Determining Final Yield Strength & Elongation from Sheet to Tube - HSLA Steel

[jmarkus]

Is there a rule or thumb, or an approximate calculation that can be performed to determine the strength increase (and elongation decrease) which occurs when roll forming a flat sheet into a circular (tubular) profile, given the thickness, outside diameter and flat sheet mechanical properties for an HSLA steel?

Thanks,
Jeff

 

[metengr]

You can use one of the forming strain formulas in ASME B&PV Code. Once you know the tensile fiber strain after forming, you can use Hollowman's power law work hardening equation to approximate the increased tensile strength.

You will need to know the strain hardening coefficient and strength coefficient (K) for HSLA steel.

 

[jmarkus]

Are the forming strain formulas only available from the B&PV Code? We don't have a copy here and I can't justify purchasing one just to use the formula. Are they referenced from other sources?

Thanks,
Jeff

 

[metengr]

Yes. You can search the internet for such formulas. They are typically based on radius and thickness before and after forming.

 

[jmarkus]

Thanks, and I assume the only way to obtain the strength coefficient (K) is to perform a tensile test and determine the true stress - true strain curve and extrapolate the value at a strain equal to 1.0? Or do steel suppliers typically provide the value for K?

Jeff

 

[EdStainless]

Test a tube. Make plugs for the end and use vee jaws in the tensile machine.
It should only take a few tests for you to build your own correlation.

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P.E. Metallurgy, Plymouth Tube

 

[jmarkus]

EdStainless,

I was hoping to be able to use tensile properties/results from the flat sheet to predict the tube properties when I don't have a tube to test. Ultimately I want to be able to use different tube thicknesses and diameters in a design and I wouldn't be able to test for every combination.

Jeff

 

[EdStainless]

Remember that your incoming material will have a fairly wide variation in properties, you need to make sure that you are working with the minimum allowed. And thinner walls will work harden less when formed.
You can assume a K, or test and find one, finding one that is reliable for your specific steel could be difficult.

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P.E. Metallurgy, Plymouth Tube

 

[tbuelna]

To form sheet into a tube requires a certain amount of deformation/stretching/thinning of the sheet depending on the size of the finished tube. The finished size the HSLA sheet can be formed to without cracking will depend on its initial hardness. Unless you want to conduct some material testing, it would be safe to assume the tensile strength of the HSLA material after forming will be similar to what it was before forming.