Rolling elements

Abstract

The pitting resistance of a gear is increased by hardening its tooth flanks through application of carburizing/quenching, bright hardening and induction hardening to a steel material capable of providing significantly improved softening resistance in tempering at a low temperature of 300 to 350° C. To this end, the steel material prepared so as to satisfy the relationship described by: 5≦4.3×Si (wt %)+7.3×Al (wt %)+3.1×V (wt %)+1.5×Mo (wt %)+1.2×Cr (wt %)×(0.45÷C (wt %)) is carburized such that the carbon concentration of its carburized surface layer is adjusted to 0.6 to 0.9 wt %; and the steel material is subjected to quenching and tempering at 300° C. or less subsequently to the carburization process, or alternatively the steel material is once cooled after the carburization process and then subjected to treatments of re-heating hardening and tempering at 300° C. or less so that a hardness of HRC 58 or more is ensured by the tempering process at 300° C.

1 . A rolling element which is made from a steel material containing at least 0.15 to 0.35 wt % C; further containing either 1.0 to 3.0 wt % Si or 0.35 to 1.5 wt % Al or alternatively, 0.5 to 3.0 wt % (Si+Al); and further containing one or more alloy elements selected from the group consisting of Mn, Ni, Cr, Mo, V, Cu, W, Ti, Nb, B, Zr, Ta, Hf, and Ca, unavoidable impurities such as P, S, N and O, and balance essentially consisting of Fe; said steel material being prepared so as to satisfy the relationship described by:  
       5≦4.3×Si (wt %)+7.3×Al (wt %)+3.1×V (wt %)+1.5×Mo (wt %)+1.2×Cr (wt %)×(0.45÷C (wt %)), and  which is formed by carburizing said steel material such that the carbon concentration of a carburized surface layer of the steel material is adjusted to 0.6 to 0.9 wt %; quenching the steel material subsequently to the carburization process and then tempering the steel material at 300° C. or less, or alternatively cooling the steel material once after the carburization process and then applying treatments of re-heating hardening and tempering at 300° C. or less to the steel material so that a hardness of HRC 58 or more is ensured by the tempering process at 300° C.    
   
   
       2 . The rolling element according to  claim 1 , wherein, in said steel material, the amount of Cr is limited to no more than 1.4 times the amount of Si, and one or more elements selected from the group consisting of 0.35 wt % or less Mo, 0.4 wt % or less V, 1.0 to 2.5 wt % (Mn+Ni) are added in an amount which satisfies the relationship described by:  
       −0.146×Si (wt %)+0.03×Mn (wt %)−0.024×Ni (wt %)+0.075×Cr (wt %)+0.043×Mo (wt %)+0.133×V (wt %)≦0.  
   
   
       3 . The rolling element according to  claim 1 , wherein said steel material contains 1.5 to 2.5 wt % Si or (Si+Al) and less than 2.0 wt % Cr to prevent precipitation of cementite during the carburization process.  
   
   
       4 . The rolling element according to  claim 1 , wherein said steel material having an Al content of 0.3 wt % or more contains 0.3 to 1.5 wt % Ni.  
   
   
       5 . The rolling element according to  claim 2 , wherein said steel material contains 1.5 to 2.5 wt % Si or (Si+Al) and less than 2.0 wt % Cr to prevent precipitation of cementite during the carburization process.  
   
   
       6 . The rolling element according to  claim 2 , wherein said steel material having an Al content of 0.3 wt % or more contains 0.3 to 1.5 wt % Ni.