aluminized strengthened steel

A method of making an article of strengthened steel comprising a combination of metallurgical processes coordinated in such a manner as to make an inexpensive ferrous article with high yield strength together with enhanced corrosion resistance especially for use as a tool downhole in oil and gas wells, the method comprising the steps of:

• (a) providing a ferrous article which is made of carbon or alloy steel having an alloying equivalent composition of from 0.43 weight percent to 0.93 weight percent of the sum of the carbon content and one tenth of the sum of the content of boron, chromium, manganese, molybdenum, nickel, silicon, and vanadium, where the alloy composition is uniformly distributed throughout the metallurgical structure;
• (b) strengthening the said ferrous article by a heat treating process consisting of heating the article to a temperature above 1341.degree. F. to within the austenitizing temperature range of the carbon or alloy steel for a period of 15 minutes or longer in order to transform sufficiently the metallurgical structure to face-centered cubic austenite and then further subsequently cooling rapidly to below 600.degree. F. in order to transform the face-centered austenite structure to body-centered tetragonal martensite thereby hardening the ferrous article to a minimum of 30 Rockwell C and further comprising tempering the martensitic structure by reheating in order to reduce residual cooling stresses and toughen the ferrous article, but where the tempering temperature is still maintained below 1341.degree. F. in order to avoid over-tempering retransformation of the martensitic tetragonal structure to face-centered cubic austenite;
• (c) aluminizing the strengthened ferrous article by applying aluminum to its surface and simultaneously or thereafter heating said to a temperature in the range of from 1000.degree. F. to below 1341.degree. F. to prevent retransforming the body-centered tetragonal martensite structure to face-centered cubic austenite structure, for a period of more than 5 minutes in order to sufficiently cause interdiffusion of the applied aluminum and the iron in the ferrous article and in order to further metallurgically cause to transform the surface and subsurface of the ferrous article to form an intermetallic layer composed of an alloy of essentially aluminum and iron which is from 0.002 inch to 0.027 inch in thickness and where the aluminizing process is carried out without reducing the yield strength of the steel below 60,000 psi.