Mechanical characteristics of tungsten-free hard alloys (review)

Abstract

Titanium carbide-or carbonitride-base tungsten-free hard alloys with a nickel-molybdenum binder, which can effectively replace tungsten alloys in metal and alloy cutting, are examined. The proper choice of the composition and grain sizes of the carbonitride component allows the bending strength of 1500 MPa and hardness of 90-91 HRA to be reached. High-temperature hardness and fracture toughness are of the same order as those of hard tungsten alloys.Sintered WC-Co tungsten alloys with their unique physicomechanical characteristics find application as a tool material for a multiplicity of uses: mining, metal and nonmetal cutting, wood working, pressing etc.However, late in the 1960s and early in the 1970s for lack of tungsten and cobalt, a number of titanium carbide-and carbonitride-base tungsten-free hard alloys with a nickel-molybdenum binder was developed and introduced into full-scale practice. Although tools from these alloys were coming into widespread use, mainly in metal working, the scope of their application was restrained by high instability of item properties. A deeper insight into the relation between the structure of tungsten-free hard alloys and their mechanical characteristics can contribute to clarifying this issue, which would favor the development and improvement of their production techniques.The present review summarizes data on this relation for major tungsten-free hard alloys produced in Ukraine and Russia.The choice of tungsten-free hard alloys for making different tools is based on their physicomechanical and electromagnetic properties. In particular, the mechanical properties are a decisive factor for cutting and noncutting shaping applications. The thermal characteristics are responsible for the thermocyclic load resistance and solderability. The data on major tungsten-free hard alloys (Tables 1 and 2) [1] suggest that a relatively low heat conduction controls the soldering behavior on fastening cutting tools in a steel holder. Due to their high linear expansion coefficient, they can be employed for gauges used in measuring the steel component geometry.The mechanical characteristics of tungsten-free hard alloys are somewhat lower than those of tungsten and titanium-tungsten ones. However, the bending strength R bm of some KNT16 alloy lots can be higher as compared to that of a T15K6 alloy. This value varies with test temperatures: it decreases up to 873 K, then increases up to 1073 K [1]. Tungsten-free hard alloys display a relatively wide scatter in the data on mechanical characteristics, which is likely to be explained by the inhomogeneous structure of prepared specimens.Transmission electron microscopy revealed [2-4] quite a different structure of TN20 alloy lots, produced by the same technology. This alloy also possesses different strength properties. In particular, R bm for some lots reaches 1400-1600 MPa [3]. It was established [5, 6] that grain forms varied from round to acute-angled, with Va carbides introduced into tungsten-free hard alloys. For one alloy, a change in the "ring" ...