Heat Treating a 47,000 Pound Gear

How to Carburize a 47,000 Pound Gear Destined for a Rolling Mill in a Steel Mill-From, Mark Podob, President, Metlab

“With the largest pit carburizing furnaces in North America, no one does it better than we do – we quenched this fabricated 47,000-pound gear yesterday afternoon. It was the reverse image with the helix in the opposite direction to one that we heat treated about a month ago.

Gear measured 12 feet in diameter and has a 30” face width. The customer left approximately 0.050” grind stock on the gear for clean-up. With proper fixturing, handling, and shimming, typical taper on a gear this size is about 0.030”. Both gear halves will be matched when grinding is complete to make a complete gear to drive a rolling mill for a steel plant.

We are expecting two smaller, 37,500-pound gears for similar heat treatment within the month. 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 1725.

After lowering the temperature of the furnace and gear to 1550, it is quenched in oil. The transfer time from the pit furnace into the quench tank is less than a minute. With proper carbon control monitored by an oxygen probe in the furnace, examination of the microstructure of a sacrificial test piece used to measure surface,  core hardness and through a hardness traverse with readings every 0.010” from the outside of the gear util core hardness is reached, shows minimal retained austenite and IGO (intergranular oxidation), well within specification requirements.

These two microstructural constituents in excess, out of range amounts, can lead to cracking and premature failure. After quench, the gear will be double tempered, sandblasted and prepared for shipment to the Midwest customer for final grinding. Mark Podob, President, Metlab, 1000 E. Mermaid Lane, Wyndmoor, PA 19038 215-233-2600 Ext. 232”

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