Nitrogen assisted hardening of an article

Abstract

A method of heat-treating an article is described. The method includes the steps of depassivating the article at a temperature in a range from about 200 degrees Centigrade to about 300 degrees centigrade, at a pressure in a range from about 0.1 millibar to about 10 millibar in an atmosphere comprising hydrogen and an inert gas, to produce a hot article, heating the hot article to a temperature in a range from about 300 degrees centigrade to about 400 degrees centigrade, at a pressure in a range from about 1 millibar to about 10 millibar in an atmosphere comprising hydrogen, inert gas, nitrogen and a hydrocarbon to produce a preliminary heat-treated article, and maintaining the preliminary heat-treated article at a temperature in a range from about 300 degrees centigrade to about 400 degrees centigrade at a pressure in a range from about 1 millibar to about 10 millibar in an atmosphere comprising hydrogen, inert gas, nitrogen and a hydrocarbon, for a time period in a range from about 10 hours to about 30 hours, to produce the heat-treated article. The heat-treated article retains the appearance of the article prior to heat-treatment.

What is claimed is: 
     
         1 . A method of heat-treating an article, comprising:
 depassivating the article at a temperature in a range from about 200 degrees Centigrade to about 300 degrees centigrade, at a pressure in a range from about 0.1 millibar to about 10 millibar in an atmosphere comprising hydrogen and an inert gas, to produce a depassivated article;   heating the depassivated article to a temperature in a range from about 300 degrees centigrade to about 400 degrees centigrade, at a pressure in a range from about 1 millibar to about 10 millibar in an atmosphere comprising hydrogen, inert gas, nitrogen and a hydrocarbon to produce a preliminary heat-treated article; and,   maintaining the preliminary heat-treated article at a temperature in a range from about 300 degrees centigrade to about 400 degrees centigrade at a pressure in a range from about 1 millibar to about 10 millibar in an atmosphere comprising hydrogen, inert gas, nitrogen and a hydrocarbon, for a time period in a range from about 10 hours to about 30 hours, to produce the heat-treated article, wherein the heat-treated article retains the appearance of the article prior to heat-treatment.   
     
     
         2 . The method as claimed in  claim 1 , wherein the article is a stainless steel article. 
     
     
         3 . The method as claimed in  claim 1 , wherein the article is a double ferrule. 
     
     
         4 . The method as claimed in  claim 1 , wherein the hydrocarbon is acetylene. 
     
     
         5 . The method as claimed in  claim 1 , wherein the temperature of the heating step is in a range from about 350 degrees centigrade to about 400 degrees centigrade. 
     
     
         6 . The method as claimed in  claim 1 , wherein the temperature of the maintaining step is in a range from about 350 degrees centigrade to about 400 degrees centigrade. 
     
     
         7 . The method as claimed in  claim 1 , wherein the depassivating step is carried out in an atmosphere comprising
 about 100 standard cubic centimeters per minute to about 150 standard cubic centimeters per minute of hydrogen, and   about 100 standard cubic centimeters per minute to about 150 standard cubic centimeters per minute of inert gas.   
     
     
         8 . The method as claimed in  claim 1 , wherein the heating step is carried out in an atmosphere comprising
 about 180 standard cubic centimeters per minute to about 250 standard cubic centimeters per minute of nitrogen;   about 10 standard cubic centimeters per minute to about 20 standard cubic centimeters per minute of hydrocarbon;   about 75 standard cubic centimeters per minute to about 1000 standard cubic centimeters per minute of hydrogen; and   about 35 standard cubic centimeters per minute to about 50 standard cubic centimeters per minute of inert gas.   
     
     
         9 . The method as claimed in  claim 1 , wherein the maintaining step is carried out in an atmosphere comprising
 about 180 standard cubic centimeters per minute to about 250 standard cubic centimeters per minute of nitrogen;   about 10 standard cubic centimeters per minute to about 20 standard cubic centimeters per minute of hydrocarbon;   about 75 standard cubic centimeters per minute to about 1000 standard cubic centimeters per minute of hydrogen; and   about 45 standard cubic centimeters per minute to about 75 standard cubic centimeters per minute of inert gas.