FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 2 2 4 6 Optimal heat treating of gears depends on several factors, including part design, fixturing, and process development. Different furnaces may offer unique advantages, such asminimizing part distortion, while operating and maintenance costs vary greatly for hardening furnaces. The challenge is to understand which furnace type can most effectively process the gear design and material grade. Protective-atmosphere furnace solutions are well-suited for hardening of gears. The process techniques include gas or vacuum carburizing, carbonitriding, and neutral hardening in a carbon-based atmosphere or in a vacuum, Fig. 1. This article looks at vacuum, controlled atmosphere, and hybrid furnace types highlighting available processes while sharing respective associated operation andmaintenance costs. CASE STUDY COMPARISONS To determine which heat treat furnace is ideal for gear heat treatment, a few case studies and cost comparisons were reviewed. It is important to note that previous processing steps such as forging, machining, stress relieving/ normalizing, hardening, and quenching processes will all affect the final dimensional variance. Elimination of unpredictable residual stresses from as many of these processing steps as possible will be required to improve the final part dimensions. With gears increasing in size, distortion challenges are greater as the same percentage of distortion equates to a much larger absolute distortion. All steps with regard to the process need to be evaluated, as the final heat treating step can make the distortion worse, but generally is not capable of improving distortion characteristics. To develop the furnace load and process it is beneficial to use available modeling programs. For this, the CHTE-bf and CarbTool programs were used. These programs were developed at the Center for Heat Treating Excellence at Worcester Polytechnic Institute (WPI). The first example is of a satellite gear. The workpiece was estimated using the models shown in Fig. 2. The furnace data was entered including materials of construction and work zone volumes. The connected heat input was varied to calculate different designs’ recovery time in between loads, utilizing load pre-heating or no pre-heating of incoming loads. This technique showed 30 minutes savTECHNIQUES AND EQUIPMENT FOR GEAR HARDENING A comparison of different heat treat processes and furnace types typically used to harden gears. Benjamin T. Bernard* Surface Combustion Inc., Maumee, Ohio ings in the batch integral quench (BIQ) furnace and 15-20% fuel savings. The carburizing cycle can be modeled as well, and in this case CarbTool was used to define the cycle per customer’s specification, again starting with the workpiece definition. Either atmosphere carburizing or vacuum carburizing calculations are possible with CarbTool. Comparisons of atmosphere versus vacuum carburizing using CarbTool were previously reported[1]. This model was for atmosphere carburizing. The estimated surface carbon and case depth were modeled for the satellite gear, Figs. 3 and 4. A BIQ furnace load with pinion gears was evaluated to document oil quenching uniformity. A 36 x 48 x 36inch load with 3200 lbs of pinion gears with multiple layers had 0.003 wt% carbon spread from top to bottom of load at a depth of 0.04 in. The capability study tested 18 parts throughout a single load. Average measured properties were surface hardness 60.8 Rc, total case depth was 0.064 in., effective case depth was 0.045 in., pitch hardness was 37.6 Rc, and root hardness was 35.2 Rc. Surface hardness and total case variation was 1%, while effective case depth, pitch and root hardness varied 5 to 6%, respectively. The quench velocity around the parts was documented and compared to product literature and customer requirements, which averaged 123 GPM (0.625 m/s). *Member of ASM International Fig. 1 — Vacuum vestibule BIQ, flameless type. Courtesy of Chugai Ro Co. Ltd. 10
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