Modern trends in increasing the quality of the steels intended for cutting and metal-working tools: II. Spray forming of billets

Abstract

Tools for cold forming of materials are used to produce articles made of metals and ceramic materials by cold drawing, embossing, drifting, forging, knurling a thread, and so on. Apart from a high strength, the tool material in this case must also have high abrasive and adhesive wear resistance. When a tool interacts with high-strength abrasive particles, abrasive wear takes place: it is characteristic of processing of ceramics, when high-strength ceramic particles penetrate into a meat surface and form microgrooves in it. In this case, large carbide particles present in a metal hinder this process better than small carbide particles [1]. Adhesive wear takes place when metal is in contact with a metal: microbonds appear between the tool and processed-metal surfaces at contact microareas because of high pressures, and metallic microparticles are torn out from the tool. Adhesive wear can combine with abrasive wear, which is related to the presence of ceramic particles in the metal to be processed. Therefore, high-hardness carbides (VC etc.) in a tool intended for cold metal and ceramic working without cutting should provide an optimum combination of hardness and abrasive wear resistance. In this case, large carbides in a tool metal are favorable for wear as compared to small carbides. However, as the carbide size increases, the metal hardness decreases. In other words, neither a traditional “ingot” metal with large carbides and a low hardness nor a powdered (granulated) metal with a high hardness and small carbides can provide an optimum combination of the functional properties of a certain tool. Therefore, in the last decade, metallurgists have focused their attention on the production of ingots by a nontraditional spray deposition method [2], namely, a spray forming (SF) process. Specifically, they use spray forming to produce a radically new type of ingots, i.e., deposited ingots. The quality of the SF metal is so high that this process is now one of the most dynamically developed high-quality metallurgy processes all over the world. The SF process is used to produce ingots from various steels and alloys, as well as from nonferrous metals. This process has attracted the particular attention of the manufacturers of tool cold-working metal, since it can form spherical primary carbides whose sizes are smaller than those in the metal of traditional ingots and large than those in the metal of gas-sprayed powders. In other words, the SF metal has the optimum combination of hardness and wear resistance that cannot be achieved by other methods of producing a tool metal. The SF process is an improved Osprey process [2, 3], which was designed at Osprey Metals Ltd. (Great Britain). In the course of the conventional Osprey process, a metal‐gas torch is directed into a mold during gas spraying, where semiliquid metal drops solidify layer-by-layer and form an ingot. The mold is fixed in this process. In the SF process, a mold or a crystallizer is absent. A metal‐gas torch is directed onto a gas-cooled rotating metallic disk, which serves as a seed for freezing (deposition) of an ingot. The seed is rotated and pulled down at a speed so that the distance between the spraying focus and the center of the deposited surface is unchanged. Although the rate of rotation was indicated, we can assume by analogy with a two-electrode VADER process that it ranges from 80 to 120 rpm.