This fact sheet series is produced by the Minnesota Association of Metal Finishers & Minnesota Technical Assistance Program for metal fabricators and their platers.
High-Temperature Processing Burns on Soils
Fact Sheet #5
Oils, coolants and many organic liquids form a varnish or paint-like coating on part surfaces when heated or aged. These coatings can form at room temperature over a period of days or weeks. They form much faster as temperatures are elevated in heat treating, buffing, drawing and some drying operations. In extreme cases, a tough carbon-based smut is formed on surfaces. Removing these coatings requires chemical or mechanical stripping rather than a milder standard cleaning operation.
Faster Processing Degrades the Surface
A medium-sized metal finisher plates a large quantity of heat-treated parts every day. Most jobs plate relatively trouble free but some are a problem every time they are processed. One particular job started easy and then became difficult.
Shafts were machined, carburized (57-62 Hc with a case depth of .015-.025 inches), finish ground, and then plated with electroless nickel. This job ran trouble free for the first six months. Then, black particles appeared on part surfaces that had not been finish ground, even after a strong alkaline bath cleaning was done according to the developed process. Parts were salvaged by polishing the areas which had not been ground before cleaning. This cost the customer an extra $0.75 per shaft.
Upon investigation, the only process change was the manufacturer had moved the carburizing to an outside heat treater. After working with three different heat treaters to resolve the surface particulate problem, the manufacturer brought the heat treating back in house. The black particulate did not recur.
Problem Sources and Solutions
The problem encountered was probably due to:
- machine fluids remaining on parts while in the oven
- the aging of these fluids before outside processing
- perhaps in part, the conditions used by the commercial heat treater
High-temperature processes can bake soils onto surfaces, forming a tough organic film or an equally tough inorganic smut or scale. These processes can include abrasive finishing processes like buffing, polishing and some types of fine, dry sanding or grinding; oven processes like heat treating or stress relief; and joining processes like welding, brazing and soldering.
Buffing & Polishing Solutions
Avoid overheating to prevent metal discoloring, distorting and warping.
- Control the feed rate and pressure used to buff and polish.
- Lubricate wheels and operate to avoid over heating. This is especially important when using synthetic fiber buffing media which can melt onto parts.
- Use an abrasive belt rather than a wheel to keep surface temperatures down by spreading work over a larger surface area.
Use liquid and waterbased buffing compounds which tend to be easier to remove and less susceptible to burning on than pastes and solids or oil-based compounds.
Consider electropolishing as an alternative to buffing before plating stainless steel. Electropolishing does not add surface contaminants, improving the bond of subsequent plated deposits. In some cases, electropolishing can also eliminate a deburring step.
Heat Treating Solutions
Clean parts before heat treating.
Filter quench baths to keep them clean.
Use inert atmospheres or a vacuum for parts made of tool steel or stainless steel to avoid forming oxides and scales.
Remove scale before plating when inert atmospheres cannot be used. Scale removal involves at least one extra step and can damage parts. Abrasive removal of scale generally roughens surfaces. Chemical scale removal (acid pickling) tends to change dimensions and surface properties (removes carbon), and can cause hydrogen embrittlement.*
Maintain consistent batch characteristics.
- Pack ovens to assure even treatment.
- Use consistent treatment times and procedures.
Use the lowest temperature for adequate heat treating. Higher temperatures can increase production rates but also create soils that are more difficult to remove.
Avoid polyvinyl alcohol (PVA) quench baths on parts-to-be-plated to prevent coating them with a resin that is hard to remove.
Welding, Brazing and Flame Processing Solutions
Clean parts before joining. Good cleaning prevents burning on organic soils that can interfere with plating. It also yields higher quality welds by either removing or preventing smut and scale layers that interfere with metal to metal bonding.
Use the proper welding rod for the alloy being joined.
Use the lowest temperature and voltage for adequate bonding. Higher temperatures can increase production rates, but they also create soils that are more difficult to remove. Make sure operators, especially on night shifts, run equipment according to design specifications rather than pushing the process during part of the shift to generate slack time later.
Use an inert shield gas where possible to avoid forming oxides. Shield gas should be at least 97% inert for stainless steel brazing.
Avoid the over-application of brazing fluxes and use acid flux where possible to avoid burning on rosin.
Avoid brazing steels with copper-zinc filler. Copper-based fillers will stain steels during plating.
Avoid tin-lead soldering of parts to be plated. Lead alloys require special plating processes to give adequate bonding of the plated coating. It also contaminates process baths. Plating and metal finishing are extra steps which enhance the decorative or mechanical properties of surfaces. If the end-use of parts justifies the extra metal finishing, care during part fabricating is also warranted.
* Hydrogen embrittlement can be corrected or eliminated by treating parts within one hour of acid pickling/chemical descaling. Soak parts in a neutral to alkaline bath or bake in an oven according to ASTM or military specifications.
Get it Plated Right Series
- Introduction: Cleaning and Design for Plating
- Fact Sheet #1: Cleaning Processes - Capabilities and Limitations
- Fact Sheet #2: Particulate Contamination on Part Surfaces - Eliminate to Reduce Plating Rejects
- Fact Sheet #3: Selecting Materials for Plated Parts
- Fact Sheet #4: Dried on Process Fluids and Fluid Combinations
- Fact Sheet #5: High Temperature Processing Burns on Soils
- Fact Sheet #6: Holes, Seams, Threads, Recesses and Tubing Assemblies
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