Estimating die wear, Tungsten on Nylon 3

[Robbyod]

Hi all, I have recently been presented with a problem posed by die wear.

The dies are used for necking nylon tubing while on a mandrel at ~250°F. They are made of cemented tungsten carbide (unsure of exact composition/grade but may have data on this in the future) and seem to have worn quite substantially during their life. I am trying to estimate the typical wear i would see per cycle of necking, but im not sure where to start.

I can think of two methods, estimation through calculation, incorporating material and usage properties, or through trial and measurement. I would think the wear levels too small to detect during a short run study so i think im left only with the former option. Not being familiar with any form of wear calculations im lost.

Any suggestions? Much appreciated

 

[MikeHalloran]

Part of my first real job was shuttling cemented carbide dies back and forth to the carbide guy for repair until we found the right combination of radii that would allow us to neck 3.5" OD x .25" WT mild steel tube to 3.25" OD for insertion into the center section of an axle housing. The other end of the tube was multipy necked, even farther, for a spring seat and a bearing housing. The dies lasted only a few strokes before they got scratched, until we got the radii right. Then they didn't need attention, and have probably been working since 1967 at a couple million strokes per year.

I.e., I can't see a carbide necking die being bothered by nylon tubing, ever, unless dirt is involved.


Mike Halloran
Pembroke Pines, FL, USA

 

[Robbyod]

Thats what i thought, and i assume thats what the original validation crew thought. Something went wrong somewhere however and the IDs have significantly increased.

I am dealing with IDs of 0.025"-0.038", with spec windows of 0.001". The observed increases so far have been up to 0.0006", which is big when its almost the width of my spec window. The work is also carried out in a clean room, so dirt is not (Should not) be a big factor.

These dies could be in use for the last 10 years, the lifecycle is not tracked

Thanks

 

[tomwalz]

I’ll go along with MikeHalloran on this. Has there been a change in the nylon or in the way the nylon is cleaned or prepped? Tungsten carbide is susceptible to mass loss in a basic environment if there is an electrical influence. It is susceptible to mass loss if there is exposure to acid.

A lot of problems, such as this, get solved about the fourteenth time you ask someone if something was changed. Quite often they don’t remember a change. Maybe purchasing made a change. Made the nylon is different. First time I ran across this was about 25 years ago. Brazing problem. Tips coming off the saw. Customer sais he hadn’t changed anything. After a couple weeks the customer finally said that all they had changed was the flux. Then he added that it couldn’t be the new flux because the salesman had said it was better than the old stuff.

If your dies are 10 years old then it is probably elemental Cobalt as a binder. Elemental Cobalt is more reactive than modern alloy binders.

Another thought is metal fatigue. Possibly the Cobalt has been through enough compression / tension (or compression / release) cycles that it has become brittle and you are getting loss from the rubbing. This is a real guess. The stuff I see in tools doesn’t go though that many cycles before other wear factors over shadow any possible metal fatigue considerations.

If your dies are 10 years old then there are much, much better grades of carbide available. Think about the transition from cathode ray tubes to flat screens in monitors and televisions for an idea.

When I do carbide wear analysis I use a list of 17 factors. See following (This is designed for analysis of cutting tools.)

Tungsten carbide is actually tungsten carbide grains cemented with a metal, usually cobalt. What follows are failure mechanisms we have seen in industry.

The following list is open for discussion but we have found it to be a useful tool for developing new grades.

1. Wear – the grains and the binder just plain wear down

2. Macrofracture – big chunks break off or the whole part breaks

3. Microfracture – edge chipping

4. Crack Initiation – How hard it is to start a crack

5. Crack propagation - how fast and how far the crack runs once started

6. Individual grains breaking

7. Individual grains pulling out

8. Chemical leaching that will dissolve the binder and let the grains fall out

9. Rubbing can also generate an electrical potential that will accelerate grain loss

10. Part deformation - If there is too much binder the part can deform

11. Friction Welding between the carbide and the material being cut

12. Physical Adhesion – the grains get physically pulled out. Think of sharp edges of the grains getting pulled by wood fibers.

13. Chemical adhesion – think of the grains as getting glued to the material being cut such as MDF, fibreboard, etc

14. Metal fatigue – The metal binder gets bent and fatigues like bending a piece of steel or other metal

15. Heat – adds to the whole thing especially as a saw goes in and out of a cut. The outside gets hotter faster than the inside. As the outside grows rapidly with the heat the inside doesn’t grow as fast and this creates stress that tends to cause flaking (spalling) on the outside.

16. Compression / Tension Cycling - in interrupted cuts the carbide rapidly goes though this cycle. There is good evidence that most damage is done as the carbide tip leaves the cut and pressure is released.

17. Tribology – as the tip moves though the material it is an acid environment and the heat and friction of the cutting create a combination of forces.

Notes:

As with any chemical reaction of this sort the acids create a salt that protects underlying binder until the salt is abraded away so grain size and binder chemistry are also important.

Electrochemical effect – erosion compounded by the differences in electrical resistivity between carbide and cobalt

Heat from rubbing can affect carbide so a slicker grade can increase life.


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[CoryPad]

Does the nylon have any fillers? Glass or carbon fibers, ceramic powders/spheres/other? That could be a source for the wear.

 

[tomwalz]

MikeHalloran,

Thanks for the heads up on this. It could well be chemical attack from the nylon.

See UncleSyd's answer from

http://www.eng-tips.com/viewthread.cfm?qid=134459&page=5

Robbyod,

Again I don't have that much experience with dies but in tool wear the wear accelerates as it progresses. E.G. If you lose one unit of sharpness in a week then you lose a second unit in six days and a third unit in another 5 days.

Do you have a way to see if there are any carbide grains embedded in the nylon? Unfortunately they will be in the micron range. May be awfully hard to find. Can you measure or other wise assess the surface profile to see if it is rougher than originally specified.

It just feels to me like someone changed procedures or materials.

tom


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[Robbyod]

Thanks for the info guys,
Regarding material/process change resulting in chemical attack, i will follow that up, but i dont know whether this has happened just recently, or has been happening gradually for years. The nylon is part of a medical device, and as such is subject to FDA req's on cleaning raw materials, and the nylon composition is validated as is, so it cannot change. Not ruling it out, but it is highly unlikely.

Judging by the factors involved and the complexity and time invloved in calculating the wear, i think it may be more beneficial to just screen all current dies and maintain screening indefinitely, gathering data at the same time.

Although i would love to go on a mission to find this root cause, cost/benefit is all wrong, so i wont be chasing it too hard.

These things were overlooked during qual/validation as i suppose no one thought that nylon would ever have an affect on Cem Tungsten Carbide dies.

I suppose the lesson from this is that our preconcieved notions of material interactions, no matter how unbalanced (ie Tungsten Vs Nylon) should not be relied on.

I may need to start a new thread for this, but i also need to measure the ID of the dies, preferably to a resolution of 0.0001" (Tolerance for the dies is 0.0002") what i have found so far, apart from contractors is this

http://dyergage.thomasnet.com/Asset/486Y A_39_Fpo.pdf

Does anyone have any rcommendations or a better method?

Thanks again for all the help and info

 

[ornerynorsk]

Are you allowed any swaging lubricant on the tube? Nylon can be very abrasive.

 

[MikeHalloran]

I'd just use gage pins to probe the die ID.



Mike Halloran
Pembroke Pines, FL, USA

 

[Robbyod]

@Mike
I dont think pin gages are suitable, a set of pin gages with a resolution of 0.0001" and ranging from 0.025"-0.038" would be a very big set of pin gages (130). Already have sets with a resolution of 0.001" but thats not nearly good enough to detect tolerance issues with the dies (+-0.0002").

I could get them in increments of 0.0005" but i really need high resolution measurements to detect the extremely small variations between dies, which is the cause of the original issue.

@ornerynorsk
No, we cannot use lubricants on the tubing, a new process using this would cost more than replacing all of the dies so its not practical.

 

[saberblue]

There is a process called cobalt leach where the chemistry in contact with the carbide(WC) will disolve the Co binder causing premature failure of the WC. There are many different compounds that can cause the leaching- with the heat present at the interface of the die and the nylon you may be creating a compound from material left on the surface of the nylon or it may be something that is being used to clean the dies - you would not believe the stuff people use to make their life easier.

A.R. "Andy" Nelson
Engineering Consultant
anelson@arnengineering.com

 

[unclesyd]

Does your Nylon have TiO2 as an opacifier?
TiO2 can be extremely abrasive to some materials at your operating temperature. There isn't much finish added to Nylon pellets or than some waxes, mainly microcrystalline. There could be some finish on the on the tubing from production requirements.

As stated above you may want to use some type finish in your production. A lot of these finishes have FDA approval and are not very expensive.

Though we used Hard Chrome or ceramics handling Nylon at these temperatures we did do some work with the micro-grain carbides for some applications. They were not ideal but better than large rain carbides. We tested both Co and Ni binders.

To gage your die bore you may want to look into air gages.

http://www.westerngage.com/Buyers_Guide/Air_Gages_for_Internal_Diameter.html

The post by saberblue concerning the dissolving the binder, either Cobalt or Nickel, is a very common problem in the chemical industry. Case in point is in the polymerization of Nylon from Nylon salt. Nylon salt will will dissolve the binder in a WC pump seal in minutes. Hexamethylenediamine, the base in Nylon salt will dissolve the binder in a WC pump seal in seconds.