A Hanson wobble feeder used to separate limestone.

The embrittlement of manganese steel is a time/ temperature reaction.

Hanson Aggregates-Eagle Crushed Stone (www.hanson.biz) of Winchester, Ohio, uses a wobble feeder with a series of 13 rotating bars to separate limestone from shale and other fines extracted in the mining process. The bars that convey and filter the limestone are subject to wear. Replacing them is expensive, and immediate availability is never guaranteed. Hard surfacing is the preferred method for repairing each bar.

In the past, operators dismantled the feeder once per year so that all 13 bars could be built back to original specifications. Because the old wire would not lay down a smooth weld, welders ground each part to achieve a smooth finish. The system requires smoothness to ensure that shale or limestone pass easily along it and to prevent the system from locking up. A "locked system" would create extra work and additional downtime.

The company increased the life of its wobble feeder bars to more than two years with a McKay Tube-alloy APO hard surfacing wire. The wire is supplied by Hobart Brothers (www.hobartbrothers.com). Along with the increased life, Hanson Aggregates saw as much as a 75-percent savings over replacement parts. The process returns worn parts from older equipment to nearly new condition and increases the bars' resistance to abrasion and impact.

Tube-alloy AP-O hard surfacing wire deposits a work hardening austenitic manganese steel alloy that can be used for build up or overlay on austenitic manganese steel, carbon steel and low alloy steel. It can also be used to join austenitic manganese steel to manganese steel, carbon steel and low alloy steel. The weld metal has higher toughness than conventional manganese steel weld metal.

Bobby Roades, plant superintendent for Hanson Aggregates, attributes a portion of the improvement in wear characteristics to the wire's ability to work harden as it is in use. The weld deposited by the wire features a hardness of 18-24 Rc (Rockwell) when first applied, and it is further hardened in service. For example: As rock impacts welds on Hanson Aggregates' wobble feeder bars, the microstructure of the welds compresses and hardens to 55 Rc. That makes the bars highly resistant to impact and abrasion wear in this application.

Roades says the hard surfacing wire also makes maintenance easy. The feeder bars are made of manganese steel. Hanson's welding operators position each bar so they can weld continuously on a flat surface because the hard surfacing wire is a self-shielded, flux-cored wire that performs best in that position. By welding on a flat, operators do not have to clean between passes and the need for shielding gas is eliminated. They are able to weld multiple passes without having to haul shielding gas cylinders around the yard.

"It's nice that we don't have to worry about having a shielding gas. So if it's real windy or there are adverse conditions, just shielding the weld puddle with a screen or putting our backs to the wind works. This wire isn't limited," Roades says. He adds that operators can build welds in two or three passes on the surface of the bars to account for wear.

When they use the hard surfacing wire, Hanson's welders adhere to optimum suggested settings of 275 amps to 350 amps and 24 volts to 27 volts for a 1.0625-in.-diameter weld to provide a deposition rate of approximately 10 pounds per hour.

Roades said users have to be careful when they hard surface austenitic manganese steels with multiple passes. Manganese steel can become brittle when the temperature is raised over 500 F for extended periods of time. To avoid this problem, Hanson's welders rely on heat sticks to keep the interpass temperature at acceptable levels.