Hardenable stainless for mildly cold application?

[Lyrl]

We are a commercial heat treater working with a potential customer on carburizing a stainless steel. The idea is to replace 4-5" diameter pipes currently made of carburized 8620 material. The pipes are failing after 3-10 weeks. They think the failure is due to corrosion, thus the move to stainless steel. The pipes have integrated bearing races that need to have high 50s Rockwell C hardness, thus the carburizing at least of the race areas (potentially the non-race areas could be masked if the carburizing lowers the corrosion resistance too much.) The core uncarburized areas have the following mechanical requirements:

A. Minimum Tensile Strength = 119,000 psi
B. Minimum Yield Strength = 95,000 psi
C. Minimum Elongation = 15%
D. Minimum Reduction of Area = 35%

And there is a requirement for a Charpy test at -20°F to have three samples all above 15 ft-lb with the average of the three at least 20 ft-lb.

They have initially come up with X4CrNiMo 16-5-1 and 431 as candidate materials. They provided 16-5-1 samples to us to use in developing a low pressure (vacuum) carburizing process. In addition, they requested that we suggest alternate materials (so they can compare price and availability, I think). Any suggestions for me to look into that I could pass on to our customer would be greatly appreciated; it would be a special bonus if any of the materials have carburizing process data available.

 

[metengr]

To provide reasonable material options requires much more information than what you have posted. You really need to question the client as to what the actual corrosion mechanism is with the current material. Was wastage a concern from wear, abrasion or actual corrosion? I hear this same approach - switch to "stainless" to improve corrosion resistance.

Is carburizing required because of specific case depth and hardness requirements for service conditions? What rides against the races? What are the operating conditions? I can go on but as I mentioned, much more background information is required to evaluate suitable materials.

 

[EdStainless]

Can they live with the thin cases from low temp nitriding? If they have high enough strength?
The first thing that comes to mind is 13-8PH. Good corrosion resistance and high strength.
You might look into SolNit-M or S3P or StaniHard for the surface treatment.

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P.E. Metallurgy, Plymouth Tube

 

[Lyrl]

The customer is not 100% sure the problem is corrosion. Potentially the failure is by galling or some other mechanism. It's been hard to get information out of them.

They had some parts nitrided (by another source, the company I'm at doesn't have nitriding equipment) and decided against nitriding. Again, it's been hard to get details out of them on why they decided not to nitride.

I understand about not having enough information - I appreciate you guys reviewing my post and commenting, it helps. Thank you also for the pointer to the stainless hardening processes, those are interesting to read about.

 

[metengr]

Agreed. Making any type of material recommendation is useless until better information is known.

 

[EdStainless]

The treatments that I referred to are not traditional nitriding.
They are process to diffuse N into the alloy without forming any nitrides.
They harden the surface without reducing corrosion resistance.

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P.E. Metallurgy, Plymouth Tube

 

[arunmrao]

Traditional nitriding has been beneficial in my experience for paper pulp processing, without sacrificing corrosion
Ion/plasma nitriding,Kolsterizing are special treatment and cost benefit analysis will have to be performed .

Ed's choice of 13-8PH is a good one.



"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[tbuelna]

If the "integrated bearing races" are for rolling element bearings, then you will need the hardness (>Rc58) and case depth provided by carburizing. The thin case from nitriding or nitro-carburizing processes will likely not be sufficient after finish grinding to support the hertzian contact stresses of rolling element bearings.

If surface corrosion is your concern, then I would recommend using a carburizing steel like 8620 or 9310 for the races, and coating them with thin dense chrome for corrosion protection. It is common practice with commercial bearings to apply thin dense chrome over E52100 steel races for corrosion protection rather than using more costly corrosion resistant alloys like 440C. Rough machine the pipe and race surfaces, carburize/heat treat them, perform an intermediate grinding operation on the race/journal surfaces, apply the thin dense chrome plating, and then perform a finish grind operation on the race/journal surfaces.

 

[mrfailure]

I think before your customer throws away a lot of good money on a change in material, they need to really have a real failure analysis performed to identify the mechanism and material characteristics - THEN they can take the appropriate corrective action which may or may not include a material change. In my experience, the use of the term "corrosion" is a red flag that more work is needed both because there are many forms of corrosion and many failure mechanisms that have the appearance of oxygen pitting corrosion (such as fretting) but are not. I try not to recommend a change in material system (e.g. from alloy to stainless steel) unless it is absolutely necessary as the difference in material properties have the potential to lead to unanticipated failures from new causes.

 

[Lyrl]

I agree the customer has neglected proper failure analysis and consideration of other better options. Their rationale seems to be "the competition switched to stainless and had improved product life so we should imitate the competition".

Unfortunately in this case our customer's interests and my company's interests diverge, because their other options would lead them to not give us any work. At this point we're focused on heat treating their chosen material as best as possible and just keeping in mind that they later might find an alternative path and pull the work from us.

I really appreciate all the comments here - I have learned a lot from them (always a good thing in itself) and they have helped me extract some additional background information from our customer.