Urgent question, Residual stress 5

[akhb76]

Hi,
I'm R&D manager of a bone implant manufacturing company,
We recently produced 1000 screws using Thread rolling method but they didn't get approved and we recalled them,
The reason was 'Residual stress' how to release the stress by non-thermal method.

Thanks

 

[tbuelna]

From the picture provided it appears there was a fairly substantial amount of cold-working involved with roll forming those threads. You did not say what the material used for the screws was. Some materials actually benefit from cold-working. Why wasn't a thermal stress relief process acceptable for these screws?

 

[akhb76]

The material is Medical Grade 316 stainless steel, Known as 316LVM (Vacuum-Arc Remelt).
my main concern regarding thermal stress relief is the possibility of unwanted transformation of metallurgical characteristics of this type of steel.
Another problem is that the core of the screw contains voids after thread rolling, while the raw material is defect free.

 

[tbuelna]

This 316 cres material will likely work harden from this thread rolling operation. Voids in the material after thread rolling will not likely be an issue, but due to the amount of material displaced you may see some problems like cold laps or tears at the thread tips. These conditions can be detected by an NDI process like dye pen inspection.

 

[mrfailure]

For bone implants, no stress relief is going to correct the problem of "voids", which would be a serious issue in bone implants due to the possibility of fracture during service. I think you really need failure analysis performed by someone experienced to tell you what is really going on.

 

[kingnero]

As tbuelna already mentioned, voids due to cold forming are very unlikely.
Better to start with an identification of the exact problem (as said, (micro-)tears and cold laps are very well possible). Once the problem is correctly identified, you can adapt your production process to avoid these defects.

 

[Tmoose]

How did you determine the residual stress, and have any screws been made previoiusly that were stress free?

 

[tbuelna]

What is the reason for stress-relieving the screw after thread rolling? Is dimensional stability or corrosion resistance an issue?

Given the degree of mechanical working involved, there may be some amount of strain-induced martensite after thread rolling. But I think it would require a full solution anneal heat treatment to transform any martensite back to austenite.

 

[EdStainless]

The internal voids could be fissures from too high of external pressure during rolling. After all cross rolling is how they pierce solid bar.
Who says that you need stress relief?
And how did they measure it?
If you need stress relief you will need a thermal treatment. You will have to stay at a temp below the sigma formation temp. There should be no risk of carbide formation as this alloy should have ultra low carbon. It may require a long time, 6-10hrs might be needed. You probably don't want to exceed 1100F.

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P.E. Metallurgy, Plymouth Tube

 

[AndrewFinAustralia]

I don't understand how the rolling process can introduce voids, unless there's delamination in the core material occurring due to friction.

Would I be correct in assuming that the issue with residual stress is stress corrosion cracking as blood is fairly corrosive due to the proteins/enzymes? I do know that for bioengineering applications, carbides in the steel are a no-no as Chromium tends to be a trifle cytotoxic when the carbides dissolve.

A stress relieve anneal would fix this, but I'm not sufficiently knowledgeable of the behaviour of this grade of stainless to comment on the interactions occurring at 450*C

 

[TVP]

akhb76,

That image shows a bone screw with a deep buttress thread form such as Type HB from ASTM F543. That type of thread form is manufactured by chip removal processes such as thread whirling, rather than deformation processes such as rolling, due to the large strains that would be required. The large amount of plastic deformation is not possible on a routine basis using today's engineered materials, as they will form defects like laps/folds, severe adiabatic shear bands, etc.

 

[EdStainless]

Look up the process of billet piercing to make hollows, such as pipe. This is a hot work process that uses strain from external rollers to nearly cause fissure down the ID, this reduces the force required to open the ID with a pointed tool. If you squeeze too hard you get many spits internally. It does not require any prior defects.

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P.E. Metallurgy, Plymouth Tube

 

[tbuelna]

Unfortunately akhb76 seems to have lost interest in the discussion.

The OP asked about "non-thermal" stress-relief methods. The only true stress-relief mechanical process I can think of is stretching, but it would not seem suitable for this situation. There are shot/laser peening processes, but these are really stress-modification rather than stress-relief techniques. There is also vibratory stress-relief, but there are questions about its efficacy. Plus I don't know if it would be suitable for batch processing of these small screws. There are cryogenic steel treatments which don't involve elevated temperatures and supposedly provide effective stress-relief. But I don't know how well the process would work on 316 cres.

I found this topic interesting, so I did a bit of reading on the subject. It does appear that bone screw threads are commonly produced by roll forming. But I wondered why you would want to stress-relieve roll formed threads. I work in aerospace, and all fatigue sensitive threads are roll formed because it leaves a very beneficial compressive pre-stress in the root fillet surface. I found one technical paper that described the significant influence of the interface between the bone and screw thread root area on maintaining a secure connection over time. It seemed to indicate that residual stress relaxation in the metal over long periods of time would degrade the quality of the metal/bone connection, possibly allowing the screws to loosen. If this is the concern, then I can understand why the screws need to be fully stress-relieved after thread rolling.

 

[Frederick]

One way of relieving some of the residual stress would be to give the fasteners a true cryogenic treatment. This would also increase the fatigue life of the fastener.

 

[EdStainless]

If these were made correctly then they are single phase, no ferrite and no martinsite.
So a cryo treatment would do nothing for them.

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P.E. Metallurgy, Plymouth Tube

 

[maxj]

I was wondering if there is an ultrasonic method to remove stress. I know ultrasonic impact treatment is used in removing residual stress in welds.

 

[Maui]

Ed, agreed. A cryo treatment would do nothing for these fasteners.

Maxj, what you are referring to is called vibratory stress relief. You can read about it here:

https://en.wikipedia.org/wiki/Vibratory_stress_relief

Maui

 

[Frederick]

The idea that cryo treatment would do nothing is false. You are making your assumption that the only thing cryogenic treatment does is create an austenite to martensite transformation. This is a false assumption. If it were true, then cryogenic treatment would do nothing for brake rotors which are cast iron and have a pearlitic micro-structure. Yet it has been proven over and over again that Deep Cryogenic Treatment (DCT) can triple the life of a rotor, prevent warping and delay cracking. If you remember from Metallurgy 101, metals are crystals. Temperature changes the concentration of point disorganization of the lattice structure. A whole lot of things happen when you reduce the temperature of a crystal. That is why DCT works on brass, aluminum, titanium, most other metals and some plastics. These claims are backed up by research.

 

[Maui]

The use of a cryogenic treatment will do nothing to change the microstructure of an austenitic stainless steel such as 316 that does not contain any significant amount of martensite or ferrite. This is one of the reasons why austenitic grades are used in cryogenic service.

Maui

 

[Frederick]

Brass, aluminum and plastics have no significant ferrite either, but show significant changes due to Deep Cryogenic Treatment. I will say again that many of the things happening at cryogenic treatments have nothing to do with retained austenite and martensite. That is the point! Many of the changes have to do with creation of small carbide practicals, the reduction of or changing of point defects, migration of alloying elements such as carbon and the reduction of bond energy in the crystal. If DCT does nothing to austenitic stainless, why does the wear resistance go up and why does the standard deviation of the hardness go down when it is treated? If you need ferrite and martensite for DCT to work, why does DCT work on aluminum, brass, silver, titanium, magnesium, and plastics?

You are thinking about micro structure when you should be thinking about crystal structure. Take your Metallurgy 101 textbook off the shelf and look into how crystal structures react with low temperature.