Hard Surfacing Tips
The least severe form of abrasive wear — low-stress scratching — results when the metal slowly wears away from the scouring action of materials across the equipment. Hard surfacing with carbide or chrome-carbide filler metals best protects against this type of wear, and, often, filler metal formulations are available to provide stress-relieving cracks that prevent spalling.
For high-stress grinding abrasion caused by repeated crushing and grinding of materials against the equipment, the best filler metals are those containing austenitic manganese, martensitic irons, or titanium carbides.
Filler metals containing high-carbide alloys and supported by austenitic manganese are the best choice when encountering gouging abrasion, as these filler metals provide good impact resistance. Gouging abrasion occurs when large objects, such as rock, press against the equipment and create grooves.
Impact wear often occurs on equipment like crusher rolls, impact hammers, and impactor bars, and results from a compressive load placing high mechanical stress on the equipment. The best protection against this type of wear is to use an austenitic manganese steel (11 to 20 percent manganese) filler metal, as it offers good work-hardening characteristics.
To protect against adhesive or metal-to-metal wear, which occurs from the non-lubricated friction of metal parts against one another, use a martensitic hard-surfacing alloy. Austenitic manganese or cobalt-based alloys also work, but they may be too soft to resist adhesive wear for as long a period of time.
When equipment repeatedly encounters high temperatures and rapidly cools afterward, it can result in high-temperature wear, also called thermal fatigue or fire cracking, which leaves deep cracks in the equipment’s base material. This type of wear is usually secondary, or in addition to the abrasion or impact wear equipment encounters. Generally, a non-ferrous alloy is best for protecting steel surfaces subject to temperatures above 1200 degrees F. For those below this range, a filler metal containing chromium-carbide or a martensitic steel filler metal with 5- to 12-percent chromium is suitable.
Corrosive wear is also a secondary type of wear that should be dealt with separately. Most filler metals provide some rust protection, but it is best to consult an equipment manufacturer or a trusted welding-supply distributor for recommendations.
4. Desired surface finish
Finally, determine the type of surface finish the equipment requires. Since hard-surfacing filler metals range from easy to difficult to grind, determine the required finish prior to choosing one. If a smooth surface is necessary, measure the time and cost of grinding to achieve this surface versus using a filler metal that has slightly less wear resistance, but provides a smooth finish. A filler metal that can be heat-treated to soften it for machining and then brought back to the hardness necessary to protect the equipment may also be an option. Or, if relief checks (small checks which do not weaken wear resistance) are an acceptable surface finish, consider a carbide alloy designed to be crack sensitive.
If the equipment encounters repeated impact, abrasion, or both, hard surfacing can offer valuable time and cost savings. If in doubt about the process, consult a reputable filler-metal manufacturer or trusted welding-supply distributor for assistance. Most of all, don’t get discouraged. It may take some time to get the hard-surfacing process and filler-metal selection right, but, in the end, it can provide stronger, longer-lasting equipment.
Chris Monroe is a training specialist with CWI/CWE, Hobart Brothers.
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