Machinery's Handbook, 31st Edition
GEAR MATERIALS
2319
GEAR MATERIALS Classification of Gear Steels.— Gear steels may be divided into two general classes—the plain carbon and the alloy steels. Alloy steels are used to some extent in the industrial field, but heat-treated plain carbon steels are far more common. The use of untreated alloy steels for gears is seldom, if ever, justified, and then, only when heat-treating facilities are lacking. The points to be considered in determining whether to use heat-treated plain carbon steels or heat-treated alloy steels are: Does the service condition or design require the superior characteristics of the alloy steels, or, if alloy steels are not required, will the advantages to be derived offset the additional cost? For most applications, plain carbon steels, heat treated to obtain the best of their qualities for the service intended, are satisfac tory and quite economical. The advantages obtained from using heat-treated alloy steels in place of heat-treated plain carbon steels are as follows: 1) Increased surface hardness and depth of hardness penetration for the same carbon content and quench. 2) Ability to obtain the same surface hardness with a less drastic quench and, in the case of some of the alloys, a lower quenching temperature, thus giving less distortion. 3) Increased toughness, as indicated by the higher values of yield point, elongation, and reduction of area. 4) Finer grain size, with the resulting higher impact toughness and increased wear resistance. 5) In the case of some of the alloys, better machining qualities or the possibility of machining at higher hardnesses. Use of Case-Hardening Steels.— Each of the two general classes of gear steels may be further subdivided as follows: 1) Case-hardening steels; 2) full-hardening steels; and 3) steels that are heat treated and drawn to a hardness that will permit machining. The first two—case-hardening and full-hardening steels—are interchangeable for some kinds of service, and the choice is often a matter of personal opinion. Case- hardening steels with their extremely hard, fine-grained (when properly treated) case and comparatively soft and ductile core are generally used when resistance to wear is desired. Case-hardening alloy steels have a fairly tough core, but not as tough as that of the full- hardening steels. In order to realize the greatest benefits from the core properties, case- hardened steels should be double-quenched. This is particularly true of the alloy steels, because the benefits derived from their use seldom justify the additional expense, unless the core is refined and toughened by a second quench. The penalty that must be paid for the additional refinement is increased distortion, which may be excessive if the shape or design does not lend itself to the case-hardening process. Use of “Thru-Hardening” Steels.— Thru-hardening steels are used when great strength, high endurance limit, toughness, and resistance to shock are required. These qualities are governed by the kind of steel and treatment used. Fairly high surface hardnesses are obtainable in this group, though not so high as those of the case-hardening steels. For that reason, the resistance to wear is not so great as might be obtained, but when wear resistance combined with great strength and toughness is required, this type of steel is superior to the others. Thru-hardening steels become distorted to some extent when hard- ened, the amount depending upon the steel and quenching medium used. For that reason, thru-hardening steels are not suitable for high-speed gearing where noise is a factor, or for gearing where accuracy is of paramount importance, except, of course, in cases where grinding of the teeth is practicable. The medium and high-carbon percentages require an oil quench, but a water quench may be necessary for the lower carbon contents, in order to obtain the highest physical properties and hardness. The distortion, however, will be greater with the water quench. Heat Treatment that Permits Machining.— When the grinding of gear teeth is not prac ticable and a high degree of accuracy is required, hardened steels may be drawn or
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