High strength steel and method of making same

Abstract

A thermally treated steel is provided comprising a steel having a microhardness of between approximately 60 and 67 HRC at a surface depth between approximately 0.002 to 0.014 inches from the steel surface and having approximately 0-36% by volume martensite converted from retained austenite at the surface depth. The steel may be used for ANSI 60 pins used in roller chain that may maintain elongation of less than 0.040 inches for up to 264 hours of wear testing. A method of making the steel pins is provided that comprises the steps of carburizing the steel in an atmosphere of 1.00% carbon or greater at a temperature between 1550° and 1750° F., quenching the steel to room temperature, tempering the steel to a temperature of 375° F. or less, cryogenically treating the steel to a temperature of −300° F. or lower and tempering the steel to a temperature of 375° F. or less.

1 . A thermally treated steel comprising: 
 a steel having a microhardness of between approximately 60 and 67 HRC from the surface to a depth of approximately 6% of the part thickness (or diameter).    
   
   
       2 . The steel of  claim 1  wherein the steel is used for pins used in ANSI 6-roller chain that has elongation of less than 0.040 inches for up to approximately 264 hours of wear testing.  
   
   
       3 . The steel of  claim 1  wherein the steel is used for pins used in ANSI 60 roller chain that takes at least 302 hours of wear testing to elongate 0.070 inches or more.  
   
   
       4 . The steel of  claim 1  wherein the steel is used for pins used in ANSI 60 roller chain that has at least 279% better wear characteristics than standard pins with respect to elongation over wear time.  
   
   
       5 . The steel of  claim 1  wherein the steel is used for pins used in ANSI 60 roller chain that has a microhardness of approximately 65.4 HRC at 0.002 inches from the surface.  
   
   
       6 . The steel of  claim 1  wherein the steel is used for pins used in ANSI 60 roller chain that has a microhardness of approximately 60.3 HRC at 0.014 inches from the surface.  
   
   
       7 . The steel of  claim 1  where the core hardness is 25% less than the surface hardness.  
   
   
       8 . The steel of  claim 1  where the steel forms a pin provided in combination with bushings, link plates and roller bearings to provide roller chain.  
   
   
       9 . A method of thermally treating a pin of an ANSI 60 roller chain, the method comprising the steps of: 
 providing the pin of steel;    carburizing the pin at a high carbon concentration; and    thermally treating the pin in order to convert retained austenite to martensite and provide a microhardness of 60 HRC or greater at a depth of between approximately 0.002 to 0.014 inches from the pin surface.    
   
   
       10 . The method of  claim 9  further comprising the step of quenching the pin to about 100° F.  
   
   
       11 . The method of  claim 9  further comprising the step of quenching the pin using fast quench oil.  
   
   
       12 . The method of  claim 9  further comprising the step of tempering the steel to a temperature of 375° F. or less.  
   
   
       13 . The method of  claim 9  wherein the carburizing step provides for carburizing the pin in an atmosphere of 1.00% carbon or greater at a temperature of between 1550° and 1750° F.  
   
   
       14 . The method of  claim 9  wherein the carburizing step may further comprises a carbonitriding process including an ammonia atmosphere.  
   
   
       15 . The method of  claim 9  wherein the thermal treatment step includes a deep cryogenic treatment of the pin at approximately −300° F. or lower in order to convert retained austenite to martensite.  
   
   
       16 . The method of  claim 15  wherein the deep cryogenic treatment occurs for approximately 24 hours.  
   
   
       17 . A thermally treated steel comprising: 
 a thermally treated steel that prior to cryogenic treatment has a microhardness of between approximately 55 and 65 HRC from the surface to a depth of approximately 6% of the part thickness (or diameter) and having approximately 15-50% by volume retained austenite near the surface.    
   
   
       18 . The steel of  claim 17  wherein after cryogenic treatment the steel has a microhardness of between approximately 60 and 67 HRC from the surface to a depth of approximately 6% of the part thickness (or diameter) and having full transformation of retained austenite to martensite near the surface.  
   
   
       19 . The steel of  claim 18  wherein the hardness of the steel at the surface is at least 25% harder than at a core of the steel.  
   
   
       20 . The steel of  claim 18  wherein the steel provides a component having a core and a case over the core with the case being harder than the core and the case having at least approximately 15% martensite converted from retained austenite during thermal treatment.