Carburizing austempering process

Abstract

A novel combination of heat treatment steps includes the steps of carburizing a component fabricated of a medium carbon alloy steel at an elevated temperature for between three and six hours, subjecting the component to an austempering bath and holding it there for between fifteen and two hundred forty minutes and finally cooling the component to room temperature to allow martensitic transformation. These steps may be followed with cryogenic treatment to reduce retained austenite if needed. The process produces components with low distortion, high surface hardness, from HRC 56 to 62, and high surface compressive residual stress.

What is claimed is: 
     
         1 . A heat treatment process for a metal component, comprising the steps of:
 carburizing the metal component at an elevated temperature for between three and six hours,   subjecting the component to an austempering bath for between fifteen minutes and four hours, and   cooling the component to room temperature thereby allowing martensitic transformation.   
     
     
         2 . The heat treatment process of  claim 1 , wherein the carburizing step is achieved by one of gas carburizing, low pressure carburizing process and carbonitriding. 
     
     
         3 . The heat treatment process of  claim 1 , wherein the austempering step is achieved in an oil or salt bath at a temperature of between 250° C. and 450° C. 
     
     
         4 . The heat treatment process of  claim 1  wherein the cooling step is achieved by exposing the component to atmospheric air. 
     
     
         5 . The heat treatment process of  claim 1 , further including the step of subjecting the component to a cryogenic bath to cool the component to a temperature of the cryogenic bath. 
     
     
         6 . The heat treatment process of  claim 5  wherein the cryogenic bath is liquid nitrogen. 
     
     
         7 . The heat treatment process of  claim 1  wherein the metal component is fabricated of SAE 4340 steel. 
     
     
         8 . A heat treatment process for a medium carbon alloy steel component, comprising the steps of:
 carburizing the medium carbon alloy steel component at an elevated temperature for between three and six hours,   subjecting the component to an austempering bath for between fifteen minutes and four hours,   cooling the component to room temperature thereby allowing martensitic transformation, and   subjecting the component to a cryogenic bath to cool the component to a temperature of the cryogenic bath.   
     
     
         9 . The heat treatment process of  claim 8 , wherein the carburizing step is achieved by one of gas carburizing, low pressure carburizing process and carbonitriding. 
     
     
         10 . The heat treatment process of  claim 8 , wherein the austempering step is achieved in one of an oil bath and salt bath at a temperature of between 250° C. and 450° C. 
     
     
         11 . The heat treatment process of  claim 8  wherein the cooling step is achieved by exposing the component to atmospheric air. 
     
     
         12 . The heat treatment process of  claim 8  wherein the medium carbon alloy steel component is fabricated of SAE 4340 steel. 
     
     
         13 . A heat treatment process for a medium carbon alloy steel component, comprising the steps of:
 carburizing a medium carbon alloy steel component at an elevated temperature for between three and six hours to produce a layer near a surface of the component having between 0.7% and 1.2% carbon,   subjecting the component to an austempering bath for between fifteen minutes and four hours to produce bainite and retained austenite on and near the surface of the component, and   cooling the component to room temperature thereby allowing martensitic transformation.   
     
     
         14 . The heat treatment process of  claim 13 , wherein the carburizing step is achieved by one of gas carburizing, low pressure carburizing process and carbonitriding at a temperature of between 850° C. and 1200° C. 
     
     
         15 . The heat treatment process of  claim 13 , wherein the austempering step is achieved in an oil or salt bath at a temperature of between about 250° C. and 450° C. 
     
     
         16 . The heat treatment process of  claim 13  wherein the cooling step is achieved by exposing the component to atmospheric air. 
     
     
         17 . The heat treatment process of  claim 13  wherein the medium carbon alloy steel component is fabricated of SAE 4340 steel. 
     
     
         18 . The heat treatment process of  claim 13 , further including the step of subjecting the component to a cryogenic bath to cool the component to the temperature of the cryogenic bath.