Hard Surfacing Makes Maintenance Easy
As one of the largest suppliers of crushed limestone for the Cincinnati area, Hanson Aggregate-Eagle Crushed Stone (located in Winchester, Ohio) relies on the welding technique of hard surfacing to keep their wobbler feeder - the equipment that separates shale from limestone - running smoothly. So far, with the help of McKay® Tube-Alloy® AP-O hard surfacing wire from Hobart Brothers, they are doing just that. In fact, since their distributor, Tom Budd of Weldco, helped them switch to the AP-O hard surfacing wire, Hanson Aggregates has increased the life of its wobbler feeder bars from one year to two or more and has eliminated multiple operating problems.
Why Hard Surfacing?
Wobbler feeders serve a critical function in limestone operations like the one run by Hanson, separating valuable limestone from the shale and other fines that are extracted in the mining process. But even with softer aggregate like limestone, the bars that convey and filter the limestone are subject to costly wear. Replacing these bars is extremely expensive and immediate availability is never guaranteed, so the preferred method for repairing each bar is hard surfacing.
So what is hard surfacing? By definition hard surfacing is the deposition of a special alloy material onto a metallic part through welding to obtain more desirable wear properties and dimensions. For about 25 to 75 percent less than the cost of replacement parts, the process returns worn parts on older equipment to nearly new condition and increases the equipment’s resistance to abrasion and impact to provide it with a longer life. In this case, Hanson hard surfaces the wobble feeder bars to gain both benefits.
Noticeable Improvements
Hanson’s wobbler feeder, with its system of 13 rotating bars, pushes limestone forward while the shale and other fine materials drop through and are sent elsewhere for processing. Prior to switching to the AP-O hard surfacing wire, operators had to dismantle the entire feeder once each year so that all 13 bars could be built back up to original specifications. Once welding operators applied the old hard surfacing wire, they then ground each part to achieve a smooth finish.
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McKay Tube-Alloy AP-O increased the life of Hanson Aggregates' wobbler feeders from one year to two or more. |
According to Bobby Roades, plant superintendent, Hanson Aggregates, they have seen drastic improvements since switching to Tube-Alloy AP-O. “The product that we were claims. “Now we’ve extended that life (of the bars) to two years or more. At this time, we’re actually at two-and-a-half years because customer demand has been so high we can’t stop to hard surface. So, from a cost standpoint, we are definitely saving time and money and we’re getting good performance.”
Roades attributes a portion of the improvement in wear characteristics to the wire’s ability to “work harden” the more it is used. Put simply, the more rock that crashes onto the bar, the harder the deposited weld gets. The wire features a hardness of 18-24 Rc (Rockwell points) when first applied, but as rock impacts it during the normal course of processing, the microstructure of the welds compress and harden to 55 Rc. This feature makes the bars highly resistant to the impact and abrasion wear experienced in this application and ultimately accounts for the added lifespan of the hard surfacing welds.
“What has also really worked well with the AP-O,” adds Roades, “is that these bars have valleys and ridges. We’re able to build the ridges and the material will stay on them without flowing off during the welding process—it makes the whole process quicker.”
Also, because Hanson’s wobbler feeder relies on a series of bars that work in time with one another to carry the material forward—each bar is egg shaped and one bar is standing while the next one in succession is lying flat—shale or limestone that does not “slide” off the preceding bar can lock up the system. “We ran into this problem of what we call ‘clamping’ quite a bit before switching wires,’” says Roades. “Our old wire wouldn’t lay down a smooth weld. We’d have to grind and do a lot of extra work to make sure that those bars didn’t clamp. Since we started using the AP-O wire, we’ve had zero downtime due to that difficulty.”
Welding Process Benefits
When hard surfacing time rolls around and once each bar has been properly cleaned of all contaminants, Hanson’s welding operators place each manganese feeder bars on a turntable so that they can continuously weld in a flat position. To their advantage, AP-O is a self-shielded, flux cored wire that performs best in this position and doesn’t require a shielding gas—a convenience that eliminates the need to haul cylinders around the yard. “It’s nice that we don’t have to worry about having a shielding gas,” says Roades. “So if it’s real windy or there are adverse conditions, just shielding the weld puddle with a screen or our putting our backs to the wind works.”
The optimum suggested settings - ones that Hanson’s welders adhere to closely - are 275-350 amps and 24-27 volts for a 1/16-in. diameter wire, parameters that provide a deposition rate of approximately 10 lbs. per hour. The fact that the AP-O wire does not require cleaning in between passes, allowing the operator to make multiple passes without stopping, is a feature Roades appreciates. “This wire isn’t limited,” says Roades. “The operator can keep on building, and we normally do two, sometimes three passes depending on wear. And in the past, we put another hard surfacing wire over top our layers (for added wear resistance), but because the AP-O work hardens, it actually works better by itself than trying to put another hard surfacing wire on top of it.”
Roades offers a word of caution, however, for companies who hard surface with multiple passes on austenitic manganese steels: watch the temperature of the work piece. Manganese can become brittle when raised over 500 degrees F for extended periods of time. To avoid this problem, Roades and his welders rely on heat sticks to keep the inter-pass temperature at acceptable levels.
Longer Life Cycles = Greater Production
By switching to Tube-Alloy AP-O, Hanson Aggregates has more than doubled the life cycle of the hard surfacing welds on its wobbler feeder and has simultaneously eliminated the clamping problem that plagued the company’s productivity. And even though much of the hard surfacing repairs took place during regularly-scheduled maintenance, Roades reports a reduction in downtime and labor thanks to the new wire. That reduction is due to the fact that they can now wait two or more years before hard surfacing again. So in an industry that constantly looks for ways to reduce “cost-per-ton,” these advantages alone justified switching to Tube-Alloy AP-O.
“The added availability of the feeding equipment,” says Roades, “and the ease of the hard surfacing application is what helps keep us up and running for such a long time.”