February 16, 2012
When done properly, hard surfacing can save you money and extend the life of your buckets and other equipment.
By Chris Monroe
Whether hard surfacing old equipment or new, when completed properly, the result is the same: less downtime for replacing worn or broken components, fewer spare parts to inventory, and longer equipment life. In short, hard surfacing is a fast, easy, and efficient way to make buckets and other equipment more wear resistant and keep them in the field longer — often for less money.
During the hard surfacing process, a filler metal (sometimes called an alloy) is bonded to the equipment’s base metal in order to obtain specific wear properties and/or dimensions. Specifically, these filler metals provide abrasion and/or impact resistance. On older equipment, hard surfacing can return worn parts to a nearly new condition for about 25 to 75 percent less than the cost of replacement parts. Hard surfacing can also lengthen the life of surfaced parts by up to 300 percent more than non-surfaced parts, especially on newer equipment.
To obtain the best results from the hard surfacing process, consider these four basic guidelines.
1. Technique and process requirements
First, determine the equipment’s hard surfacing needs: build-up, overlay, or a combination of the two. The build-up technique (placing layers of welds on top of each other) returns older equipment back to its original dimensions after it has been worn by impact and/or abrasion. Overlay is the addition of a weld layer that protects the equipment against metal loss. A combination of build-up/overlay can also extend equipment life and may be used repeatedly, provided the part or equipment remains sound.
The size, shape, and location of the equipment or parts that require hard surfacing will determine which welding process should be used, as will the welding operator’s skill set and the availability of specific equipment. Typically, hard surfacing uses either stick (SMAW) or flux-cored (FCAW) welding processes, but some companies that weld large, thick components on a regular basis may choose the submerged arc welding (SAW) process.
Each welding process has advantages and disadvantages. For example, stick welding is highly portable, making it ideal for hard surfacing in the field. There are also many types of stick electrodes available, each of which can be used in all welding positions and are able to weld on relatively thick materials. Stick electrodes, however, have a low efficiency (due to stub loss), a relatively low deposition rate (approximately 1 to 7 pounds/hour), and may require several weld layers to obtain maximum wear properties. On the other hand, hard surfacing with flux-cored wire offers better deposition rates (approximately 4 to 25 pounds/hour), and the process is easy to use. FCAW often requires minimal training to become adept. Unlike stick welding, however, flux-cored welding is limited to flat and horizontal positions.
2. Base material considerations
Consider the equipment’s base material. Carbon or low alloy steels are probably the most commonly hard-surfaced materials. As a word of caution, those materials containing higher amounts of carbon and/or alloy content tend to be more brittle and may require pre- or post-heat or stress relieving to prevent cracking. Thicker base materials require similar heating considerations, as well.
Austenitic manganese steels can also be hard surfaced, and these, too, can become brittle during the welding process. Unlike carbon or low-alloy steels, austenitic manganese steels should not be pre-heated unless the temperature of the part is less than 50 degrees F. During the hard surfacing process, the base-metal temperature should remain under 500 degrees F, as exceeding this temperature barrier for an extended period of time increases the steel’s brittleness. Austenitic manganese steels with higher carbon and lower manganese content accelerate this time/temperature reaction.
Regardless of the base material that needs to be hard surfaced, remember to pre-clean the part prior to welding. First, wipe it free of all contaminants, including grease, dirt, rust, and oil. Then, if necessary, remove old hard-surfacing layers, as well as cracks, via arc (or plasma) gouging or grinding.
3. Equipment wear factors
Consider the type of wear the bucket or equipment encounters, as this will be a significant factor in determining the best filler metal to use. Abrasion accounts for roughly 55 to 60 percent of equipment wear, and there are three main types: low-stress scratching, high-stress grinding, and gouging. Impact and adhesive wear (also called metal-to-metal wear) are also common. Secondary types of wear include high-temperature and corrosive wear.
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.