Physical Metallurgy of Tool Steels

Abstract

The steels employed for various tooling purposes in performing cutting and machining operations, as well as the steels which are used in manufacturing of dies for various metal working operations like rolling, forging, wire drawing and extrusion fall under the category of tool steels. These steels are required to possess high strength, high toughness with excellent resistance to wear. In many cases of material removal operations, the steels are needed to tolerate an excessive rise in temperature at the tool stock interface; this necessitates a good high temperature property in tool steels. Tools are required to be produced with closed dimensional tolerance; so, machinability in annealed state should be quite good. In view of its required reliability in operations, a very high level of cleanliness is demanded in tool steels. For this reason, secondary steel making viz, Electro-slag refining or Vacuum-arc melting are quite often employed in the production of tool steels. To meet the requirements of tool steels, such as high hardness and high strength at high fracture toughness levels, high wear resistance and very good resistance to softening at elevated temperatures, special precautions are necessary in processing of tool steels. Tool steels available with various compositions and mechanical properties are classified into several groups like shock resistance tool steels, cold work tool steel, hot work tool steel, high speed steels, mold steels, special purpose steels, water hardening tool steels. In order to achieve the desirable microstructures, controlled thermal and mechanical treatments of tool steels are carried out; this insures the attainment of specified properties of the steels. In air hardening steels, slow and uniform heating is accomplished to dissolve the second phase particles in austenite after which the steels are cooled in still air. In most of the cases annealing is recommended for attaining the homogeneous microstructure of the steels for subsequent heat treatments. High speed steels, shock resisting tool steel, cold and hot work tool steel and most of mold steels are air hardened and hence prior normalizing is not required. The austenitising heat treatment of tool steels are extremely important so far as the extent of dissolution of carbides in austenite, grain growth and compositional homogeneity of austenite are concerned. Quenching of tool steels can be done in air, oil or water depending upon the composition of the steels. Due to high alloy content in tool steels, the quenched microstructure contains a good amount of retained austenite; the removal of retained austenite calls for multiple tempering after quenching or sub-zero treatment preceding tempering. Many of the tool steels experience a high-volume change during phase transformation; quite often this leads to objectionable distortion or warpage in heat-treated steels. Therefore, special care is warranted to control distortion in tool steels. To further enhance the wear resistance property at the surface, tool steels are subjected to plasma nitriding and, or coating of nitrides of Cr, Ti, V, etc. Tribological characteristics of tool steel can be significantly increased in TiAlN or CrN coated steels. Recent trend has indicated that vacuum heat treatment of tool steel yields better properties than the conventionally heat-treated tool steels.