minimum carbon in API 5L pipe steel 1

[pipelinermike]

A pipe mill is proposing a minimum level of 0.04% carbon for API 5L X65 steel. This is for subsea pipe that will be operating around (+) 40 deg. F, subjected to near 0.25% strain levels during installation. Anyone have an opinion on if this is too low? The client specification stated a minimum of 0.07%.

 

[stanweld]

I expect that you will have problems with meeting the tensile requirements in welding procedure qualification with such low carbon content - without further steel chemistry info.

 

[grampi1]

Issue is low strength long seam HAZ of what I assume is TMCP plate long seam welded into pipe. Laying strains will concentrate in tbe low strength region and could fail/collapse there first.

The mill wants to shoot for 0.05 to 0.06C. The 0.07min means actual would be 0.08-0.09 which is quite high for TMCP. The client thinks they can avoid the problem by specing 0.07min. And for an offshore laying job, it is a real concern.

If you have suppliers that meet all requirements, honor the clients requirements. If not or if big $$ difference, ask the pipe mill why they want the lower carbon and then see if the client will go for it.

 

[stanweld]

Agree with grampi1 as well. I would ask the pipe manufacturer to provide weld qualification records to verify tensile properties after welding. Too low Yield strength in the girth seam HAZ could lead to a major fishing expedition!

 

[pipelinermike]

The pipe is seamless any may possibly be reeled. As I can tell, there is no good reason the client had specified a minimum carbon level of 0.07%. Thanks for the info on the girth weld HAZ, that may lead to an interesting discussion.

 

[grampi1]

Note 0.07C is actually quite low for a seamless. Am guessing this is for an X60-X70 grade, which usually means Q&T for seamless. Even 0.07 will have trouble making the enhanced strength. If 0.07C or less and strength from Q&T, watch out for high microalloy additions (V and Nb).