Machinery's Handbook, 31st Edition
434 Tool Steels To permit a better appraisal of the actual causes of failure and possible corrective action, a general, although not complete, list of common tool faults, resulting failures, and corrective actions is shown in Table 2a through Table 2d. In this list, the potential failure causes are grouped into four categories. The possibility of more than a single cause being responsible for the experienced failure should not be excluded. Note: Examples of tool failures from causes such as those listed in Table 2a through Table 2d may be found in “The Tool Steel Trouble Shooter” handbook, published by Bethlehem Steel Corporation. Finally, it must be remembered that the proper usage of tools is indispensable for obtain ing satisfactory performance and tool life. Using the tools properly involves, for example, the avoidance of damage to the tool, overloading, excessive speeds and feeds, the applica tion of adequate coolant when called for, a rigid setup, proper alignment, and firm tool and work holding. Table 2a. Common Tool Faults, Failures, and Cures Improper Tool Design Fault Description Probable Failure Possible Cure
Make such parts of two pieces or use an air-hardening tool steel that avoids the harsh action of a liquid quench. Apply fillets to the corners and/or use an air-hardening tool steel. The use of round keyways should be preferred when the general configuration of the part makes it prone to failure due to square keyways. Use taper transitions, which are better than even generous fillets. Assure solid support, avoid unnecessary play, adapt travel length to operational conditions (e.g., punch to penetrate to four-fifths of thickness in hard work material). Adapt clearances to material conditions and dimensions to reduce tool load and to obtain clean sheared surfaces.
Drastic section changes—widely different thicknesses of adjacent wall sections or protruding elements Sharp corners on shoulders or in square holes
In liquid quenching, the thin section will cool and then harden more rapidly than the adjacent thicker section, setting up stresses that may exceed the strength of the steel. Cracking can occur, particularly in liquid quenching, due to stress concentrations. Failure may arise during service and is usually considered to be caused by fatigue. Due to impact in service, pneumatic tools are particularly sensitive to stress concentrations that lead to fatigue failures. Excessive wear or breakage in service may occur.
Sharp cornered keyways
Abrupt section changes in battering tools Functional inadequacy of tool design—e.g., insufficient guidance for a punch
Improper tool clearance, such as in blanking and punching tools
Deformed and burred parts may be produced, excessive tool wear or breakage can result.
The Effect of Alloying Elements on Tool Steel Properties.— Carbon (C): The presence of carbon, usually in excess of 0.60 percent for nonalloyed types, is essential for raising the hardenability of steels to the levels needed for tools. Raising the carbon content by different amounts up to a maximum of about 1.3 percent increases the hardness slightly and the wear resistance considerably. The amount of carbon in tool steels is designed to attain certain properties (such as in the water-hardening category where higher carbon content may be chosen to improve wear resistance, although to the detriment of toughness) or, in the alloyed types of tool steels, in conformance with the other constituents to produce well-balanced metallurgical and performance properties. Manganese (Mn): In small amounts, to about 0.60 percent, manganese is added to reduce brittleness and to improve forgeability. Larger amounts of manganese improve hardenability, permitting oil quenching for nonalloyed carbon steels, thus reducing defor mation, although, with regard to several other properties, manganese is not an equivalent replacement for the regular alloying elements. Silicon (Si): In itself, silicon may not be considered an alloying element of tool steels, but it is needed as a deoxidizer and improves the hot-forming properties of the steel. In combination with certain alloying elements, the silicon content is sometimes raised to about 2 percent to increase the strength and toughness of steels used for tools that have to sustain shock loads.
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