As a local heat treater told me years ago, heat treating is great, but only do it when you need to. Because heat treating does not generally make the parts prettier or straighter.

A significant part of my consulting services over the years has been related to helping sort out what the source of the “not prettier” stains might be. Many of these stains appear after heat treating. Of course it’s not just heat treating that might be the problem. Heat treating is not the first process to be performed on most parts. Any substance that touches a part may leave residue that is objectionable to someone (or at least might be considered objectionable to someone).

Part geometry greatly affects how easy it is to clean effectively. Inside surfaces of small-diameter features can be particularly problematic. And performing analysis on these parts is particularly challenging, especially when the contaminant is at the bottom of a closed-end feature (e.g., cup or blind hole). Of course it’s not the round geometry that is the problem; it’s the ratio of the depth to the size of the opening. Small openings to deep holes are often a problem.

That’s because the test method of choice for a completely unknown substance is usually EDS. This is an attachment to a scanning electron microscope (SEM) that analyzes X-rays that are given off by the sample as it is bombarded with electrons. The energy level of the X-rays is measured by the EDS detector and matched to a list of energy levels associated with each atomic species.

EDS can’t determine the type of molecule. It only looks at the basic electron structure of each type of atom. If you need information on molecules, which is generally the case if you are trying to sort out different types of oils or oily substances, you need to use a different instrument: a Fourier Transform Infrared (FTIR) analyzer. Some FTIR instruments have microscopes to help zoom in on a tiny area, but you generally still can’t look down to the bottom of a deep, skinny hole. FTIRs are not currently available as attachments to SEMs.

Whenever possible, I prefer to analyze attached contaminants on whatever surface it is attached to. If it is not attached or can be readily removed (and it’s big enough), that removes problems related to figuring out what the contaminant is and what is the base metal. If there is loose debris, great. I can analyze it and not get confused by the surrounding materials.    

When forced to analyze the contaminant in place, I will analyze the “dirt” and an adjacent, cleaner area nearby and look for the differences. Having a known good, clean part is also very helpful.