How Much Stress Needed To Induce SCC? 1

[tc7]

Let me start my question by stating that I am fully aware that austenitics SS are not the best selection for seawater applications - use of 316/316L was decided by others against the advice of many.....now to the question:

When welding a series of branch connections (typically weldolets or similar boss attachments) to a thin wall 316/316L SS pipe, we will perform cold straightening operations to the pipe to take out the distortion that occurs during welding. The distortion can be considerable, as much as ~2 1/2 inch bow in a 10 ft length of 3.5 inch O.D. x .125" wall thickness pipe.

I am wondering how to quantify the amount of stress induced by this bending (straightening) operation? To straighten the bowed pipe, we bend it in the opposite direction beyond it's yield point so that it restores and retains straightness. Insodoing, are there residual stresses now built in? Or have all stresses been relieved once yielding begins? In any event, I would guess some areas of the pipe is no longer in it's solution annealed condition as it was when delivered from the mill.

A second likely area of residual stress is that induced around the solidifying weld puddles which supposedly can be taken as equal to the yield stress of the base metal or weld metal.

We perform no pwht to the SS weldments. We will passivate.

How valid is any of the above concerns and if valid, are they sufficient to cause SCC in a corrosive (warm seawater) environment?


Thankyou for your interest.

 

[SJones]

Without a stress strain curve for the material in its bowed state, it will be difficult to work out exact stresses even neglecting stress redistributions at attachments. You have rightly picked out that the welds are an issue as they are not included in the deformation and could have residual stresses approaching yield strength. It's not the residual stress alone; it's the pitting that will serve to locally elevate stress intensity which is of concern. If the seawater is constantly flowing, it may reduce the cracking risk; but, if there are any quiescent periods, the risk is quite high. If the arrangement cannot be changed, perhaps someone can think to apply cathodic protection and hope that the deformations haven't brought about a susceptibility to hydrogen embrittlement.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[EdStainless]

The local stresses from the welds are going to be higher than from the straightening.
Just because the original pipe was annealed doesn't mean that it has low stress.
I would wager that the residual tensile stress in the pipe equals the yield strength.
to your original question, in 316 my testing has shown that you don't get a reduction in SCC sensitivity until the stress is below 20% of the UTS or 33% of the Yield.

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