Alloys or metallic compositions – Ferrous – Chromium containing – but less than 9 percent
Reexamination Certificate
2001-12-05
2004-03-09
Yee, Deborah (Department: 1742)
Alloys or metallic compositions
Ferrous
Chromium containing, but less than 9 percent
C420S119000, C420S108000, C420S109000, C420S113000, C420S129000, C148S319000
Reexamination Certificate
active
06702981
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to high speed steels and, more particularly, to a carburizable high speed steel having low carbon and chromium levels for use as roller bearings, taps and other applications where hardness and fracture resistance are required.
2. Description of the Related Art
It is well-known that stress state on or below a bearing race subjected to alternating contact loads can have a major influence on service life. Most tapered roller bearings are made from carburizing grades of steels. When components made from these grades are carburized and heat treated, some of the advantages realized relative to through hardened components are: fewer quench cracks in heat treating, a decreased sensitivity to grinding injury, and improved toughness or resistance to catastrophic failure which provide a more reliable product exhibiting fewer in-service problems.
For carburized components, the compressive residual stresses are developed during heat treating. The absorption of carbon into a component during carburizing creates a carbon gradient wherein the carbon level is highest near the surface and decreases as the distance away from the surface increases. Thus, the core of the component contains the nominal carbon content of the alloy. When steel components are quenched from the austenitizing temperature, martensite is formed. The transformation of austenite to martensite is accompanied by a volume expansion which is directly proportional to the carbon content of the alloy. When quenched, the surface of a component cools more rapidly than the inner portion of a component. In addition, the Ms temperature (the temperature at which austenite transforms to martensite) decreases with increasing carbon content. Thus, for a carburized component, relative to the core, the surface or so-called “case” transforms to martensite at a lower temperature than would occur for a component of uniform composition. Consequently, these two effects operating in unison cause a relatively high compressive residual stress to be formed on the surface layer or case. Comprehensive details of this type of processing are contained in “Carburizing and Carbonitriding”,
American Society for Metals
, Materials Park, Ohio (1997).
This surface residual stress effect does not occur in components having a uniform composition, i.e., non-carburized components. The enhanced performance created by compressive surface residual stresses has resulted in processes being developed to carburize high carbon bearing alloys, such as that of U.S. Pat. No. 4,191,599 granted Mar. 4, 1980. The '599 patent discloses the carburizing of high carbon steels such as 52100 and M50 in carburizing atmospheres containing higher carbon potentials than used for standard carburizing steels. The carbon gradients produced lead to reasonable surface compressive residual stresses when these steels are quenched and tempered.
Another factor where compressive residual surface stresses are beneficial involves the press fitting of bearing components onto shafts. It is well-known that press fitting of bearings on shafts can create a tensile stress on the bearing. It has been demonstrated that the press fitting of through hardened AISI 52100 steel definitely has an adverse effect on fatigue life. However, similarly, press fit bearings fabricated from carburized AISI 8620 were found to perform satisfactorily. It was concluded that under press-fitting conditions, carburized AISI 8620 steel had superior fatigue characteristics compared to AISI 52100. This work was reported by T. E. Hustead, “Consideration of Cylindrical Roller Bearing Load Rating Formula”,
SAE Preprint
569
A
(Sept. 1962).
By way of example, consider a LM12749 bearing cone inner race made from carburized 8119 steel compared with the same inner race made from a through hardened 1.0% carbon 46100 steel having a uniform carbon gradient (uncarburized). The compressive stresses in the carburized bearing cone vary from −48.1 ksi on the surface to −22.6 ksi at a depth of 0.030″ below the surface. For the bearing cone made from the through hardened 46100 high carbon alloy steel, the stress from the surface to 0.030″ below the surface was, at most, only −3.8 ksi.
While carburized bearings fabricated from low carbon alloy steels have better properties than through hardened bearings fabricated from high carbon alloy steels, neither of these types of alloys performs well at continuous temperatures in excess of 400° F. Furthermore, brief exposures to temperatures of 500° F. or greater can significantly soften components manufactured from most alloy steels. In demanding applications such as jet engine main bearings, high speed steels (sometimes referred to as “HSS”) such as M50 are selected. High speed steels have higher compressive yield stresses than alloy steels. The high compressive yield stresses of these steels are a direct result of the high carbon content of the HSS alloys and a presence of alloying elements such as chromium, molybdenum, vanadium and tungsten.
The heat treatments used for high speed steels are different from the heat treatments used for alloy steels. For example, a typical heat treating cycle for an alloy steel such as AISI 4340 would be to austenitize the material at 1550° F. until the entire component was equilibrated for one hour at the austenitizing temperature. The material would then be rapidly removed from the furnace and quenched into oil. After the material cooled to approximately 150° F., it would be removed from the quench bath. The alloy would then be tempered for approximately two hours at a temperature of less than 1320° F. For maximum hardness and strength, the alloy would be tempered at or below 350° F. However, if toughness was important, a tempering temperature of 1150° F. would be selected. For a bearing alloy such as AISI 52100, the austenitizing may be 1525° F. After quenching, a tempering temperature of approximately 350° F. would be used. Low temperature tempering would be used for any bearing fabricated from an alloy steel. This would ensure that the resulting component would be hard and have as high a compressive yield stress as possible. Tempering temperatures exceeding 350° F. will lower the hardness and, consequently, the compressive yield stress of bearings made from through hardened steels.
For all alloy steels, after being austenitized and then oil quenched, increasing the tempering temperature is found to decrease the alloy's hardness. Steels having this type of tempering response are referred to as “class 1” types of steels, depicted in FIG.
1
.
The heat treating procedures used for high speed steels typically begin with a preheat of approximately 1450° F. to 1550° F. Components fabricated from HSS are equilibrated at the preheating temperature for at least one hour. Following the preheat, high speed steel alloys are then quickly placed in an austenitizing furnace that is at a higher temperature. Depending on the alloy, the high austenitizing temperature may range from 2000° F. to 2125° F. The HSS components are only held at the austenitizing temperature for a brief amount of time, say, 3 to 10 minutes. Following austenitization, the material is quenched into a salt bath at 1000° F. After equilibrating in the salt bath, the components are allowed to air cool to at least 150° F. If an oil quench is employed, the material should be removed when it reaches 900° F., after which, cooling to 150° F. in still air is recommended.
Following quenching, high speed steel alloys contain untempered martensite, alloy carbides and retained austenite. Tempering HSS must accomplish two things. The martensite needs to be tempered, and the retained austenite has to be transformed to martensite. The general procedure employed for tempering high speed steels is to heat the alloys to approximately 1000° F. for two hours and then air cool to room temperature. The cycle is then repeated one more time. Most high speed steels show “class 3” tempering response, of the type
The Timken Company
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
Yee Deborah
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