Repairing Bearing Surfaces and Shafts Using Thermal Spray
We use this process for repairing bearing journals and shafting at our shop. We would turn the worn journals down on the lathe about a1/4" less than the desired finish size and turn deep grooves (like a course thread) in the surface. The filler material came in a plastic can which screwed on top of the torch head. With the shaft turning slowly in the lathe we would heat the shaft to the recommended temp. Using heat indicating crayons to check the temp. Heating was done with the torch turned so that the filler metal can was below the torch. When the desired temp. was reached you would turn the torch over to allow the filler powder to flow down into the torch and spray onto the shaft. Most of the filler we used was a hard surfacing metal. We would then grind the bearing journal back to the desired size. It takes a bit of practice to put on a good even spray but once you get the hang of it, it gives way better results than building up with stick. It's faster and primarily you are not putting as intense heat into the shaft which would cause warping. We also use a machinable coating that can be machined back down with lathe tooling, and file, polish to fit finish.
Some History of the Development of Thermal Spray Hard Surfacing and Protective Coatings
Techniques and equipment developed since 1940 make it possible to spray-apply welded surfaces or wear-resistant nickel and cobalt alloys
By W.P. Clark Sr.
What is today “thermal spray” process has, in the past been termed “metal spraying” is also referred to as “flame spraying” and “metallizing.” The literature shows the process was known in Europe during the days just before World War I and that its invention has generally been credited to a “Dr. Schoop.”
During its early days, thermal spraying, according to today’s terminology, was used mainly for depositing coatings on metals for corrosion or heat resistance as well as for decorative purposes and for the repair of surface defects. Also, as the literature states, no claim was made that “sprayed molten metal coating are either welded or brazed to the metal.” As presently defined by the American Welding Society, with thermal spraying “finely divided metallic or non metallic particles are deposited in a molten or semi-molten condition to form a spray deposit” (coating).
Hard surfacing – or hard facing as it is officially known – also dates back to earlier days in the history of welding. It involved the use of oxyacetylene fusion welding procedures to deposit alloy layers on base metals to provide specific properties not possessed by the base metals – namely, good impact properties or a desired degree of corrosion or abrasion resistance. By the mid-1930’s arc welding procedures were being utilized in hard facing operations, and – in 1939 – it was noted that tungsten carbides with their very high hardness and very high resistance to wear by abrasion and erosion were unlike other alloys used for hard facing in that the tungsten carbides, simply stated, are “wetted by the molten metals and thus become fused in place – much as a lump of tinned steel might be soldered to a piece of copper.”
Hard facing has been defined by the American Welding Society as a “particular form surfacing in which a coating or cladding is applied to a substrate for reducing wear or loss of material by abrasion, impact, erosion, galling, and cavitation.” Also, surfacing is the “deposition of filler metal (material) on a base metal (substrate) to obtain desired properties or dimensions.”
New Alloy – A New Company
Thermal spray hard surfacing as discussed in this paper combines the attributes of both “thermal spraying” and “hardfacing.” Its development began in the year 1940, shortly after Mr. A. F. Wall purchased a California company, Colmonoy, Inc., from Mr. Norman Cole and Mr. Walter Edmonds. These two men had patented a nickel-chrome-boron hard surfacing alloy, and had named it “Colmonoy” (from their own names, and that of the product; “Col” from Cole, “mon” from Edmonds, and “oy” from alloy.) They sold it in oxyacetylene welding rod form, primarily to those engaged in rebuilding pumps and valves for the petroleum industry. This new alloy contained certain chromium boride crystals that were nearly as hard as diamonds, which provided excellent abrasion resistance. The nickel-chromium matrix alloy supplied the ability to survive in corrosive media.
The alloy had one more quality that was intriguing – it was “self-fluxing.” The boron (and silicon) content acted to remove surface oxides on metal surfaces to which the alloy was being applied. This meant the base metal surface did not have to be made molten to get overlays to bond. It also meant lower application temperatures.
Mr. Wall had an interest in metal spraying since the 1920’s. He reasoned that the self-fluxing characteristics of the new alloy should make it possible to use metallizing techniques to spray-apply the material, followed by the use of a torch to weld it to the base metal. This would be faster than rod application; it would also reduce the amount of material required, because spraying could be done to fairly close limits. There would also be a great reduction in finishing costs as the sprayed overlays would be closer to finish dimensions, and much more evenly distributed than hand welding with rod and torch.
What is Thermal Spraying,
With this process, a specially designed gun is used, having a nozzle (similar to a welder's heating torch) which burns Oxygen and Acetylene achieving temperatures up to 5500°F. A powder is fed through the center of the nozzle into the flame where it is melted. Compressed air is concentrated around the flame atomizing the molten material into fine spherical particles and propelling these particles at high velocity onto a specially prepared substrate.
An Intro to Thermal Spray
Thermal spraying, like weld cladding or chrome plating, is a coating process. In thermal spray, wire or powder is melted by a flame or electricity and sprayed onto the workpiece. During the actual process, the spray torch makes successive passes across the workpiece to produce a coating. Like all industrial processes, thermal spraying has its advantages and limitations. These have to be kept in mind in order to take proper advantage of thermal-sprayed coatings. The following are some of the benefits of thermal spray coatings.
- Reduced Cost. In lieu of making the entire part out of an expensive material, a high-performance material is sprayed onto a low-cost base material.
- Low Heat Input. Thermal-sprayed coatings do not impact the substrates' microstructure. The coating does not penetrate the base material, i.e., there is no heat-affected zone.
- Versatility. Almost any metal, ceramic, or plastic can be sprayed.
- Thickness Range. Coatings can be sprayed from 0.001 in. to more than 1 in. thick, depending on the material and spray system. Coating thickness generally range from 0.001 to 0.100 in.
- Processing Speed. Spray rates range from 3 to 60 lb/h depending on the material and the spray system.
With today’s advanced alloys and equipment, the thermal spray hard surfacing of almost any metal having a melting point above 1950°F (1065°C) is practical. Four basic steps are involved
1.Preparation. The base metal is blasted with angular chilled iron grit so that the sprayed overlay can achieve the mechanical bond with the part necessary until after fusing.
2. Spraying. Most cylindrical work is done on a lathe where it is rotated while the pistol, mounted on the tool post, moves the length of the overlay. Parts of other shapes may be sprayed by a handheld pistol.
3. Fusing. This changes the mechanical bond of the sprayed particles into the metallurgical bond of a fused or welded overlay. An oxyacetylene torch is the preferred method of fusing. Controlled atmosphere furnaces or induction heating coils may also be used.
4. Finishing. All sprayed-and-fused overlays can be ground. This is best done wet, using light, fast cuts. Many can also be machined with tungsten carbide tools.
“Puddle Torch.” During the 1950’s a simpler kind of powder spraying torch was introduced by several manufacturers. Generally known now as a “puddle torch,” it consists of a small oxyacetylene torchwith a powder-holding hopper attached between the nozzle and butt. The powdered alloy is added to the gas stream before it leaves the tip. Spraying and welding are accomplished alternatively, using an on-off powder flow valve; the spray pattern is quite precise.
The Thermal Spray Processes
In the flame-spraying process, oxygen and a fuel gas, such as acetylene, propane, or propylene, are fed into a torch and ignited to create a flame. Either powder or wire is injected into the flame where it is melted and sprayed onto the workpiece.
Flame spraying requires very little equipment and can be readily performed in the factory or on site. The process is fairly inexpensive and is generally used for the application of metal alloys. With relatively low particle velocities, the flame spray process will provide the largest buildups for a given material of any of the thermal spray processes. Low particle velocities also result in coatings that are more porous and oxidized as compared to other thermal spray coatings. Porosity can be advantageous in areas where oil is used as a lubricant. A certain amount of oil is always retained within the coating and thus increases the life of the coating. The oxides increase hardness and enhance wear resistance. With regard to hardfacing, self-fluxing alloys are typically applied by flame spraying and then fused onto the component. The fusing process ensures metallurgical bonding to the substrate, high interparticle adhesive strength, and very low porosity levels.
Surface Preparation for Thermal Spray Coatings
An essential feature of any coating system is the bond between the coating and the substrate. Thermal Spray operations are typically based on the materials being applied to the substrate in the plastic (non-molten) state. Therefore, the bond is not due to fusion between the coating and the substrate. In addition, there is usually little or no chemical reaction between the coating and the substrate, so the bond is not chemical in nature. What is the bond mechanism?
Coatings applied using thermal spray processes typically depend on a mechanical (interlocking) bond. The nature of the substrate surface is therefore a key to quality Thermal Spray Coatings. For successful coatings, the substrate surface needs to rough and pitted to provide a “foot-hold” (Splat-Hold) for each splat of powder that impacts the substrate. In addition, the surface needs to be clean and free from contamination that would fill the pits and prevent locking of the splats. How is this achieved?
Grit blasting is popular for surface preparation, which is simply pressurizing an abrasive media with compressed air and aiming the stream of accelerated particles at the surface being prepared. Many are familiar with grit blasting for cleaning surfaces prior to painting. However, grit blasting for thermal spray is quite different since more than removal of oxides is needed; instead, pits and crevices need to be formed where the molten thermal spray particles “splat” into the rough surface and adhere.
We also use a angle grinder to rough up the repair area or turn small grooves into the work surface while work piece is in the lathe.
I hope this is helpful to anyone who is interested in repairing shafts and bearing surfaces.
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