Hard Surfacing: Basic Considerations for Choosing the Right Alloy
By definition, hard surfacing "is the deposition of a special alloy material on a metallic part, by various welding processes, to obtain more desirable wear properties and/or dimensions." More importantly, for those of you who are new to the idea, hard surfacing is a welding technique that can help you save time and money. By hard surfacing your new and worn equipment, you can increase your equipment's resistance to abrasion and impact to provide it with a longer life.
This article explores some of the basic reasons for choosing hard surfacing for your needs and how to choose the right welding consumable for your application.
Why Hard Surfacing?
Companies choose hard surfacing for a number of reasons, most of which directly relate to reducing the overall cost of operation. On older equipment, hard surfacing returns worn parts to a nearly new condition for about 25 to 75 percent less than the cost of replacement parts.Depending on the application, hard facing can also lengthen the life of surfaced parts by 30 to 300 percent more than non-surfaced parts. Repairing worn equipment and extending equipment life reduces the amount of downtime needed to replace broken or worn parts and eliminates the need to maintain a large inventory of spare parts. This technique also allows for the use of more inexpensive base metals on equipment and can help reduce power consumption.
Uses for Hard Surfacing
There are two main uses for hard surfacing equipment: build-up and overlay. In certain instances, you may choose to use a combination of the two for increased protection of your equipment. When hard surfacing, you will generally weld against the flow of the base material and create a herringbone, waffle or dot pattern according to your application and whether you desire a build-up or overlay.
Older equipment that has been worn by impact and/or abrasion can be hard surfaced as a means of saving on parts cost. Using the build-up technique-where layers of welds are placed on the equipment to return the metal parts to their original dimensions-provides excellent protection against impact, but offers low abrasion resistance. You can also restore older equipment through a combination ofbuild-up/overlay; overlay occurs when you place an additional, protective layer of welds onto the part after it has been built up. Adding an overlay will provide lower impact resistance, but will increase the abrasion resistance.
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Hard surfacing on buckets helps extend parts life. |
Whichever method you use, the rebuilt part often exhibits higher material strengths than even the original, provided that the procedure has been done correctly. And as long as the part remains sound, you can continue to use the build-up or build-up/overlay hard surfacing method repeatedly to extend the life of your equipment and return it to its original dimensions.
You can also protect new equipment against the loss of metal by using the overlay method of hard surfacing where the parts are most susceptible to wear. Because overlay welding consumables have a higher alloy content than those used for hard surfacing build-up, the overlay provides improved resistance over that of the base material of your equipment. Using a hard surfacing overlay can increase the life of your new equipment and parts by two or more times that of a part that has not been hard surfaced.
Choosing an Alloy
To determine what welding alloy is right for your application, there are several factors that you need to consider-four to be precise: 1) the preferred and available welding process; 2) the base metal of the equipment; 3) the wear factors on the equipment; and 4) the surface finish required on the equipment. These factors are especially important because unlike most other welding techniques, there are no specifications for hard surfacing, nor are there any AWS (American Welding Society) classification for hard surfacing alloys. Finding the right hard surfacing alloy is simply a matter of taking the above factors into consideration to ensure that you are achieving the best impact and/or resistance protection for your application.
Process Considerations
First, you will need to determine which welding process you prefer: Stick (SMAW), flux cored (FCAW) or submerged arc (SAW) welding. Oxyfuel and GTAW or TIG welding processes can also be used for hard surfacing, but they are not recommended as they have lower deposition rates. Determining which welding process you use for hard surfacing depends largely on the availability of specific welding equipment and the skill of the operators who perform the technique. You also need to consider the size, shape and location of the parts needing to be hard surfaced before deciding which process will best meet your needs.
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Stick welding is the most common process for hard surfacing, due to the high availability of alloys. |
Stick or SMAW
The Stick or SMAW process is the most common welding used for hard surfacing. Its appeal is due in part to the high availability of welding alloys and to the fact that most of these electrodes lend themselves to welding on a variety of material thicknesses. Many hard surfacing Stick electrodes are also available for out-of-position work and can be readily transported for outside use. However, hard surfacing with Stick electrodes may require several layers to reach maximum wear properties, as these welding alloys tend to have a lower efficiency (due to stub loss) and a lower deposition rate (approximately 1 to 7 lbs/hr).
Flux Cored or FCAW
Flux cored welding is also a common process for hard surfacing and, similar to Stick electrodes, there are many alloys available. Hard surfacing with flux cored wire provides increased deposition rates (approximately 4 to 25 lbs/hour) compared to Stick electrodes, while also improving deposition integrity. These alloys are generally easy to use, require minimal operator training and can be used outdoors. Unlike Stick electrodes, however, hard surfacing flux cored wire are generally optimized for flat and horizontal applications only and for maximum wear properties, you will need to create two to three layers.
Submerged Arc or SAW
Though not as common, hard surfacing can be done with the submerged arc welding process where only build-up is needed (hard surfacing alloys are generally not available for overlay). One advantage to this process is that it is easily automated, making the rebuilding of larger parts more economical. The process also creates smooth, strong welds and requires minimal operator training. Submerged arc hard surfacing creates a more worker friendly environment where there is minimal risk of arc exposure (or arc flash). Even so, alloys for hard surfacing with the submerged arc process are quite limited, as is the welding position; you can only weld in the flat position or on cylindrical parts.
Additionally, you can only hard surface on larger parts because the submerged arc process requires high heat input and can distort the metal on smaller, thinner parts. Submerged arc hard surfacing also has an extremely high dilution and requires multiple layers to obtain the desired wear properties, both factors that can increase overall costs for purchasing the flux needed for this operation.
Base Metal Considerations
Generally, carbon or low alloy steels and austenitic manganese steels are the two types of base materials that are hard surfaced, but there is hard and fast rule for using alloys on either one.
Base materials containing a high content of carbon and/or alloy content have a tendency to be brittle and susceptible to cracks-both risks increase as the carbon and alloy content also increase. On these base materials, you may need to pre- or post-heat, slow cool or stress relieve the welds to ensure that you have solid, long lasting weld beads. Pre-heating especially reduces the chances of developing cracks, distortion, porosity and other weld discontinuities.
As a note: the higher the carbon and alloy content of a base material, the higher the pre-heating temperatures need to be to gain the fullest benefit.
On austenitic manganese steels, you need to take special precautions to prevent brittleness when hard surfacing. Even though this base metal is strong and hardens under impact, pre-heating should not be done unless the part is less than 50 degrees Fahrenheit. During welding, the base metal temperature should remain under 500 degrees Fahrenheit. As this temperature barrier is exceeded for increasingly longer periods of time, austenitic manganese steels gradually become more brittle. Base metals with higher carbon and lower manganese accelerate this time/temperature reaction.
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A dot pattern helps with impact abrasion. |
Whether you are hard surfacing on carbon or low alloy or austenitic manganese steels, you need to clean your base material properly, ensuring that it is free of all contaminants, including grease, dirt, rust and oil. Old hard surfacing layers, as well as cracks, can be removed through arc (or plasma) gouging or grinding prior to adding new layers.
Wear Factors
There are four major types of wear that can occur on equipment. In order to choose an appropriate hard surfacing alloy to protect and/or rebuild your parts, it is important to determine the type of wear to which you subject your equipment. And while corrosion may be considered a fifth type of wear factor, its presence is usually a secondary factor that needs to be dealt with separately.
Abrasion
Abrasion is responsible for most wear on equipment-approximately 55 to 60 percent-and is caused by the rubbing of foreign materials against parts. There are three types of abrasion, each of which cause a different degree and type of wear on equipment. Depending on whether your equipment is subject to low-stress scratching, high-stress grinding or gouging abrasion, the hard facing alloy you choose will vary.
Low-stress scratching abrasion, or the slow wearing away of parts due to the repeated scouring action of materials across the equipment, is the least severe type of abrasion. Alloys with carbide (especially chrome-carbide) are a good choice for protecting against this type of abrasion, as impact resistance is not a factor. Additionally, many of these alloys are designed to develop stress-relieving cracks that help prevent spalling; less relief cracks occur as the percentage of carbides is lower, but there will also be less abrasion resistance.
For high-stress grinding abrasion caused by abrasive materials repeatedly crushing or grinding against a part, there are a wide variety of alloy options. Hard surfacing with alloys containing austenitic manganese, martensitic irons or titanium carbides are the most viable options.
Alloys containing carbide and supported by austenitic manganese are the best choice when encountering gouging abrasion. This type of abrasion occurs when large objects, such as rock, are pressed against the equipment and create gouges and grooves. High carbide alloys create toughness against impact and weight and will resist gouging.
Impact, Adhesion and High Temperatures
For parts that are subject to repeated impact, it is important to find an alloy that can resist the high mechanical stress caused by large, compressive loads of materials. Alloys with work hardening characteristics, such as those containing 11 to 20 percent austenitic manganese steel, are the best choice for hard surfacing high-impact equipment.
Another form of wear-adhesive, or metal-to-metal wear-results from the non-lubricated friction of metal parts against one another, and is responsible for about 15 percent of all equipment wear. To prevent the roughening and wear of parts through adhesive wear, you can use a martensitic hard surfacing alloy. Austenitic manganese or cobalt based alloys will also work well, but it is important not to overmatch too soft of an alloy on too hard of a surface as this combination will not resist adhesive wear for as long.
On equipment subject to extreme temperature, the appearance of surface cracking and spalling is not uncommon. Thermal fatigue, or "fire cracking" on parts that have been repeatedly heated and quickly cooled causes parts to expand and contract until the metal can no longer return to its original properties. To prevent this resulting cracking, use a martensitic steel alloy with 5 to 12 percent chromium for hard surfacing, as these alloys maintain resistance up to 1200 degrees Fahrenheit.
Surface Finish
The final consideration you need to make before choosing a hard surfacing alloy is what type of surface finish you need and/or require for your equipment. If you need a smooth surface, you have to determine whether the cost of grinding still makes hard surfacing a viable cost saving measure for your operation. Since hard surfacing alloys range from easy to difficult in terms of grinding, you may want to determine your required finish prior to choosing an alloy. For example, you could opt for an alloy that has slightly less wear resistance, but one that provides you with the desired surface finish.
You may also consider using an alloy that can be heat-treated to soften it for machining, and then brought back to the hardness necessary to protect your equipment. Likewise, if relief checks (small checks which do not weaken wear resistance) are acceptable on your surface finish, using a carbide alloy designed to be crack sensitive may be a good option.
Conclusion
Whether your equipment encounters repeated impact, abrasion or both, using hard surfacing to protect parts can be a viable cost savings measure for your overall operation. There are, of course, many factors to consider when determining the best alloy for your hard surfacing needs. Once you have determined the type of welding process you want to use, the base material you want to protect and the surface finish you desire, the benefits become clear. By implementing hard surfacing into your daily operations, you can reduce your inventory management and find yourself with stronger, longer lasting equipment.