I need some help from a metallurgist or similar. Failing stainless wire link.

Nathan the Machinist

Nathan the Machinist

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This is a link used in a product that I'm involved in the manufacture. The link articulates a valve. It is breaking in a bend after about 6000 cycles, apparently a fatigue issue. Obviously it needs a design change. However, I'm wondering if there is a HT solution for the existing parts?

broken_link.jpg


The material is either 302 or 304 stainless (most likely 302). It is 11 gauge .120". I have bent some in a vice and it feels half hard.

Obviously there is a stress riser issue. It needs a larger fillet and a radial draw type bend rather than a compression bend. However, I suspect the material in the bend is significantly work hardened and the lack of ductility is making that already weak area weaker. I think I need to temper or anneal the wire.

I don't think I want to make the wire dead soft or it may actualy take a bend in service. I want to draw it back a bit. I want to avoid issues such as tempering embrittlement or other pitfalls I may not be aware of.

I have never heat treated a work hardening alloy before.

Any suggestions?
 
I'm not makeing them. They're already made. Thousands of them.
 
The only HT you can do is anneal. Can you see any damage done in the bending operation ? Nicks, scratches that might have caused the fatigue failure ? Since they're already made you can't upgrade to another material.
 
Yeah, there are stress risers where they dented it in the bend. But the parts are made and paid for...

What do you recommend for the anneal?

I've never annealed 302.

Would a soak at 1200 do it?

Thanks
 
A 1200 F would give you a better stress relief instead of a full anneal .
A smooth bend with a larger radius would be the only improvement with that material.
 
Yeah. I'm not sure I want a full anneal. I'm worried about deforming the parts during HT, I don't want to risk precipitating carbides with a less than perfect technique, and I don't think I want the finished part dead soft.

I'm wanting to "draw it back" a little to improve ductility, but I don't want to accidently artificially age it and inadvertently precipitation harden it or anything like that.

I was leaning towards a one hour soak at 750, but I want to avoid any kind of embrittlement range, so I was wondering if 1200 would be smarter?

I don't know enough about this to take an intelligent stab... I do know that a soak at 2000F and a water quench (the recommended anneal for 302, isn't that some sh!t) doesn't sound like a good idea to me.
 
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The fast quench from annealing is there to prevent any intergranular corrosion from chromium carbides in the grain boundaries.Water quench would be necessary only on large parts .For the wire air cooling wouldbe fine.
I think anywhere from 750-1200 F would do the job . I haven't found any info on stress relieving 302.
Next time use a better alloy . My amateur radio antennas for my car are made from 17-7 PH stainless .You can take the 8' whip and bend it end touching end and it will spring back to the original shape.
 
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Just to add to mete's advise.
What I have on stress relieving 302 stainless is:
.....Heating to 445-750F and holding for several hours, depending on section size ( yours is small). This will result in no change to metallographoic structure.

....Aerospace annealing is at 1650F and water quench for thicker sections, air quench for .10" and less sections.

.....As an alternative, parts can be stress relieved from annealing temps (1850-2050F)....


It advises against stress relieving parts from half hard and strain hardened condition without first checking with the engineers who designed the part. ( I assume because they may be softer).

Stacy
 
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