Test failures after machining A4-80 Strain hardened stainless steel 4

[Leh_002]

Hi I’m wondering if someone might be able to point me in the right direction with an problem I am seeing.

Recently I have been seeing a lot of failures of A4-80 SS following machining. There is a consistently large drop in strength from the starting properties. Sometimes this is as much as 300MPa lower.

I’ve tested samples of some raw bar (multiple casts involved) and it passes testing at D/4, but fails at D/2. The material is being machined down to bolts so the shank area has seen a significant size reduction.

I’m assuming that this is down to the nature of the cold drawing process which works the surface of the bar more than the center and the resulting internal stresses.

I’ve looked at the data for all passed and failed tests but can’t see any clear relationship between the starting mechanicals and risk of failure.

I did wonder if cold forging would give better results, but this may be limited by the specification.

If anyone has any suggestions and ideas of how to combat this they would be very welcome as this issue is driving me mad and I can’t see a way around it!

 

[EdStainless]

What is the actual alloy?
What UTS and Yield is it worked to?
And yes, it is the nature of the cold work to be more severe near the surface. If they were forged to the same strength then you see the exact same pattern. It doesn't matter if they they are drawn, rolled, or forged.
But you say you can measure the 300MPa when you machine down to what diameter?
It isn't clear to me what you mean by D/4 and D/2, clarify please.

We used to make some lightly cold worked rod that that was then cold formed into bolts with rolled threads.
One trick that is used with stainless when you need deep strength is to warm work it. I have had bar warm (175F) rotary forged when I needed deep strength.

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P.E. Metallurgy, consulting work welcomed

 

[btrueblood]

"One trick that is used with stainless when you need deep strength is to warm work it. I have had bar warm (175F) rotary forged when I needed deep strength."

Ed, can you explain that to us semi- or non-metallurgists?

 

[Leh_002]

Hi Ed,

Thanks for your reply.

A4-80 is an austenitic stainless steel with the same chemistry as 316. Class 80 is UTS if 800MPa min and PS of 600MPa. My initial thought was to start with stronger material but in some cases even material which started at over 1000MPa has failed after machining.

Generally the machining reduces the diameter by in the region of 30-50% depending on the fastener head type.

By D/2 and D/4 I meant at half diameter and quarter thickness for where the test pieces were taken from. I believe this is how the companies drawing the bar are able to get the results to pass, however with bar turning we have no option but to test at D/2.

BTrueBlood, my background is in hot forging and as rotary forging is generally a cold operation I haven’t had a lot of experience with it, hopefully Ed can share some more details. Warm operations are generally done between 25 and 60% of the melting temp of the alloy. This avoids the recrystallisation and annealing that happens during hot working resulting in higher strength.

 

[mfgenggear]

weird,it appears the outer diameter vs the inner diameter is work hardening at different hardness, thus different UT & YT.
be interesting to verify raw bar stock and make test samples of the inner and outer material. but would not solve the issue, but better under stand the issue.
have tried contacting the mill if they have a work around?
may be ED can enlighten us all.

 

[mfgenggear]

I wondering if forged bar vs cold rolled bar would be better for this application

 

[hendersdc]

I assume you have already thought of this, but are you introducing any stress concentrating geometry when you are machining your test samples? Can you provide pictures of failed parts?

 

[EdStainless]

In these meta-stable austenitic grades when you work harden then they strengthen by two mechanisms. One is the induced strain effects, and the other is martensite transformation. If you want to see this directly measure the ferromagnetism of your material, that is the martensite.
When you warm these alloys (340 and 316 mostly) slightly you keep them above the critical temperature and they do not form martensite when deformed. Then you can cold work them more, with less effort, and still retain higher ductility.
While I have used this trick when cold drawing tube, we would put tubes on a heated drying rack before cold draw. But I first learned of this when I needed some high strength shafting back in the 90's. We were using a rotary forge and working to about 1000MPa/900MPa and wanted over 18% elongation.
I have seen this method work with some chemistries as low as 125F, but some require a slightly higher temp. I have never seen 175F not be hot enough to suppress the deformation driven martensite transformation.
I don't think that it matters If you roll, draw, or forge. This isn't a lot of cold work that you are looking at. I would guess that about 25% reduction would enough to do this.
And yes, a mid-radius sample would be the traditional location for a tensile check.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[mfgenggear]

Thank You Ed I gave you star

 

Thanks Ed, I leaned something new here.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[btrueblood]

3rd star from me, Ed. If I'm following correctly - you can get higher UTS if you let the martensite develop in strain hardening, but at the cost of lower ductility?

 

[mfgenggear]

the other question is and I can not wrap in my simpleton mind is how an austenitic steel develops martensite?
which is common for low alloy steels. or 400 series steels, am I missing something.
ED your golden for sharing this knowledge.

 

[Leh_002]

That’s really interesting Ed, I hadn’t really considered the effect of strain induced martensitic transformation.

I’m still wondering what my best option is to prevent future failures, especially as this is a pretty common grade for bolting and I haven’t come across any others having a similar problem.