Maximum temperature for API 5LX Pipe

[SteelWil]

Does anyone know why the maximum temperature in B31.3 for API 5LX (X56 in particular) is 400F? Is it due to microalloying with Ti, V, or Nb?

Since this grade is thermomechanically rolled or formed without PWHT, and will performance of a stress relief heat treatment at 1150 - 1200F change the maximum temperature?

 

[metengr]

SteelWil;
Besides the stress lines did you carefully review the Notes (51,55,71,77) in Appendix A in B31.3? Note (55) Pipe produced to this specification is not intended for high temperature service. This means the committee has established by testing that the strength/toughness properties of the pipe are unpredictable above 400 deg F if B31.1 is the design Code for service. The Notes in Appendix A do not reflect the use of stress relief to permit use of this pipe above 400 deg F.

 

[SteelWil]

Yes. I read the notes. I was curious about WHY it is not intended for high temperature service. Is there some kind of embrittlement that can occur at elevated termperatures due to the microalloying?

 

[metengr]

The material will exhibit a loss of strength and toughness at elevated service temperature. Microalloying promotes strength and fine grained microstructures to improve low temperature toughness properties. Upon exposure to elevated temperature you have softening and reduced toughness, which are not good for high strength low temperature line pipe.

 

[redpicker]

SteelWil,

First, 5LX is obsolete. The high strength grades are now covered by 5L. I mention this because there have been a lot of changes in 5L over the past few years and if you aren't using the latest edition, you could get into an “awkward situation”.

Second, you are reading too much between the lines here. The use of micoralloy elements is not required by the standard. These elements are controlled by limiting maximums, but they are not required by the standard (there are no minimums listed).

metengr had it right in one; the temperature limit of 400F is there because the strength changes with temperature and the committee has decided that 400F is a practical limit that is useful in design and that the changes in properties up to 400F is not significant. It is not meant to indicate that use above 400F is unsafe, only that use of these materials above 400F is not addressed.

rp