Classification of gear steel Gear steel can be divided into two categories - ordinary carbon steel and alloy steel. Alloy steels are used to some extent in industry, but heat-treated plain carbon steels are more common. The use of untreated alloy steel on gears is rarely, if ever, justified, and only in the absence of heat-treating facilities. The main points to consider when deciding whether to use heat-treated plain carbon steel or heat-treated alloy steel are: Does the service conditions or design require the superior properties of the alloy steel, or, if the alloy steel is not required, will the advantages gained justify the additional cost? For most applications, plain carbon steel heat treated to obtain the best quality for the intended service is satisfactory and fairly economical. The advantages of using heat-treated alloy steel instead of heat-treated ordinary carbon steel are as follows:
1. Increased surface hardness and hardness penetration depth for the same carbon content and quenching.
2. The same surface hardness can be obtained with a less severe quench, and in the case of some alloys, the quench temperature is lower, so the deformation is smaller.
3. Higher yield point, elongation and area reduction values indicate increased toughness.
4. Finer grain size, resulting in higher impact toughness and higher wear resistance.
5. Possibility of better machining quality or higher hardness machining in the case of certain alloys.
Use of case hardened steel
Each of the two broad categories of gear steels can be further subdivided as follows:
1) case hardened steels;
2) fully hardened steels;
3) steels that are heat treated and drawn to a hardness that can be machined.
The first two - case-hardened steel and full-hardened steel - are interchangeable in some services, and the choice is often a matter of personal opinion. When wear resistance is required, a case-hardened steel with an extremely hard, fine-grained (when properly treated) case and a relatively soft and ductile core is typically used. Case hardened alloy steel has a fairly tough core, but not as tough as fully hardened steel. To get the most from the core properties, case hardened steel should be double quenched. This is especially true for alloy steels, as the benefits derived from their use rarely justify the extra expense unless the core is refined and toughened by a second quench. The price that extra refinement has to pay is increased distortion.
Use "through hardened" steel
Through hardened steels are used when high strength, high endurance limit, toughness and impact resistance are required. These qualities depend on the type and treatment of the steel used. Relatively high surface hardnesses can be obtained in this group, although not as high as case-hardened steels. For this reason, the wear resistance is not as great as it is possible to obtain, but this steel is superior to others when it is required to combine wear resistance with high strength and toughness. Hardened steel deforms to some extent as it hardens, and the amount of deformation depends on the steel and quenching medium used. For this reason, through hardened steel is not suitable for high-speed gears where noise is a major factor, or for gears where accuracy is critical, unless of course: where teeth can be ground. Oil quenching is required for medium and high carbon percentages, but water quenching may be required for lower carbon contents for maximum physical properties and hardness. However, the deformation will be greater during water quenching.
Heat treatment to allow machining
When grinding of gear teeth is impractical and high precision is required, hardened steel can be drawn or tempered to a hardness that allows cutting teeth. This treatment provides a highly fine structure, excellent toughness, and excellent wear resistance despite low hardness. The lower strength is partially compensated by eliminating incremental loads due to inaccuracy-induced shocks. When treating steels with low hardness penetration from the surface to the core in this way, the design cannot be based on physical properties corresponding to surface hardness. Since physical properties are determined by hardness, a decrease in hardness from the surface to the core results in a decrease in the physical properties of the root, where the pressure is greatest. The quenching medium can be oil, water or brine, depending on the steel used and the desired hardness penetration. Of course, the amount of deformation doesn't matter because machining is done after heat treatment.
Make pinions harder than gears to balance wear
By making the pinion harder than the gear, there are beneficial results from a wear point of view. A pinion has fewer teeth than a gear, each tooth naturally does more work, and the difference in hardness between the pinion and gear (depending on the ratio) is used to balance the wear rate. Harder pinion teeth correct errors in the gear teeth to some extent by initial wear, and then smooth the gear teeth and increase their ability to withstand wear due to cold working of the surface. In applications with high gear ratios and no severe shock loads, case-hardened pinions that run with oil-treated gears, treated to Brinell hardness of cuttable teeth, are a good combination.