Chemical activation of self-passivating metals

Abstract

A method for treating a workpiece made from a self-passivating metal and having a Beilby layer is disclosed. The method comprises exposing the workpiece to the vapors produced by heating a reagent having a guanidine [HNC(NH2)2] moiety and complexed with HCl to activate the workpiece for low temperature interstitial surface hardening.

We claim: 
     
         1 . A mixture of a first reagent and a second reagent for activating and/or hardening of an alloy, wherein the mixture forms an azeotrope of the first and second reagents and wherein at least one of the reagents includes a guanide-containing reagent. 
     
     
         2 . The mixture of  claim 1  having an evaporation point that is lower than the evaporation point of the first reagent. 
     
     
         3 . The mixture of  claim 2  wherein at least one of the first and second reagents comprises melamine. 
     
     
         4 . The mixture of  claim 3  wherein at least one of the first and second reagents comprises at least one of Biguanide HCl, Dimethylbiguanide HCl, and Guanidine HCl. 
     
     
         5 . The mixture of  claim 1  wherein a weight ratio of the first reagent to the second reagent in the mixture is one of 5 to 95%, 10 to 90%, 25 to 75%, and 50 to 50%. 
     
     
         6 . The mixture of  claim 1  wherein:
 the mixture is formed by fusing or melting the first and second reagent below a boiling point of the first reagent and a boiling point of the second reagent; and 
 the mixture further includes a petroleum distillate and the petroleum distillate is evaporated leaving a dry mixture of the first and second reagents. 
 
     
     
         7 . A method for treating a workpiece made from a self-passivating metal and having a Beilby layer comprising:
 exposing the workpiece, at an exposing temperature below a temperature at which coarse nitride and/or coarse carbide precipitates form in the workpiece, to vapors produced by heating one or more non-polymeric N/C/H compounds to activate the workpiece for low temperature interstitial surface hardening, wherein the one or more N/C/H compounds:
 (a) is solid or liquid at 25° C. and atmospheric pressure; 
 (b) has a molecular weight of ≤5,000 Daltons; and 
 (c) can be either uncomplexed or complexed with a hydrohalide acid, and further wherein:
 (i) if the non-polymeric N/C/H compound is uncomplexed, any halogen atoms replace one or more labile hydrogen atoms of the non-polymeric N/C/H compound, and 
 (ii) if the non-polymeric N/C/H compound is complexed, any halogen atoms form a part of the hydrohalide complexing acid. 
 
   
     
     
         8 . The method of  claim 7 , wherein at least one of:
 the exposing temperature is between 500-700° C.;   the non-polymeric N/C/H compound has a molecular weight of ≤500 Daltons; and   an exposing time is 1 hour or less.   
     
     
         9 . The method of  claim 7 , wherein the self-passivating metal comprises at least one of:
 a titanium-based alloy;   an iron-based, nickel-based, cobalt based or manganese-based alloy which contains at least 10 wt. % Cr; and   a stainless steel containing 10 to 40 wt. % Ni and 10 to 35 wt. % Cr.   
     
     
         10 . The method of  claim 7 , wherein the exposing temperature is about 600° C. or less. 
     
     
         11 . The method of  claim 7 , wherein the exposing temperature is about 550° C. or less. 
     
     
         12 . A method for treating a workpiece made from self-passivating metal and having a Beilby layer comprising:
 exposing the workpiece to the vapors produced by heating a reagent having a guanidine [HNC(NH 2 ) 2 ] moiety and complexed with HCl to activate the workpiece for low temperature interstitial surface hardening;   applying a heating protocol during the exposing that ramps from a lower to a higher temperature during the exposing to enhance decomposition of the reagent and/or surface harden the workpiece, wherein the heating profile is as follows:
 maintain a temperature of substantially 500° C. for approximately 15 minutes; 
 ramp the temperature from approximately 500° C. to approximately 480° C.; 
 maintain a temperature of 480° C. for approximately 15 minutes; 
 ramp the temperature from approximately 480° C. to approximately 470° C.; and 
 maintain a temperature of 470° C. for approximately 30 minutes. 
   
     
     
         13 . A workpiece made according to a method of  claim 7 .