First off, you might want to know what the heck surfacing is? Surfacing is the process of rebuilding and/or protecting metals by adding metal alloys or ceramics. These materials can be applied to a metal as either a powder or a solid. Surfacing might seem like a needless, time-consuming task when you want to just grab your gun and weld, but it has some real advantages. Surfacing a part using a metal alloy or other material can:
- Maintain certain dimensions
- Increase part service (by using low alloy materials and only coat the areas with the expensive alloy where there would be extensive wear)
- Lower production costs
- Lower parts cost
There are many good reasons to surface metals. Surfaces wear because of abrasion, fatigue, chemical or atmospheric corrosion, and metal transfer or adhesion. Bearing surfaces, metal rock crusher surfaces, and earth moving equipment are examples of machines with components that wear very fast.
Metal surfaces can be rebuilt by adding metals and metal alloys. These materials are applied by either hard facing or thermal spraying. There are various methods of hard facing. One such method is done with materials enclosed in a hollow metal wire using flux core arc welding (FCAW) process. Thermal Spraying is done using a solid or powder of metallic or non-metallic materials, and is generally applied with a Plasma arc process.
Preparing your base metal prior to surfacing is very important. The base metal must be cleaned or preheated, depending on the type of metal. Preheating can be done using a large oven, torch, or other heat source. Cleaning the metal can be done using solvents, acids, or some form of grit blasting.
This might beg the question: Which metals are more appropriate to preheat as opposed to clean?
It all comes down to alloys. Look at metals like carbon steel or aluminum alloy. For carbon steel, you’re likely to find rust, if anything. With aluminum alloy? Oxide buildup. Both are simply cleaned by wiping down with a gentle cleaning agent, or you could use a mechanical process like sandblasting.
But, depending on the alloy, preheating might be better. Cast iron, for instance, is something you’d preheat. It’s a thick, brittle metal that’s properties need to be slowly transformed in order to weld effectively with. In fact, any material that is thicker needs to be preheated to a certain temperature to help facilitate the weld. Also, if you weld two alloys that are extremely hard would need to be preheated. An example of that would be Inconel, which is a super-alloy with a high nickel-chromium concentration.
One way to test the hardness of materials are the Rockwell Scale and Brinell Scale. The Rockwell method tests ferrous material and the Brinell method generally measures the hardness of non-ferrous material. The higher the number read on both scales, the harder the material is. For example, a hardened tool steel would read as 60 on the Rockwell scale, and 600 on the Brinell scale.
Today, surfacing plays a huge role in the industrial market place. Without this process, large, expensive, unrepairable parts would have to be replaced. As equipment costs rise, more industries that have wear parts are repairing them using the hard surfacing or thermal arc process. Leading manufacturers are also hard surfacing to increase the life of the part.