Even in the midst of troubled economic times, forward-looking companies expand their operations to better support their customer’s needs and grow the business.

Fig. 2. Typical large gear product

Fig. 1. Pit carburizing furnace line

Merit Gear LLC, a precision gearing company, has invested in a major plant expansion to accommodate in-house machining and heat-treating equipment. The company was founded in 1952 to service tractor replacement and general machine applications. Merit Gear has continuously expanded their capabilities and operations to service and broaden their customer base. They include new industries that utilize gearboxes in rigorous applications such as mining, water-well drilling, oil field, fire trucks and the rapidly growing wind-energy market.

This recent investment consisted of a 52,000-square-foot building expansion, of which 10,000 square feet was dedicated to a heat-treating cell and additional machining equipment. Merit Gear served as the general contractor.

The expansion has allowed Merit Gear to increase their machining capability for processed parts up to 132 inches (3.35 meters) in diameter. The in-house heat-treating capability, which was previously limited to small-batch integral-quench furnaces, has been dramatically increased to include large atmosphere, pit-type furnaces and auxiliary equipment capable of processing 30,000-pound loads up to 90 inches in diameter (Fig. 1). The new heat-treating equipment was designed to process large gear and pinion products that require excellent temperature and atmosphere control to meet the challenging process requirements of Merit Gear’s customers.

The heat-treating-facility expansion was designed, manufactured and installed by Surface Combustion, Inc. of Maumee, Ohio. The heat-treating cell features three radiant-tube-heated, atmosphere-type, pit carburizing furnaces, two direct-fired pit temper furnaces and two companion oil quench tanks. The commissioning of the equipment and the training of Merit Gear’s personnel was completed in January of 2009.

In addition to the heat-treating equipment, auxiliary equipment to complete the expansion included two 25-ton building cranes, a washer system and a shot-blast system.

Fig. 3. Test load leaving carburizing furnace

Heat-Treating Equipment Design Features

The new pit carburizing furnaces include many features and provide consistent, repeatable process cycles, resulting in high-quality parts (Fig. 3). The furnaces are effectively heated by radiant tubes surrounding the outside diameter of the workload area. These tubes use non-recuperated Surface Combustion natural gas burners. This design approaches the system efficiency of recuperated designs due to the correct application of the burners on long heat-treating cycles required for deep case depths. As an added feature, “plunge cooling” (accelerated cooling of the large furnace loads prior to quenching) can be accomplished by passing cool air through the inside of the radiant tubes.

Each pit carburizing furnace is configured to operate with Merit Gear’s existing nitrogen/methanol atmosphere system. The system includes a high/low flow feature to allow atmosphere conservation during the long heat-treating cycles. A closed-loop carbon-potential control system is included for precise furnace atmosphere control. This system utilizes a process controller and oxygen-probe arrangement to monitor carbon levels in the furnace. As an enhancement to the control system and its capabilities, a sample-part test port is included in the furnace cover to allow in-process evaluation of the carburizing process. The test port provides for the removal of test pins for quenching and review of the results prior to completion of the cycle for the load being processed. This evaluation allows for the long furnace cycles to be adjusted in real-time as needed for precise part carbon profiles.

To provide uniform temperatures and atmosphere flow to the work being processed, each furnace is provided with Surface Combustion’s patented vibration-dampening fan design mounted on the furnace cover. The fan works in conjunction with an internal alloy shroud for improved atmosphere and heat circulation. Thermal profiles performed during the commissioning period of the furnaces showed a temperature uniformity within the effective workload area of the furnace to be well within ±10°F between the temperatures of 1550°F and 1750°F.

After heating and processing of a load of parts in the carburizing furnaces, the overhead building crane is utilized to remove the load and transfer it into one of the 30,000-gallon oil quench-tank systems. Each quench tank includes multiple agitators working in conjunction with a fully baffled workload area to direct high-velocity oil around the parts being quenched. The quench system includes an air-to-oil heat-exchanger system and a gas-fired immersion radiant-tube heating system to either cool or heat the oil and provide precise control of the quench-oil temperature for the critical quenching process (Fig. 4).

At the completion of the quench portion of the heat-treating process, the parts and fixture are transferred to the batch washer unit. The washer is an integrated cleaning unit that uses a gas-heated, alkaline-type system. It incorporates a high-pressure spray arrangement to remove residual quench oil from the parts prior to the tempering portion of the cycle.

Two direct gas-fired, pit temper furnaces are provided, not only to provide the post-quench temper requirements, but also to allow for load preheating prior to loading into the pit carburizing furnaces. The furnaces have a wide operating temperature range to service both the preheat and temper portions of the overall process requirements. The maximum operating temperature is 1200°F.

The furnaces are an extension of Surface Combustion’s patented high-convection design, which utilizes a circular wind flow along the inside of the furnace wall in conjunction with a second circular pattern in the center of the furnace. These patterns are fed back to the cover-mounted fan unit and burners to promote overall temperature uniformity throughout the entire work area of the furnace. With this design, the measured temperature uniformity in the effective workload area was well within ±10°F.

Fig. 4. Test load being quenched in oil quench tank

Specific Heat Treatments

As previously stated, components heat treated in Merit Gear’s new pit furnace equipment include very large gears and pinion products (Fig. 2). The largest gear products may exceed 12,000 pounds per gear, with gross loading capacity in the furnaces being 30,000 pounds for multiple part loads. The typical carburizing process cycles require very deep effective case depths between 0.080 and 0.120 inches. Cycles to obtain these case depths can require days.

The current and future growth of the energy market requires ever-increasing part sizes, specifically larger and better-engineered gearboxes. With the investment that Merit Gear has made in equipment and technology, they are positioned to produce these products efficiently for many years to come. IH

For more information: Contact Merit Gear LLC, 810 Hudson Street, Antigo, WI 54409; tel: 800-756-3748; fax: 715-623-2290; e-mail: info@meritgear.com; web: www.meritgear.com or Surface Combustion, Inc., 1700 Indian Wood Circle, Maumee, OH 43537; tel: 419-891-7150; fax: 419-891-7151; e-mail: info@surfacecombustion.com; web: www.surfacecombustion.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: wind energy, heat-treating cell, pit-type furnace, atmosphere carburizing, radiant tube, temperature uniformity