Net or near net shape powder metallurgy process

Abstract

In a hot isostatic pressing process or hot uniaxial pressing process for producing a net or near net shape product, a diffusion filter comprising boron nitride is provided between a graphite former and metal powder to be pressed thereagainst. The diffusion filter allows a controlled amount of carbon to diffuse into the surface of the pressed component. The boron nitride is conveniently applied as an aqueous slurry by spraying. In order to obtain adherence between the coating and the surfaces of the former, one or more thin ghost coat layers of slurry are applied to the surface of the graphite former before one or more layers of normal strength slurry are applied. Each layer of coating is allowed to dry before the next layer is applied, and the former may be heated to dry each layer. Pressed components of length greater than 2m can be processed, relative contraction of the component and former during cooling of the component being accommodated by the boron nitride coating on the former. The thickness of the coating can be determined by trials to achieve a controlled diffusion of carbon into the surface of the pressed component, and the dimensions of the former chosen to accommodate that thickness of coating.

1 . A hot isostatic pressing process or hot uniaxial pressing process for producing a net or near net shape product in which a diffusion filter, comprising boron nitride is provided between a graphite former and metal powder to be pressed thereagainst. 
     
     
         2 . A process as claimed in  claim 1  in which the diffusion filter is configured to allow a controlled amount of carbon to diffuse into the surface of the pressed component. 
     
     
         3 . A process as claimed in  claim 1  in which one or more layers of a coating material comprising boron nitride are applied to the surface of the graphite former prior to metal powder being arranged next to the former. 
     
     
         4 . A process as claimed in  claim 3  in which the layer or layers are applied to the surface of the graphite former by application of a slurry of the coating material to the surface. 
     
     
         5 . A process as claimed in  claim 4  in which the slurry is an aqueous slurry. 
     
     
         6 . A process as claimed in  claim 4  in which the slurry is applied by spraying. 
     
     
         7 . A process as claimed in  claim 4  in which one or more thin ghost coat layers of slurry are applied to the surface of the graphite former before one or more layers of normal strength slurry are applied. 
     
     
         8 . A process as claimed in  claim 7  in which each layer of coating is allowed to dry/is dried before the next layer is applied. 
     
     
         9 . A process as claimed in  claim 8  in which the former is heated to dry at least one of the layers. 
     
     
         10 . A process as claimed in  claim 4  in which the metal powder is a PM Nickel based alloy and in which the layer or layers when dry are of thickness 1 μm to 2 μm. 
     
     
         11 . A process as claimed in  claim 1  used to create a pressed component of length greater than 2 m, relative contraction of the component and former during cooling of the component being accommodated by the boron nitride coating on the former. 
     
     
         12 . A process as claimed in  claim 1  in which the alloy has the following composition:
 C1.0, Cr15, Si 4.0, B3.5, Fe4.5 Ni Balance. 
 
     
     
         13 . A process as claimed in  claim 1  in which the alloy has the following composition:
 C 0.13, Si 0.20, Mn 0.50, P 0.020, S 0.020, Mo 0.18, Ni 3.40 
 
     
     
         14 . A graphite former for use in a hot isostatic pressing process or hot unixial pressing process, the former being provided with a coating on a surface thereof that presses against metal powder in use of the former, the coating comprising boron nitride to act as a diffusion filter, the thickness of the coating having been determined by trials to achieve a controlled diffusion of carbon into the surface of the pressed component. 
     
     
         15 . A component produced by a process in accordance with  claim 1  in which the surface of the component incorporates a controlled amount of carbon that has diffused into the surface from the graphite former during pressing, the carbon content of the surface of the finished component having been determined by trials of how the carbon content varies with the thickness of the boron nitride coating on the former, and by choosing the dimensions of the uncoated former in accordance with the required coating thickness and required final dimensions of the pressed component.