Process for Heat Treatment of Steel or Cast Iron Workpieces

Abstract

A method for the heat treatment of workpieces made of steel or cast iron involves quenching the workpiece to a temperature above the martensite starting temperature (i.e., the temperature below which martensite is formed) after a holding period at or above the austenitizing temperature. In the ensuing time period, austenite is transformed into bainite. The temperature of the workpiece is reduced during the time period when transformation occurs and the transformation of austenite into bainite is thus continued.

1 . Process for heat treatment of a steel or cast iron workpiece, comprising: quenching the workpiece, after a holding interval at or above the austenitizing temperature, to a temperature above a martensite start temperature, martensite being formed at a temperature below this martensite start temperature, and in a following time interval transforming the austenite into bainite, and lowering the temperature of the workpiece during the transformation as the transformation of austenite into bainite is continued. 
     
     
         2 . Process as claimed in  claim 1 , wherein the temperature of the workpiece is lowered to values below the temperature which is achieved by quenching at least during a time portion of 20% of a duration of the transformation. 
     
     
         3 . Process as claimed in  claim 1 , wherein the temperature of the workpiece is lowered depending on the martensite start temperature. 
     
     
         4 . Process as claimed in  claim 1 , wherein the temperature of the workpiece is lowered to values above the martensite start temperature. 
     
     
         5 . Process as claimed in  claim 1 , wherein the temperature of the workpiece is lowered according to a given temperature profile. 
     
     
         6 . Process as claimed in  claim 1 , wherein the temperature of the workpiece is lowered in increments. 
     
     
         7 . Process as claimed in  claim 6 , wherein the workpiece is exposed to media of varied temperature to lower the workpiece temperature. 
     
     
         8 . Process as claimed in  claim 1 , wherein the temperature of the workpiece is continuously lowered. 
     
     
         9 . Process as claimed in  claim 8 , wherein the temperature of the workpiece is lowered according to a behavior of the martensite start temperature. 
     
     
         10 . Process as claimed in  claim 1 , wherein the temperature of the workpiece is lowered by one or more controllable facilities. 
     
     
         11 . Process as claimed in  claim 1 , wherein the workpiece is heated to a higher temperature after partial transformation of austenite into bainite in order to accelerate transformation. 
     
     
         12 . Process as claimed in  claim 1 , wherein the time interval of transformation is dimensioned such that complete transformation of austenite into bainite does not take place and the workpiece is cooled to room temperature following the transformation time interval, held briefly there and then tempered. 
     
     
         13 . Process as claimed in  claim 1 , wherein the workpiece is made from a through-hardening rolling bearing or heat treatment steel. 
     
     
         14 . Process as claimed in  claim 1 , wherein the workpiece is a rolling bearing component. 
     
     
         15 . Process as claimed in  claim 6 , wherein the workpiece is placed in baths kept at varied temperatures to lower the workpiece temperature. 
     
     
         16 . Process as claimed in  claim 1 , wherein the temperature of the workpiece is lowered by one or more controllable furnaces. 
     
     
         17 . Process for heat treating a steel or cast iron workpiece comprising:
 heating the workpiece to at least an austenitizing temperature;   quenching the workpiece which has been heated to at least the austenitizing temperature to a temperature above a martensite start temperature, the martensite start temperature being a temperature below which martensite is formed;   transforming austenite in the workpiece into bainite; and   lowering the temperature of the workpiece during transformation of the austenite into bainite.   
     
     
         18 . Process as claimed in  claim 17 , wherein the lowering of the temperature of the workpiece during transformation of the austenite into bainite comprises lowering the temperature of the workpiece to a temperature above the martensite start temperature. 
     
     
         19 . Process as claimed in  claim 17 , wherein the lowering of the temperature of the workpiece during transformation of the austenite into bainite comprises lowering the temperature of the workpiece in increments. 
     
     
         20 . Process as claimed in  claim 17 , wherein the lowering of the temperature of the workpiece during transformation of the austenite into bainite comprises continuously lowering the temperature of the workpiece.