wall thickness variations in low-alloy steel seamless tube 1

[AndrewW]

This is my first post here so if I make a mistake, please just point it out and I'll fix it. Been reading here for a while but never posted before, having more to learn than to teach.

My understanding is that wall thickness variation is hot-rolled seamless tube in introduced in piercing, when the piercing point "wanders" radially in the billet. If there is another source I'd like to know.

To minimize wall variation, I read 1) start with uniform billet, 2) hold billet temperature uniform, 3) maintain piercing point. Again, if there are additional approaches / actions, please let me know. Is initial billet length an issue? Speed of piercing? Billet temperature (actual temp. aside from uniformity)? This is the crux of my question.

We are trying to qualify a vendor that is having a hard time holding wall tolerances on 4" - 5" OD tube, about 0.300" - 0.500" wall (several sizes in this general range).

All my knowledge of this is from books, nothing practical. I am not qualified to advise them - their mill, they ought to know how to run it - but am asked to help, so I'm turn to y'all for advice / assistance.

Thanks in advance.

Regards,
Andrew

 

[metengr]

As an end user of tube and pipe products, I have performed a number of quality/metallurgical audits on tube mills. Most of the reputable mills can handle this OD size with minimal variation in wall thickness.

I am curious. What is your position or association with the tube mill? As a customer, you should have no business to tell the mill or interfere with the mill as to how to make adjustments to their process. They need to figure it on their own. They should have a quality control program, production and engineering departments to handle these process problems.

 

[metengr]

Andrew;
One last bit of advice; if you can't qualify the mill or vendor, walk away from them and find the next one. This is why they call it a qualification process.

 

[AndrewW]

metengr,

Thanks for your reply! I'll explain my position, you will see why am was so diffident about asking advice.

The company for which I work buys tube for use in a product. I'm the metallurgist here, and most of my work centers around advice to mechanical engineers and designers about what to make things out of, and failure analysis when they make the wrong choices - or when the parts get abused and break. I have never worked in a seamless mill.

Given my choice, I would not walk away from this producer - I would run away! Given for example a 0.300" wall 4" OD tube - when I measure (UT thickness gauge) everywhere from 0.325" to 0.275" at twenty spots on a grid on a 5-foot length of pipe, I think there is a serious process problem.

We do buy good material from a couple of well-established mills, and from a technical standpoint we do not need these new people. From a business standpoint, someone thinks we do - and I've been directed to help them meet our requirements, never mind that it is beyond my scope.

Your point about my not having any business telling the mill how to run their process is spot on! It is like telling a heat treater what austenitizing or tempering temperature to use - they don't want to hear that at all, they just want the alloy and hardness range desired.

Thanks again for your input. I'm still seeking some knowledge, if only to tell them where to start looking for their problem.

Regards,
Andrew

 

[metengr]

Andrew;
Be real careful here because ethics should be first and foremost. If you have been charged with a vendor evaluation and they cannot meet your quality requirements, it is over, unless they can correct their nonconformance.

Regarding hot rolling, most mills start with rounds that are pierced followed by roller expanding over a center mandrel. It just may be that the current vendor because of equipment limitations cannot hold tolerance on larger size product form. Do not get in over your head!

 

[AndrewW]

metengr,

Again, thanks! I am already in over my head but not, so far, in an ethical bind.

This is not so much a vendor evaluation as a situation where management wants a supplier in a certain geographical region for logistics reasons. They want to buy material from this mill, and only the prospect of undesirable consequences has held them back so far. If I can help the mill improve their process that will smooth the way forward, but I lack the hands-on knowledge of piercing mill practice to do so.

I think their mill is new, modern, and well-equipped. I also think they need to hire some experienced people to run it.

Thanks for your time and guidance on this. I'm still hoping someone who may have retired from a hot mill, or who has some insight, will follow up with suggestions to pass along to the mill.

Regards,
Andrew

 

[TubeSled]

Andrew,
What are your specifications to the mill? What are the tolerances that you have given them? Is this a hot-finshed product or cold drawn?
Most seamless tube mills generally produce to ASTM A519 as a norm, and within this spec, wall tolerance can vary from +/- 7.5% to 12.5%. In the situation you described above, (4.000" x .300"), the readings you have are 8.3% wall variation, which falls withing the requirements of the spec if this is a Hot-finished product.
Make sure in your specification/requirements to the mill you spell out exactly what you need, and if they fall out of this requirement, then you have reason to react. Most mills will only change or try to improve if there is a real reason to (read-$$$).
In any case you may have a hard time finding a mill that would guarantee a wall tolerance much better than +/- 7.5% or 6% on this size.
To provide a little insight into your first question about eccentricity in a seamless mill. From what I have experienced, everything you mentioned above and more can have an influence on eccentricty depending on the type of seamless mill and production process. It can be an extremely complex issue.

 

[AndrewW]

TubeSled,

Thanks for your reply. It is hot-rolled and they are within tolerace, which is +/- 10% of nominal. However, we have never seen such variation within such a short distance.

Generally, the tube from other suppliers is much more uniform. The tolerance in that case becomes the range of acceptable sizes - the particular lot is on the high side of nominal or on the low side. On any individual tube they hold around +/- 5% of nominal, and the tube is on the order of 20 ft. long.

In this case, even though they are at about +/- 8%, the measurements use up that entire range in 5 ft. of length.

Part of this is I want to understand what causes the variation for my own sake, and part of it is I need to make suggestions to the mill about what to try, to improve wall uniformity from one end to the other.

Thanks again. This is a great Forum; I will be looking for questions that I am qualified to answer, so the interaction is give-and-take instead of just one-way.

Regards,
Andrew

 

[TubeSled]

Andrew,
Depending on their process, you will get this "entire" tolerance in a short distance. If a tube is running at 10% wall eccentricity normally in the same cross section sample you will get one side of the wall at +10% and then -10% on the other. So there is your entire tolerance used up in one cross section.
But like I said, it all depends on their process and how they produce this product, seamless tubing can be produed by several difffent methods.
Are they rotary piercing? What kind of piercing? Any type of mill after that?
Wall variation can be affected by all of the below and more: Billet Temperature, Uniform temperature, piercing point, piercing bar, bar steadiers, feed angle, speed of mill, etc.

 

[metengr]

Go to this web site for a detailed explanation regarding steel tube and pipe manufacturing. Good information

http://www.mrw.de/downloads/stahlrohre_engl.pdf

 

[AndrewW]

metengr,

Tried the link, got:

Die Seite wurde nicht gefunden
Die von Ihnen gesuchte Seite wurde eventuell gelöscht, der Name wurde geändert oder die Seite ist derzeit nicht verfügbar.

I will click around on the Mannesmann site and see if I can find it. Thanks!

TubeSled,

Sorry, I owe you a reply. All I know is that thay are hot rotary piercing, nothing more. The mill is remote from where I am, and there is a language barrier as well as distance.

Thank you gentlemen again for your time.

Regards,
Andrew

 

[metengr]

http://www.mrw.de/english/frames_7-sitemap.html

 

[AndrewW]

metengr,

Got it! Thank you!