Metal treatment – Compositions – Heat treating
Patent
1989-04-13
1990-05-01
Rutledge, L. Dewayne
Metal treatment
Compositions
Heat treating
148 16, 148316, 148319, 428627, C21D 178
Patent
active
049210254
DESCRIPTION:
BRIEF SUMMARY
This application is a continuation-in-part of application Ser. No. 135,775, filed Dec. 21, 1987, and now abandoned. This application was filed through the PCT Office as International Application No. PCT/US88/04470 on Dec. 14, 1988.
TECHNICAL FIELD
This invention relates generally to metal heat treatment and more particularly to case hardening with a gas containing carbon and a resulting article.
BACKGROUND ART
Carburizing is an effective method of increasing the surface hardness of low carbon, unalloyed, or low carbon, low alloy steels. Typically, steel articles are placed in an atmosphere containing carbon in an amount greater than the base carbon content of the steel and heated to a temperature above the austenite transformation temperature of the steel. After the desired amount of carbon has been diffused into the article, hardness is induced by quenching.
Gas carburizing is efficient, controllable, and one of the most widely used methods of generating a carbonaceous atmosphere for carburizing. However, most commonly used gas mixtures typically contain small amounts of oxygen which tend to form surface oxides with one or more of the steel elements which have a strong affinity for oxygen, such as silicon, chromium and manganese. Oxides that form along grain boundaries, i.e., intergranularly, extend inwardly from the surface and have a harmful effect on the mechanical properties of the carburized article.
Conventional carburizing processes typically avoid the formation of case carbides, and generally produce an essentially carbide free martensitic structure. Even when case carbides are involuntarily formed, characteristically there is a thin surface layer void of carbides. This is due to the oxidation of carbide forming elements in the surface layer.
The detrimental effects of oxidation during gas carburizing have been known for a long period of time. Heretofore, when designing a carburized article, it has been necessary to consider the reduction in bending fatigue strength attributable to intergranular surface oxides. To avoid reduction in bending fatigue properties, it has been necessary to physically remove intergranular surface oxides formed during carburizing by machining or grinding, or prevent such surface formations by removing oxygen compounds from the carburizing media. These alternatives are costly. A 1978 article authored by Ruth Chatterjee-Fischer, "Internal Oxidation During Carburizing and Heat Treating," Metallurgical Transactions A, published by American Society for Metals and the Metallurgical Society of AIME, Vol. 9A, Nov. 1978, pp. 1553-1560, reported that, when employing conventional carburizing processes, the presence of silicon in the parent metal is a prime contributor to the formation of oxides at the surface of the article. However, while identifying that lowering silicon is a solution to the problem of surface oxidation, the paper fails to recognize any interrelationship between low silicon amounts and the enhanced formation of surface carbides by nonconventional carburizing methods.
A low silicon carburizing steel, developed by Sanyo Special Steel Co., Ltd. is described in Japanese Patent Publication No. 57-23741. The Sanyo reference teaches that low silicon, i.e., 0.06% to 0.12%, when combined with relatively high carbon and chromium amounts will accelerate carbon diffusion and thereby reduce carburizing time. This reference limits the amount of chromium in the steel composition to intentionally avoid the formation of case carbides. Furthermore, this reference fails to link the influence of a low silicon composition with the formation of surface oxides and carbides during carburizing. The Sanyo low silicon steel composition described in the above publication is no longer in commercial production.
Another low silicon carburizing steel intended for use in applications wherein the formation of case carbides is purposefully avoided, was developed by Kobe Steel, Ltd. Kobe's composition is described in Japanese Patent Publication No. 61-253346 and, as described therein, was dev
REFERENCES:
patent: 4202710 (1980-05-01), Naito et al.
patent: 4836864 (1989-06-01), Murakami et al.
Chemical Abstracts, vol. 107, No. 4, Jul. 27, 1987, (Columbus, Ohio, USA), p. 214, Abstract 26771e.
Naito, Takeshi et al., "Fatigue Behavior of Carburized Steel with Internal Oxides and Nonmartensitic Microstructure Near Surface", 7/84, Metallurgical Transactions A, vol. 15A, pp. 1431-1436.
Biltgen Gary L.
Gromer David R.
Holloway Gregory S.
Keil Gary D.
Tipton Sheryl A.
Caterpillar Inc.
McFall Robert A.
Phipps Margery S.
Rutledge L. Dewayne
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