Machinery's Handbook, 31st Edition
Tool Steels
435
Table 2b. Common Tool Faults, Failures, and Cures Faulty Condition or Inadequate Grade of Tool Steel
Fault Description
Probable Failure
Possible Cure
Improper tool steel grade selection Typical failures:
Choose the tool steel grade by following recommendations and improve selection when needed, guided by property ratings. Obtain tool steels from reliable sources and inspect tool material for detectable defects. Provide allowance for stock to be removed from all surfaces of hot-rolled tool steel. Recommended amounts are listed in tool steel catalogs and vary according to section size, generally about 10 percent for smaller and 5 percent for larger diameters. Bars with large diameter (above about 4 inches or 10 cm) tend to be prone to nonuniform carbide distribution. Choose upset forged discs instead of large-diameter bars. Upset forged discs made with an upset ratio of about 2 to 1 (starting to upset thickness) display radial grain flow. Highly stressed tools, such as gear-shaper cutters, may require the cross forging of blanks.
Chipping—insufficient toughness. Wear—poor abrasion resistance. Softening—inadequate “red hardness.” When not recognized during material inspection, tools made of defective steel often prove to be useless. Cracking may originate from the decarburized layer or it will not harden (“soft skin”).
Material defects—voids, streaks, tears, flakes, surface cooling cracks, etc. Decarburized surface layer (“bark”) in rolled tool steel bars
Brittleness caused by poor carbide distribution in high-alloy tool steels
Excessive brittleness can cause chipping or breakage during service.
Improper grain flow of the steel used for milling cutters and similar tools can cause teeth to break out.
Unfavorable grain flow
Tungsten (W): Tungsten is one of the important alloying elements of tool steels, particu larly because of two valuable properties: it improves “hot hardness,” that is, the resistance of the steel to the softening effect of elevated temperature, and it forms hard, abrasion- resistant carbides, thus improving the wear properties of tool steels. Vanadium (V): Vanadium contributes to the refinement of the carbide structure and thus improves the forgeability of alloy tool steels. Vanadium has a very strong tendency to form a hard carbide, which improves both the hardness and the wear properties of tool steels. However, a large amount of vanadium carbide makes the grinding of the tool very difficult (causing low grindability). Molybdenum (Mo): In small amounts, molybdenum improves certain metallurgical properties of alloy steels, such as deep hardening and toughness. It is used often in larger amounts in certain high-speed tool steels to replace tungsten, primarily for economic rea sons, often with nearly equivalent results. Cobalt (Co): As an alloying element of tool steels, cobalt increases hot hardness and is used in applications where that property is needed. Substantial addition of cobalt, how ever, raises the critical quenching temperature of the steel with a tendency to increase the decarburization of the surface, and reduces toughness. Chromium (Cr): This element is added in amounts of several percent to high-alloy tool steels and up to 12 percent to types in which chromium is the major alloying element. Chromium improves hardenability and, together with high carbon, provides both wear resistance and toughness, a combination valuable in certain tool applications. However, high chromium raises the hardening temperature of the tool steel and thus can make it prone to hardening deformations. A high percentage of chromium also affects the grind ability of the tool steel. Nickel (Ni): Generally in combination with other alloying elements, particularly chro mium, nickel is used to improve the toughness and, to some extent, the wear resistance of tool steels.
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