Fig. 1. Green internal furnace-refractory contamination due to excess cleaning compound on parts

As heat treaters, one of our New Year’s resolutions should be to clean up our act! Cleaning parts before and after heat treatment is a critical part of what we do. Most of us feel we clean parts “good enough” and feel we know what cleaning is, but few actually do. Let’s learn more.

Cleaning is the application of time, temperature, chemistry and energy to remove contamination from the surface of a part to a level appropriate for the intended application. In other words, cleaning is simply moving contaminants from where they are not wanted (on the parts) to where they should be (in the waste-disposal system). If all four aspects of the cleaning process are not working together, the parts will not be properly cleaned. Although heat treating demands only a moderate level of cleanliness compared to many industries, contamination left on parts can cause significant problems in our equipment (Fig. 1) and on the parts themselves (Fig. 2).

All cleaning systems depend on one or a combination of three basic actions:
  • A physical action – mechanical force – such as spray agitation, dunking, ultrasonics or even hand (abrasive) cleaning to remove the contaminants from the part surface
  • A thermal action to improve the activity of the cleaning solution and increase the kinetic energy of the system
  • A chemical action to allow contaminants to be either desorbed from the part surfaces with the aid of surface active agents or dissolved by an action of absorption and dilution

Fig. 2. Soft spots on carburized parts after shot peening

Aqueous Cleaning

Aqueous cleaning is the dominant approach used in our industry. Its simplicity, ease of use and overall flexibility is what makes it an attrac-tive process. Aqueous cleaning uses detergents to lift contaminants from the surface of the parts; heat to make the detergents more com-patible with the contaminants and to soften them; fluid force to dislodge the contaminants from the parts and to collect the insoluble con-taminants in some removal systems; and time to allow the process to take effect.

Aqueous cleaning is not perfect, however. It often leaves a surface residue or “film” on the parts, which may interfere with certain processes such as brazing or nitriding and thus require subsequent removal. Aqueous cleaners don’t dry well and the solution is often difficult to remove from internal part surfaces such as holes, crevices and recessed areas. Finally, aqueous cleaners evaporate slowly, requiring large amounts of energy to dry them, and have been known to damage certain sensitive parts.

Solvent Cleaning

Cleaning in a solvent offers a level of simplicity and forgiveness not seen in aqueous methods. Solvent cleaning involves three basic steps: wash, rinse and dry. Washing is where the parts are immersed in or contact the typically boiling solvent to help the removal process. The purpose of rinsing is to bring “fresh” or clean solvent in contact with the parts. The aim is to dilute the contaminated solvent present on the surface of the parts from washing. It is important to remember that the rinse solvent must be kept clean. Contaminated solvent is a very common problem and will only reintroduce contaminants back onto the surface. The drying step evaporates the solvent and separates the rinse solvent from the parts.

Solvent cleaning has a negative connotation in the heat-treating industry primarily due to environmental concerns, safety and cost issues. The emergence of vacuum vapor degreasing in a sealed vessel offers an attractive alternative that takes advantage of the best aspects of solvent cleaning. The size and amount of residual contaminants are reduced while meeting the most stringent cleaning requirements and avoiding the traditional problems of open degreasing systems.

Cleaning Tips

1. Clearly understand what your cleaning needs are. That is, what types of contaminants are present and in what quantities. Also, know to what extent they must be removed. In other words, answer the question, how clean do I need to be?

2. If you currently own cleaning equipment, spend your time making sure the system is well maintained, operating properly and performing up to expectation.

3. If you are considering the purchase of a cleaning system, narrow the choices to one or two suppliers, using published information and referrals, then witness a cleaning trial on your parts by the supplier you most prefer. Finally, understand that your time is best spent making the selection work.

4. Rinse the parts thoroughly. Rinsing is needed to separate the dirty chemistry from the more pure chemistry. The benefits of a good rinse should not be underestimated.

5. Dry the parts thoroughly. Drying is separation of parts from pure cleaning agent. Dry only to the extent necessary and consider non-evaporative methods of separating the cleaning agent from the parts.

6. Measure your cleaning effectiveness constantly to ensure the system is not degrading. If you are using an aqueous system, invest in a good oil skimmer. Be sure that it is well maintained and operates consistently over time.

7. Don’t re-contaminate clean parts.

8. Clean the cleaning equipment – thoroughly and often. Replace the bath on a routine basis and don’t try to extend its life.

9. Do all cleaning, rinsing and drying at as low a temperature as practical. Balance process efficiency and cost of cleaning.

10. Recognize that the average life expectancy of a cleaning system is three to five years. Replace your equipment regularly to keep the cleaning process working at optimum efficiency.

How Long and How Clean is Clean?

Cleaning time depends to a large extent on the system and the parts. Typically, an aqueous-based process needs to run for 10-20 minutes while solvent-based techniques need only 5-10 minutes to complete the cleaning process. As a rule, it is more difficult to clean clean parts than dirty ones. A key question is always how do we know when the parts are clean?

There are a large number of tests to measure cleaning effectiveness. These include visual inspection, macroscopic (5-50X) inspection, “white glove” inspection, observation under ultraviolet (black) light, tape sampling, “water break” tests, surface-tension test fluids, the Nordtest method and gravimetric methods. A number of technical organizations including ASTM ( offer cleaning standards, often based on the type of material to be cleaned. Remember, clean is generally observed, not measured, and cleaning effectiveness is established by answering the question, can we do what we need to do next? However, cleaning tests provide quantification of the nature of the part surface so that the influence of the remaining contaminants can be factored into the heat-treatment operation as well as subsequent manufacturing operations.

Other Types of Cleaning

Blast cleaning, pressure washing, steam cleaning, abrasive cleaning and other mechanical methods can be used to “clean” part surfaces. Fluidized beds have also been used for years to remove contaminants. In cases where it is beneficial to remove imperfections such as stains or surface corrosion, heat discoloration, oxide films, weld marks, scratches and particles of all sizes, electropolishing can be used. These “nontraditional” cleaning approaches have value and present their own unique set of challenges.

Where the Industry is Headed

It is important to recognize that both aqueous and solvent cleaning processes can be made to clean almost any parts requiring subsequent heat treatment. Therefore, the choice between them will most often be made by factors other than expected part cleanliness. The focus today is on improving physical action (force and volume) in combination with a chemistry choice balanced for the type of cleaning required.

Finally, take a systems approach. Consider the manufacturer of the cleaning system and the supplier of the cleaning agent as partners in the long-term success of any cleaning operation.IH