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QUENCHING AFTER VACUUM CARBURIZING This Article Discusses the Ommon Applications for Satisfy All of These Requirements

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QUENCHING AFTER VACUUM CARBURIZING This Article Discusses the Ommon Applications for Satisfy All of These Requirements ecm.qxp 9/3/2004 11:49 AM Page 1 A USER’S GUIDE TO QUENCHING AFTER VACUUM CARBURIZING This article discusses the ommon applications for satisfy all of these requirements. vacuum carburizing in- Arriving at the best choice of factors heat treaters have clude automobile power quench medium requires careful transmission and fuel consideration of a number of factors, to consider when deciding system components, light- including: when to apply oil and heavy-duty truck and • Economics/cost (initial invest- quenching or gas off-highway vehicle trans- ment, maintenance, upkeep, life) Cmission components such as ring • Health and safety (codes, regu- quenching for cooling gears and pinions, aerospace trans- lations, exposure) mission and actuator systems, and • Minimization of distortion low-pressure/vacuum various industrial products such as (quench system) carburized steel parts. hydraulic pump cams, bearings, and • Performance (cooling rate/ valves. quench severity) by Dennis Beauchesne Today, most vacuum carburizing • Versatility (controllable cooling is performed at temperatures of 1700 rates) ECM USA Inc. to 1800°F (930 to 980°C), with effec- • Environmental issues (waste dis- Kenosha, Wis. tive case depths of 0.010 to 0.080 in. posal and noise, for example) (0.25 to 2.05 mm) or greater. Typical Aymeric Goldsteinas production load weights range from Why use gas quenching? ECM France 500 to 1000 lb (225 to 455 kg). One of the reasons for the intense Grenoble, France Vacuum carburizing often eliminates interest in vacuum carburizing com- the need for slow cooling, reheating, bined with gas quenching is the and subsequent press or plug ability to achieve dramatic reductions quenching. And a copper electro- in part distortion (dimensional vari- plate or stop-off paint can be used to ation), especially when compared prevent the carburization of selected with atmosphere oil quenching. For surfaces. example, automatic transmission Vacuum carburizing systems must pinion gear lead profiles were com- be rugged, versatile, and cost effec- pared after atmosphere carburizing tive since the parts processed in them and oil quenching in an integral- may change tomorrow in size, shape, quench furnace and after low-pres- material, required properties, or sure carburizing and 15-bar nitrogen throughput. In addition, the ability gas quenching (Fig. 3). to easily switch from one quench The lead charts in Fig. 3 show the medium to another is critical. The angle error (if any) from the top face systems shown in Figures 1 and 2 of a tooth to the bottom face, meas- Oil quench High-temperature cell Gas quench Transfer cart Low-temperature cell Fig. 1 — Advanced vacuum carburizing system using vari- Fig. 2 — Schematic of expandable high-production vacuum carburizing able-speed drive (VSD) technology in its oil- and gas-quench cells. furnace system having both gas- and oil-quench cells. HEAT TREATING PROGRESS • SEPTEMBER/OCTOBER 2004 41 ecm.qxp 9/15/2004 10:05 AM Page 2 some equipment currently being 2.4 supplied with oil-quench capability will need to be converted to gas quenching in the future. Another reason why gas quench- ing may be a good choice is the ability to alter the cooling rate — ac- celerating or decelerating it by pres- 21.6 sure change (densification), changing the type of cooling gas (heat transfer), and varying the speed of the gas (mass flow). Note: Current thinking Left 18 13 6 1 1 6 13 18 Right is to use the lowest possible gas- quench pressure to reduce distortion. 2.4 Why use oil quenching? In certain instances, oil quenching is the best method of producing ac- ceptable results, especially for parts having large cross sections and for 21.6 low-alloy steels. Oil-quench cells typ- ically use oil heated in the 130 to 375°F (55 to 190°C) range to mini- Bottom Top Bottom Top Bottom Top Bottom Top mize distortion. Arecent study (Table 1) compared Fig. 3 — Lead profiles (in mm) after oil quenching, top, and gas quenching, bottom. distortion after atmosphere and ured at the pitch line (middle of the vacuum oil quenching, with the ad- tooth) surface. Gas quenching re- vantage going to the vacuum method. sulted in lead profiles that are all in the same direction, making it easier Fixturing and distortion to predict tooth displacement at final One factor that affects the distor- machining. The oil-quenched gears, tion of parts, if not the biggest factor, however, have lead profiles that are is how the parts are placed and sup- not predictable, making machining ported in the furnace by the fixturing. more difficult. Conclusion: quieter Any heat treating process that re- gears can be more easily and eco- quires heating the part to above the nomically produced by adopting gas transformation temperature causes quenching. the part to lose most of the strength Fig. 4 — Load of automobile transmis- Full loads of 850 parts (Fig. 4) were that it has at room temperature. Apart sion pinion gears used in lead profile study run in both cases. Oil- and gas- subjected to extended times at ele- (Fig. 3). quenched parts met all the metallur- vated temperature (as in carburizing) gical specifications including core experiences creep and plastic defor- hardness (>35 HRC), surface hard- mation due to its own weight unless ness, and case depth. properly fixtured and supported. Other benefits: Reducing distor- Single parts are better than stacked tion without sacrificing metallur- parts. If parts have to be stacked, sup- gical, mechanical, or physical prop- port is important between the layers, erties is important. The ability to especially for thin-wall parts. Parts transform the microstructure to one also should have their surfaces that is identical to that produced readily accessible during both using another quenching medium heating and cooling. For gas (oil or salt, for example) is manda- quenching in particular, it is neces- tory. We believe it is highly likely that sary to offset parts so that layers Fig. 5 — Carbon fiber composite (CFC) fixturing for gas quenching of ring gears. Table 1 — Gear distortion: atmosphere oil quench vs. vacuum oil quench Parameter Before heat treating, in.* Atmosphere, in. Vacuum, in. Average 5.6923 5.6915 5.6910 Standard deviation 0.0005 0.0015 0.0009 Three-sigma 0.0015 0.0045 0.0027 Predicted high 5.6938 5.6960 5.6937 Predicted low 5.6908 5.6870 5.6883 Predicted range 0.0030 0.0090 0.0054 * Measurement over wires. Values before heat treatment = 5.692 to 5.690. 42 HEAT TREATING PROGRESS • SEPTEMBER/OCTOBER 2004 ecm.qxp 9/15/2004 10:06 AM Page 3 above or below are not directly on perature, quality) Motor + turbine top of each other. This helps guar- • Quench tank antee both uniform core hardness design (volume, and repeatable distortion patterns. agitation) While there is no general rule for Oil quench: In the furnace load design, proper oil quenching, the spacing and orientation of parts (ver- size of the quench Load tically and/or horizontally) are crit- tank influences Heat ical to minimizing distortion. Other both the instanta- exchanger factors include the furnace used, the neous and max- part geometry, and the quench imum rate of rise medium. It should also be noted that of the bath as well similar parts may need to be loaded as localized effects. Fig. 6 — Photo and schematic of a typical gas-quench cell. differently depending on the distor- (It does little good tion that develops after they are heat to have a large-capacity tank if parts maximizing heating efficiency and treated. are exposed to only a small fraction maximizing cooling efficiency). Composite fixtures: To take full ad- of the quenchant.) Equipment-in- Cell volume should be as small as vantage of gas quenching for chal- duced variables include circulation possible, typically around 100 ft3 (3 lenging parts such as ring gears, and method (agitator or pump), circula- m3) to limit gas consumption, espe- to ensure they remain consistently flat tion pattern, method of heating, and cially if the gas is not recycled (as is load after load, full support is neces- tank capacity (rate of rise). Other often the case when using nitrogen), sary. The use of carbon fiber com- variables include oil type, heat or to reduce the size of the recycling posite (CFC) or carbon/carbon (C/C) transfer characteristics, initial tem- system (compressor and storage composite fixturing (Fig. 5) achieves perature, and bath cleanness (con- tanks). Design and arrangement of this goal. It is important to purchase taminant type and percentage). internal components must be simple material having fibers oriented in Tip: When oil quenching follows yet rugged for high reliability and to three dimensions for a tighter weave vacuum carburizing, a controlled keep the cost of the vessel, which is and added strength. This ensures that pressure applied to the surface of the subject to the ASME Pressure Vessel the fixturing will not sag over time oil in the quench cell can influence Code, as low as possible. and contribute to distortion. Due to the vapor phase that forms and help Tip: Cooling fan motors should be its high cost and the extra care needed control dimensional change. powerful for efficient gas circulation in handling, composite fixturing is an Gas quench: Equipment-induced and water cooled to limit their size. ideal candidate for robotic loading/ variables in gas quenching include unloading of parts. hot- or cold-chamber quenching, size Using variable-speed drives In some instances, fixturing is im- of the quench cell, arrangement of The use of variable-speed drive practical. Large gears, for example, internal components, motor horse- (VSD) technology gives the gas- may have to be placed directly on a power, heat exchanger capacity, gas quench cell more flexibility in con- grid.
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