Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
1999-10-28
2001-03-20
Jenkins, Daniel J. (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S660000, C148S662000, C148S612000
Reexamination Certificate
active
06203634
ABSTRACT:
This application is based on and claims priority under 35 U.S.C. §119 with respect to German Application No. P 19849681.8 filed on Oct. 28, 1998, the entire content of which is incorporated herein by reference.
FIELD OF THE INVENTION
The present invention generally relates to a method for heat-treating steel or cast iron components. More particularly, the present invention pertains to a method for heat-treating through hardened bearing steel components.
BACKGROUND OF THE INVENTION
Rolling bearing manufacturers decide upon the type of heat treatment to be used for rolling bearing components depending on the particular application for the rolling bearing, or the types and sizes of the roller bearings. For through hardened bearing steel components, two heat-treating methods are available—martensite hardening or austempering. Component properties such as hardness, microstructure, retained austenite content, and dimensional stability are associated with or affected by the particular type of heat treatment employed.
The following table provides an overview and comparison of various component properties associated with different heat treatment methods.
Retained
Dimensional
Hardness
Austenite
Stability
Method
HRC
%
&Dgr; D/D
Martensite
62 . . . 55
8 . . . 16
+60 &mgr;m/100 &mgr;m
Hardening
(tempered
normally)
Martensite
58 . . . 62
≦3
+15 &mgr;m/100 &mgr;m
Hardening
(stabilized)
Austempering
58 . . . 63
≦3
+15 &mgr;m/100 &mgr;m
Austempering and martensite hardening (stabilized) do not differ significantly with respect to hardness, retained austenite and dimensional stability. Austempering, however, has better toughness properties than martensite hardening and also has a different residual stress state. Both methods also suffer from various disadvantages.
At present, both methods are always embodied completely. That is, either a martensite transformation or a bainite transformation takes place. In addition, time-temperature combinations, quenching and transformation in the bainite stage are done in the manner specified in time-temperature transformation diagrams such as
FIG. 1
from the
Atlas zur Wärmebehandlung der Stähle,
the Atlas of Heat-Treatment of Steels.
The time required to attain the desired component properties is relatively long. For through hardened bearing steel components, the time required can be more than four hours, both for austempering and in martensitically hardened stabilized components.
The previously known methods described above preclude one another and so it is not possible to combine the properties of austempering and martensite hardening. Nor is it possible with current methods to reduce the total process time.
A need thus exists for a method of heat-treating a steel or cast iron component, including a through hardened bearing steel component, in a shorter time yet with the component possessing the desired properties.
SUMMARY OF THE INVENTION
The present invention provides a method for heat-treating steel components, especially through hardened bearing steel components, to establish arbitrary intermediate states of the microstructure between martensite and bainite, and to adapt the product properties of the components to suit the demands or requirements of a particular application, while at the same time reducing the treatment time. The method involves implementing various process parameters relating to austenitization of the component, the quenching of the component from the austenitizing temperature, the temperature at the onset of bainite transformation and the time period for which the component is held in the bainite state, the cooling down of the component after partial bainite transformation, and the time until the tempering treatment.
According to one aspect of the invention, a method for heat-treating a component of steel or cast iron, particularly a through hardened bearing steel component, involves heating the component to the austenitization temperature and holding the component at the austenitization temperature to achieve austenitization, rapidly quenching the component to approximately the martensite start point or M
S
temperature, and holding the component at the bainite transformation temperature until partial bainite transformation prior to complete transformation occurs. After partial bainite transformation, the component is cooled down to and briefly held at room temperature, followed by short-cycle tempering.
In accordance with another aspect of the invention, a method for heat-treating a steel or cast-iron component includes heating the component to the austenitization temperature and holding the component generally at such temperature, quenching the component and holding the component at a temperature to effect partial bainite transformation, rapidly cooling the component to room temperature after partial bainite transformation in the component has occurred, holding the component at room temperature, and short-cycle tempering the component.
REFERENCES:
patent: Re. 28645 (1975-12-01), Aoki et al.
patent: 4203783 (1980-05-01), Economopoulos et al.
patent: 4204892 (1980-05-01), Economopoulos
patent: 4295902 (1981-10-01), Economopoulos
patent: 4992111 (1991-02-01), Yamada et al.
patent: 5292384 (1994-03-01), Klueh et al.
patent: 5334269 (1994-08-01), Khare et al.
patent: 5672217 (1997-09-01), Hengerer et al.
patent: 4007487 (1991-09-01), None
Burns Doane Swecker & Mathis L.L.P.
Coy Nicole
Jenkins Daniel J.
SKF GmbH
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