Designing to higher material properties then published by ASME

[pugheng]

Hello. I am wondering if it is acceptable practice to specify a minimum material yield strength in fabrication drawings? In particular, I am designing an open top tank (non pressure vessel) out of SA-240-316L and 304L plate and HSS materials. ASME section II specifies a yield strength of 25 KSI (19 KSI at 300F), however, typical MTR's for 1/4" plate quote yield strengths above 45 KSI. I would like to specify something above the published 25 KSI to reduce cost and weight.

Does anyone have experience doing this? Also, is there an accepted temperature de-rating factor that can be applied against actual yield test results?

I have asked suppliers and a steel mill to quote reasonably expected yield strengths so as to ensure I won't have a problem finding suitable materials. Still waiting for answers on that.

Thanks in advance for your help.

 

[metengr]

Why don't you use ASME Section II, Part D, Table Y? This table contains actual tensile yield strength values (from known testing) as a function of condition of material and service temperature. Some austenitic stainless steel plate material may be supplied in a cold worked condition, which would be the reason for different values.

You will still need to decide on how much margin you want below the SMYS either from Table Y or using MTR values. Your call for a non-code vessel.

 

[pugheng]

Thanks for the information.

I would like to design to a higher yield strength than that specified by ASME section II, Part D, table Y. All of the MTR's for this material that I have ever seen are above 45 KSI (SA-240-316L and 304L, 1/4" and under). I will be using plate and HSS (formed from this plate) which I understand are all hot rolled. I was hoping I could specify on the drawings that all materials have MTR's which show a yield strength to my specifications (greater than 25 KSI, but not too high so as to ensure ease of supply). The reason to do this is to cut down on weight and complexity by designing to a more realistic yield strength for the material.

As for a safety factor, yes, an appropriate one will be applied to the design.

Thanks again

 

[TGS4]

Make sure that the actual yield values that you are getting are from multiple directions. During cold-forming, there is the tendency of the material to harden preferentially in one direction over another.

You may have to obtain your own elevated temperature yield and ultimate values at temperature, as the yield-temperature curves (or an inferred temperature knock-down factor) in Table Y-1 may not be applicable if you are using higher yield values.

Out of curiosity, what design margins will you be using on your design - and against what failure modes will you be protecting you design against failure from?

 

[pugheng]

I'm considering a safety factor of 3 or so.

The plate is annealed after rolling (as described in the MTR). Would this remove any preferential hardening direction? As for failure modes, this is a square open top tank, so most of the stress is due to bending of the plate walls and reinforcement structure.

Thanks for the help

 

[metengr]

You do realize that the current ASME design margin is 3.5 for design by rule rather than design by analysis? Why not use the tabular values in Table Y knowing that these are minimum values for SMYS and that the material will actually come in higher than this value. The question is how much higher and your reliance on how much higher.

I doubt after all is said and done you are going to save much in the way of cost, and you have peace of mind knowing that the minimum values for SMYS in Table Y are indeed minimum values for design purposes.

 

[SnTMan]

pugheng, a little extra steel is the cheapest insurance you can get.

Regards,

Mike

 

[pugheng]

Thanks for all of your input. Certainly some food for thought...

 

[TGS4]

pugheng and metengr - actually, the margin against yield is 1.5 for ASME Section VIII, Division 1 AND 2. The margins against ultimate are 3.5 for Div. 1, 3 for the OLD Div. 2, and 2.4 for the new Div. 2.

As far as failure modes go, for a square open-top tank, I would be thinking:
- general bending of the sides
- localized bending at the corners
- buckling (the bending stresses may overcome the tension caused by hydrostatic pressure near the top, leading to a buckling-type failure). Note that a stronger steel won't help you with elastic buckling...
- fatigue (will this thing cycle?)

How much do you really think that you can save with your approach (pounds of steel and fabrication and shipping dollars)?

 

[metengr]

Thanks, TGS4 for the clarification.

 

[pugheng]

Thanks TGS4. That's helpful information.