Method of heat treating high carbon alloy steel parts to develop surface compressive residual stresses
AssigneeFord Motor Company
InventorsCharles A. Stickels, Adam M. Janotik
Life of this patent
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Abstract
A method of developing compressive residual stresses in the surface region of a high carbon steel alloy article is disclosed. The article is made of an alloy having 0.8-1.6% C, 0.2-5% Cr, 0-20% ingredients selected from the group consisting of M.sub.N, V, Mo, W, Si, and the remainder Fe. The article is heated in a carburizing atmosphere at 800.degree.-950.degree. C. for 1-2.5 hours, and then quenched to cool the central core of the article at a rate sufficiently fast to suppress the formation of non-martensitic austenite decomposition products, thereby establishing a residual compressive stress gradient proceeding from the surface to a depth of 0.007-0.03 inches.
Claims22
A method of developing compressive residual stresses in the surface region of a high carbon steel alloy article, said method comprising:
- (a) constituting said steel alloy article to contain 0.8-1.6% carbon, 0.2-5% chromium, and 0-20% alloying ingredients selected from the group consisting of manganese, vanadium, molybdenum, tungsten, silicon, the remainder being iron;
- (b) heating said article to a temperature of 800.degree.-950.degree. C. (1472.degree.-1742.degree. F.) for 1-2.5 hours in a carburizing atmosphere effective to generate a differential in retained austenite, primary carbides and carbon between the surface region and core region of said article;
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- (c) immersing said article in a cooling medium to quench the central core of said article at a rate sufficiently fast to effectively suppress the formation of non-martensitic austenite decomposition products, thereby establishing a residual compressive stress gradient proceeding from the surface region of said article to a depth between 0.007-0.03 inches, the residual compressive stress being in the range of 5-40 Ksi.
A high carbon steel alloy article having gradients of compressive residual stress, and at least one of a carbon gradient and hardness gradient proceeding from the surface region of said article to its core, said article being characterized by a microstructure consisting essentially of tempered martensite, retained austenite and a carbide phase, said article having a chemical content consisting essentially of 0.8-1.6% carbon, 0.75-25% alloying ingredients including 0.2-5% chromium, the remainder being essentially iron, said article having a compressive residual stress level at its surface region of at least 10,000 psi, and tensile stresses at the article core, said article having a hardness differential between its surface and core of at least 2.0 R.sub.c, and a volume fraction of primary carbides at its surface region of at least 0.18.
A method of making bearing elements comprising:
- (a) shaping a body of SAE 52100 steel into a desired bearing configuration, (b) heating said body in a carburizing atmosphere to a temperature of about 815.degree. C. for about 1 hour, (c) quenching said heated body in oil maintained at about 55.degree. C., (d) tempering said body at 150.degree. C. for about 1.5 hours, and (e) finish grinding said heat treated product to a depth of less than 0.005", said resultant bearing being characterized by a microstructure consisting essentially of tempered martensite, a primary carbide phase which is at least 0.18 at its surface region and a retained austenite volume fraction of about 25% at the surface.
In a method for preparing bearing components wherein a hot-formed shape of low alloy steel containing carbon in the range of 0.8-1.5% and containing alloying ingredients in an amount to achieve a hardening response from heat treatment throughout said shape, said alloying ingredients being selected from a group consisting of Cr, Mn, Ni, Cu and Mo, said shape being subjected sequentially to a spheroidizing-anneal treatment, a rough forming treatment, and a hardening austenitizing treatment, the improvement comprising:
- (a) carrying out said hardening-austenitizing treatment in a carburizing atmosphere having a carbon potential sufficiently high to saturate a 0.0025" thick iron foil in 30 minutes and at a temperature level of between 800.degree.-900.degree. C. for a period of time between 1-2.5 hours to establish both carbon additions and an increase in the primary carbide and retained austenite phases of said steel in the surface layers of the steel;
- (b) establishing a compressive residual stress gradient in said steel by quenching said steel at a rate such that the core experiences a cooling rate of at least 300.degree. F. per second when it is passing through the temperature zone of 1300.degree.-700.degree. F., said resultant article having a compressive stresses in the surface region which range between 5-40 KSI and tensile stresses in its core.
Dependencies are inferred from the claim text.