Re: But why? The question of ASTM A312-TP321

From: <Christopher>
Date: Wed Mar 17 2004 - 10:37:00 EST

>I am not an expert in such subject, but I wanted to reform the question.
>How a material can have two different strengths depending on the thickness?.
Strength variations with material thickness depend on processing variables. In the case of austenitic stainless, most likely the effect of varying cold work. With heat treatable materials like A-564 bolts, the variation is likely due to the fact that the interior of thicker sections cools more slowly during heat treatment. Look through your ASTM specs. You'll see that it's quite common for thicker sections to have lower minimum mechanical properties.

After a quick look through an older Section IID of the Code, I note that A-312 Gr 321 seamless pipe was not permitted at all for thicknesses greater than 5/8, with no thickness limitation for welded pipe. Newer versions now include thicker seamless pipe although with lower allowables as you've noted. I suspect that the additional reduction needed to produce thinner walls gave 321 a higher ultimate strength in smaller thicknesses.

Just a guess, but either seamless Gr 321 pipe wasn't available until recently in the greater thickness or perhaps if it was available no one needed thicker material badly enough to pay for the testing required to get it into the Code.

Christopher Wright P.E. |"They couldn't hit an elephant at <a href="/group/PipingDesign/post?postID=KcHrLlxlR4klehVtw5yMXABKxLGC_1H2m6q7DHh9X7eMmUQTGxDzV_tHDagLsqvtuPZorqrLD6Dovg">chrisw@skypoint.com</a> | this distance" (last words of Gen.

___________________________| John Sedgwick, Spotsylvania 1864)

<a href="http://www.skypoint.com/~chrisw">http://www.skypoint.com/~chrisw</a> Received on Wed Mar 17 10:37:00 2004