We continue with our discussion on the heat treatment of fasteners by offering additional tips for the heat treater.

Tip #5: Clean and Dry Parts
Many fastener operations heat treat poorly cleaned parts. While most heat-treat processes can tolerate wet or oily parts, especially with respect to atmosphere control, this is no excuse for not taking the time to understand how to better clean your parts.  

Cleaning is simply moving contaminants from where they are not wanted (on the parts) to where they should be (in the waste disposal system). The process parameters that are important for cleaning are the application of time, temperature, chemistry and energy so as to remove contamination from the surface of a part to a level appropriate for the intended application. If all four aspects of the cleaning process are not working together, the parts will not be properly cleaned. Although heat treating of fasteners demands only a moderate level of cleanliness compared to many industries, contamination left on parts can cause significant problems in our equipment (e.g., atmosphere variability, deterioration of internal components) and on the parts themselves (e.g., shallow or irregular case depths, spotty hardness).  

All cleaning systems depend on one or a combination of three basic actions:
  • A physical action (i.e. a 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.
Aqueous cleaning is the dominant approach used in our industry. Its simplicity, ease of use and overall flexibility is what makes it an attractive process. Aqueous cleaning uses detergents to lift contaminants from the surface of the parts; heat to make the detergents more compatible with the contaminants and to soften them; fluid force to dislodge them from the parts and to collect the insoluble contaminants 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 that may interfere with the case-hardening process. Aqueous cleaners don’t dry well, especially in continuous systems, which is why wet and/or oily fasteners often wind up entering the furnace. Also, the solutions are difficult to keep clean and oils tend to emulsify and redeposit on the parts. Threaded areas may be susceptible to buildup of residues. Finally, aqueous cleaners evaporate slowly, requiring large amounts of energy to dry them.  

Next time, we’ll cover cleaning tips.