A USER S GUIDE TO QUENCHING AFTER VACUUM CARBURIZING

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1 A USER S GUIDE TO QUENCHING AFTER VACUUM CARBURIZING This article discusses the factors heat treaters have to consider when deciding when to apply oil quenching or gas quenching for cooling low-pressure/vacuum carburized steel parts. by Dennis Beauchesne ECM USA Inc. Kenosha, Wis. Aymeric Goldsteinas ECM France Grenoble, France Common applications for vacuum carburizing include automobile power transmission and fuel system components, lightand heavy-duty truck and off-highway vehicle transmission components such as ring gears and pinions, aerospace transmission and actuator systems, and various industrial products such as hydraulic pump cams, bearings, and valves. Today, most vacuum carburizing is performed at temperatures of 1700 to 1800 F (930 to 980 C), with effective case depths of to in. (0.25 to 2.05 mm) or greater. Typical production load weights range from 500 to 1000 lb (225 to 455 kg). Vacuum carburizing often eliminates the need for slow cooling, reheating, and subsequent press or plug quenching. And a copper electroplate or stop-off paint can be used to prevent the carburization of selected surfaces. Vacuum carburizing systems must be rugged, versatile, and cost effective since the parts processed in them may change tomorrow in size, shape, material, required properties, or throughput. In addition, the ability to easily switch from one quench medium to another is critical. The systems shown in Figures 1 and 2 satisfy all of these requirements. Arriving at the best choice of quench medium requires careful consideration of a number of factors, including: Economics/cost (initial investment, maintenance, upkeep, life) Health and safety (codes, regulations, exposure) Minimization of distortion (quench system) Performance (cooling rate/ quench severity) Versatility (controllable cooling rates) Environmental issues (waste disposal and noise, for example) Why use gas quenching? One of the reasons for the intense interest in vacuum carburizing combined with gas quenching is the ability to achieve dramatic reductions in part distortion (dimensional variation), especially when compared with atmosphere oil quenching. For example, automatic transmission pinion gear lead profiles were compared after atmosphere carburizing and oil quenching in an integralquench furnace and after low-pressure carburizing and 15-bar nitrogen gas quenching (Fig. 3). The lead charts in Fig. 3 show the angle error (if any) from the top face of a tooth to the bottom face, meas- Oil quench High-temperature cell Gas quench Transfer cart Fig. 1 Advanced vacuum carburizing system using variable-speed drive (VSD) technology in its oil- and gas-quench cells. Low-temperature cell Fig. 2 Schematic of expandable high-production vacuum carburizing furnace system having both gas- and oil-quench cells. HEAT TREATING PROGRESS SEPTEMBER/OCTOBER

2 Left Right Fig. 3 Lead profiles (in mm) after oil quenching, top, and gas quenching, bottom. Fig. 4 Load of automobile transmission pinion gears used in lead profile study (Fig. 3). Fig. 5 Carbon fiber composite (CFC) fixturing for gas quenching of ring gears. ured at the pitch line (middle of the tooth) surface. Gas quenching resulted in lead profiles that are all in the same direction, making it easier to predict tooth displacement at final machining. The oil-quenched gears, however, have lead profiles that are not predictable, making machining more difficult. Conclusion: quieter gears can be more easily and economically produced by adopting gas quenching. Full loads of 850 parts (Fig. 4) were run in both cases. Oil- and gasquenched parts met all the metallurgical specifications including core hardness (>35 HRC), surface hardness, and case depth. Other benefits: Reducing distortion without sacrificing metallurgical, mechanical, or physical properties is important. The ability to transform the microstructure to one that is identical to that produced using another quenching medium (oil or salt, for example) is mandatory. We believe it is highly likely that some equipment currently being supplied with oil-quench capability will need to be converted to gas quenching in the future. Another reason why gas quenching may be a good choice is the ability to alter the cooling rate accelerating or decelerating it by pressure change (densification), changing the type of cooling gas (heat transfer), and varying the speed of the gas (mass flow). Note: Current thinking is to use the lowest possible gasquench pressure to reduce distortion. Why use oil quenching? In certain instances, oil quenching is the best method of producing acceptable results, especially for parts having large cross sections and for low-alloy steels. Oil-quench cells typically use oil heated in the 130 to 375 F (55 to 190 C) range to minimize distortion. A recent study (Table 1) compared distortion after atmosphere and vacuum oil quenching, with the advantage going to the vacuum method. Fixturing and distortion One factor that affects the distortion of parts, if not the biggest factor, is how the parts are placed and supported in the furnace by the fixturing. Any heat treating process that requires heating the part to above the transformation temperature causes the part to lose most of the strength that it has at room temperature. Apart subjected to extended times at elevated temperature (as in carburizing) experiences creep and plastic deformation due to its own weight unless properly fixtured and supported. Single parts are better than stacked parts. If parts have to be stacked, support is important between the layers, especially for thin-wall parts. Parts also should have their surfaces readily accessible during both heating and cooling. For gas quenching in particular, it is necessary to offset parts so that layers Table 1 Gear distortion: atmosphere oil quench vs. vacuum oil quench Parameter Before heat treating, in.* Atmosphere, in. Vacuum, in. Average Standard deviation Three-sigma Predicted high Predicted low Predicted range * Measurement over wires. Values before heat treatment = to HEAT TREATING PROGRESS SEPTEMBER/OCTOBER 2004 Bottom Top Bottom Top

3 above or below are not directly on top of each other. This helps guarantee both uniform core hardness and repeatable distortion patterns. While there is no general rule for the furnace load design, proper spacing and orientation of parts (vertically and/or horizontally) are critical to minimizing distortion. Other factors include the furnace used, the part geometry, and the quench medium. It should also be noted that similar parts may need to be loaded differently depending on the distortion that develops after they are heat treated. Composite fixtures: To take full advantage of gas quenching for challenging parts such as ring gears, and to ensure they remain consistently flat load after load, full support is necessary. The use of carbon fiber composite (CFC) or carbon/carbon (C/C) composite fixturing (Fig. 5) achieves this goal. It is important to purchase material having fibers oriented in three dimensions for a tighter weave and added strength. This ensures that the fixturing will not sag over time and contribute to distortion. Due to its high cost and the extra care needed in handling, composite fixturing is an ideal candidate for robotic loading/ unloading of parts. In some instances, fixturing is impractical. Large gears, for example, may have to be placed directly on a grid. If the grid is warped, the part will attempt to conform to its shape during carburizing. Similarly, furnace trays are another often overlooked cause of distortion. Recently developed and commercialized nickel aluminide materials have the potential to dramatically reduce dimensional change in trays and fixtures and lengthen their useful life. The quench equipment factor Equipment-induced variability is perhaps the least recognized yet most significant factor with respect to producing consistency in heat treating. Understanding and controlling this aspect of the overall process results in predictable (and repeatable) distortion patterns. Areas of primary focus are: Load size/weight (uniformity) Part orientation Heating rate Soak time and temperature Process choice (hardening method, carbon source) Quenchant choices (type, temperature, quality) Quench tank design (volume, agitation) Oil quench: In oil quenching, the size of the quench tank influences both the instantaneous and maximum rate of rise of the bath as well as localized effects. (It does little good to have a large-capacity tank if parts are exposed to only a small fraction of the quenchant.) Equipment-induced variables include circulation method (agitator or pump), circulation pattern, method of heating, and tank capacity (rate of rise). Other variables include oil type, heat transfer characteristics, initial temperature, and bath cleanness (contaminant type and percentage). Tip: When oil quenching follows vacuum carburizing, a controlled pressure applied to the surface of the oil in the quench cell can influence the vapor phase that forms and help control dimensional change. Gas quench: Equipment-induced variables in gas quenching include hot- or cold-chamber quenching, size of the quench cell, arrangement of internal components, motor horsepower, heat exchanger capacity, gas and water systems, and fan design. The quench cell (Fig. 6) should be a separate (cold) chamber isolated from the heating cell. Not only is the quench faster due to heat transfer to the black body, but the design also can be optimized for quenching (no compromise is necessary between Temperature, o C Heat exchanger Motor + turbine Fig. 6 Photo and schematic of a typical gas-quench cell. maximizing heating efficiency and maximizing cooling efficiency). Cell volume should be as small as possible, typically around 100 ft 3 (3 m 3 ) to limit gas consumption, especially if the gas is not recycled (as is often the case when using nitrogen), or to reduce the size of the recycling system (compressor and storage tanks). Design and arrangement of internal components must be simple yet rugged for high reliability and to keep the cost of the vessel, which is subject to the ASME Pressure Vessel Code, as low as possible. Tip: Cooling fan motors should be powerful for efficient gas circulation and water cooled to limit their size. Using variable-speed drives The use of variable-speed drive (VSD) technology gives the gasquench cell more flexibility in controlling hardness and distortion. The effect of VSD on quenching of a load of manual transmission shafts is shown in Fig. 7. Particularly noteworthy is the improvement in homogeneity with VSD. Aload of the parts tested is shown in Fig. 8. Continued Nitrogen 4th gear root 2nd gear pitch 2nd gear root 1st gear pitch Time, s Fig. 7 Effect of gas quenching on cooling homogeneity with, top, and without, bottom, variable-speed drive (VSD) technology. HEAT TREATING PROGRESS SEPTEMBER/OCTOBER Load

4 Fig. 8 Loads of manual transmission shafts were used to develop the gas-quench cooling homogeneity curves in Fig. 6. Fig. 9 Ring gear load used in tests of the effect of VSD technology on distortion (Table 2). Fig. 10 Truck transmission shafts, top, were oil quenched, while the automobile axle pinion gears, bottom, were high-pressure gas quenched. Table 2 Effect of variable-speed drive (VSD) technology on distortion* Number of readings in range Dimensional Gas quench Gas quench change, µm Oil quench (without VSD) (with VSD) x x xxx x xxx x xxx x x x x x x xx xx xxx xx x xx xxx xx x x x xx 9 12 x x x * Tooth flank profile data for ring gears. A comparison (Table 2) of oil quenching and gas quenching (with and without VSD) for loads of ring gears (Fig. 9) quantifies the distortion reduction resulting from use of VSD. Role of cooling water The heat exchanger in a gasquenching system must be designed to allow high gas flow and good heat transfer. When 20-bar quenching of a massive load having a high surface area, the heat extraction rate during the first few seconds of the quench is extremely high. Thus, improper water flow, low pressure, or flow restrictions can result in vaporization/ boiling of the water in the heat exchanger. A slack quench can result or, even worse, the heat exchanger or piping could be damaged due to a rapid buildup of pressure. Core hardness: Controlling water temperature is very important for consistent results, not only during a given cycle but throughout the year warmer water running in the heat exchanger during summer can lower the core hardness by a few Rockwell points. This is particularly true for low-hardenability material. A water temperature of 80 F (27 C) is generally sufficient for gas quenching. However, for massive parts or problem alloys, cold water at 60 F (15 C) needs to be provided to the heat exchanger to ensure sufficient heat extraction for high core hardness. Tip: Monitoring the exit temperature of the water during the quench is a very good indication of quenching efficiency. To reduce the instantaneous demand for cold water, consider a system that incorporates a chiller. These systems produce enough cold water to satisfy the demands of the gas-quench cell during the first 2 to 3 minutes of the quench. A switch can then be made to regular cooling tower water for the remainder of the quench. Oil- and gas-quench examples Selection of the quench medium should be made only after the part s end-use performance properties are understood, and the design and material have been chosen. These factors dictate whether a low-pressure or vacuum carburizing process should be combined with oil or gas quenching to develop the required properties. 44 HEAT TREATING PROGRESS SEPTEMBER/OCTOBER 2004

5 For example, large truck transmission shafts (Fig. 10, top) made of AISI 8620 low-alloy steel (20NiCrMo2) must be oil quenched to develop the required properties. The core hardness of 25 HRC at mid-radius is only achievable by quenching in 195 F (90 C) oil. The effective case depth of in. (1.3 mm) was obtained in an overall cycle time of 5 hours. Load weight: 1000 lb (450 kg). Gas quench: By contrast, automobile axle pinion gears (Fig. 10, bottom) made of AISI 4320M (M = modified) were high-pressure gas quenched using 10-bar nitrogen. A core hardness of 37 HRC and an effective case depth of in. (0.75 mm) were achieved in a total cycle time of 3 hours. Gross load weight: 800 lb (360 kg). Many low-pressure/vacuum carburizing applications are candidates for gas quenching (Table 3). Why nitrogen is favored Nitrogen is the lowest cost and safest gas for high-pressure gas quenching. It is used in almost all (99%) of today s production equipment. Gas pressure systems up to 20 bar are common. Quench cell designs that offer extremely fast cooling rates take nitrogen s thermodynamic properties, including its density, into consideration. The resultant cooling rate is effective for quenching most common steels, including those used for auto powertrain gears. Nitrogen quench gas can be recovered and recycled. However, a return on investment (ROI) analysis comparing the cost of recovered nitrogen and discharged nitrogen should be made for each individual project. The analysis should take into account the recovery system purchase price, cost to operate the system, and compressor maintenance. Often, the price of nitrogen may be such that discharging the gas after every quench is the most cost-effective solution. High-pressure (30 bar) liquid nitrogen systems are now being proposed by gas producers as an alternative to the use of a costly compressor. At this pressure, the nitrogen can be directly supplied to the surge (accumulator) tanks. Gas mixture speeds quench Extensive research by ECM and Air Liquide into alternative quench Table 3 Case hardening quench medium selection guide 1 Material Quench Gas pressure, Core hardness, grade 2 medium 3 bar HRC Process 1018 Oil Carburizing 1030 Oil Carburizing 1117 Oil Carbonitriding 12L14 Oil Carbonitriding 3310 Gas Carburizing 4027 Gas Carburizing 4118 Gas Carburizing 4120 Gas Carburizing 4142M Oil 50 Carburizing 4320 Gas Carburizing 4615 Gas Carburizing 4620 Gas Carburizing 4820 Gas Carburizing 5115 Oil/Gas Carburizing 5120 Gas Carburizing 8620 Gas Carburizing 8822 Gas Carburizing 9310 Gas Carburizing Ferrium C61 4 Gas Carburizing Ferrium C61S 4 Gas Carburizing Ferrium CS62 4 Gas Carburizing Ferrium C69 4 Gas Carburizing Pyrowear 53 5 Gas Carburizing Pyrowear Gas Carburizing 1. Data for section thickness up to 1 in. (25 mm). 2. AISI/SAE designations except for Pyrowear and Ferrium alloys. 3. Gas-quench medium is nitrogen. 4. Alloys developed by QuesTek Innovations LLC, Evanston, Ill., using its Materials by Design technology. Ferrium is a registered trademark of QuesTek Innovations LLC. 5. Pyrowear Alloy 53 (UNS K71040, AMS 6308) and Pyrowear 675 Stainless (AMS 5930). Pyrowear is a registered trademark of CRS Holdings Inc., Subs. Carpenter Technology Corp., Wyomissing, Pa. gases has produced a joint patented technology for a mixture of carbon dioxide (CO 2 ) and helium (He) that produces cooling rates typically 30% faster than a pure nitrogen quench. Advantages of this technology over 100% helium quenching are reduced costs and conservation of a limited helium supply. Recycling systems have already been designed for use with the CO 2 -He gas mixture. The new mixture also has the same density as pure nitrogen. This means that a quench cell optimized for use with 100% nitrogen can utilize the new technology without needing to be modified. In one test, 660 lb (300 kg) of AISI 5120 ring gears weighing 7 lbs (3.2 kg) each and arranged in eight layers were quenched in various gas mixtures to determine the gas pressure required to fully transform the steel. Cooling rate for a CO 2 -He mixture was 50% higher than that for pure nitrogen (Fig. 11). In another example, a 15% improvement in core hardness was recorded for 26 lb (12 kg) AISI 8620 ring gears when Cooling rate, C/s N 2 He CO 2 -He Fig. 11 Average cooling rate observed in gas quenching of AISI 5120 low-alloy steel ring gears using nitrogen, helium, and a CO 2 - He mixture. HEAT TREATING PROGRESS SEPTEMBER/OCTOBER

6 they were quenched in a CO 2 -He mixture. Gas quenching s future Gas quenching is proven, user friendly, and helps simplify manufacturing. Gas quenching plus vacuum carburizing has a bright future and will play an increasingly important role as products are designed to take advantage of the unique benefits of the combination process. Trends: To achieve more predictable results and lower part distortion, vacuum carburizing users and potential users are actively working with materials researchers to develop less costly steels having better hardenability. Even within the same steel grade a user often can specify, with no cost premium, material at the upper end of the hardenability band. This provides greater flexibility in quenching and helps minimize distortion. This strategy has been found to be particularly useful in gas quenching, and accounts for a trend toward lower gas pressures. Variable-speed drive (VSD) technology is being applied to both oiland gas-quench cells to enhance load homogeneity and load-to-load consistency. Optimum placement of quench-tank internal components is being determined by using computational fluid dynamics (CFD) to model the quench tanks. Further improvements in gasquench cooling speed and homogeneity can be expected in the future. However, oil quenching will remain a good alternative, particularly when parts are too massive or the material is too lean in alloy content to produce good results any other way. Finally, the emergence of quenching simulation software will help guide the user to load parts more efficiently and to develop more-effective quenching recipes, including interrupted quenching, as dictated by the material performance requirements. For more information: Mr. Beauchesne is general manager, ECM USA Inc., th Ave, Suite 900, Kenosha, WI ; tel: 262/ ; fax: 262/ ; dennisbeauchesne@ecm-usa.com; Web: Mr. Goldsteinas is process engineer, ECM France, 46 Rue Jean Vaujany Technisud, Grenoble, France; tel: +33 (0) ; fax: +33 (0) ; a.goldsteinas@ecm-ip.com; Web: Selected references Modular Vacuum Thermal Processing Installation : U.S. Patent 6,065,964, May 23, Vacuum Carburizing A Technology Whose Time Has Come, by Dennis Beauchesne and Xavier Doussot: Industrial Heating, September 2003 (special insert). Experience in Low Pressure Vacuum Carburizing, by T. Hebauf and Aymeric Goldsteinas: Industrial Heating, January 2003, p Low Pressure Carburizing Using the INFRACARB Process, by A. Goldsteinas: Technical Seminar for Heat Treatment, Tokyo, Japan, February New High Pressure Gas Quenching Solutions, by L. Lefevre: presented at the ASM Heat Treat 2003 Conference, Indianapolis, Ind., Sept , 2003 Circle 31 or visit