In this column, we will discuss the selection of a cleaner for prewashing prior to carburizing. The choice is critical to avoid forming a stop-off that prevents proper carbon penetration and removes the various soils from operations prior to heat treatment.
Cleaning is the removal of dirt and soil from a surface. A cleaner is a formulated chemical system or product dispersed in water, which removes unwanted dirt, soil, grease, and other similar matter from a surface.
Cleaners are used in nearly all industrial plants where metal-working operations are found. Almost all soils need to be removed to some extent. Thus, if a plant uses coolants, drawing compounds, rust preventatives, or heat-treating oils, then the plant will likely have a strong requirement for cleaning.
To establish quality cleaning of parts, all process variables should be considered. This includes previous process and soils from previous processes. The subsequent process should also be considered, as the residues and soils from the previous operation may not be tolerable. The washer capability should be understood in terms of the types of soils it can remove. The existing water quality should be considered, including whether water treatment should be considered (excessive water hardness). The biostability of the cleaner should be reviewed to avoid the growth of bacteria and fungus, and to avoid the use of biocides and fungicides. The part orientation should also be considered. The part orientation evaluation should include examination of nesting of parts, and the orientation of the spray nozzles with relationship to the parts. Finally, the capacity of the washer should be taken into account. The washer should have adequate skimmers to remove the soils and not be overloaded with soils. If the washer is too small, it will not clean the soils from the part. The washer should be sized appropriately for the number of parts to be processed between the dump intervals.
The soil is an important factor in determining the type of cleaner to be used as well as conditions for cleaning. For carburized gears, the most common soils are machining coolants. Neat oils, 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.
Machining debris is typically made up of large fines and chips mixed with grit and machining swarf. This type of debris is readily dislodged from parts using either spray or soaking. Agitation during the soak helps remove the debris.
In general, surface residues are classified as either inorganic or organic. This does not include particulate or fines that can be dislodged by agitation. Typical organics that are found in either prior operations or in cleaners include silicates, sulfonates, phosphates, carbonates, borates, and chlorides. Sodium or potassium in any form is also found. These inorganic compounds burn onto the part and possibly attack the surface at austenitizing temperatures. They do not burn off. Organic residues, such as oils or amines, will burn off at the austenitizing temperatures. They can, however, leave a gummy carbonaceous deposit on the part.
Rinsing the part after cleaning is necessary to remove any soil and residual cleaner film.
The diffusion of carbon into the surface of the part can be blocked if residues are present on the part during carburizing. It is the goal of the cleaner to remove these residues so that proper carburizing can occur. Failure to remove these residues, whether from the prior machining operations or residues of the cleaner, will result in soft spots in the carburized case, or non-uniform carburizing. In some cases, the diffusion of carbon is completely blocked, with the residues acting as a carburizing “stop-off.”
There are eight criteria for selecting the proper cleaner. These will be discussed individually.
Product form. It is generally recommended that the product form for cleaners be a liquid. The first reason is that it is easier to add to the washer. Concentration can be easily controlled using a small pump and a Dosatron™. Powders require operator handling, with the potential for inhalation of the powders. It is also much easier to overcharge a system with powders, resulting in a high concentration. The high concentration could lead to elevated pH, and increased soap residues on the part.
Alkaline with pH < 10.5. Often, parts are handled or examined after washing, or during the washing process. To minimize risk to operators, the pH of the washer solution should be below 10.5. The neat cleaner solution will run higher. However, it really should not be higher than 11 to prevent possible operator injury during transfer of the cleaner to the washer. As with any chemical solution, proper personal protective equipment should be worn.
Low foaming. The foaming potential of the cleaner should be low. This prevents foam from overflowing the washer and potentially spilling over to the floor. This is a potential slip and fall hazard, as well as providing poor cleaning during the wash cycle.
Rust inhibition. The cleaner should incorporate adequate rust inhibitors to protect the parts being washed, but also the washer. The cleaner should not stain or attack the parts or the washer. Rust inhibitors are amine type and protect ferrous and non-ferrous materials. A combination of different amines may be present to provide adequate protection.
Operator acceptance. The washer operator must be considered in the selection of a cleaner. The washer shouldn’t emit strong unpleasant odors or go rancid. The biostability of the cleaner should be considered to avoid the use of dangerous biocides. The operator is the one who is responsible for the process — proper selection of a cleaner should make their job as easy as possible.
Hazardous materials. EPA and individual plant restrictions for hazardous chemicals shall be followed. This means getting approval by the Environmental Health and Safety person (or committee) for any cleaner brought into a plant. In general, these cleaners should not contain any phenols, phosphates, nitrites, or chlorinated solvents; however, individual plants may vary on allowed chemicals.
Residues. The cleaner should not leave a residue on the part. If a residue is present, it should be rinsed. Rinsing parts after washing is always recommended practice. When washing dishes, the soap is always rinsed thoroughly, so as not to leave a soap residue. Otherwise, you would taste the soap during dinner. In the same fashion, parts should be thoroughly rinsed so that any residue is removed from the parts. Failure to do so can result in caustic burn, staining, or non-uniform carburizing. Silicates, chromates, barium, sodium hydroxide, potassium hydroxide, and borates can all leave residues on parts that can act as a “stop-off” and prevent uniform carburizing. Silicate and borate residues can form a low temperature glass that reacts with the surface scale and create a very adherent layer than prevents carbon diffusion. Hydroxides can create thick oxide at the surface that can impede carbon diffusion.
In this short article, the types of soils present prior to heat treatment are discussed. The reasons for removing these soils to prevent staining and carbon “stop-off” during carburizing and non-uniform carburizing were presented. Finally, a series of selection criteria for a pre-heat treatment cleaner was provided.
As always, should you have any comments of this or any other column, please contact the writer, or the editor. I welcome any topic suggestions for new articles.