Is cleaning a must in heat treat?
The answer is neither a simple Yes nor a simple No – it depends.
In the past, cleaning has not been given much attention to in heat treatment, and the step is often bypassed. However, the attitude is slowly changing. On one hand, product specifications and higher quality requirements are driving the demand for cleaner products – both visually and qualitatively. On the other hand, evolving materials used in upstream manufacturing has amplified the need for cleaning. Environmental-friendly cutting fluids, for example, can go deep into the substrate where their complete removal is necessary to ensure successful thermal treatment.
But still, the question remains: should you clean or not clean?
To answer that, we need to differentiate between cleaning pre heat treat, and cleaning post heat treat.
The main goal of cleaning pre heat treat is to remove upstream contaminants such as cutting oils, coolants, chips and dust to ensure a clean, smooth surface. Their remnants could otherwise become baked on surfaces which might require costly processes to remove. Pre cleaning is also key to protecting the furnaces. It prevents the formation of smoke and oil vapors resulting from burned oils, which in itself is also an environmental and worker safety issue.
Pre-heat treat cleaning key to nitriding and carburizing
The reality though is that the majority of heat treaters have not fully recognized the need for cleaning prior to heat treat (with the exception of brazing). And this is of particular concern for demanding applications such as nitriding and carburizing, where a cleaned surface is fundamental to achieving good heat treat results.
Insufficient cleaning can lead to challenges such as non-uniform layers, soft spots and stop-off paint issues. Particularly with nitriding, spotty nitriding layers may not be obvious to the eyes and can only be detected under microscope. Phosphate additives in corrosion inhibitors can also work themselves deep into the surface of deep drawn parts, which can cause spotty nitriding patterns if not removed properly.
When it comes to cleaning post heat treat, cleaning is a standard step after oil quenching where most heat treaters rely on water-based systems to do the job. Of course, it is common knowledge that water and oils do not mix well, so residues could still remain. How clean the parts need to be then will very much depend on the end-use application (e.g. will the parts be shipped to customers? Or will they be machined further?) as well as customer quality requirements. Medical applications, for example, would have strict residual particle size limitations in place.
The real (hidden) risk of not cleaning
Tackling a heat treat failure where cleaning is an apparent contributing factor needs not be problematic. The real challenge lies in cases where the heat treat process seems to have worked – while in fact it has not.
Complaints about unexpected nitriding/carburizing layers, or component problems with equipment, can arise when parts are in the final assembly, are already in use, or are even out there for many years. The link between these issues and (the lack of /insufficient) cleaning can become very hard to detect by then.
Hence, the pain point for heat treaters does not have to be “in the present”. Customer claims issues that come back to bite in the future represent a far greater risk – precisely because of their unpredictability. This is why cleaning should not be taken lightly because it can mitigate future problems that heat treaters are not necessarily aware of at this point.
The bottom line is this ...
Cleaning is good house keeping. Cleanliness conveys quality, standard and care, while also offering protection to critical furnace equipment. For certain heat treat applications, including gas nitriding, ferritic nitrocarburizing and low pressure carburizing, cleaning is almost non-negotiable. For others, cleaning could be a competitive advantage that helps differentiate your products.
When assessing your need to clean, consider these questions:
What would be the cost of not cleaning – what damage could potential claims cause in terms of money, time, delays, customer trust and corporate reputation?
What are the costs to maintain/replace your furnace?
What are the technical justifications?
What are your customer expectations in terms of quality, aesthetics, and applications?
What are your corporate standards in terms of health, safety and the environment (HSE)?
Are you producing parts for high-value manufacturing sectors such as aviation, automotive, or medical devices, where your product quality can make or break your business?
Needless to say, cost will be a key driving factor. Do you have a high enough utilization rate to justify the cost of investing in a cleaning system? If not, outsourcing to job shops could be a potential option.
If you do have the cost argument to invest in an in-house solution, by all means – do not cut corners! Don’t be tempted to choose the cheapest cleaning option available. Companies do that and realize years later that the system is not working properly, and have to shell out another large sum to upgrade their equipment. Because it is not always obvious that your heat treat failures are a direct result of poor cleaning, as a best risk mitigation policy, take a step above rather than a step below – no one loves paying twice!