Gear Materials and their Heat Treatment
Gears play an essential role in the performance of many products that we rely on in our everyday lives. When we think about gears we generally separate them into two categories – motion-carrying and power-transmission.
Motion-carrying gears are generally nonferrous or plastics, while load-bearing power-transmission gears (Fig. 1) are usually manufactured from ferrous alloys and are intended for heavy-duty service applications.
Power-transmission gears involve a wide variety of steels and cast irons. In all gears, the choice of material must be made only after careful consideration of the performance demanded by the application end-use and total manufactured cost, taking into consideration such issues as pre- and post-machining economics.
Key design considerations require an analysis of the type of applied load – whether gradual or instantaneous – and the desired mechanical properties, such as bending fatigue strength or wear resistance, all of which will define core strength and heat-treating requirements.
It is important for the designer to understand that each area in the gear-tooth profile sees different service demands. Consideration must be given to the forces that will act on the gear teeth, with tooth bending and contact stress, resistance to scoring and wear, and fatigue issues being paramount. In the root area, for example, good surface hardness and high residual compressive stress are desired to improve endurance or bending fatigue life. At the pitch diameter, a combination of high hardness and adequate subsurface strength are necessary to handle contact stress and wear and to prevent spalling. As an example, some of the factors that influence fatigue strength are:
Hardness distribution, a function of:
- Case hardness
- Case depth
- Core hardness
Microstructure, a function of:
- Retained-austenite percentage
- Grain size
- Carbide size, type and distribution
- Non-martensitic phases
Defect control, a function of:
- Residual compressive stress
- Surface finish and geometry
- Intergranular toughness
Thus, in the total manufacturing scheme, a synergistic relationship must exist between the material selection process, engineering design and manufacturing (including heat treatment). A balance of the priorities in each discipline must be reached in order to achieve the optimization necessary for the ultimate performance of the gear design. This is often not an easy task.
Atmosphere Heat-Treatment Methods
Atmosphere carburizing is the most common of the case-hardening methods in use today and can handle a diverse range of part sizes and load configurations (Fig. 2). In general, a properly carburized gear will be able to handle somewhere between 30-50% more load than a through-hardened gear. Examples of commonly carburized gear steels include SAE grades 1018, 4320, 5120, 8620 and 9310 as well as international grades such as 20MnCr5, 17CrNiMo6, 18CrNiMo7-6 and 20MoCr4.
Atmosphere carburizing is typically performed in the temperature range of 870-955°C (1600-1750°F), although temperatures approaching 1010°C (1800°F) are used for deep-case work. Carburizing case depths vary over a broad range, typically 0.13-8.25 mm (0.005-0.325 inches).
Carbonitriding is a modification of the carburizing process, not a form of nitriding. This modification consists of introducing ammonia into the carburizing atmosphere in order to add nitrogen to the carburized case as it is being produced. Examples of gear steels that are commonly carbonitrided include AISI 1018, 1117 and 12L14.
Typically, carbonitriding is done at a lower temperature than carburizing – between 700-900°C (1300-1650°F) – and for a shorter time. Combine this with the fact that nitrogen inhibits the diffusion of carbon and what generally results is a shallower case than is typical for carburized parts. A carbonitrided case is usually between 0.075-0.75 mm (0.003-0.030 inches) deep.
Nitriding is another surface-treatment process that has increasing surface hardness as its objective. One of the appeals of this process is that rapid quenching is not required. Hence, dimensional changes are kept to a minimum. It is not suitable for all gear applications. One of its limitations is that the extremely high surface-hardness case produced has a more brittle nature than that produced by the carburizing process. Despite this fact, nitriding has proved to be a viable alternative in a number of applications. Examples of commonly nitrided gear steels include SAE 4140, 4150, 4340 and Nitralloy 135M.
Nitriding is typically done in the 495-565°C (925-1050°F) temperature range. Case-depth and case-hardness properties vary not only with the duration and type of nitriding being performed but also with steel composition, prior structure and core hardness. Typically, case depths are between 0.20-0.65 mm (0.008-0.025 inches) and take from 10 to 80 hours to produce.
Nitrocarburizing is a modification of nitriding, not a form of carburizing. In the process, nitrogen and carbon are simultaneously introduced into the steel while it is in a ferritic condition (i.e. at a temperature below that at which austenite begins to form during heating). A very thin “white” or “compound” layer is formed during the process as well as an underlying “diffusion” zone. Like nitriding, rapid quenching is not required. Examples of gear steels that are commonly nitrocarburized include SAE grades 4140, 5160, 8620 and certain tool steels.
Nitrocarburizing is normally performed at 550-600°C (1025-1110°F) and can be used to produce a 58 HRC minimum hardness, with this value increasing dependent on the base material. White-layer depths range from 0.0013-0.056 mm (0.00005-0.0022 inches) with diffusion zones from 0.03-0.80 mm (0.0013-0.032 inches) being typical.
Vacuum Heat-Treatment Methods
Vacuum processing can be used for most of the atmosphere treatments mentioned above, including carburizing (Fig. 3). Progress continues to be made in low-pressure carburizing (LPC) of advanced applications in the aerospace, automotive, off-highway and motorsports markets,[1,3] particularly in teh development of carburizing cycles for high-performance materials (Table 1).
Effective case depths for most grades range from 2.0-3.0 mm (0.080-0.120 inches) without significant sacrifice of microstructure (Fig. 4). Furnace variables such as temperature uniformity (±3°C or ±5°F), control-of-cycle parameters for carbide control (flow rate, pressure, hydrocarbon type) and surface-carbon selection (0.42-1.05% C) can produce case uniformities of ±0.05 mm (±0.002 inches). Where permitted, the range of carburizing temperatures now includes the use of high-temperature techniques. All of these advanced materials required extensive development testing to produce custom-designed recipes to optimize cycle parameters. Also, quenching methods have improved, allowing us to achieve desired core properties with quenching-paramter selection (high-pressure gas or oil) for distortion-sensitive and distortion-prone part geometrics.[4,6]
Induction Hardening Methods
Various methods of hardening by use of applied energy are utilized in the manufacture of gears, including flame hardening, laser surface hardening and induction.
Of the various types of applied-energy processing, induction hardening is the most common. Induction heating is a process that uses alternating electrical current to induce a magnetic field, causing the surface of the gear tooth to heat. The area is then quenched, resulting in an increase in hardness within the heated area. This process is typically accomplished in a relatively short time. The final desired gear performance characteristics are determined not only by the hardness profile and stresses but also by the steel’s composition and prior microstructure. External spur and helical gears, bevel and worm gears, racks and sprockets are commonly induction hardened. Typical gear steels include SAE grades 1050, 1060, 1144, 4140, 4150, 4350, 5150 and 8650 to name a few.
The hardness pattern produced by induction heating (Fig. 5) is a function of the type and shape of inductor used as well as the heating method. Quenching or rapidly cooling the workpiece can be accomplished by spray or submerged quench. The media typically used for the quench is a water-based polymer. The severity of this quenchant can be controlled by the polymer’s concentration. Cooling rates are usually somewhere in between what would be obtained from pure water and oil. In some unusual situations, compressed air is used to quench the workpiece.
The most common methods for hardening gears and sprockets are by single shot (Fig. 6) or the tooth-by-tooth method (Fig. 7a). Single shot often requires large kW power supplies but results in short heat/quench times and higher production rates. This technique uses a copper inductor (coil) encircling the workpiece. An inductor, which is circumferential, will harden the teeth from the tips downward.
While the single-shot method is acceptable for splines and some gearing, the larger heavier-loaded gears (where pitting, spalling, tooth fatigue and endurance are an issue) need a hardness pattern that is more profiled like those produced with carburizing. This type of induction hardening is called tooth-by-tooth hardening. This method is limited for gear tooth sizes up to 5 or 6 DP using frequencies from 2 to 10 kHz and about 10 DP using a range of 25 to 50 kHz. The lower the frequency, the deeper the case depth. This is a slow process due to the number of teeth and index times, and it is usually reserved for gears and sprockets that are too large to single shot due to power constraints. The process involves heating the root area and side flanks simultaneously while cooling each side of the adjacent tooth to prevent temper-back on the backside of each tooth (Fig. 7b). The induction system moves the coil at a pre-programmed rate along the length of the gear. The coil progressively heats the entire length of the gear segment while a quench follower immediately cools the previously heated area. The distance from the coil to the tooth is known as coupling or air-gap. Any variation in this distance can yield variation in case depth, hardness and tooth distortion.
The gear is indexed after each tooth has been hardened, often skipping a tooth. This requires at least two full revolutions in the process to complete the hardening of all teeth. Straight, spur and helical gears up to 210 inches and 15,000 pounds have been processed with this method. The entire process yields a repeatable soft tip of the tooth with hard root and flank. In other applications, the tip and both flanks can be hardened simultaneously and yield a soft root.
Today, the design engineer has the good fortune of being able to choose from a number of heat-treatment technology choices for any given type of gear material and gear design. The secret to success is to understand the advantages and limitations of each technology and take these into consideration when determining the overall cost of gear manufacturing. IH
For more information: Contact Dan Herring, president of THE HERRING GROUP Inc., P.O. Box 884 Elmhurst, IL 60126; tel: 630-834-3017; fax: 630-834-3117; e-mail: email@example.com; web: www.heat-treat-doctor.com. Dan’s Heat Treat Doctor columns appear monthly in Industrial Heating, and he is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.