The demands of heat-treated goods are increasing. Not only do they have to meet property and straightness requirements, but they also have to meet increasingly stringent appearance specifications. It is not only important that parts perform as expected, but they must look good doing so. The perception is that clean parts are quality parts. In my experience, staining of parts and cleanliness issues is the No. 1 problem experienced by heat treaters, followed by distortion. Property issues are far down the list of potential problems.
Cleanliness is a relative term. The total process will dictate the degree of cleanliness. For example, a part is being cleaned during various states of manufacture to remove chips or machinery fluids. The cleaner will leave behind a residual rust protective film. Although there is residual film on the part, it is considered clean. In the case of electroplating, blackening, or enameling, the part must be chemically clean. This is often referred to by the term “water break free.” After cleaning, if a part is rinsed in clear water, the water should run down the part in a continuous unbroken film. This indicates a water break free part. If the film is interrupted, this indicates some soil is remaining on the surface. Cleanliness should be determined by the customer. During a cleaner trial, it is good policy to allow the customer to make the initial comments concerning cleanliness. A part that may appear to be dirty to an operator could be quite acceptable to the final customer. Typical soils for heat-treating processes are shown in Figure 1.
Basic Factors to Consider
With few exceptions, there are certain principles that apply to all types of cleaning. These include:
- Increased temperature usually improves cleaning.
- Agitation to move the soil (rather than the cleaner) improves cleaning. Agitation to move the cleaner is important when the layer of cleaner next to the surface has become heavily contaminated with soil or has cooled off.
- A minimum concentration of cleaner is needed for cleaning; above this level cleaning improves with increased concentration, but each increment of cleaner has a lesser effect than the previous. A point of diminished returns is eventually encountered.
- Adequate time must be provided for detergency or reaction of cleaner with soil. Otherwise, agitation or mechanical removal effects become more important.
- Rinsing away of soil and cleaner is necessary and must take into consideration factors such as: The amount that can be left behind without harm; how much cleaner residue can be tolerated; and pressure rinse water or an agitated rinse is far more effective than a “still” rinse.
- Soil must be kept from redepositing on the work. This might take the form of cleaner components to suspend the soil or design of the vessel holding the cleaner to provide room for soil to settle to the bottom (away from the cleaning area) or an overflow for oil to float away from the surface of the rinse.
The general effect of process variables on cleaning is shown in Figure 2.
There are three different scenarios for cleaning prior to heat treatment:
- Prior to carburizing or neutral hardening.
- Prior to vacuum processing.
- Prior to induction hardening.
Carburizing and Neutral Hardening
The presence of various soils, such as coolants, if improperly removed, can have a drastic effect on the carburized case. If the coolant contains significant quantities of sodium borate or sodium tetraborate, residues of coolant can form a low temperature glass on the surface of the part, which effectively blocks carburizing. Highly chlorinated oil-based coolants can result in carbonaceous deposits forming on parts with chlorine attacking the surface of the part.
Highly sulfurized parts can attack not only parts, but the fumes from the breakdown of the coolant can attack alloy within the furnace and form low-melting temperature NiS compounds at the grain boundaries of alloy. This shortens the life of burner tubes and supporting alloy racks and fixtures. The formation of nickel sulfide at grain boundaries can also lead to catastrophic failure of burner tubes.
It is important to properly clean parts prior to heat treatment. As a general rule, the organic components will burn off (but possibly leave a carbonaceous film). Inorganic constituents will burn onto the part, creating sites for subsequent rusting or blocking the diffusion of carbon.
There are several factors that should be considered preparing work pieces for vacuum heat treating or brazing. Cleanliness of the work piece is extremely important to prevent staining of parts or damage to parts.
These parts should be free of oil, dirt, and other contaminants prior to austenitizing. Compounds that contain sulfur can cause discoloration of the part and also result in poor braze alloy flow.
These contaminants can be either water-based or oil-based. Different cleaners are required for the different contaminants. Aqueous cleaners are required for water-based contaminants, and solvent cleaners should be used for oil-based contaminants. Generally, solvent-based cleaners are the first choice for vacuum-processed parts. However, solvent-based cleaners do not properly clean water-based contaminants. In this case, the use of alkaline cleaners and at least one rinse is recommended. Often, multiple rinse cycles are required. The rinse tank should be dumped on a regular basis to prevent re-contaminating the part.
Parts should be inspected for contaminants in deep recesses or holes for entrapped lubricants or coolants, as well as metal chips. Tags and wire used to attach the tags should be verified to make sure they are not of a low melting point alloy such as aluminum.
Some materials require special cleaning practice. Titanium and zirconium alloys should never be cleaned in chlorinated solvents such as trichloroethylene or methyl chloride. Chlorine residues can result in stress-corrosion-cracking (SCC) when heated to above 280°C. These alloys should be cleaned in non-chlorinated solvents such as acetone or alcohol. Alkaline water-based cleaners can also be used. Care must be taken to properly rinse the parts. Many drawing lubricants contain sulfur and possibly lead. Both of these elements can attack nickel alloy surfaces and form a low melting temperature eutectic that will embrittle nickel alloys. Nickel-based parts must be thoroughly cleaned and rinsed to ensure the removal of drawing or stamping compounds. After cleaning, the parts must be dried to prevent degradation of vacuum during the process cycle.
In induction hardening, parts are heat-treated using a strong electromagnetic field to create eddy-currents within the part. This process is commonly used for medium carbon steels to form a hard case. After heat treatment, the part is flooded with quenchant, typically containing a polymer quenchant at some nominal concentration. This process is often highly automated and is in-line with machining operations. The quench tanks on many induction machines are quite small, typically less than 500 liters. Induction hardening is fast and capable of processing hundreds or thousands of parts per day.
Minor carry-over of coolant on parts to the induction hardener is a common occurrence in the heat-treating shop. Coolants and hydraulic fluids are often found on parts prior to induction hardening. Over time, these contaminants build up in the quench tank, leading to changes in cooling curve behavior. This can contribute to cracking or inadequate properties. Figure 3 is an example of a poorly controlled polymer quenchant where no washing of parts prior to heat treatment occurs.
How can consistent properties be expected, when the contamination of the quench bath exceeds that of the polymer quenchant?
Proper cleaning involves not only removing the various soils from the part but also removing any residual cleaner residue by proper rinsing. This is a critical step and is often the cause of many problems. One-stage washers, with only a wash cycle, can leave a cleaner residue on the parts. The residue must be removed to prevent staining and attack of parts by the residual cleaner fluid. A good analogy would be a dishwasher full of dishes. The soap must be rinsed off the plates so as not to impart a chemical taste to food. Parts must be rinsed to remove any residual cleaner from parts. Failure to remove cleaners during the cleaning cycle can result in problems such as caustic burn or rusting (Figure 4).
In this brief overview, the impact of proper cleaning and rinsing on part quality was discussed and illustrated. The possible soils to be cleaned from parts prior to heat treatment were discussed and their impact of parts during heat treatment was shown.
For high quality parts and parts meeting increasing customer demands, it is imperative that proper washing and rinsing of parts be accomplished. Your local cleaner representative can help you identify improvements to your process.