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 quenching 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 typically 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. If the grid is warped, the part and water systems, and fan design. trolling hardness and distortion. The will attempt to conform to its shape The quench cell (Fig. 6) should be effect of VSD on quenching of a load during carburizing. Similarly, fur- a separate (cold) chamber isolated of manual transmission shafts is nace trays are another often over- from the heating cell. Not only is the shown in Fig. 7. Particularly note- looked cause of distortion. Recently quench faster due to heat transfer to worthy is the improvement in homo- developed and commercialized the “black body,” but the design also geneity with VSD. A load of the parts nickel aluminide materials have the can be optimized for quenching (no tested is shown in Fig. 8. potential to dramatically reduce di- compromise is necessary between Continued mensional change in trays and fixtures and lengthen their useful life. Nitrogen 800 4th gear root 2nd gear pitch The quench equipment factor 600 Equipment-induced variability is 2nd gear root 1st gear pitch perhaps the least recognized yet 400

most significant factor with respect C o 200 to producing consistency in heat treating. Understanding and controlling this aspect of the overall process 800 results in predictable (and repeat- Temperature, Temperature, able) distortion patterns. Areas of pri- 600 mary focus are: • Load size/weight (uniformity) 400 • Part orientation 200 • Heating rate

• Soak time and temperature 1 10 100 1000 • Process choice (hardening meth- Time, s od, carbon source) Fig. 7 — Effect of gas quenching on cooling homogeneity with, top, and without, bottom, • Quenchant choices (type, tem- variable-speed drive (VSD) technology. HEAT TREATING PROGRESS • SEPTEMBER/OCTOBER 2004 43 ecm.qxp 9/3/2004 11:50 AM Page 4

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) 77–80 73–76 69–72 65–68 x 61–64 x 57–60 xxx 53–56 49–52 x 45–48 xxx x Fig. 8 — Loads of manual transmission shafts were used to develop the gas-quench 41–44 xxx x cooling homogeneity curves in Fig. 6. 37–40 x x 33–36 x x x 29–32 xx xx xxx 25–28 xx x 21–24 xx xxx 17–20 xx x 13–16 x x xx 9–12 x x x 5–8 0–4 * Tooth flank profile data for ring gears. Fig. 9 — Ring gear load used in tests of the effect of VSD technology on distortion A comparison (Table 2) of oil (27°C) is generally sufficient for gas (Table 2). quenching and gas quenching (with quenching. However, for massive and without VSD) for loads of ring parts or problem alloys, cold water gears (Fig. 9) quantifies the distor- at 60°F (15°C) needs to be provided tion reduction resulting from use of to the heat exchanger to ensure suf- VSD. ficient heat extraction for high core hardness. Role of cooling water Tip: Monitoring the exit temper- The heat exchanger in a gas- ature of the water during the quench quenching system must be designed is a very good indication of quench- to allow high gas flow and good heat ing efficiency. transfer. When 20-bar quenching of To reduce the instantaneous de- a massive load having a high surface mand for cold water, consider a area, the heat extraction rate during system that incorporates a chiller. the first few seconds of the quench is These systems produce enough cold extremely high. Thus, improper water to satisfy the demands of the water flow, low pressure, or flow re- gas-quench cell during the first 2 to strictions can result in vaporization/ 3 minutes of the quench. A switch boiling of the water in the heat ex- can then be made to regular cooling changer. A slack quench can result tower water for the remainder of the or, even worse, the heat exchanger quench. or piping could be damaged due to a rapid buildup of pressure. Oil- and gas-quench examples Core hardness: Controlling water Selection of the quench medium temperature is very important for should be made only after the part’s consistent results, not only during end-use performance properties are Fig. 10 — Truck transmission shafts, top, a given cycle but throughout the year understood, and the design and ma- were oil quenched, while the automobile axle — warmer water running in the heat terial have been chosen. These fac- pinion gears, bottom, were high-pressure gas exchanger during summer can lower tors dictate whether a low-pressure quenched. the core hardness by a few Rockwell or vacuum carburizing process points. This is particularly true for should be combined with oil or gas low-hardenability material. quenching to develop the required A water temperature of 80°F properties. 44 HEAT TREATING PROGRESS • SEPTEMBER/OCTOBER 2004 ecm.qxp 9/3/2004 11:50 AM Page 5

For example, large truck transmis- Table 3 — Case hardening quench medium selection guide1 sion shafts (Fig. 10, top) made of AISI Material Quench Gas pressure, Core hardness, 8620 low-alloy steel (20NiCrMo2) 2 3 must be oil quenched to develop the grade medium bar HRC Process required properties. The core hard- 1018 Oil – 20–24 Carburizing ness of 25 HRC at mid-radius is only 1030 Oil – 20–24 Carburizing achievable by quenching in 195°F 1117 Oil – 22–26 Carbonitriding (90°C) oil. The effective case depth 12L14 Oil – 22–26 Carbonitriding of 0.050 in. (1.3 mm) was obtained in 3310 Gas 10–15 32–36 Carburizing an overall cycle time of 5 hours. Load 4027 Gas 15–20 33–35 Carburizing weight: 1000 lb (450 kg). 4118 Gas 15–20 33–35 Carburizing Gas quench: By contrast, automo- 4120 Gas 15–20 33–35 Carburizing bile axle pinion gears (Fig. 10, 4142M Oil – 50 Carburizing bottom) made of AISI 4320M (M = modified) were high-pressure gas 4320 Gas 12–20 33–35 Carburizing quenched using 10-bar nitrogen. A 4615 Gas 12–20 28–34 Carburizing core hardness of 37 HRC and an ef- 4620 Gas 10–20 30–35 Carburizing fective case depth of 0.030 in. (0.75 4820 Gas 12–20 32–38 Carburizing mm) were achieved in a total cycle 5115 Oil/Gas 12–20 32–36 Carburizing time of 3 hours. Gross load weight: 5120 Gas 15–20 28–32 Carburizing 800 lb (360 kg). 8620 Gas 20 32–40 Carburizing Many low-pressure/vacuum car- 8822 Gas 20 38–42 Carburizing burizing applications are candidates 9310 Gas 5–10 38–44 Carburizing for gas quenching (Table 3). Ferrium C614 Gas 2–3 50–54 Carburizing Ferrium C61S4 Gas 2–3 50–52 Carburizing Why nitrogen is favored Ferrium CS624 Gas 2–3 50–52 Carburizing Nitrogen is the lowest cost and 4 safest gas for high-pressure gas Ferrium C69 Gas 2–3 50–52 Carburizing 5 quenching. It is used in almost all Pyrowear 53 Gas 5 36–40 Carburizing (99%) of today’s production equip- Pyrowear 6755 Gas 20 40–44 Carburizing ment. Gas pressure systems up to 20 1. Data for section thickness up to 1 in. (25 mm). bar are common. 2. AISI/SAE designations except for Pyrowear and Ferrium alloys. 3. Gas-quench medium is nitrogen. Quench cell designs that offer ex- 4. Alloys developed by QuesTek Innovations LLC, Evanston, Ill., using its Materials by Design technology. Ferrium tremely fast cooling rates take ni- is a registered trademark of QuesTek Innovations LLC. trogen’s thermodynamic properties, 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. including its density, into consideration. The resultant cooling rate is ef- gases has produced a joint patented 14 technology for a mixture of carbon fective for quenching most common 12 12 steels, including those used for auto dioxide (CO2) and helium (He) that 11 powertrain gears. produces cooling rates typically 30% 10 Nitrogen quench gas can be re- faster than a pure nitrogen quench. 8 covered and recycled. However, a Advantages of this technology over 8 return on investment (ROI) analysis 100% helium quenching are reduced comparing the cost of recovered ni- costs and conservation of a limited 6 trogen and discharged nitrogen helium supply. Recycling systems Cooling rate, °C/s 4 should be made for each individual have already been designed for use project. The analysis should take with the CO2-He gas mixture. 2 into account the recovery system The new mixture also has the same density as pure nitrogen. This means purchase price, cost to operate the N He CO -He system, and compressor mainte- that a quench cell optimized for use 2 2 nance. Often, the price of nitrogen with 100% nitrogen can utilize the Fig. 11 — Average cooling rate observed may be such that discharging the new technology without needing to in gas quenching of AISI 5120 low-alloy steel gas after every quench is the most be modified. ring gears using nitrogen, helium, and a CO2- cost-effective solution. In one test, 660 lb (300 kg) of AISI He mixture. High-pressure (30 bar) liquid ni- 5120 ring gears weighing 7 lbs (3.2 trogen systems are now being pro- kg) each and arranged in eight posed by gas producers as an alter- layers were quenched in various gas native to the use of a costly mixtures to determine the gas pres- compressor. At this pressure, the ni- sure required to fully transform the trogen can be directly supplied to the steel. Cooling rate for a CO2-He mix- surge (accumulator) tanks. ture was 50% higher than that for pure nitrogen (Fig. 11). In another Gas mixture speeds quench example, a 15% improvement in Extensive research by ECM and core hardness was recorded for 26 Air Liquide into alternative quench lb (12 kg) AISI 8620 ring gears when HEAT TREATING PROGRESS • SEPTEMBER/OCTOBER 2004 45 ecm.qxp 9/7/2004 2:52 PM Page 6

they were quenched in a CO2-He nology is being applied to both oil- a good alternative, particularly when mixture. and gas-quench cells to enhance load parts are too massive or the material homogeneity and load-to-load con- is too lean in alloy content to produce Gas quenching’s future sistency. Optimum placement of good results any other way. Gas quenching is proven, user quench-tank internal components is Finally, the emergence of friendly, and helps simplify manu- being determined by using compu- quenching simulation software will facturing. Gas quenching plus tational fluid dynamics (CFD) to help guide the user to load parts vacuum carburizing has a bright fu- model the quench tanks. more efficiently and to develop ture and will play an increasingly im- Further improvements in gas- more-effective quenching recipes, in- portant role as products are designed quench cooling speed and homo- cluding interrupted quenching, as to take advantage of the unique ben- geneity can be expected in the future. dictated by the material per- efits of the combination process. However, oil quenching will remain formance requirements. Trends: To achieve more predictable results and lower part distortion, vacuum carburizing users For more information: Mr. Beauchesne is general manager, ECM USA Inc., 5732 95th Ave, Suite 900, Kenosha, WI 53144-7801; tel: 262/605-4810; fax: 262/605-4814; e-mail: and potential users are actively [email protected]; Web: www.ecm-usa.com. Mr. Goldsteinas is process working with materials researchers engineer, ECM France, 46 Rue Jean Vaujany – Technisud, 38100 Grenoble, France; tel: to develop less costly steels having +33 (0)4 76 49 65 60; fax: +33 (0)4 38 49 04 03; e-mail: [email protected]; Web: better hardenability. Even within the www.ecm-ip.com same steel grade a user often can specify, with no cost premium, ma- Selected references terial at the upper end of the harden- “Modular Vacuum Thermal Processing Installation”: U.S. Patent 6,065,964, May 23, ability band. This provides greater 2000. flexibility in quenching and helps “Vacuum Carburizing — A Technology Whose Time Has Come,” by Dennis Beauch- minimize distortion. This strategy esne and Xavier Doussot: Industrial Heating, September 2003 (special insert). “Experience in Low Pressure Vacuum Carburizing,” by T. Hebauf and Aymeric Gold- has been found to be particularly steinas: Industrial Heating, January 2003, p. 29–33. useful in gas quenching, and ac- “Low Pressure Carburizing Using the INFRACARB® Process,” by A. Goldsteinas: counts for a trend toward lower gas Technical Seminar for Heat Treatment, Tokyo, Japan, February 2003. pressures. “New High Pressure Gas Quenching Solutions,” by L. Lefevre: presented at the ASM Variable-speed drive (VSD) tech- Heat Treat 2003 Conference, Indianapolis, Ind., Sept. 15–17, 2003

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