Abstract:
A sinterable material for use as sealing strips for rotary piston engines, the material being based on a known mixture of iron and vanadium carbide and further including a material which will be liquid at the sintering temperature and which will wet the carbide grains and partially dissolve them to give them a rounded form in the finished strip.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of copending Application Ser. No. 424,802, filed December 14th, 1973, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a sinter material composed of a powder mixture of vanadium carbide and iron, particularly for use as sealing strips for rotary piston engines. 
     Sealing strips in pistons for rotary piston engines are subjected to rapid wear due to the very high operating pressures and temperatures, high relative speeds between sliding parts, difficult or insufficient lubrication between the contacting surfaces, and the abrasiveness of the countersurface which usually consists of an electrodeposited nickel layer with embedded silicon carbide. 
     Very hard materials are known which meet these requirements regarding wear. However, the particles in the matrix of these materials have poor stability. These particles are subject to wear and their unfavorable grain configuration causes the particles to break out during operation of the rotary piston engines. These particles then cause an abrasive action between the sealing strip and the countersurface, with the result that more particles break out and the wear is accelerated more and more. 
     On the other hand, ceramic oxide materials are known whose particle stability and wear resistance are very good. However, the tensile strength of sealing strips made of these materials is poor. As a result, the sealing strips often break through operation of the rotary piston engine, in which case, the operational dependability of the sealing strips of such engines would be no longer assured. 
     It presently appears that rotary piston engines will be used increasingly in automotive vehicles. Thesefore, any improvement in the operating reliability and durability of these engines represents a great benefit to the motoring public. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a material which can be made into sealing strips, for rotary piston engines, that have good wear resistance combined with excellent sliding behavior against nickel and a high tensile strength. 
     These and other objects are achieved according to the present invention by the provision of a sinter material based on a known powder mixture of iron or an iron alloy and vanadium carbide or vanadium carbide mixed with other carbides, as disclosed, for example, in German Pat. No. 659,534 and German Pat. No. 764,144. The material according to the present invention further includes powdered additives which are in the liquid phase at the sintering temperature. The material according to the invention can be pressed, or otherwise formed, and sintered in the usual manner. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Sinter material according to the invention is characterized in that the additive is of a composition which in the liquid state thoroughly wets the vanadium carbide during the sintering process and simultaneously modifies it by partially dissolving it. Such additive may be, for example, constituted by one of the following compounds or alloys: Fe-B, Ni-Mn-B, V-Al, Cu-Mn and/or Cu 3  P. 
     It has been found that the partial dissolution process occurring during sintering of a composition according to the invention causes the originally sharp-edged vanadium carbide crystals to be changed into rounded grains. Thus, in an engine operating with a sealing strip according to the invention, there are no sharp-edged vanadium carbide crystals to attack the countersurface and wear is reduced substantially. The accelerated wear or abrasion previously observed with other materials becomes impossible. 
     At the same time, a composition which is liquid at the sintering temperature better wets the individual vanadium carbide particles. As a result, the particles are more firmly incorporated into the matrix and individual carbide bodies no longer break out to produce the annoying abrasive wear between the sealing strip and the trochoidal countersurface. 
     Additionally, the incorporation of such substances also improves the stability and, in particular, the breaking and bending strength of the sealing ledge, eliminating one source of adverse influence on the operational dependability of a rotary piston engine. 
     It has been found that the best sealing strip characteristics are obtained with materials according to the invention in which the carbide component concentration is 15 to 45 percent, by weight. If the carbide proportion is increased beyond this value, the liquid phases, at the sintering temperature, no longer wet all grains so that not all of the carbide crystals are converted to a rounded grain form and annoying particle break-outs will occur. In addition, the flexure strength of the material decreases above this value. 
     If, however, the carbide content drops below 15 percent by weight, the wear resistance is reduced and the substance, particularly when the carbide proportion is less than 3 percent by weight, can no longer be used as a material for sealing strips. 
     In our experiments, a sintered alloy containing 22 percent by weight vanadium carbide, 4 percent by weight of material which will be in the liquid phase at sintering temperature, and the remainder iron has been found to be particularly suitable. In one version, 74 parts of iron powder, 22 parts of VC and 4 parts Ni-Mn-B are taken and thoroughly mixed in a milling process. The mixture is placed in a mold, the size of an apex seal and compressed under a pressure of 8 metric tons/cm 2 . This is followed by sintering for 1 hour at 120° C in an atmosphere of hydrogen. 
     In order to increase the wear resistance of the metallic matrix, one or a plurality of elements contributing to the hardening of the material, such as, for example, the metals chromium, molybdenum, vanadium and/or titanium may be present in the powder mixture. It is unimportant whether these metals are already alloyed to the iron powder or are added individually or together to the powder mixture in powdered form. 
     Instead of vanadium carbide, it is also possible to use mixtures of carbides of vanadium with, for example, carbides of tungsten, molybdenum, chromium, titanium or zirconium. 
     Apex seals are manufactured by the above-described process from sinter metals with the following compositions: 
     1. 75 parts of an iron powder containing 3.2% by weight of C, 2.4% by weight of Si, 2.4% by weight of Cr and 4.8% by weight of Mo; 22 parts VC and 3 parts Ni--Mn--B; 
     2. 75 parts of an iron powder with 3.2% by weight C, 2.4% by weight Si, 2.4% by weight Cr and 4.8% by weight Mo; 22 parts carbide with 50% VC and 50% WC and 3 parts Ni--Mn--B; 
     3. 65 parts of an iron powder with 3.1% by weight C, 1.8% by weight Si, 4% by weight W and 4% by weight Cr; 30 parts VC and 5 parts Ni--Mn--B; 
     4. 55 parts iron powder with 3.1% C, 1.8% by weight Si, 4% by weight W and 4% by weight Cr; 40 parts VC and 5 parts Ni--Mn-B. 
     It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.