1. FIELD OF THE INVENTION
The present invention relates to a forging die excellent in durability and usable for semi-hot working or hot working.
2. DISCUSSION OF BACKGROUND
In forging steel workpieces under a semi-hot or hot condition, a fast precise forging method has been employed in order to reduce finishing steps after the forging. In the forging method of this kind, since a product after forging is required to have highly accurate dimensions, tool steel for hot usage or a cemented carbide (WC-Co hard material) having an excellent strength upto a high working temperature and hardwearing properties is used for the forging die.
However, high speed steel or the tool steel for hot usage had such problems that the hardness and strength of the material were insufficient in the high working temperature and heat cracking or deformation was apt to occur. Accordingly, the service life of the forging die was too short to be processed with high accuracy.
In a case of using a single cylinder type forging die made of cemented carbide having insufficient toughness, there was a problem that cracking occurred in the die at the initial stage. In order to eliminate the problem, a forging die comprising an inner cylinder made of a cemented carbide and an outer cylinder made of steel wherein the inner cylinder and the outer cylinder are in interference fit by means of shrink fitting or press fitting is used. Such construction prevents the occurrence of cracking and the hardwearing properties can be assured by the surface of the inner cylinder made of the cemented carbide having high hardness.
However, the forging die in which the inner cylinder of cemented carbide is used, still has problems of cracking especially at a diameter reducing portion (a land portion) having a large reducing taper and surface roughening due to the peeling-off of the surface layer. As a result, the shape of workpieces became irregular or flaws were introduced in them and the yield rate of the workpieces was reduced.
In order to solve these problems, an improvement of the cemented carbide itself has been proposed. For instance, there has been proposed a material having improved strength upto a high working temperature and toughness by coexisting a metallic carbide such as TaC, NbC or the like or a complex ceramic phase such as (W,Ta)C with the WC ceramic phase, or by alloying Ni or Cr in the metal phase of Co.
However, in the forging die having the interference-fit structure of the inner cylinder of a cemented carbide and the outer cylinder of steel, when a working temperature exceeds 500.degree. C., the WC ceramic phase is oxidized resulting in surface roughening. Further, the effect of the interference fit is reduced by the difference of the thermal expansion coefficients between the inner and the outer cylinders, and conspicuous cracking may result by the reduction of the strength of the cemented carbide material.