400 series stainless steel welds strengthing over time 3

[sampsonr]

Hi Guys,

I'm currently doing a research and development project in laser welding thin (0.5mm) stainless steel using a fibre welder (Rofin 2kW 1070nm CW) and have notice a phenomenon in which the weld becomes up to 20% more ductile as its aged over a couple of days verses tested straight away. We have statistic proven this on 2 different grades of 400 series stainless steels and can confirm that it does not happen on mild steel.

Has anyone had any experience with this? Does anyone know what could cause this? Our method of testing is a ball cupping tester where we clamp the sample and drive a ball into it until the material or weld fails, we can then read of a relevant displacement.

If anyone can help or direct me to someone that can then that would be greatly appreciated

Regards,
Roy

 

[EdStainless]

What gases are you using?
What alloys (ferritic or martensitic)?
I can think of a couple of mechanisms.

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

 

[sampsonr]

Hi there,

Thanks for the reply.

Gas has not played any role as we see it in samples of no shield gas, argon shield gas and nitrogen shield gas. I should of said before but we get this using both fibre and CO2 lasers.

Regards,
Roy

 

[sampsonr]

Samples have all been ferritic.

 

[EdStainless]

My first hunch is that you should check the hydrogen levels in the metal right after welding.
As a second, I am guessing that these are stabilized alloys, is that your welding times may be so short that they don't re-stabilize and that some of this is happening over time. SEM should be able to tell you that.

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

 

[sampsonr]

Interesting, do you have any more information on this at all or know where I could find some?

Weld times are very short and concentrated, Weld speeds are around 100mm/s so you maybe right with the stabilization. The element that makes it hard to detect is that within the first hour the strength has greatly increase so any kind of test needs to be straight away. I'm not sure what kind of sample prep is required with a sem but I'll look into it.

Thanks,
Roy

 

[EdStainless]

Is the strength changing or the ductility?
You need to pull a series of tensile coupons, either straight across the weld or at angle. Could you laser cut them? That would be fast and they would be uniform.

If you do forming tests right away do you get any cracking?
After some time does the failure mode change?
If you warm the sample to 100C for 30min do you get the full ductility improvement?

For SEM work a very quick prep involving punching out a sample of material, gluing it to a base, and grinding and polishing the surface. You should be able to get a sample into the SEM withing 15 min, then watch for structure changes. Maybe map the stabilizer (Ti or Nb) to see where it is an look there.

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

 

[sampsonr]

Hard to say for sure, I believe it has an element of both. The test we do is call the Erichsen cupping displacement test in which we deform the material until we detect a crack, see the attached image to see the process. The failure mode is the same. We do Tensile testing but our samples generally fail in the parent material so you don't really get a point of difference between samples like you do with the cupping test.

Would warming the samples speed up the stabilization?

Thanks for your input.

 

[EdStainless]

Then cut tensile coupons that have the weld at 45deg across the gage length.
If the strength is decreasing and the ductility is increasing then I would suspect hydrogen embrittlement.
If the strength stays the same and the ductility increases then I would lean toward stabilization.

Heating will speed up both.
Are the failures always transverse to the weld? Do they initiate in the weld or the HAZ?

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

 

[metengr]

I think before jumping the gun, I would at least examine the failed cup coupons to confirm fracture mode and exact failure location between as-tested coupons and "aged" after several days. This can be done by any competent metallurgical lab and will provide first hand information. You might be surprised at the results.

Second, do a little background search of information, like the paper below;

http://www.saimm.co.za/Journal/v089n08p243.pdf

 

[mcguire]

The only logical explanation is the delayed stabilization in that nitrogen and hydrogen are the only species which are mobile at that temperature.

Michael McGuire