Heat Treat 101: a Primer by Frederick J
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Heat Treat 101: A Primer By Frederick J. Otto and Dan Herring The manufacture of precision gearing depends, to a great extent, on heat treating as a core competency. Gears play an essential role in the performance 1), represented as a series of interlocking rings control, durability, and reliability. Heat treating of many products that we rely on in our everyday underscoring the interdependence of each represents a significant portion—about 30%— lives. When we think about gears, we generally element in the model. We see that the end use of of a typical gear manufacturing cost (Figure 2). separate them into two categories: motion- performance capability of the product is defined If not properly understood and controlled, it carrying and power transmission. Motion- by its mechanical, physical, and metallurgical can have a significant impact on all aspects of carrying gears are generally nonferrous or properties, which are determined by the part the gear manufacturing process (Figure 3). plastics, while load-bearing power transmission microstructure, produced by a specific heat Ggears are usually manufactured from ferrous treatment process. PROCESS SELECTION alloys. The focus here is on heat treating gears It is clear from this model that the manufacture Pre-hardening Processes intended for heavy duty service. of precision gearing depends, to a great Several heat treatments are normally performed To understand why heat treating is important, extent, on heat treating as a core competency. during the gear manufacturing process to consider the model of material science (Figure Its contribution is vitally important for cost prepare the part for the intended manufacturing 40 | Thermal Processing for Gear Solutions PERFORMANCE PROPERTIES MICROSTRUCTURE PROCESS Figure 1: Model of material science outside the furnace to relieve residual stresses Materials (10%) Finishing (5%) in a gear blank, and for dimensional stability. Normalizing is often considered from both a thermal and microstructural standpoint. In the thermal sense, normalizing is austenitizing followed by cooling in still- or slightly-agitated Heat Treating (30%) Machining (55%) air or nitrogen. In a microstructural sense, normalizing produces a more homogenous Figure 2: Typical gear manufacturing costs structure. A normalized part is very machinable but harder than an annealed part. Normalizing of carbon in the steel; the depth of hardness also plays a significant role in the control of depends on the hardenability of the steel as dimensional variation during carburizing. well as the quench severity. Through hardening can be performed either Stress Relieving before or after the gear teeth are cut. When gear Stress Relieving involves heating to a teeth are cut after the part has been hardened, temperature below the lower transformation surface hardness and machinability become temperature, as in tempering, holding long important factors, especially in light of the fact enough to reduce residual stress and cooling that machining will remove some or most of slowly enough, usually in air, to minimize the the higher hardness material into the austenitic steps. These are essential to the manufacture of development of new residual stresses. Stress range, typically 815 to 875°C (1500 to 1600°F), a quality gear. relief heat treating is used to relieve internal followed by quenching and tempering. stresses locked in the gear as a consequence of Annealing a manufacturing step. Case Hardening Annealing consists of heating to and holding at Case hardening produces a hard, wear resistant a suitable temperature, followed by cooling at HARDENING PROCESSES case or surface layer on top of a ductile, shock an appropriate rate, primarily intended to soften Various heat treatment processes are designed resistant interior, or core. The idea behind case the part and improve its machinability. Super- to increase gear hardness. These usually involve hardening is to keep the core of the gear tooth critical or full annealing involves heating a part heating and cooling and are typically classified at a level around 30 to 40 HRC to avoid tooth above the upper critical temperature (AC3)— as through hardening, case hardening (carbu- breakage while hardening the outer surface that is the temperature at which austenite begins rizing, carbonitriding, nitriding, nitrocarburiz- to increase pitting resistance. The higher the to transform to ferrite during cooling, and then ing) and hardening by applied energy (flame, surface hardness value, the greater the pitting slowly cooling in the furnace to around 600ºF. laser, induction). resistance. Bending strength increases for Intercritical annealing involves heating the part surface hardness up to about 50 HRC, after to a temperature above the final transformation Through or Direct Hardening which the increase in bending strength is offset temperature (AC1), the temperature at which Through or direct hardening refers to heat by an increase in notch sensitivity. austenite begins to form during heating. Subcriti- treatment methods, which do not produce a cal annealing heats the part to just below the AC1 case. Examples of commonly through hard- Carburizing point, followed by a slow cool in the furnace. The ened gear steels are AISI 1045, 4130, 4140, Carburizing is the most common of the case rate of softening increases rapidly as the anneal- 4145, 4340, and 8640. It is important to note hardening methods. A properly carburized gear ing temperature approaches the AC1 part. that hardness uniformity should not be as- will be able to handle between 30-50% more sumed throughout the gear tooth. Since the load than a through hardened gear. Carburiz- Normalizing outside of a gear is cooled faster than the inside, ing steels are typically alloy steels with approxi- Normalizing involves heating the part above the there will be a hardness gradient developed. mately 0.10% to 0.20% carbon. Examples of upper critical temperature and then air cooling The final hardness is dependent on the amount commonly carburized steels include AISI thermalprocessing.com | 41 Figure 3: Current gear manufacturing processes 10148, 4320, 5120, 8620, and 9310 as well ture than the surface produced by carburiz- as international grades such as 20MnCr5, ing. Despite this, nitriding has proved to be a 16MnCr5, ZF-7B, 20MoCr4, and V2525. viable alternative for numerous applications. Carburizing can be performed in the Commonly nitride gear steels include AISI temperature range of 1475-2000°F (800- 4140, 4150, 4340, 7140, 8640, and AMS 1090°C). Common industry practice today 6475 (Nitralloy N). finds the majority of carburizing operations Nitriding is typically done in the 925- taking place at 1600-1850°F (870 to 1010°C). 1050°F (495-565°C) range. Three factors that Carburizing case depths can vary over a are extremely critical in producing superior broad range, 0.005 to 0.25 in. (0.13 to 8.25 and consistent nitride cases and predictable mm). However, it is common to use the carbo- dimensional change are steel composition, nitriding process for case depths below 0.015 prior structure, and core hardness. Case in. (0.4 mm). depth and case hardness properties vary not only with the duration and type of nitriding Carbonitriding being performed but are also influenced by Carbonitriding is a modification of the car- these factors. Typically, case depths are be- burizing process, not a form of nitriding. This tween 0.008-0.025 in. (0.20-0.65 mm) and modification consists of introducing ammo- take from 10 to 80 hours to produce. nia into the carburizing atmosphere to add nitrogen to the carburized case as it is being Nitrocarburizing produced. Examples of gears steels that are Nitrocarburizing is a modification of nitrid- commonly carbonitrided include AISI 1018, ing, not a form of carburizing. Here, nitrogen 1117, and 12L14. and carbon are simultaneously introduced Typically, carbonitriding is done at a lower into the steel while it is in a ferritic condi- temperature than carburizing, or between tion, that is, at a temperature below that at 1330-1650°F (700-900°C), and for a shorter which austenite begins to form during heat- time. Because nitrogen inhibits the diffusion ing. A very thin “white” or compound layer is of carbon, what generally results is a shallow- formed during the process, along with an un- er case than is typical for carburized parts. A derlying diffusion zone. Like nitriding, rapid carbonitrided case is usually between 0.003- quenching is not required. Examples of gear 0.030 in. (0.075-0.75 mm) deep. steels commonly nitrocarburized include AISI Figure 4: Model of gear engineering 1018, 1141, 12L14, 4140, 4150, 5160, 8620, Nitriding and certain tool steels. APPLIED ENERGY HARDENING Nitriding is another surface treatment pro- Nitrocarburizing is normally performed at Various methods of hardening by use of ap- cess that increases surface hardness. Since 1025-1110°F (550-600°C) and can be used plied energy are used in the manufacture of rapid quenching is not required, dimensional to produce an equivalent 58 HRC minimum gears, including flame hardening, laser surface changes are kept to a minimum, which is a hardness, with this value increasing depen- hardening, and induction. major advantage. It is not suitable for all dent on the base material. White layer depths gear materials. One of its limitations is the range from 0.00005-0.0022 in. (0.0013 to Flame Hardening extremely high surface hardness or “white 0.056 mm) with diffusion zones from 0.0013- Flame hardening can be used for both small