Method for heat treating with a gradual temperature profile

Abstract

A method for heat treating a crankshaft for a vehicle propulsion system, or other work piece, includes heating at least a portion of the crankshaft to a gradual temperature profile. The temperature profile has gradually lower temperatures from the surface to the core of the crankshaft. The temperature profile includes a midpoint temperature at a midpoint between the surface and an innermost part of the core, the midpoint temperature being at least 50% of the surface temperature as measured in the Celsius scale. The surface temperature is within a transformation range of the crankshaft material. The method further includes quenching the surface of the crankshaft journal. The material of the crankshaft is preferably a carbon steel alloy having a DI less than 1.7 and having greater than 0.3 wt % carbon.

What is claimed is: 
     
         1 . A method for heat treating a crankshaft for a vehicle propulsion system, the crankshaft being formed of a crankshaft material and having an outer surface, the method comprising:
 heating at least a portion of the crankshaft to a temperature profile having a surface temperature at the outer surface, the temperature profile including gradually lower temperatures from the outer surface to a core of the crankshaft, the temperature profile including a midpoint temperature at a midpoint between the outer surface and an innermost part the core, the midpoint temperature being at least 50% of the surface temperature as measured in the Celsius scale, the surface temperature being within a transformation range of the crankshaft material; and subsequently quenching the outer surface of the crankshaft.   
     
     
         2 . The method of  claim 1 , wherein heating the portion of the crankshaft includes induction heating the portion of the crankshaft. 
     
     
         3 . The method of  claim 2 , wherein the temperature profile includes a 25th percentile temperature at a 25 th  percentile point that is halfway between the midpoint and the outer surface, the 25 th  percentile temperature being within 10% of the surface temperature as measured in the Celsius scale. 
     
     
         4 . The method of  claim 3 , wherein the temperature profile includes a 75 th  percentile temperature at a 75 th  percentile point that is halfway between the midpoint and the innermost part of the core, the 75 th  percentile temperature being at least 50% of the surface temperature as measured in the Celsius scale, the midpoint temperature being at least 70% of the surface temperature as measured in the Celsius scale. 
     
     
         5 . The method of  claim 4 , wherein the midpoint temperature is in the range of 70% to 80% of the surface temperature as measured in the Celsius scale. 
     
     
         6 . The method of  claim 5 , wherein the 75 th  percentile temperature is in the range of 60% to 70% of the surface temperature as measured in the Celsius scale. 
     
     
         7 . The method of  claim 6 , wherein the outer surface is a round bearing journal surface of the crankshaft and the temperature profile extends along a radius of the bearing journal surface to the innermost part of the core. 
     
     
         8 . The method of  claim 7 , further comprising providing the crankshaft material as a steel having an ideal critical diameter (DI) less than 1.70. 
     
     
         9 . The method of  claim 8 , further comprising providing the steel as a carbon steel having at least 0.3 wt % carbon. 
     
     
         10 . The method of  claim 9 , wherein heating the portion of the crankshaft includes applying a plurality of inductive field pulses to the crankshaft. 
     
     
         11 . The method of  claim 10 , wherein heating the portion of the crankshaft includes applying a first inductive pulse having an intensity in the range of 2.0 to 2.5 J/C*mm 2 , applying a second inductive pulse having an intensity in the range of 2.0 to 2.5 J/C*mm 2 , and applying a third inductive pulse having an intensity in the range of 2.0 to 2.5 J/C*mm 2 , and pausing between the application of each of the first, second, and third inductive pulses. 
     
     
         12 . The method of  claim 11 , further comprising applying the first inductive pulse for a first period, applying the second inductive pulse for a second period, and applying the third inductive pulse for a third period, each of the first, second, and third periods being in the range of 8 to 12 seconds. 
     
     
         13 . The method of  claim 12 , wherein inductively heating the portion of the crankshaft comprises applying an alternating current to a coiled conductor. 
     
     
         14 . A method for forming a crankshaft for a vehicle propulsion system, the method comprising:
 forming the crankshaft of a crankshaft material;   forming a round bearing journal surface about the crankshaft;   applying a series of inductive heating pulses to the crankshaft until the crankshaft has a gradual temperature profile extending normally from the round bearing journal surface to a core of the crankshaft, the gradual temperature profile including a surface temperature at the round bearing journal surface, the surface temperature being within a transformation range of the crankshaft material, the gradual temperature profile including a midpoint temperature at a midpoint between the round bearing journal surface and an innermost part of the core, the midpoint temperature being at least 50% of the surface temperature as measured in the Celsius scale; and   quenching the round bearing journal surface to a temperature below the transformation temperature to harden the round bearing journal surface.   
     
     
         15 . The method of  claim 14 , wherein the gradual temperature profile includes:
 a 25 th  percentile temperature at a 25 th  percentile point that is halfway between the midpoint and the round bearing journal surface, the 25 th  percentile temperature being within 10% of the surface temperature as measured in the Celsius scale; and   a 75 th  percentile temperature at a 75 th  percentile point that is halfway between the midpoint and the innermost part of the core, the 75 th  percentile temperature being at least 50% of the surface temperature as measured in the Celsius scale, the midpoint temperature being at least 70% of the surface temperature as measured in the Celsius scale.   
     
     
         16 . The method of  claim 15 , wherein the midpoint temperature is in the range of 70% to 80% of the surface temperature as measured in the Celsius scale, and wherein the 75 th  percentile temperature is in the range of 60% to 70% of the surface temperature as measured in the Celsius scale. 
     
     
         17 . The method of  claim 16 , further comprising providing the crankshaft material as a carbon steel having at least 0.3 wt % carbon and an ideal critical diameter (DI) less than 1.70. 
     
     
         18 . The method of  claim 17 , wherein applying the series of inductive heating pulses comprises:
 applying a first inductive pulse having an intensity in the range of 2.0 to 2.5 J/C*mm 2 ;   applying a second inductive pulse having an intensity in the range of 2.0 to 2.5 J/C*mm 2 ;   applying a third inductive pulse having an intensity in the range of 2.0 to 2.5 J/C*mm 2 ; and   pausing between the application of each of the first, second, and third inductive pulses.   
     
     
         19 . The method of  claim 18 , further comprising applying the first inductive pulse for a first period, applying the second inductive pulse for a second period, and applying the third inductive pulse for a third period, each of the first, second, and third periods being in the range of 8 to 12 seconds, and wherein pausing includes deactivating the induction for a period between 1 and 3 seconds between the application of each of the first, second, and third inductive pulses. 
     
     
         20 . A method of induction hardening a work piece, the method comprising:
 providing a work piece formed of a work piece material and having an outer surface and a center;   applying a series of inductive heating pulses to the work piece until the workpiece has a gradual temperature profile extending normally from the outer surface to the center of the work piece, the gradual temperature profile including a surface temperature at the outer surface, the surface temperature being within a transformation range of the work piece material, the gradual temperature profile including a midpoint temperature at a midpoint between the outer surface and the center, the midpoint temperature being at least 50% of the surface temperature as measured in the Celsius scale; and   quenching the outer surface to a temperature below the transformation temperature to harden the outer surface.