Proof Strength of 316 Stainless Steel Bolts 4

[RackleyJ]

Why do 316 Stainless Steel bolts not have a proof stress value? I'm in the process of developing a torque chart. I've found a wealth of information online already, but I've hit a snag with stainless steel bolts. The most generic torque calculations use the clamp load (or Bolt Tension), which is the load it takes to stress the bolt a percentage of it's proof stress (which is some factor of yield stress). Grade 5 and Grade 8 has their proof stress listed, but I can't seem to find a value for 316 stainless steel. Instead most torque charts show lab tested torque values for stainless steels in dry conditions. What's so special about stainless steel that you can't calculate the torque the same way as Grade 5 or Grade 8 bolts?

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Would it be appropriate to take the tested values of "dry" 316SS bolts and multiply them by 0.15/0.20 to obtain torque at "wet" conditions?

The formula I use is T=KDF
T = torque
K = Nut Factor = 0.2 for "dry" and 0.15 for "wet"
D = Bolt Nominal Diameter
F = Bolt Tension or Clamp Load = 75% of the load it takes to stress the bolt up the proof strength. I've seen 90% as well for non-reusable applications.

I understand that this method is very generic and prone to error, but the components are not critical. My theory is that I would rather error on the low side and not stress the bolt enough (which is why I used 75%), rather than have a possibility of bolt heads snapping.

As I mentioned earlier, stainless steel bolts don't have a proof strength (I can't find it anyways). If we consider that the only difference between dry and wet conditions is the nut factor, then the ratio 0.15/0.2 should be the difference in the torque value.

 

[TVP]

Proof load is really only used for steels that have been quenched and tempered to produce a martensitic microstructure, because they exhibit a fairly consistent elastic response up to the onset of yielding. In general, austenitic stainless steels and nonferrous materials exhibit a more gradual yielding behavior, and thus use yield strength, which for fasteners is defined in ASTM F606 as the stress at an offset equal to 0.2% strain. You should be able to use a value of 50-85% of the yield strength for stainless steel fasteners with the T = KDF formula.

 

[unclesyd]

Here is summation of about all the information readily available for Stainless Steel fasteners.

http://www.ssina.com/view_a_file/fasteners.pdf

 

[RackleyJ]

thank you both.

 

[desertfox]

hi RackleyJ

This site might be of interest:-

http://www.bssa.org.uk/topics.php?article=1

desertfox

 

[Chiaveinglese]

There is a general lack of consistency in the use of the terms 'proof' and 'yield', (and in the case of Machinery's Handbook a lack of consistency between adjacent pages), so it is often difficult to interpret suggestions on preload stresses.

The following is a link to NASA document MSFC-STD-486B - Torque Limits For Standard Threaded Fasteners - which uses a tangent modulus of 2/3 of the elastic modulus to define the 'yield' (what is generally called the proof) stress. It gives sensible answers for a couple of materials I have tried,(86% of 0.2% proof stress for IN718 and 89% for Waspaloy using MMPDS properties) and as a bonus gives torque tightening values for 300 Series CRES fasteners in their passivated, Ag plated and lubricated states