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Urgent question, Residual stress
5

Urgent question, Residual stress

Urgent question, Residual stress

(OP)
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

RE: Urgent question, Residual stress

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?

RE: Urgent question, Residual stress

(OP)
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.

RE: Urgent question, Residual stress

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.

RE: Urgent question, Residual stress

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.

RE: Urgent question, Residual stress

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.

RE: Urgent question, Residual stress

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

RE: Urgent question, Residual stress

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.

RE: Urgent question, Residual stress

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.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Urgent question, Residual stress

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



RE: Urgent question, Residual stress

2
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.

RE: Urgent question, Residual stress

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.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Urgent question, Residual stress

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.

RE: Urgent question, Residual stress

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.

RE: Urgent question, Residual stress

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

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Urgent question, Residual stress

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.

RE: Urgent question, Residual stress

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_rel...

Maui

RE: Urgent question, Residual stress

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.

RE: Urgent question, Residual stress

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

RE: Urgent question, Residual stress

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.

RE: Urgent question, Residual stress

We are talking about 316 stainless steel.

Maui

RE: Urgent question, Residual stress

I've read your book. I have also bought it for my employees. Maybe you should take ASM Handbook Volume 4A off the shelf and read pages 382 to 386. I wrote that.

RE: Urgent question, Residual stress

Good for you. Can you provide a single objective and unbiased peer reviewed article that demonstrates unambiguously that 316 stainless steel which does not contain any significant amounts of ferrite or martensite is altered or impacted in any significant way by a cryogenic treatment? If so, please reference it.

Maui

RE: Urgent question, Residual stress

Here is one. It was on the web and took about 3 minuets to find. I will submit it to the CSA database for inclusion.

International Journal of Engineering Research and Reviews ISSN 2348-697X (Online)
Vol. 2, Issue 4, pp: (18-23), Month: October - December 2014, Available at: www.researchpublish.com
Page | 18
Research Publish Journals
A Review Paper on Methods of Improvement of
Wear, Corrosion and Hardness Properties of
Austenitic Stainless steel 316L
P.Elango
PG Student, Department of mechanical Engineering, Bharath University, Chennai –India

My own experience with the use of 319 stainless has shown a huge increase in life over 319 with a hard coating. The use was a screen used in the manufacture of highly abrasive plastics. DCT replaced a hard coating that was many times more expensive and many times less effective. We are now specified on the print for DCT to be done because it saves the plastics company so much money. That in my eyes is better than a research paper.

We have to rethink wear resistance in light of what DCT is showing us. I see huge increases in fatigue life on almost all metals. Top drag racers are using DCT to give them six times the life on valve springs. They do not take this lightly as a broken valve spring can cause an engine to detonate and kill the driver.

You have to look at the crystal lattice structure for the benefits, not the microstructure. To relate to this thread, the release of residual stresses DCT can create and the refining of the crystal structure may be what is needed in the referenced screw. DCT is MUCH more than conversion of retained austenite.

RE: Urgent question, Residual stress

Maui-

The significant amount of mechanical working applied to the 316L material during the thread rolling process might result in some strain-induced martensite formation. Here is a tech paper that discusses the issue. Don't know if cryo stress-relief would be of any benefit though.

RE: Urgent question, Residual stress

Tbuelna, thank you for referencing a peer reviewed article. Yes, that is a possibility. Martensite can form in 316 during cold working if the resulting deformation is severe enough. But this is usually only a concern if the cold working is done at a relatively low temperature. Unless the material has been processed to induce it, when cold worked at room temperature the formation of martensite is usually minimal in this stainless grade. This is discussed, for example, in the following article,

http://www.sciencedirect.com/science/article/pii/S...

Fred, what you provided is a review by a college student of several different investigations, one of which is entitled "A study on wear and corrosion characteristics of marine propeller shaft material - AISI 316L stainless steel subject to cryogenic treatment" by J. Suresh and S. Suthagar. This is not a peer reviewed investigation - it is a conference proceeding from the 2nd international conference on science, engineering and management. And the summarized results that are claimed raise several questions. The author claims that by cryogenically treating 316L material he improved the wear characteristics according to his pin test results, dramatically improved the corrosion resistance, and the material was 16% harder (whatever that means). This is all supposed to be accomplished by inducing a martensitic transformation in the 316L by cryogenically treating it at -186 C which is referred to by P. Elango as the liquid nitrogen temperature. At atmospheric pressure nitrogen boils at -195.79 C (77 K, or -320 F), not -186 C. The formation of martensite would be expected to reduce, not enhance, the corrosion resistance of the material. And since I am not able to locate a copy of the paper by J. Suresh I am unable to review the details of his investigation beyond what was summarized by P. Elango.

And none of this suggests that the residual stress that was the original topic of discussion by the OP would be relieved by a cryogenic treatment.

Maui

RE: Urgent question, Residual stress

Frederick,

I just don't believe you comments concerning the movement if voids and other crystal structure changes due to DCT and I have never seen a paper which was based on good science to explain these phenomena.

Most diffusion processes are thermally activated so how can very low temperatures and allow alloy diffusion this to me is counter intuitive - Re-visit Fick's Laws if you disagree and explain to my mistake.

RE: Urgent question, Residual stress

Maui:
The scientific community generally accepts the beginning of the cryogenic temperature zone to be -244F, so the research you are criticizing was done well under cryogenic conditions. As a practical matter it takes a lot of liquid nitrogen to get a chamber down to -320F so a lot if not most research in the field is done at -300F or slightly lower. There is nothing sacred about the boiling point of liquid nitrogen.

Here is some research done to 316 which is focused on residual stress.
Relief of tensile residual stress in girth joint of AISI 316 steel
by deep cryogenic treatment
Qiongqi Wang, Weize Wang, Fu-Zhen
Xuan, Zhengdong Wang, Shan-Tung Tu
*
School of Mechanical and Power Engineering, East China Un
iversity of science and tec
hnology, Shanghai 200237,

Abstract
Although it has been reported that deep cryogenic treatment can improve the wear and fatigue resistance of some metals and alloys, less work was done to investigate the effect of such processing on weld joint of austenitic stainless steels. In this study, phase trans-formation of 316 austenitic stainless steels was studied by the magnetic measurement before and after deep cryogenic treatment, and residual stress in fusion zone and heat-affected zone of girth joint of AISI 316 stainless steel was measured by the electric discharge cutting and static state strain gauge after deep cryogenic treatment. The results indicated that the deep cryogenic treatment can greatly alleviate tensile residual stress of girth joint of austenitic stainless steels greatly.

I did not have time to read the entire paper, but you can see that cold can be used to reduce residual stress. Also the work of Victor Sloan at Victor Aviation has indicated that most metals will have residual stresses relieved at cryogenic temperatures. This is why very sensitive satellite components are treated to -400F so that they do not move in use.

Fennlane


Collins (Cryogenic Treatment of Tool Steels
Collins, D. N.
Advanced Materials and Processes
December 1998, pp. H23-H29)

Collins concludes that the formation of fine carbides are initiated during the hold at -300F and they actually form as the temperature comes up. The diffusion of carbon atoms is happening at cold temperatures. The longer you hold, the more carbides are formed. As to research,you have to realize that the diffusion of point defects is taught in Metallurgy 101. The colder you go the fewer point defects will be in equilibrium. (See Structure-Property Relations in Nonferrous Metals, Alan M. Russell, Wiley, 2005. pages 18-19. The trick is to lower the temperature slowly as not to "freeze in" the point defects.

Something that I don't believe is how supposedly astute metallurgists can make blanket statements without looking into the claims being made. More and more research is being done in the world on this process that can save industry huge amounts of money, and it is not being done here in the USA. Yet my company has shown auto parts makers six to nine time life on their carbide tooling. Those are their numbers, not mine. Why does DCT work on carbide when there is no retained austenite in it? Why did a recent paper show that plastics respond to DCT? Something is happening. Go to the CSA www.Cryogenictreatmentdatabase.org and read before you tell me you "don't believe." This is not a religion.







RE: Urgent question, Residual stress

Fred,

In the first paper you referenced I pointed out that the student referred to -184 C as the temperature of liquid nitrogen. This very basic statement is incorrect. At atmospheric pressure nitrogen boils at -195.79 C. If the author gets something this fundamental wrong, then it leads the reader to question what else in the paper might be in error. This is the main reason why I mentioned it.

In the other paper you listed above entitled, "Relief of tensile residual stress in girth joint of AISI 316 steel
by deep cryogenic treatment" the author discusses the benefits of using cryogenic treatment to relieve the residual tensile stresses produced in welding 316 stainless steel. But the component that the OP described was not welded. It was cold rolled. And the residual stresses produced during cold rolling are completely different from the residual stresses produced by welding 316. This study simply does not appear to be relevant to the problem at hand. I am not inferring or suggesting that cryogenic treatment cannot be beneficial when used correctly in the appropriate application. My point is that for this particular problem it would not be beneficial in resolving the issue that the OP described.

Maui

RE: Urgent question, Residual stress

The paper I noted states:

" In a Deep Cryogenic treatment the material is first allowed to cool from room temperature
to a temperature of -186 C by introducing the test piece in Liquid Nitrogen (LN2) controlled flow chamber up to 2-3
hours duration to take, then maintained in a cooling chamber with the above temperature up to 24 hrs and retain back to
room temperature, it takes 6 hrs. "

Nowhere does it state a boiling point for liquid nitrogen. What he is saying is that the part was put into a chamber where a controlled flow of liquid nitrogen slowly reduced the temperature to -186 C. He then maintained the temperature at -186 for up to 24 hours and brought it back to room temperature in 6 hours. So he did not get anything fundamentally wrong. You are probably assuming the piece was immersed in LN2 which is a common (but usually wrong) assumption that people make about DCT.

Regarding the stresses from welding being different from the stresses of cold rolling, you have a point. But that doesn't mean that DCT would not help the screw in question. DCT has been shown to relieve residual stresses in many materials. Savvy die makers use it to reduce time on EDM machining because it reduces the movement in plates being machined. The suggestion to try DCT on the subject screw would be an inexpensive process to try out that did not involve high temperatures. DCT is a valid process that can be used on almost all metals and some plastics. It can do a lot of things and can save this country considerable time and money if we can get it passed the uninformed naysayers. I know it is not intuitive that you can modify something with cold and we have been using heat to modify metals for over 8000 years. But we have only had industrial quantities of cold for a little over 100 years and we are just scratching the surface.

RE: Urgent question, Residual stress

Has composition been consistent batch to batch?
Heavily worked 300 stainless steels are sensitive to composition, particularly nickel.
Long ago I encountered an unlisted Cu grade (4% Cu IIRC) intended just for fasteners, to withstand the severe strain.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

RE: Urgent question, Residual stress

Brimstoner,

Even listed grades can have high copper. ISO 3506-1 A4 allows up to 4 % copper, and 302HQ (UNS 30430) is another example.

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