Help in Understanding the Stronger Grades of 304 Stainless Steel 2

[Maxham1]

We're looking into using a higher grade of 304 stainless steel tubing than the standard grade. My question is about the 1/8 hard, 1/4 hard, etc. grades. They have the desired yeild strength (60-75 ksi) but is the yield strength constant, or consistent throughout the thickness or is the increase in strength just a surface characteristic?

Thanks,

Dave

 

[MikeMet]

Depending on the thickness, ID to OD ratio, and amount of work put into the tubing, it can be through wall. The thinner the wall, the more likely to get through wall hardening and homogeneous mechanical properties. Autoclave tubing is a good example. Small diameter tubing that has very high strength through wall. I don't know the exact wall thickness above which you begin to worry, but it is an issue.

 

[EdStainless]

If you are thinking about machining into a piece of work hardened 304, don't. The residual stress will cause severe distortion.
Commonly work hardened SS is available as sheet (strip), wire, and tube. The Old 1/8 and 1/4 hard only applies to sheet. There are wire specs of their own, and tube is all custom specified.
In tube we don't see any property issues with 1.385" od x 0.208" wall. We have turned samples to half of the wall and the tensile is almost the same.

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Plymouth Tube

 

[Maxham1]

Thanks for the helpful info Mike and Ed - much appreciated!

For those interested, here's a little background on the situation: The part is a band about 2 inches long with an ID of 4 inches and OD of 4.2 inchs (both dims are nominal). We will be making circumferential grooves in the band's outer surface. These grooves have constant width of .25 inches and are about .05 inches deep. They're rounded on each end. It's precisely these cuts that I'm concerned about.

I've done a finite-element analysis which shows that I'll have von Mises stresses of about 30 ksi in the majority of the band and about 50 ksi in the bases of the grooves (the bases of the grooves are flat and .25 inch wide). Hence the reason for considering the hardened version of 304.

Question: The reference trade book that I have only shows "1/8 hard 304," so I appreciate your remark Ed about that being "old" - and I thought the book was current . So, what is the current grade of hardened 304 that I should consider - is it "full hard?"

Thanks, Dave

 

[EdStainless]

If you go full hard you give up almost all usable ductility. There is a spec A666 that covers cold worked stainless sheet and strip. It gives strength levels for various degrees of cold work.
If you had material with a minimum yield strength of about 80ksi or so you should be alright. Don't go any stronger than you need to. The ductility of SS is what gives you the toughness.

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Plymouth Tube

 

[Maxham1]

thanks for the addtional info - it's very helpful. I bought the A666 spec from ASTM ($37) and have selected 1/4 hard, which give us a yield strength of 75 ksi and ultimate of 125.

 

[EdStainless]

There is also a minimum elongation right? I would also set a max UTS or yield. Something just below the next strength level.

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Plymouth Tube