heat treat pit furnace

A Pennsylvania company recently heat treated and quenched a fabricated 47,000-pound gear. In addition to this gear, about a month ago, its reverse image (with the helix in the opposite direction) was also heat treated. The halves will be matched when grinding is complete to make a complete gear to drive a rolling mill for a steel plant.

The gear, the largest Metlab has ever treated, measured 12' in diameter and has a 30” face width. Material was 18CrNiMo6-7, and case depth required was a nominal 0.275” effective case depth with a surface hardness of HRC 58 – 62. Typical taper on a gear this size is about 0.030”. Carburizing time to achieve the required case depth is about 10 days in the furnace at 1725⁰F. After lowering the temperature of the furnace and gear to 1550⁰F, it is quenched in oil. The transfer time from the pit furnace into the quench tank is less than a minute.

After quench, the gear will be double tempered, sandblasted and prepared for shipment to the Midwest for final grinding. Mark Podob, president at Metlab, commented, "To our knowledge, the furnace used to heat treat this series of four gears is the largest pit carburizing furnace in the United States . . . . [T]he two 47,000 pound gears are the largest that Metlab has carburized and hardened in its 25 year history."

Photographs show the gear at 1550°F being lowered into the oil tank.

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