Sliding camshaft and method of manufacturing

Abstract

A sliding camshaft and a method of manufacturing a lobe pack for a sliding camshaft are provided. The lobe pack is formed from a steel alloy having a carbon content in the range of 0.48-0.53 wt %. The lobe pack is heated to a predetermined austenitizing temperature that is below a carburizing temperature of the steel alloy for a time period at least long enough to heat the entire lobe pack to the predetermined austenitizing temperature. The lobe pack is quenched to a martempering temperature that is greater than a martensite start temperature and within 50 degrees Celsius of the martensite start temperature. The lobe pack is held at the martempering temperature for a martempering time period that is long enough to cool the entire lobe pack to the martempering temperature. The lobe pack may have a hardness in the range of 56 to 60 HRC throughout the lobe pack.

What is claimed is: 
     
         1 . A method of manufacturing a lobe pack for a sliding camshaft of an internal combustion engine, the method comprising:
 providing the lobe pack formed from a steel alloy having a carbon content in the range of 0.48 to 0.53 weight percent;   heating the lobe pack to a predetermined austenitizing temperature that is below a carburizing temperature of the steel alloy for a heating time period, the heating time period being at least long enough to heat the entire lobe pack to the predetermined austenitizing temperature;   quenching the lobe pack to a martempering temperature, the martempering temperature being greater than a martensite start temperature and within 50 degrees Celsius of the martensite start temperature; and   holding the lobe pack at the martempering temperature for a martempering time period, the martempering time period being at least long enough to cool the entire lobe pack to the martempering temperature.   
     
     
         2 . The method of  claim 1 , wherein the step of quenching is performed directly after the step of heating without heating the lobe pack above the carburizing temperature of the steel alloy. 
     
     
         3 . The method of  claim 1 , further comprising providing the steel alloy having:
 a manganese content in the range of 0.70 to 0.90 weight percent;   a silicon content in the range of 0.15 to 0.35 weight percent; and   a chromium content in the range of 0.70 to 0.90 weight percent.   
     
     
         4 . The method of  claim 1 , further comprising providing the steel alloy as a 5150 steel alloy. 
     
     
         5 . The method of  claim 4 , the martempering temperature being in the range of about 285 to about 330 degrees Celsius. 
     
     
         6 . The method of  claim 5 , wherein the predetermined austenitizing temperature is at least 800 degrees Celsius. 
     
     
         7 . The method of  claim 6 , wherein the carburizing temperature of the steel alloy is at least 900 degrees Celsius. 
     
     
         8 . The method of  claim 7 , wherein the martempering time period is at least 25 minutes. 
     
     
         9 . The method of  claim 8 , wherein the heating time period is at least 45 minutes. 
     
     
         10 . The method of  claim 9 , wherein the steps of quenching the lobe pack to the martempering temperature and holding the lobe pack at the martempering temperature includes submersing the lobe pack within a salt bath. 
     
     
         11 . The method of  claim 9 , further comprising cooling the lobe pack to an ambient temperature after holding the lobe pack at the martempering temperature for the martempering time period. 
     
     
         12 . A sliding camshaft for an internal combustion engine, the sliding camshaft comprising:
 an inner rod extending along a cam axis;   a lobe pack circumferentially disposed about the inner rod, the lobe pack having an inner side disposed in sliding contact with the inner rod and an outer surface bearing a plurality of cam lobes, the lobe pack being formed of a steel alloy having a carbon content in the range of 0.48 to 0.53 weight percent, wherein a majority of a microstructure of the lobe pack is martensite, the lobe pack having a hardness of at least 56 HRC on the Rockwell hardness scale through a depth of the lobe pack.   
     
     
         13 . The sliding camshaft of  claim 12 , the steel alloy further comprising:
 manganese in the range of 0.70 to 0.90 weight percent;   silicon in the range of 0.15 to 0.35 weight percent; and   chromium in the range of 0.70 to 0.90 weight percent.   
     
     
         14 . The sliding camshaft of  claim 13 , further comprising:
 phosphorus in an amount not exceeding 0.030 weight percent; and   sulfur in an amount not exceeding 0.040 weight percent.   
     
     
         15 . The sliding camshaft of  claim 14 , wherein the steel alloy is a 5150 steel alloy. 
     
     
         16 . The sliding camshaft of  claim 15 , wherein the lobe pack has a hardness of no greater than 60 HRC on the Rockwell hardness scale through the depth of the lobe pack. 
     
     
         17 . The sliding camshaft of  claim 16 , the lobe pack being hardened via martempering by raising the lobe pack to a predetermined austenitizing temperature that is below a carburizing temperature of the steel alloy and then quenching the lobe pack to a martempering temperature that is above a martensite start temperature. 
     
     
         18 . The sliding camshaft of  claim 12 , wherein the lobe pack is formed by:
 heating the lobe pack to a predetermined austenitizing temperature that is below a carburizing temperature of the steel alloy for a heating time period, the heating time period being at least long enough to heat the entire lobe pack to the predetermined austenitizing temperature;   quenching the lobe pack to a martempering temperature, the martempering range being greater than a martensite start temperature and within 50 degrees Celsius of the martensite start temperature; and   holding the lobe pack at the martempering temperature for a martempering time period, the martempering time period being at least long enough to cool the entire lobe pack to the martempering temperature.   
     
     
         19 . The sliding camshaft of  claim 18 , wherein the step of quenching is performed directly after the step of heating without heating the lobe pack above the carburizing temperature of the steel alloy, wherein the lobe pack is further formed by cooling the lobe pack to an ambient temperature after holding the lobe pack at the martempering temperature for the martempering time period. 
     
     
         20 . The sliding camshaft of  claim 19 , the martempering temperature being in the range of about 285 to about 330 degrees Celsius, the predetermined austenitizing temperature being at least 800 degrees Celsius, the carburizing temperature of the steel alloy being at least 900 degrees Celsius, the martempering time period being at least 25 minutes, the heating time period being at least 45 minutes.