ABSTRACT
Tool and Bearing Steels 165
A brief description of the groups of alloys follows. Water-Hardening Steels. These are inexpensive high-carbon steels that are hardened to shallow depths (low hardenability). They are somewhat prone to cracking and distortion during heat treatment. Small amounts of chromium and vanadium are added in some W-type steels to improve hardenability and wear resistance. Shock-Resistance Steels. These steels are used for applications where repetitive impact stresses are encountered, and hence their carbon content is limited to about 0.5%. S I and S2 steels are water-quenched, while S5, a popular low-price generalpurpose tool steel, has a medium hardenability and is hardened by oil quenching. S7 has the ability to fully harden when quenched in still air. The shock-resisting properties (toughness) of most of these alloys are developed through the use of silicon as an alloying element. Cold-Work Tool Steels. These steels are used for cold-work and for die applications where toughness and resistance to wear are both important. They are limited to applications that do not involve prolonged heating above room temperature. These include both air-hardening (A type) and oil-hardening (0 type) steels plus the high-carbon and high-chromium D type. The latter were originally developed for high-speed cutting operations but were later replaced by the better high-speed steels. The D type have now been found to be useful as cold-work die steels, hence the D symbol. 03 is an oil-hardening steel, while most other D types may be airhardened. Hot-Work Tool Steels. These steels contain 3% to 5% of chromium, tungsten or molybdenum in order to resist softening at elevated temperatures. Most of the chromium type are air-hardened. The tungsten types may also be air-hardened, but they are usually quenched in oil in order to minimize scaling. The only molybdenum hot-work steels of significance are the H42 and H43 steels, which are less expensive than the tungsten-type steels. High-Speed Steels. These steels are named because of their ability to machine other hard materials at relatively high rates of speed. They are very complex ironbased alloys with high-carbon levels. Other alloying elements include chromium, vanadium, molybdenum, tungsten or cobalt in various combinations. The alloying elements form carbides that confer very excellent wear resistance and also resistance to softening at elevated temperatures. These carbides also impart a rather unique characteristic called secondary hardness. The term expresses the steel's ability to obtain a hardness as high or higher than the original as quenched hardness when they are tempered at a critical temperature. For example M42 high speed will show an as-quenched hardness of about 66HRC, but when tempered at I ooooF (538°C) will obtain a hardness of 69HRC. Powder Metallurgy Tool Steels. As the alloying elements added to an iron-based material become very high and the structure becomes complex, it is extremely difficult, and sometimes impossible, to obtain a satisfactory product through the conventional processes. In view of this, the powder process was developed. The required alloy is prepared by blending the necessary components in powder form. It is then compacted through various methods until the needed size and form are obtained. A variety of high-speed steels and some highly wear-resistant tool steels are available through this method of manufacture.
