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This modification process provides a method to a soft layer on surface of sintered material only where it is necessary for machining without deteriorating the material's strength properties. The soft layer is created on the objected surface through permeating aqueous solution of boric oxide to prevent the diffusion of graphite during sintering. Since the retained graphite is dispersed in the pore of soft ferrite matrix and gives a lubrication action during machining, it was confirmed that the machinability of developed material was greatly improved in compared to the material without the modification.

The demands on valve seat insert materials, in terms of providing greater wear-resistance at higher temperatures, enhanced machinability and using non-environmentally hazardous materials at a reasonably low cost have intensified in recent years. Due therefore to these strong demands in the market, research was made into the possibility of producing a new valve seat insert material. As a result a high speed steel based new improved material was developed, which satisfies the necessary required demands and the evaluation trials, using actual gasoline engine endurance tests, were found to be very successful.

Recently, sintered high speed steel has attracted great attention for superior performance over that of conventional wrought high speed steel. Sintered high speed steel is also widely being used for not only cutting tools but also wear resistant parts. Two of the most popular methods of producing high speed steel are the HIP process and the billet process. In order though, to save material loss and reduce labor costs, we selected water atomized M2 class alloy powder, applying the preform process as the main part of our experiments at Hitachi Powdered Metals Co., Ltd.