440 Stainless Age Hardening or Work Hardening? 1

[SandroCJW]

Is it possible for mechanical properties of a single heat lot of 440 stainless to change over time? My company makes thread rolls, that cold-work a thread form onto the OD of a cylindrical blank. One of our customers has been experiencing a steady decline in roll life in the last 6 months, on a die we have successfully supplied them for over 4 years. We are confident the tooling we supply has not changed over the last year, but our customer is equally confident that their production methods or process parameters have not changed. In my experience, usually when a part fails to cold-work properly, after having worked before, there is usually a metallurgical difference in the new material. This rule has held true 90% of the time (the other 10% has been traced back the effects of prior part processing). This part is fairly simple, with a center less grind operation as the only one preceding the rolling. The customer has informed us that the material being currently run is the same mill run and heat lot as the last 3 years, and is (was) certified to the 440 stainless spec when it was received. My question is can 3 year old 440 stainless react differently to cold-working operations than material that was more recently produced? Could the simple act of storing multiple bundles of 1/8" diameter 440 stainless rods stacked 10 feet high be changing the microstructure of the material at the bottom of the stack? I am not a metallurgist, but I'm at a loss to explain this phenomenon. Any enlightenment would be appreciated.

 

[metengr]

My question is can 3 year old 440 stainless react differently to cold-working operations than material that was more recently produced?

No, not for the same heat of material. In other words, if the same heat of 440 stainless is indeed being used over a 3 year period, there should be no change in mechanical properties from 3 years ago to now.

Could the simple act of storing multiple bundles of 1/8" diameter 440 stainless rods stacked 10 feet high be changing the microstructure of the material at the bottom of the stack?

No.

Obviously, either the 440 stainless is not the same as was supplied 3 years ago or a process parameter has changed. Have you performed a check of your own material used to manufacture the thread rolling dies?

If I were you, I verify your own product is within specification and share this with your customer using an outside lab. If nothing has changed, I would request the customer to send a sample of their 440 stainless steel and confirm what was reported. Use a metallurgical lab to provide an independent analysis for creditability.

 

[SandroCJW]

metengr,

Thank you for the reply. Early on in our troubleshooting, I did double-check our vendor certifications for die material, and our internal certs. We make hundreds of dies every day, and check every bar of material used in our tooling production for chemistry, grain structure, and sample hardenability, in-house, before it is released into our production stream. I have also checked worn-out or failed rolls after they came back to us for chemistry and hardness. There is nothing here that indicates a problem with our tooling. I have since learned that our customer is having intermittent problems getting the material to even roll-up into the die, and fill the form. So first they were getting diminishing die life, and now they can roll any parts. Again, with supposedly the same process parameters, the same heat lot of material refuses to flow up into the dies, and fill on the crest of the tooth form. That is usually an indication of material that is too hard, or is work-hardening before filling due to insufficient penetration rates. But again, the customer is insistent the process parameters have not changed. We have suggested they send several sample from different bundles, along with samples of in-spec, and out-of-spec finished product to a lab to have it checked for microstructure, and micro-hardness. I have yet to hear if they have acted on this. We want to help them get production back on track, but if they are truly not changing anything on their end, I'm at a loss as to what could be happening here.

 

[swall]

As I understand the problem, the customer is trying to roll form threads on 440 stainless. What alloy and hardness are the thread rolls?

 

[metengr]

Sounds like you have your position covered.

 

[EdStainless]

"we haven't changed anything" means that they aren't controlling things. If they were they were controlling the process then they would be flooding you with data and samples. Small changes in lubrication and pressure will result in huge differences in performance.

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Plymouth Tube

 

[CoryPad]

Excellent advice from metengr and EdStainless.

 

[TVP]

"we haven't changed anything" means that they aren't controlling things.

Truer words have never been spoken...

 

[SandroCJW]

A visit to our customer, and a few alterations to their machine parameters (to get things back to best practices) and the parts are back in-tolerance again, and die life back up to 30% of their peak. Still no word on the test samples, but I'm getting the feeling the problem has been process related all along. Now they just need to figure out exactly what changed, now matter how trivial it may seem to them.

Thank you all for your advice.