Machinery's Handbook, 31st Edition
Tool Steels 431 Because tool steels are generally heat treated to make them adaptable to the intended use by enhancing the desirable properties, the behavior of the steel during heat treatment is of prime importance. The behavior of the steel comprises, in this respect, both the resistance to harmful effects and the attainment of the desirable properties. The following are considered the major properties related to heat treatment: Safety in Hardening: This designation expresses the ability of the steel to withstand the harmful effects of exposure to very high heat and particularly to the sudden temperature changes during quenching, without harmful effects. One way of obtaining this property is by adding alloying elements to reduce the critical speed at which quenching must be car ried out, thus permitting the use of milder quenching media, such as oil, salt, or just still air. CORNERS DO DO NOT DO DO NOT
SECTION BALANCE Projections
Internal Corners
Use inserts, not projections, if sharp angles are necessary
Use fillets & radii, not sharp corners
Heavy and Light Sections
Machine Parts
Do not design heavy and light sections as a unit
Use fillets & radii, not sharp corners
Layout Design
Splines or Keyways
Maintain uniform sections, do not crowd openings into small clusters
Use fillets & radii, not sharp corners
Blanking Die Balance
Embossing Dies
Balance sections by adding openings in unbalanced areas
Use round, not sharp, corners and edges
Notch Effect
Recessed Screw or Bolt Holes
Space cutting edges for uniform strength; do not put "stress raisers" in line
Use fillets & radii, not sharp corners
Fig. 1. Tool and Die Design Tips to Reduce Breakage in Heat Treatment Courtesy of Society of Automotive Engineers, Inc.
Parts made of tool steel suffer most commonly during heat treatment from the development of cracks. In addition to the composition of the steel and the applied heat- treating process, the configuration of the part can also affect the sensitivity to cracking. The preceding figure illustrates a few design characteristics related to cracking and warpage in heat treatment; the observation of these design tips, which call for generous filleting, avoidance of sharp angles, and major changes without transition in the cross- section, is particularly advisable when using tool steel types with a low index value for safety in hardening. In current practice, the previously mentioned property of tool steels is rated in the order of decreasing safety (i.e., increasing sensitivity) as Highest, Very High, High, Medium, and Low safety, expressed in Table 6 through Table 11 by the letters A, B, C, D, and E. Distortions in Heat Treating: In parts made from tool steels, distortions are often a consequence of inadequate design (See Fig. 1) or improper heat treatment (e.g., lack of stress relieving). However, certain types of tool steels display different degrees of
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