Machinery's Handbook, 31st Edition
Tool Steels 433 Table 1. Relative Grindability of Selected Types of Frequently Used Tool Steels AISI Tool Steel Type H41 H42 H43 Other H D2 D3 D5 D7 A Types O Types L Types F Types Relative Grindability Index B BBABBBCA A A B High-Speed Tool Steel Type M1 M2 M3 (1) M3 (2) M4 M7 M8 M10 M15 M36 M43 T1 T2 T3 T5 T6 T15 Relative Grindability Index ABCCDBA B D B B ABCBBD Hot Hardness: This property designates the steel’s resistance to the softening effect of elevated temperature. This characteristic is related to the tempering temperature of the type of steel, which is controlled by various alloying elements, such as tungsten, molybde num, vanadium, cobalt, and chromium. Hot hardness testing is typically performed using a Rockwell or Vickers hardness tester. Hardness is measured over time, with the samples at high temperature (often 2100°F or 1200°C). Hot hardness is a necessary property of tools used for hot-work, like forging, casting, and hot extrusion. Hot hardness is also important in cutting tools operated at high-speed, which generate sufficient heat to raise their temperature well above the level where ordinary steels lose their hardness; hence the designation high-speed steels , which refers to a family of tool steels developed for use at high cutting speeds. Frequently it is the degree of the tool steel’s resistance to softening at elevated temperature that governs important process data, such as the applicable cutting speed. In the ratings of Table 6 through Table 11, tool steel types having the highest hot hardness are marked with A. Subsequent letters express gradually decreasing capacity to endure elevated temperature without losing hardness. Wear Resistance: The gradual erosion of the tool’s operating surface, most conspicu ously occurring at the exposed edges, is known as wear. Resistance to wear prolongs the useful life of the tool by delaying the degradation of its surface through abrasive contact with the work at regular operating temperatures; these temperatures vary according to the type of process. Wear resistance is observable experimentally and measurable by comparison based on a specific and unchanging set of test parameters. As a result, these ratings must be used with caution. Certain types of metallic carbides embedded into the steel matrix are considered to be the prime contributing factors to wear resistance, besides the hardness of the heat-treated steel material. The ratings of Table 6 through Table 11, starting with A for the best to E for poor, are based on conditions thought to be normal in operations for which various types of tool materials are primarily used. Toughness: Toughness is amount of energy a material can absorb without fracture. For tool steels, this is the material’s ability to withstand shocks, major impacts, and sudden loading and unloading. Steels used for making tools also must be able to absorb such forces with a minimum of elastic deformation and without permanent deformation that would interfere with proper functioning of the tool. Certain types of tool steels, particularly those with high carbon content and without beneficial alloying constituents, tend to be the most sensitive to shocks. They can be made functionally tougher when they are used in tools that permit a hardened case to be supported by a soft core. Also, tempering improves toughness, while generally reducing hardness. In Table 6 through Table 11, an A rating is for steels with the highest toughness, while an E rating is for steels that are the most sensitive to shocks. These ratings apply to tools heat- treated to hardness values normally used for the particular type of tool steel. Such relative ratings often are used when comparing materials, but toughness also is a measurable property with units of energy. Results of standardized testing methods can be affected by material variations and may not represent performance in real-world applications. Common Tool Faults and Failures.— The proper selection of the steel grade used for any particular type of tool is of great importance, but it should be recognized that many of the failures experienced in common practice originate from causes other than those related to the tool material.
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