As the means for improving the corrosion resistance and wear resistance of a metal member constituting a machine or apparatus, there has been adopted a method in which a constituent material such as stainless steel is quenched or the surface of a metal member is modified by deposition or the like.
Instead of the above-mentioned surface treatment method, there are recently proposed various methods in which specific hard particles are added to a metal material constituting a metal member. For example, Japanese Unexamined Patent Publication No. 58-181470 proposes a method in which hard particles of WC, NbC, TiC, CrC, VC or the like are added into a matrix metal composed of an Ni-based, Co-based or Fe-based alloy or to the surface of the base metal to improve the hardness or wear resistance of the matrix metal. We confirmed that this method is effective to some extent for improving the wear resistance but the method is still unsatisfactory in that the toughness of the alloy is reduced. For example, hard particles of NbC (having a melting point of 3480.degree. C.) or TiC (having a melting point of 3180.degree. C.) have a high melting point and they are hardly fusible. Therefore, it is difficult to avoid addition of the hard particles in the unfused state. Accordingly, hard particles are left in the matrix metal in the relatively large form at the step of forming a corrosion-resistant and wear-resistant layer, and these relatively large particles cause reduction of the toughness of the metal material.
Furthermore, even if the added carbide is fused in the matrix metal, the problem of embrittlement of the material often arises. For example, if MoC or WC is fused together with an Ni-based, Co-based or Fe-based alloy, as is well-known, a brittle compound of the M.sub.6 C type is formed as a crystal. Formation of the crystal of the brittle compound does not make any contribution to improvement of the wear resistance but embrittles the matrix metal.
Japanese Unexamined Patent Publication No. 63-157796 discloses a method for the preparation of a roll for hot rolling. According to this method, it is intended to obtain a roll having a wear resistance, a crack resistance and a high toughness by adding a VC powder to a ferrous alloy. In this method, it is an indispensable requirement that the VC powder should be added in an amount of at least 10% by weight to the ferrous alloy. The object of this method is to improve the hardenability of the roll, but we confirmed that in the case where an Fe-type alloy is used as the base metal, when VC is added to this alloy, a plurality of kinds of carbides such as WC and CrC are inevitably formed in large quantities and the intended effect is not always attained. This method is irrelevant to the technique of re-crystallizing or re-depositing VC in a specific state, and the method is not effective for improving both of the wear resistance and corrosion resistance.
Furthermore, in the conventional methods, it is necessary to control the crystallization of the above-mentioned brittle compound by controlling the elevation of the temperature to a lowest level necessary for fusion bonding to the matrix metal at the addition of hard particles, and for this purpose, it is necessary to increase the particle size of the hard particles to be added more or less. However, if the particle size is increased, uniform dispersion becomes difficult and as the result, the toughness of the metal material is reduced. If coarse and distorted hard particles are present in the matrix metal, as pointed out hereinbefore, the wear resistance is reduced because these particles are readily dropped at a frictional movement.
The problem of the deviation of distribution of hard particles or falling of the hard particles caused by this deviation is made serious by the difference of the specific gravity between the matrix metal and hard particles. For example, in the case where an Fe-based alloy is used as the matrix metal and particles of Al.sub.2 O.sub.3, SiC or TiC are used as the hard particles, agglomeration of the hard particles or deviation of the distribution is especially readily caused. When Cr carbide particles are used as the hard particles, the chromium carbide re-crystallized or re-deposited at the cooling step conducted after heating agglomerates and grows into coarse carbide grains, with the result that a problem of falling of hard particles often arises.
USPTO Backgrounds
USPTO Backgrounds