Second in a series » Among the many specifications applied to finished heat-treated goods, the cleanliness of those goods is increasingly important in appearance, performance, and customer satisfaction. In this series of articles, I’ll break down the why and how of cleaning for heat treatment.
In the last column, I discussed the important factors or variables affecting industrial cleaning. These variables were: concentration of the cleaner; time required for cleaning; temperature; agitation; and water quality (hardness).
In this column, the various soils on heat-treated parts will be described, and the importance of eliminating these soils.
There are two types of soils on heat-treated parts. These are soils that come from prior manufacturing processes (pre-heat treatment), and the soil that comes from the quenchant (post-heat treatment).
Pre-heat treatment soils
The soil is an important factor in determining the type of cleaner to be used, as well as the conditions for cleaning. Pre-heat treatment soils can be broadly classified into five groups:
Drawing Compounds. With pigmented drawing compounds, the presence of these pigments makes cleaning difficult, especially after the heat generated by a drawing operation and the impacting of pigment into the metal. Mechanical cleaning methods such as ultrasonic, electrolytic, and spray cleaning aid in the removal of such compounds.
With un-pigmented drawing compounds, oils, and greases, hot alkaline cleaning normally removes such soils. lf high viscosities are encountered, pre-cleaning in a solvent or solvent emulsion cleaner will dissolve some soil and lower the viscosity, thus enabling the alkaline cleaner to perform more efficiently.
Machining Lubricants. Soluble oils, synthetic, and emulsion cutting fluids are readily removable with synthetic and alkaline cleaners. The presence of sulfur and chlorine components may make the soil more difficult to remove with alkaline cleaners, and solvents or emulsions are sometimes necessary. The presence of fatty acids in the cutting fluids promotes better cleaning through reaction with the alkaline cleaner.
Polishing and Buffing Compounds. Buffing compounds in general are difficult to remove because of the heat generated during the buffing operation. Aging and over-buffing are to be avoided. Aging causes polymerization and over-buffing leaves excessive amounts of compounds on the work. Products containing high degrees of detergency and solvency are normally recommended.
Carbonaceous Soils. Dry, finely divided carbon particles are some of the more difficult soils to remove. Direct agitation, such as high-pressure spray impingement, sometimes removes such soils. An emulsion cleaner is often effective as is ultrasonic cleaning.
Rust Scale and Paint. Removal of these soils is considered a specialty area. High caustic alkaline cleaners may be used for removal of light rust, paint, lacquer, etc. Heavy scales must be removed with acids. Paint removal from aluminum and zinc metals must be accomplished with a paint stripper.
Besides the types of soils considered above, there are additional soil variables that should be considered to determine the proper method of cleaning. These soil variables are:
Physical form of the soil. This includes the viscosity of the liquid and the melting point of any solids.
Fine particulate. The presence of finely divided particles can make soil removal very difficult.
Corrosion or oxidation products in the soil. Proper neutralization of these soils is important so that damage will not occur to the part.
Post-heat treatment soils
These soils are usually just the oil or polymer quenchant. In this case, either emulsion or splitting type cleaners are used.
Emulsion cleaners create an oil emulsion and leave a film of oil on the parts. This can be beneficial for rust protection but can cause smoking issues during the tempering operation. Depending on the temperature, carbonaceous deposits can form on parts or inside the furnace. These types of cleaners are not used very often.
Splitting type cleaners have special additives to properly remove the oil film and have the oil report to the surface. These are usually alkaline type cleaners, that contain a blend of alkaline salts and surfactants. In general, these must be properly rinsed to avoid leaving any trace of the alkaline cleaner on the part. Failure to do so can result in rusting of the parts, as the surface is chemically active.
Polymer quenchants usually only require an ambient agitated rinse of water to remove any residual polymer films. If the film is left on the part, and is tempered, the polymer can leave a carbonaceous film and create acrid fumes.
Special practices for vacuum furnaces
The purpose of heat treatment under a vacuum is to leave a bright, shiny surface after heat treatment. Any contaminants left on the part prior to heat treatment will stain the parts, defeating the purpose of vacuum heat treatment.
The contamination on the parts prior to heat treatment (coolant, chips, swarf, etc.) will dictate the cleaning processes used. Often there are multiple steps used for cleaning parts. First, there is a cleaning step to remove debris and inorganic contaminants. Often these are water-based alkaline cleaners and a thorough rinse. Second, there is a second step to remove organic contamination, such as stamping lubricants or cutting oils. Often solvent or vapor degreasing is used. Acetone or alcohol are most often used. Chlorinated solvents are never used, especially on titanium or zirconium alloys. Chloride residues can result in stress-corrosion-cracking (SCC) at temperatures above 280°C.
With nickel alloys, drawing compounds often contain sulfur. Residues at temperature can form low melting temperature eutectics which can attack and embrittle the base metal. This also includes baskets and fixtures. They must be thoroughly cleaned with alkaline cleaners and often solvent cleaning to remove these soils.
After cleaning, the parts should be properly rinsed to remove any solvent or cleaner, and thoroughly dried. Any solvent or water residue can volatilize and contaminate and degrade the vacuum and the vacuum pumps.
Conclusions
In this article, I have described the typical soils that occur before and after heat treatment. With soils occurring before heat treatment, any inorganics present will not burn off, but will burn onto the part, resulting in part staining and sites for corrosion to occur. With post-heat treatment cleaners, the soil must be properly removed to prevent smoking and fumes in the heat-treating shop. We also described some of the special practices needed for vacuum heat-treating operations. In all cases, rinsing parts after cleaning is a critical step to prevent potential cleaner residues occurring on the part.
Should there be any questions regarding this article, or suggestions regarding new topics, please contact the writer or the editor.
