It used to be thought that “we can burn off any surface contamination on the steel.” The problem now is that the statement is still believed to be correct. If the part is to have a uniformly formed surface case with a uniform surface hardness, it will be necessary to have a clean part that is free of surface contaminations in order to produce the uniformly formed case – free of soft spots. Therefore, surface cleanliness is of paramount importance prior to the heat-treatment procedure.
How do we clean the surface? The surface can be successfully cleaned in many ways. There are a number of proprietary surface cleaning agents available. Without adequate surface cleaning, it will be contaminated and will produce substandard surface metallurgy.
The simple cleaning solution is, of course, boiling (or close to it) water followed by a good rinse capability. Some industrial washing/cleaning systems use the dunk and spray method, which is fine as long as the recirculation system is functioning correctly.
Because of the importance that has emerged for appropriate pre-cleaning prior to any of the surface treatments, the wash unit has become a most import asset in the plant equipment inventory. Without adequate pre-cleaning and washing, there is a very strong likelihood of surface soft spots occurring on the processed work. If soft spots are being experienced on the work (be it nitriding or carburizing), then there is a strong probability that the component part surface was not cleaned in an appropriate manner to remove the surface contaminant.
Unfortunately, the post-cleaned surface does not always show that there is a surface contaminant present. So, it is most important that the wash system is a well-maintained unit.
Remember, it is the quality of washing or pre-cleaning of the work to be processed that will assist you in producing clean work and a uniform metallurgical surface condition.