Chlorinated Paraffin/Vacuum Heat Treating

What is confusing me is the apparent rust pattern that develops on these parts. Can you advise me on this?


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Q. Good Afternoon, David. A local metal stamper is stamping carbon steel parts using a forming fluid containing chlorinated paraffin. The stamper uses a high-alkaline, high-heat soak and spray method to clean the parts, then rinses them with water, applies an amine carboxylate corrosion inhibitor, and dries.

The heat treater then receives the parts for vacuum heat treating. When parts come out of the vacuum furnace, they have what appears to be a rust stain on them (also occasional black spots). This happens before they can get the parts into a corrosion inhibitor. The appearance of the stain is similar to the appearance of the dirty parts at the metal stamper prior to cleaning.

I have a suspicion that the metal stamper is not removing all of the chlorinated paraffin and that the chlorinated paraffin is having an adverse effect on the steel parts during or shortly after the vacuum heat treat.

Considering that the heat treatment is being done in a vacuum, I don’t think any paraffin is being decomposed, but rather cracked into smaller alkanes and alkenes.

What is confusing me is the apparent rust pattern that develops on these parts. Can you advise me on this?

It has been an ongoing issue for this heat treater, and I would love to solve it for them.

I am waiting on the heat treater to receive more parts from the metal stamper. Once I have a sampling of these parts, I will wash them down with chloroform and boil it off.

If I can get enough residue, I can put it on another customer’s SEM and look for chlorine.

That test would be definitive for me, but in the meantime, I am hoping that you can help or suggest another source. T.H.

 

A. I appreciate the thorough background to your problem, T.H. I think you are doing a good job isolating and tracking the situation. There are a couple of things I would recommend in addition to what you have been doing.

The primary area to understand in this case is the transformation of the lubricant throughout the process steps.

I agree with you in that I do believe the problem lies with the lubricant, so the first question you should ask is if the stamper really needs that sort of lubricant. The chlorinated hydrocarbon paraffinic additive lubricants are usually reserved for some of the most demanding and difficult forming operations such as deep drawing of steel.

If it appears to you that this may not be that difficult of a forming application, it could be that the stamper has a more difficult job in the shop, but to minimize inventory of lubricants, uses the most robust lubricant no matter if the job is heavy or light. So although it is unlikely that they will change, it is worth asking the question to investigate that as a potential quick fix to the problem.

Following that, the next step would be to understand the amount of storage time between stamping and cleaning. The chlorinated paraffin hydrocarbon is what is known as an extreme pressure (EP) additive. It is incorporated into the lubricant and will break down at elevated temperatures and pressures in difficult forming applications. It is a typical additive in deep drawing applications. One of the breakdown products is hydrochloric acid, which will actually react with the metal surface.
Naturally, the longer this lubricant residue is left on the surface, the more attack will occur to the base metal and the more difficult it is to remove. If at all possible, the parts should be cleaned as soon as possible after forming. In-process storage time needs to be kept to an absolute minimum.

You also mentioned that the stain pattern on these parts is pre-existing, or similar to what is seen on the parts at the stamper. This is a good observation and indicates that what you are seeing is an artifact of the lubricant. If the stamper cannot decrease the in-process storage time and effectively eliminate this problem, then they will need to use an even more aggressive cleaning process that, in all likelihood, would involve a mineral acid to etch the base material and remove the stain by metal removal.

The vacuum heat treating operation would still break down any lubricant you had on the surface that was not removed by the cleaning process. Although you do not specify the vacuum and temperature levels, most lubricants will break down not from oxidation, but simply by boiling off and decomposing at the elevated temperature and low pressure.

Finally, I have seen some vacuum heat treating or brazing operations remove surface oxides, especially if they are superficial and relatively light. So while your heat treating temperature is “set” by your metal and application requirements, it may be possible to increase the vacuum level to potentially eliminate or reduce the problem. However, the attack from the residual lubricant may be deep enough to render this ineffective.

I will summarize since this is not a short or insignificant problem:

Determine if the chlorinated paraffinic lubricant is absolutely necessary, or if it can be changed to something less aggressive. Phosphonated EP additives are not as effective, but if the operation does not require the chlorinated EP additive, it may perform equally.

Find out about in-process storage time. The stamper should work to minimize this as much as possible. Perform a trial with parts immediately after stamping to determine the efficacy of this method. If effective, that could be a good means to motivate the stamper to modify its in-process storage times.

If neither of the above steps can be implemented, then it may be necessary to initiate a more aggressive cleaning process. This process would likely be an acid-based cleaning that would aggressively go after base metal as well as the stained portions.

It may be a stretch, but increasing the vacuum level of the heat treating process could remove some of the inorganic stains (i.e., oxides) through sublimation.

The last, and probably least desirable, alternative is to implement a cleaning process at the heat treater to remove the rust or stains.

As far as analytical methods, I think there would be benefit to performing SEM investigations following: 1) the cleaning process to check for residual surface chlorine and 2)post-heat treating process. I would think the post-heat treating process would show primarily oxidation product which may still have some residual chlorine on the surface. 

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