Diffusion promoters for low temperature case hardening

Abstract

Low temperature carburization of a workpiece surface is accomplished faster by impregnating the surface with a diffusion promoter prior to or during the low temperature carburization process.

1 . A metal product produced by subjecting a metal workpiece to a low temperature diffusion-based surface treatment to produce an altered surface layer containing an increased concentration of a diffusing element in an amount sufficient to alter the properties of the metal forming the surface layer, the altered surface layer also being free of precipitates of compounds of the diffusing element, wherein the altered surface layer further contains an increased concentration of at least one promoter element capable of enhancing the rate at which the diffusing element diffuses into the surface of the workpiece to be altered during the low temperature diffusion-based surface treatment. 
   
   
       2 . The metal product of  claim 1 , wherein the metal workpiece is made from a carbon or low alloy steel, a nickel-based alloy, a chromium-steel alloy, a titanium based alloy or an aluminum-based alloy. 
   
   
       3 . The metal product of  claim 2 , wherein the altered surface layer is produced by the low temperature carburization of a stainless steel workpiece to produce an altered surface layer containing interstitially diffused carbon atoms, the altered surface layer being harder than the base metal from which the product is formed and free of carbide precipitates. 
   
   
       4 . The metal product of  claim 2 , wherein the altered surface layer is produced by carburizing, nitriding or carbo-nitriding a carbon or low alloy steel, a chromium-steel alloy, stainless steel, or a nickel-based alloy. 
   
   
       5 . The metal product of  claim 2 , wherein the altered surface layer is produced by carburizing, nitriding or carbo-nitriding a titanium-based alloy. 
   
   
       6 . The metal product of  claim 2 , wherein the altered surface layer is produced by infusing atomic nitrogen, carbon, boron or mixtures of these elements into an aluminum-based alloy. 
   
   
       7 . A low temperature carburized stainless steel product having an altered surface layer which (a) contains an increased concentration of carbon atoms relative to the base metal from which the product is made, (b) is harder than the base metal from which the product is made, (c) is free of carbide precipitates, and (d) contains an increased concentration of at least one promoter element selected from Mo, Ni, Cr, Ti, V and Nb relative to the base metal from which the product is made. 
   
   
       8 . The process of  claim 7 , wherein the stainless steel product is made from an AISI 400 series stainless steel. 
   
   
       9 . A process for enhancing the rate at which a diffusing element diffuses into the surface of a metal workpiece in a low temperature diffusion-based surface treatment to produce an altered surface layer containing an increased concentration of the diffusing element in an amount sufficient to alter the properties of the metal forming the surface layer, the altered surface layer also being free of precipitates of compounds of the diffusing element,
 the process comprising treating the workpiece to increase the concentration of at least one promoter element in the surface to be altered, the promoter element being capable of enhancing the rate at which the diffusing element diffuses into the surface of the workpiece to be altered during the low temperature diffusion-based surface treatment.   
   
   
       10 . The process of  claim 9 , wherein the concentration of the promoter element in the surfaces to be altered is increased
 (a) by applying a coating containing the promoter element, or a compound capable of decomposing to yield the promoter element, to these surfaces and then heating the applied coating to drive the diffusing element into these surfaces, or   (b) by a gas phase diffusion process in which the promoter element, or a compound capable of decomposing to yield this element, is contacted with the workpiece surface in the form of a gas, or   (c) by contacting the workpiece with a molten bath of a fluoride salt containing the promoter element, or   (d) by treating the surfaces to be altered to make them porous, impregnating these porous surfaces with the promoter element, or a compound capable of decomposing to yield this element, and then heating these impregnated surfaces to elevated temperature to promote diffusion of the promoter element into these surfaces.   
   
   
       11 . The process of  claim 9 , wherein the metal workpiece is made from a carbon or low alloy steel, a nickel-based alloy, a chromium-steel alloy, a titanium based alloy or an aluminum-based alloy. 
   
   
       12 . A process for altering the physical properties of the surface layer of a metal workpiece by a low temperature diffusion-based surface treatment in which the metal workpiece is contacted with a gas containing a diffusing element at a treatment temperature which is high enough to cause the diffusing element to diffuse into the workpiece surfaces thereby producing an altered surface layer, the treatment temperature also being low enough to prevent formation of precipitates of compounds of the diffusing element,
 wherein the workpiece is treated to increase the concentration of at least one promoter element in the surface to be altered, the promoter element being capable of enhancing the rate at which the diffusing element diffuses into the surface of the workpiece to be altered during the low temperature diffusion-based surface treatment.   
   
   
       13 . The process of  claim 12 , wherein the low temperature diffusion-based surface treatment comprises carburizing, nitriding or carbo-nitriding a titanium-based alloy. 
   
   
       14 . The process of  claim 12 , wherein the low temperature diffusion-based surface treatment comprises infusing atomic nitrogen, carbon, boron or mixtures of these elements into an aluminum-based alloy. 
   
   
       15 . The process of  claim 12 , wherein the low temperature diffusion-based surface treatment comprises carburizing, nitriding or carbo-nitriding an iron-based alloy, a chromium-based alloy or a nickel-based alloy. 
   
   
       16 . The process of  claim 15 , wherein a stainless steel workpiece is low temperature carburized to produce an altered surface layer containing interstitially diffused carbon atoms, the altered surface layer being harder than the base metal from which the product is formed and free of carbide precipitates. 
   
   
       17 . The process of  claim 12 , wherein the workpiece is treated to increase the concentration of at least one promoter element in the surface to be altered prior to the start of the low temperature diffusion based surface treatment. 
   
   
       18 . The process of  claim 12 , wherein the workpiece is treated to increase the concentration of at least one promoter element in the surface to be altered during the low temperature diffusion based surface treatment by interrupting this surface treatment, treating the workpiece to increase the concentration of at least one promoter element in the surface to be altered during this interruption, and thereafter resuming the low temperature diffusion based surface treatment.