Metal treatment – Stock – Carburized or nitrided
Reexamination Certificate
1999-06-16
2002-07-02
King, Roy (Department: 1742)
Metal treatment
Stock
Carburized or nitrided
Reexamination Certificate
active
06413328
ABSTRACT:
TECHNICAL FIELD
The present invention relates to high surface pressure resistant steel parts and producing methods thereof. The high surface pressure resistant steel parts are suitably used as power transmitting parts which are required to have contact fatigue strength and wear resistance and examples of which are rolling members (e.g., gears and bearings), the races of a rolling member and cam components.
BACKGROUND ART
In recent years, mechanical reduction gears and transmissions are increasingly required to have high power transmitting capability to meet the trend toward high output power, light weight and compactness. More compactness and higher surface pressure strength are required particularly in gears and bearings.
High contact fatigue strength is also required in gears and bearings used as power transmitting elements in automotive and construction machinery applications. As a measure for enhancing surface pressure strength, a treatment such as carburization or nitriding is widely applied to gears for the purpose of surface hardening. Another measure such as addition of Mo to steel is also taken to increase surface pressure strength, whereby the hardness of the surface as well as resistance to softening caused by tempering is increased. A method widely used in recent years is such that a carburization or carburization/carbonitriding treatment is applied to steel, and then, quenching and shot peening are carried out in order to significantly increase surface hardness, while providing considerable compressive residual stress to the steel.
There has been reported a method in which a high density cementite phase is precipitated on the surface of steel through carburization thereby increasing surface hardness, tempering softening resistance, and therefore surface pressure strength.
There has also been reported a development of highly clean steel which is designed to reduce the amount of inclusions with a view to the prevention of destruction due to contact fatigue, which occurs with inclusions as a starting point.
As noted earlier, a method, in which increased surface pressure strength is achieved by carburization of steel to which Mo (tempering-softening resistant element) has been added in a larger amount than the conventional steels, is known as an attempt to increase surface hardness and to restrict a decrease in hardness due to exothermic reaction caused by minute shear deformation resulting from friction and contact stress (Hertzian stress) which occur during rolling movement or rolling movement accompanied with sliding. In practice, this method, however, presents the following drawbacks: In spite of enhancing tempering-softening resistance, the thickness of an oil film formed on the contact surface decreases with increases in contact stress, resulting in a significant increase in wear because of the degradation of lubricating properties. This further promotes exothermic reaction and contact stress, which is a cause of creation of destructive shearing stress. Therefore, the desired, satisfactory improvement in surface pressure strength cannot be expected. Furthermore, the addition of large amounts of a tempering-softening resistance enhancing element such as Mo entails a considerable increase in the production costs of steel materials.
A known method, in which intensive shot peening is applied to the surface of a carburized article to allow the martensitic transformation of residual austenite which exists in the region extending from the outermost surface to a depth of about 200 &mgr;m below the surface so that higher surface hardness and greater compressive residual stress are achieved, thereby improving surface pressure strength, does not necessarily have versatility for the following reasons. Microscopic defects are created by shots to a grain boundary oxidation layer (defective layer) which has been created during carburization. When the steel article is in the initial stage of rolling operation, these defects bring about wear chip powder generation and surface roughness, resulting in an increase in wear factors. Another reason is that, in the case of gears, a chip in a tooth attributable to the buildup of strong residual stress as well as the presence of compressive residual stress adversely affects spalling resistance and, therefore, surface roughness causes an increase in wear factors, which results in a decrease in surface pressure strength on the contrary.
There is a case where a gear is subjected to high-carbon carburization or high concentration carburization in which a high density of cementite phase is precipitated on the surface layer of the gear by carburizing in a different manner. There is also a case where the hardness of a bearing surface is increased basically by the effect of cementite precipitation similarly to the case of bearing steel such as SUJ2 in which cementite is granulated and finely dispersed, while tempering-softening resistance is improved by the effect of particle dispersion. However, where a high density of cementite is precipitated by the above high carbon carburization process, the precipitated cementite is large in size, namely 5 to 10 &mgr;m, so that the agglomeration of cementite is likely to occur and an extremely large scale of precipitation appears along grain boundaries. As a result, the agglomerated cementite is destroyed by a shearing force generated from contact stress, forming starting points from which surface defects will occur. If this method is applied to manufacture of gears, the strength of the dedendum will be decreased.
Attempts have been made to fine cementite and prevent the cementite agglomeration by an improved high carbon carburization process or by a choice of adequate alloy elements for use in steel. For instance, Japanese Patent Publication (KOKAI) Gazette No. 4-160135 (1992) discloses a method according to which the concentration of Cr is increased to 2 to 8 wt %, one or more elements selected from the group consisting of 0.5 to 4 wt % Ni, 0.01 to 0.5 wt % Nb, 0.1 to 2 wt % V, and 0.05 to 1 wt % Mo are added, and the surface carbon content after carburization is increased to 2.0 wt % or more, whereby the carbides and carbonitrides of V and Cr of 5 &mgr;m or less are precipitated in the region extending from the surface to a depth of 150 &mgr;m below the surface. This method is, however, costly, because of the addition of large amounts of Cr for the purpose of facilitating cementite precipitation during a carburizing phase and the addition of V for the purpose of restraining the agglomeration/grow of precipitated cementite. Additionally, the concentration of Cr, V, Mo, Mn and the like in the precipitated cementite causes a decrease in the concentration of these alloy elements in the parent phase of austenite, which leads to formation of an imperfect quenched layer due to a lack of quenching ability after carburization. In order to prevent the formation of an imperfect quenched layer, Ni, which hardly concentrates in a carbide, is added and, in consequence, the material becomes more expensive.
Japanese Patent Publication (KOKAI) Gazette No. 8-120438 (1996) discloses a method and material for restraining formation of an imperfect quenched layer in a quenching process while employing less expensive alloy designs. In this publication, surface carbon content is established at 1.5 wt % or less in order to prevent the growth and agglomeration of precipitated carbides having grain size exceeding 5 &mgr;m. Since the optimum carbon content is 1.5 wt % or less, the amount of precipitated carbides is rather small, that is, approximately 7% by volume or less. In addition, permeating nitrogen is not effectively utilized in precipitating carbides or carbonitrides but is mostly dissolved in the parent phase of austenite to be utilized only for preventing formation of an imperfect quenched layer during quenching.
The method disclosed in the publication 8-120438, however, reveals the following disadvantages. The method of the publication does not precipitate a large amount of a carbide (cementite) like the conventional h
Hamasaka Naoji
Takayama Takemori
Armstrong Westerman & Hattori, LLP
King Roy
Komatsu LTD
Wilkins, III Harry D.
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