Machinery's Handbook, 31st Edition
454 Special Tool Steels Properties and Applications of Frequently Used Mold Steel Types: AISI P3 and P4: Essentially, both types of tool steels were developed for the same special purpose, that is, the making of plastics molds. The application conditions of plastics molds require high core strength, good wear resistance at elevated temperature, and excellent surface finish. Both types are carburizing steels that possess good dimensional stability. Hobbing is the process of sinking a cavity by pressing a punch that is the inverse replica of the cavity into the tool material, and is the process by which many plastics mold cavities are produced. Good “hobbability” is an important requirement of tool steels used for this purpose. The different chemistry of these two types of mold steels is responsible for the high core hard- ness of P4, which makes it better suited for applications requiring high strength at elevated temperature. AISI P6: This nickel-chromium-type plastics mold steel has exceptional core strength and develops a deep carburized case. Due to the high nickel-chromium content, the cavi ties of molds made of this steel type are produced by machining rather than by hobbing. An outstanding characteristic of this steel type is the high luster that is produced by polishing of the hard case surface. AISI P20: This general-type mold steel is adaptable to both through hardening and car burized case hardening. In through hardening, an oil quench is used and a relatively lower, yet deeply penetrating, hardness is obtained, such as is needed for zinc die-casting dies and injection molds for plastics. After the direct quenching and tempering, carburizing produces a very hard case and comparatively high core hardness. When thus heat treated, this steel is particularly well adapted for making compression, transfer, and plunger-type plastics molds. Special-Purpose Tool Steels.— These steels include several low-alloy types of tool steels that were developed to provide transitional types between the more commonly used basic types of tool steels, and thereby contribute to the balancing of certain conflicting properties, such as wear resistance and toughness. They offer intermediate depth of hard- ening and are less expensive than the higher-alloyed types of tool steels. Properties and Applications of Frequently Used Special-Purpose Types: AISI L6: This material is a low-alloy-type special-purpose tool steel. Comparatively safe hardening and fair nondeforming properties, combined with the service advantage of good toughness in comparison to most other oil-hardening types, explain the acceptance of this steel with a rather special chemical composition. L6 is used for tools whose toughness requirements prevail over abrasion-resistant properties, such as forming rolls and forming and trimmer dies in applications where combinations of moderate shock- and wear-resistant properties are sought. The areas of use also include structural parts, like clutch members, pawls, and knuckle pins, that must withstand shock loads and still display good wear properties. AISI F2: This carbon-tungsten type is one of the most abrasion-resistant of all water- hardening tool steels. However, it is sensitive to thermal changes, such as those involved in heat treatment, and it is also susceptible to distortions. Consequently, its use is limited to tools of simple shape in order to avoid cracking in hardening. The shallow hardening char acteristics of F2 result in a tough core and are desirable properties for certain tool types that, at the same time, require excellent wear-resistant properties. Water-Hardening Tool Steels.— Steel types in this category are made without, or with only a minimum amount of, alloying elements, and their heat treatment needs the harsh quenching action of water or brine, hence the general designation of the category. Water-hardening steels are usually available with different percentages of carbon, to provide properties required for different applications; the classification system lists a carbon range of 0.60 to 1.40 percent. In practice, however, the steel mills produce these steels in a few varieties of differing carbon content, often giving proprietary designations to each particular group. Typical carbon content limits of frequently used water-hardening tool steels are 0.70–0.90, 0.90–1.10, 1.05–1.20, and 1.20–1.30 percent. The appropriate group should be chosen according to the intended use, as indicated in the steel selection
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online