Machinery's Handbook, 31st Edition
PRODUCTION FORMS OF Tool Steels 457 Hot-Rolled Plates and Sheets, and Cold-Rolled Strips: Such forms of tool steel are gen erally specified for the high-volume production of specific tool types. Tool Bits: These pieces are semifinished tools and are used by clamping in a tool holder or shank in a manner permitting ready replacement. Tool bits are commonly made of high-speed types of tool steels, mostly in square, but also in round, rectangular, and other shapes. Tool bits are made of hot-rolled bars and are commonly, yet not exclusively, supplied in hardened and ground form, ready for use after the appropriate cutting edges are ground, usually in the user’s plant. Hollow Bars: These bars are generally produced by trepanning, boring, or drilling of solid round rods and are used for making tools or structural parts of annular shapes, like rolls, ring gages, bushings, etc. Tolerances of Dimensions.— Such tolerances have been developed and published by the American Iron and Steel Institute (AISI) as a compilation of available industry experi - ence that, however, does not exclude the establishment of closer tolerances, particularly for hot-rolled products manufactured in large quantities. The tolerances differ for vari - ous categories of production processes (e.g., forged, hot-rolled, cold-drawn, centerless ground) and of general shapes. Allowances for Machining.— These allowances provide freedom from soft spots and de - fects of the tool surface, thereby preventing failures in heat treatment or in service. After a layer of specific thickness, known as the allowance, has been removed, the bar or other form of tool steel material should have a surface without decarburization and other surface defects, such as scale marks or seams. The accepted industry-wide machining allowance values for tool steels in different conditions, shapes, and size ranges are spelled out in AISI specifica - tions and are generally also listed in the tool steel catalogs of the producer companies. Decarburization Limits.— Heating of steel for production operation causes the ox- idation of the exposed surfaces, resulting in the loss of carbon. That condition, called decarburization, penetrates to a certain depth from the surface, depending on the applied process, the shape and the dimensions of the product. Values of tolerance for decarburiza- tion must be considered one of the factors for defining the machining allowances, which must also compensate for expected variations of size and shape, the dimensional effects of heat treatment, and so forth. Decarburization can be present not only in hot-rolled and forged, but also in rough turned and cold-drawn conditions. Advances in Tool Steel-Making Technology.— Significant advances in processes for tool steel production have been made that offer more homogeneous materials of greater density and higher purity for applications where such extremely high quality is required. Two of these methods of tool steel production are of particular interest. Vacuum-melted tool steels: These steels are produced by the consumable electrode method, which involves remelting of the steel originally produced by conventional pro cesses. Inside a vacuum-tight shell that has been evacuated, the electrode cast of tool steel of the desired chemical analysis is lowered into a water-cooled copper mold where it strikes a low-voltage, high-amperage arc causing the electrode to be consumed by gradual melting. The undesirable gases and volatiles are drawn off by the vacuum, and the inclu sions float on the surface of the pool, accumulating on the top of the produced ingot, to be removed later by cropping. In the field of tool steels, the consumable-electrode vacuum- melting (CVM) process is applied primarily to the production of special grades of hot- work and high-speed tool steels. High-speed tool steels produced by powder metallurgy: The steel produced by conven tional methods is reduced to a fine powder by a gas atomization process. The powder is compacted by a hot isostatic method with pressures in the range of 15,000 to 17,000 psi (103 to 117 MPa). The compacted billets are hot-rolled to the final bar size, yielding a tool- steel material which has 100 percent theoretical density. High-speed tool steels produced by the PM method offer a tool material providing increased tool wear life and high impact strength, of particular advantage in interrupted cuts.
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