How to relief internal stress after cold wire drawing? 6

[xlns]

Hello!

First of all, I'd like to apologize for my poor knowledge of terms, conventions and, possibly, even basics of material science. I will try to compensate that with extensive description of my problem. What I am trying to do is to fill up a steel tube (10mm outer and 8mm inner diameter, length ~10-20cm) with a certain powder. After closing tube with aluminium foil tampons, I'd like to draw this tube to a wire with as small inner diameter as possible using tube swaging machine. Ideally, my target inner diameter would be somewhere at 1.5mm. While drawing, especially at low diameters, steel becomes very hard and, my worst problem, cracks emerge along the wire, allowing powder content to spill and making my sample useless. Cracks are narrow and 1-10 cm in length. I can take photos of cracks, should that help. I started digging through net (this forum especially) and found this thermal stress relief process that I, as a theoretical physicist, estimate it may help me. As I understand that, keeping my piece of steel for some time at a certain temperature provide enough energy for atoms to resettle thus regaining some of steel's previous capacity to form and resistivity against crack widening. Now, here is the catch: due to powder content, I can *not* go above 500C - 550C (932F - 1022F) tops! Do note that I can heat my steel wire in a inert Ar atmosphere at any temperature up to 1200 C(2192 F). So, here are few questions, for which I am very grateful should someone try to answer any of them:

1) Am I even looking at the right direction? Can thermal stress relief at some point help me achieving low diameters? I was thinking about heating my tube at maybe 500C for 15 minutes at diameters of ~5mm may help my with drawing. When at 5mm diameter, wire is already quite hard.

2) I am not sure which information from steel composition declaration are important, so I am gonna just list them

St 37.4 DIN 1630/1984/ (fully killed steel)
Chemical composition(%) : C 0.1, Mn 0.43, Si 0.24, P 0.012, S 0.015, Al 0.026

As I understand, there is a minimum temperature below which there is no stress relief. How can I determine that temperature for my steel? How do I determine amount of time I should keep steel in furnace? Any link to additional information would be really great! Of course, if any other type of steel can serve my purpose better, I am more than ready to switch. I saw this 'Iron Carbon Diagram' diagram and, for an example, it looks to me that steel with C content of 0.8 may be better than my present steel. Is this correct? Naturally, I am not trying to recrystallize the steel, but I figured maybe that high C content lowers stress relief temperature as well?

3) Can I trade temperature for time of relief? Because I don't want to jeopardize powder content, I'd like to do stress relief at as low temperature as it makes sense. Can I compensate that by prolonged stress relief? I saw some so-called T-T diagrams with stress relief but they don't make much sense to me

Thank you for any answer or comment.

 

[TVP]

Yes, you are looking in the right direction. First, a little terminology. Stress relieving does not result in a change in microstructure, just a reduction in the internal stress. Annealing is the term used to describe thermal treatment processes that result in a change in microstructure, and hence a change in the mechanical properties (yield strength and tensile strength are reduced, elongation is increased).

A typical annealing temperature for this grade of steel would 660-725 C. Below this range you will really only get stress relief, and not a change in microstructure. This means that you will not see much of an improvement in ductility/formability. Increasing the carbon content is not a good idea, because these grades do not have as much ductility, and therefore will be more prone to cracks. Given your temperature constraint, about the only thing I can think of is to use a steel grade that is even more formable, one with the following composition limits:

C = 0.08 maximum (preferably 0.06 or less)
Mn = 0.30 maximum
Si = 0.10 maximum
Al = 0.020 minimum

 

[xlns]

Hello!

Excellent post. Let me see if I got this right: no matter what steel do I use, my temperature constrains basically deny me a possibility to improve steel performance by annealing. All I can do is to try with another steel, complying with limits you've kindly provided, that may prove more resistant to cracks forming? In any case, thank you very much!

 

[EdStainless]

control of your starting tube in important. The finer the grain size and lower the carbon the better off you will be.

a stress relief might be helpful but it will not result in a large change.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[stanweld]

xlns,
I think that you misinterpreted TVP's post. The stress relieving temperature that you proposed, which is below the recrystallization temperature of the material, will not be of real benefit to further drawing. The process anneal performed above the recrystalization temperature will be useful in restoring ductility for additional drawing (cold reduction). The chemistry that TVP suggested will provide greater drawability than the alloy you listed.

 

[btrueblood]

You might consider changing to a different material, e.g. aluminum, which can be annealed at a lower temperature.

 

[IRstuff]

Your ID reduction is from 8mm to 1.5mm. Is there no possibility of starting at a smaller ID?

TTFN

FAQ731-376

 

[xlns]

Hello and thank you all for valuable replies. As a collective reply, what we are going to do is try with steel with less dopands, as TVP suggested, and with smaller ID. And hope for the best. A personal note: in last two weeks I've discovered how vast and complex material science is and you people are lucky to have this agile community of experts ready to help. Thank you, once again.

 

[Compositepro]

Why are you using steel? Use a metal that does not work harden at all. Pure iron might work. So would gold or lead or pure aluminum.

 

[xlns]

All of those metals melt at too low temperatures or react with powder content at high temperature. Just today, we were talking about pure iron - we are probably gonna give it a try. Thanks for suggestion!

 

[CoryPad]

Work hardening is not the problem, plastic strain capacity is. Unalloyed iron can be strained more than a steel due to its higher strain capacity, not because of lower work hardening.