Selective hardening treatments

Selective hardening treatments, for example, flame hardening... 

Surface hardening involves selective hardening of just the surface of a part or changing the composition of the surface by diffusing a small atom into the surface (usually C or N), followed by heat treatment of the surface. An example is case hardening of steel by increasing the carbon content of the surface by heating the part for about one hour in contact with some source of carbon (such as methane gas) followed by heat treatment. 

Use for heat treating is also small compared to the past. It has been supplanted to a large extent by furnaces using special atmospheric conditions. Heat treatment salts containing sodium cyanide are used for small metal parts when selective case hardening is required. 

Hardness as resistance of a material against penetration of another body marks a decisive material characteristic. The hardness of important construction materials can be influenced or selectively set by special hardening processes. Hardening is based upon different principles, e.g., the formation of martensite in steels as a result of thermal treatment For nonferrous metals, precipitatiOTi hardening plays an important role. Alloying elements are deposited by a multilevel thermal process. Their phase boundaries and size influence the increase of hardness and stability decisively. This attribute improvement is based upon the hindrance of the motion of dislocation (Bargel and Schulze 1988). 

It is known that after high-temperature treatment silsesquioxanes obtain a structure, close to inorganic glass with spheres of regulation due to formation of three-dimension siloxane cubic structures and selective sorption of one of the composite elements is possible on the filler surface in the hardening composite [5]. 

Fig. 6 Hardness profile of Tl-22V-4AI after various surface treatments. Because the cold-forged TI-22V-4AI beta alloy was being evaluated as a candidate material for mass-produced valve-spring retainers, oxygen hardening in air was ultimately selected. Source M. Mushiake etal., Development of Titanium Valve Spring Retainers, SAE Technicai Paper Series No. 910428, SAE, 1991...

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