Thermo-mechanical Process for Martensitic Bearing Steels

Abstract

A method for treating a steel bearing component includes subjecting the steel bearing component to a scouring treatment to impart compressive residual stress to a surface region thereof and subsequently nitriding the steel bearing component 1. The resulting steel bearing component exhibits a nitrided case depth of about 0.002-0.014 inches and a compressive residual stress value greater than −120 ksi is achieved at a depth of 0.002 inches and −20 ksi to a minimum depth of 0.010″. The steel bearing component may be a race and/or a rolling element, such as a ball.

We claim: 
     
         1 . A method for treating a steel bearing component, comprising:
 subjecting the steel bearing component to a scouring treatment to impart compressive residual stress to a surface region thereof; and   subsequently nitriding the steel bearing component.   
     
     
         2 . The method according to  claim 1 , wherein the steel bearing component is nitrided to a depth of about 0.002-0.014 inches. 
     
     
         3 . The method according to  claim 1 , wherein the steel bearing component is nitrided to a depth of 0.006-0.010 inches. 
     
     
         4 . The method according to  claim 1 , wherein the nitriding step is performed at one or more temperatures between 842° F. and 968° F. 
     
     
         5 . The method according to  claim 1 , wherein the nitriding step is performed until a compressive residual stress value greater than −120 ksi is achieved at a depth of 0.002 inches and −40 ksi to a minimum depth of 0.010″. 
     
     
         6 . The method according to  claim 1 , wherein the nitriding step is performed until a compressive residual stress value greater than −160 ksi is achieved at a depth of 0.002 inches. 
     
     
         7 . The method according to  claim 1 , wherein the steel bearing component has a hardness at its surface between HRC 64-72 after the nitriding step. 
     
     
         8 . The method according to  claim 1 , wherein the steel bearing component is a race and/or a rolling element. 
     
     
         9 . The method according to  claim 8 , wherein the steel bearing component is a ball. 
     
     
         10 . The method according to  claim 1 , wherein the scouring step is performed by:
 placing the ball in a tumbling barrel having internally-secured paddles and   rotating the tumbling barrel.   
     
     
         11 . The method according to  claim 1 , wherein, prior to the scouring step, the steel bearing component is hardened and tempered to provide a martensitic microstructure having a hardness in the range of HRC60-64 and retained austenite of less than 3% by volume. 
     
     
         12 . The method according to  claim 1 , wherein the steel bearing component is comprised of, in weight percent, about 0.80-0.85% carbon, about 4.00-4.25% chromium, about 4.00-4.50% molybdenum, about 0.15-0.35% manganese, about 0.10-0.25% silicon, about 0.9-1.10% vanadium, 0.015% max. phosphorus, 0.010% max. sulfur, 0.15 max. nickel, 0.25% max. cobalt, 0.25% max. tungsten, 0.10 max. copper and the balance being essentially iron. 
     
     
         13 . The method according to  claim 1 , wherein the steel bearing component is comprised of, in weight percent, about 0.11-0.15% carbon, about 4.00-4.25% chromium, about 4.00-4.50% molybdenum, about 0.15-0.35% manganese, about 0.10-0.25% silicon, about 3.20-3.60% nickel, about 1.13-1.33% vanadium, 0.015% max. phosphorus, 0.010% max. sulfur, 0.25% max cobalt, 0.25% max. tungsten, 0.10 max. copper and the balance being essentially iron. 
     
     
         14 . The method according to  claim 3 , wherein the nitriding step is performed at one or more temperatures between 842° F. and 968° F. and until a compressive residual stress value greater than −160 ksi is achieved at a depth of 0.002 inches; and
 wherein the steel bearing component has a hardness at its surface between HRC 64-72 after the nitriding step. 
 
     
     
         15 . The method according to  claim 14 , wherein
 the steel bearing component is a ball; and   the scouring step is performed by:   placing the ball in a tumbling barrel having internally-secured paddles and   rotating the tumbling barrel.   
     
     
         16 . The method according to  claim 15 , wherein, prior to the scouring step, the ball is hardened and tempered to provide a martensitic microstructure having a hardness in the range of HRC60-64 and retained austenite of less than 3% by volume. 
     
     
         17 . A steel bearing component, exhibiting:
 a nitrided case depth of about 0.002-0.014 inches; and   a compressive residual stress value greater than −120 ksi at a depth of 0.002 inches.   
     
     
         18 . The steel bearing component according to  claim 17 , wherein:
 the nitrided case depth is 0.006-0.010 inches;   the compressive residual stress value is greater than −160 ksi at a depth of 0.002 inches and −40 ksi to a minimum depth of 0.010″, and   the steel bearing component exhibits a hardness at its surface between HRC 64-72.   
     
     
         19 . The steel bearing component according to  claim 18 , wherein the steel bearing component is a ball, primarily has a martensitic microstructure and has a retained austenite content of less than 3% by volume. 
     
     
         20 . The steel bearing component according to  claim 19 , wherein the ball is comprised of, in weight percent:
 about 0.80-0.85% carbon, about 4.00-4.25% chromium, about 4.00-4.50% molybdenum, about 0.15-0.35% manganese, about 0.10-0.25% silicon, about 0.9-1.10% vanadium, 0.015% max. phosphorus, 0.010% max. sulfur, 0.15 max. nickel, 0.25% max. cobalt, 0.25% max. tungsten, 0.10 max. copper and the balance being essentially iron, or   about 0.11-0.15% carbon, about 4.00-4.25% chromium, about 4.00-4.50% molybdenum, about 0.15-0.35% manganese, about 0.10-0.25% silicon, about 3.20-3.60% nickel, about 1.13-1.33% vanadium, 0.015% max. phosphorus, 0.010% max. sulfur, 0.25% max cobalt, 0.25% max. tungsten, 0.10 max. copper and the balance being essentially iron.