excessive yield drop after pipe forming 2

[synodc]

We are having considerable drop in yield strength once we have steel coil formed into pipe. We are seeing average drop of 15-18 ksi, where normally only 2-5 ksi is expected. Without having seen steel mill test data, what could be causing such drop in yield???

 

[EdStainless]

Was the yield high to start with?
Or better yet, was the yield where it was to start with because of residual stress from cold work?

When you form it you are redistributing the stresses.
We used to do this with wire all of the time, deform it in order to lower the yield. But it only works on materials with high residual stress levels to start with.

The steel may have been too soft, so they cold rolled it in order to get it to pass.

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Plymouth Tube

 

[synodc]

The information we have is that yields at the steel mill were in the 70-75 ksi range and we are now seeing between 60-65 ksi once pipe is formed. The steel mill numbers are average and not higher or lower than we would expect . This is API 5L X65 material. We haven't seen test data from steel mill so having to go off verbal confirmation only at this point.

 

[CoryPad]

One problem I have seen, with tubulars in particular, is the Bauschinger effect.

 

[EdStainless]

Good though Cory, but wouldn't that also imply significant residual stress (from prior strain) in the material.

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Plymouth Tube

 

[CoryPad]

Yes, that is logical. My point being sometimes the material property reduction is in a different direction, which can be non-intuitive.

 

[metengr]

There are plenty of technical papers on the reduction of YS in line pipe steels from the Bauschinger effect. Several mention the effect based on samples removed from formed line pipe and stress applied in sample preparation (cold flattening) prior to tensile testing.

 

[synodc]

I noticed that the Carbon max requirements are different for X65 vs X56. X65 max is .12 while X56 is .22. How much carbon must be added in order to see a significant change in yield strength?

 

[EdStainless]

You have to keep in mind that the yield strength is a construct, it has no real physical existence it is a definition.
Therefore any difference in stress levels, strain rates, and a dozen other things can change the apparent yield strength without having any impact on the UTS and elongation at fracture.


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Plymouth Tube