Machinery's Handbook, 31st Edition
430
Tool Steels TOOL STEELS Overview
As the designation implies, tool steels serve primarily for making tools used in manufac turing and in the trades for the working and forming of metals, wood, plastics, and other industrial materials. Tools must withstand high specific loads, often concentrated at ex - posed areas, may have to operate at elevated or rapidly changing temperatures and in continual contact with abrasive types of work materials, and are often subjected to shocks, or may have to perform under other varieties of adverse conditions. Nevertheless, when employed under circumstances that are regarded as normal operating conditions, the tool should not suffer major damage, untimely wear resulting in the dulling of the edges, or be susceptible to detrimental metallurgical changes. Tools for less demanding uses, such as ordinary handtools, including hammers, chisels, files, mining bits, etc., are often made of standard AISI steels that are not considered to belong to any of the tool steel categories. The steel for most types of tools must be used in a heat-treated state, usually hardened and tempered, to provide properties needed for a particular application. Adaptability to heat treatment, with minimum harmful effects, dependably resulting in intended beneficial changes in material properties, is another requirement for tool steels. To meet such varied requirements, steel types of different chemical composition, often produced by special metallurgical processes, have been developed. Due to the large num ber of tool steel types produced by the steel mills, which generally are made available with proprietary designations, it is rather difficult for the user to select those types that are most suitable for any specific application, unless the recommendations of a particular steel pro ducer or producers are obtained. Substantial clarification has resulted from the development of a classification system that is now widely accepted throughout the industry, on the part of both the producers and the users of tool steels. The tool steel classification system establishes seven basic categories of tool and die steels. These categories are associated with the predominant applicational characteristics of the tool steel types they comprise. A few of these categories are composed of several groups to distinguish between families of steel types that, while serving the same general purpose, differ with regard to one or more dominant characteristics. The classification categories are given in Table 3. To provide an easily applicable guide for the selection of tool steel types best suited for a particular application, the subsequent discussions and tables are based on the previously mentioned application-related categories. As an introduction to the detailed surveys, a concise discussion is presented of the principal tool steel characteristics that govern the suitability for varying service purposes and operational conditions. A brief review of the major steel alloying elements and of the effect of these constituents on the significant characteristics of tool steels is also given in the following sections. The Properties of Tool Steels.— Tool steels must possess certain properties to a higher than ordinary degree to make them adaptable for uses that require the ability to sustain heavy loads and perform dependably even under adverse conditions. The extent and types of loads, operating conditions, and expected performance with regard to both duration and level of consistency are the principal considerations, in combination with aspects of cost, that govern selection of tool steels for specific applications. Although it is not possible to define and apply exact parameters for measuring signif - icant tool steel characteristics, certain properties can be determined that may assist in appraising suitability of various tool steels for specific uses. AISI standards define the chemical composition of tool steels, but the quality of ingredients (such as element pu- rity and initial particle size) and processing of materials (manufacturing methods and heat-treatment parameters) can result in different properties. Material testing and con- trolled processes help ensure that desired properties are obtained.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online