Bearing part and fabrication method therefor
In a heat treatment wherein a blank (intermediate material) formed with a lubrication hole is heated and quenched, the quenching of the blank is terminated at the point of time that a temperature at a portion corresponding to a depth from a raceway surface (surface of a bearing part) of an outer ring (the bearing part), to which depth the maximum shear stress affects, is lowered to an MS point.
1. A bearing part having a hole portion opening into a surface thereof and hardened at least at said surface by a heat treatment, wherein
said surface has a hardness of HRC60 or more, and a residual compressive stress of said surface is made of 30 MPa or less in order to obviate failure in a peripheral area of the hole portion during heat treatment, wherein
said bearing part is an inner ring or an outer ring,
said bearing part is a large bearing part having an outside diameter of 300 mm or more, and
said hole portion is a lubrication hole formed so as to intercommunicate an inner peripheral surface and an outer peripheral surface of the bearing part.
2. A bearing part as claimed in claim 1 , wherein said bearing part is a large bearing part for use in a steel rolling mill.
3. A fabrication method for the bearing part of claim 1 , the method comprising a heat treatment step following a drill work for drilling a steel intermediate material thereby forming the hole portion opening into the surface thereof, wherein the intermediate material is heated to a predetermined temperature and then is quenched and wherein the quenching is terminated at the point of time that a temperature at a portion corresponding to a depth from the surface of said bearing part, to which depth the maximum shear stress affects, is lowered to an MS point at which martensitic transformation starts and then, the intermediate material is air cooled.
4. A fabrication method for the bearing part as claimed in claim 3 , wherein provided that the maximum thickness of said bearing part is expressed as t, the quenching is carried out until the temperature at the portion corresponding to a depth of 0.5 t or less from the surface of the bearing part is lowered to said MS point.
5. A fabrication method for the bearing part as claimed in claim 3 , wherein said MS point is in the range of 220 to 2300° C.
6. A bearing part having a hole portion opening into a surface thereof and hardened at least at said surface by a heat treatment, wherein
said surface has a hardness of HRC60 or more, and a residual compressive stress of said surface is made of 30 MPa or less in order to obviate failure in a peripheral area of the hole portion during heat treatment,
said bearing part is an inner ring or an outer ring,
said bearing part is a large bearing part having an outside diameter of 300 mm or more, and
said hole portion is a bolt hole opening into an inner peripheral surface or an outer peripheral surface of the bearing part.
7. A bearing part as claimed in claim 6 , wherein said bearing part is a large bearing part for use in a steel rolling mill.