Increased demand for large, high-precision gears requires specialized heat-treating equipment. In addition to size, exacting metallurgical requirements and repeatable process parameters affect furnace selection.
Most large gears are processed in pit furnaces or large, batch integral-quenching furnaces. Vacuum carburizing has not been commonly used because large cross sections of most typical alloys cannot be effectively quenched with pressurized gas. Stack Metallurgical Services in Portland, Ore., installed a vacuum-carburizing, oil-quenching furnace capable of processing large gears. Precise control of process gases, uniform heating and process repeatability make vacuum carburizing an excellent choice for gear applications.
This two-chamber, low-pressure-carburizing, oil-quenching vacuum furnace was manufactured by SECO/WARWICK. Load capability is 70 inches wide x 70 inches deep x 80 inches high with a maximum total load weight of 10,000 pounds. Loads are suspended from a CFC overhead trolley and are not set on an internal hearth. Processes are PLC controlled and highly repeatable. The hot zone is insulated by 50 mm of graphite, allowing operating temperatures up to 1950°F (1066˚C) and producing furnace uniformity of ±15°F at temperatures of 1100-1950°F (593-1066˚C). Process gases are introduced through mass flow meters, giving very precise control of the carburizing environment.
Quenching is done in a 9,500-gallon tank of oil. The agitation system was upgraded with four additional motors to improve oil flow and provide adequate heat removal for very large loads. Uniform fluid flow is critical for uniform quenching of the entire load. Because the entire furnace is under vacuum during the quench, heat loss is minimal during load transfer and surface condition of the parts is not degraded.
Vacuum carburizing is a non-equilibrium process that uses a series of boost and diffuse cycles to introduce carbon into the surface of a part. Process gas composition is very carefully controlled to match the requirements of each alloy and case depth. Intergranular oxidation, case uniformity and process predictability are advantages of this method.
Surface intergranular oxidation (IGO) is caused by oxygen present during the carburizing cycle. This creates defects at the surface that can be initiation sites for cracking (Fig. 1). Most standard practice requires a post-heat-treating operation in the root, typically grinding or shot peening. Vacuum carburizing eliminates IGO. Air is pumped out of the furnace prior to heat-up and carburizing, minimizing the presence of oxygen during processing. This results in higher integrity at the surface of the carburized case and improved wear and fatigue resistance. Post-heat-treat machining or shot peening of tooth roots can be reduced or eliminated.
Vacuum carburizing provides excellent case uniformity for complex geometries. In a standard endothermic furnace, parts are introduced with carburizing gas present and the furnace at a high temperature. As the load heats, the thin cross sections of gears at the tooth tips reach carburizing temperatures well before thicker sections at the root. This yields a wide variation in case depths across the tooth profile.
During vacuum carburizing, the parts are heated to a uniform temperature before the process gases are introduced. As a result, multiple runs processed using the same set of parameters produced a very narrow range of case depths. A series of loads run with a case aim point of 0.100 inches had a spread of +0.007 inches to -0.005 inches, which is much less than standard endothermic gas processes (Fig. 2).Gear root to flank case-depth ratios are improved to as much as 90% compared to conventional methods, which achieve 65-70% ratios. Higher strength at the root for a given flank case depth can help improve overall gear design or reduce heat-treat process time for a given design requirement. In addition, high hardness can be maintained deep into the case (Fig. 3).
Stack is currently using a simulation program for process development. All process parameters, including material chemistry, can be adjusted. A given case requirement can be simulated with numerous parameter changes before the first test load is performed. The results of the test can then be used to adjust the simulation, allowing the program to precisely predict outcomes for a given furnace and alloy. Process development for new alloys and case depths is shortened, which improves heat-treating lead times.
The furnace controls allow precise alloy-specific process parameters. This yields consistent, predictable mechanical properties in the case, control of carbide presence and size, and accurate control of carbon content. Improved consistency and control can result in improved fatigue resistance.
Process Predictability – Distortion
Distortion control of complex parts like gears is the greatest challenge facing the heat treater. Symmetrical heating and cooling are critical to minimizing the movement of a part during processing. Residual stresses, alloy uniformity, prior thermal history and machining practices all contribute to the final outcome but are not controlled by the heat treater. Gear manufacturers can help decrease distortion in their parts by understanding how to reduce each of these factors prior to sending them to their heat treater. Process design, fixtures, furnace uniformity and quenching must be carefully considered and controlled by the heat treater, however, to minimize or reduce distortion.
A unique feature of this furnace type is the elimination of the hearth. The load is suspended during processing, which provides both advantages and challenges. Fixture design allows the parts to contact the quench oil without creating excessive turbulence, providing very uniform quench flow and heat removal. Part-specific fixtures provide the best opportunity to successfully quench a part. For example, flatness on a 32-inch-diameter gear improved from 0.100 inches total to less than half that amount. Ovality was also maintained within very acceptable limits (Fig. 4).
Size change is an unavoidable part of heat treatment. When a carburized gear is quenched and the material transforms to martensite (molecular structure of hardened steel), the volume of the steel increases in proportion to the carbon present. This causes a net size change that affects the overall distortion. If the part is carburized uniformly over the entire surface, the resulting distortion is minimized. Complex geometries like gears are difficult to uniformly carburize in conventional endothermic furnaces. Therefore, distortion can be greater.
Vacuum (low-pressure) carburizing offers many advantages for the processing of large gears. These advantages include precise control of process gases for the elimination of intergranular oxidation (IGO), and the uniform heating provides carburized-case consistency. The repeatable and uniform nature of the vacuum-carburizing process results in a predictable process with minimum distortion of the complex gear geometry. IH
For more information: Contact Nels Plough, president/general manager, Stack Metallurgical Services, Inc, 5938 N. Basin Avenue, Portland, OR 97217-0176; tel: 503-285-7703; fax: 503-285-2785; e-mail: firstname.lastname@example.org; web: www.stackmet.com