I have a weld joint of SA335-P91 that is proving difficult to weld due to magnetisim in the pipe. can anyony tell me what the curie temp of this material is?
Heating will not solve your problem, and will adversely affect the mechanical properties of Grade 91 material! I have come across this problem routinely with steam lines, pump and turbine shell welding. You need to counter act the residual magnetism using either a degauss machine or welders cables that are wrapped across the weld joint BEFORE welding.
You need to vary the number of wraps and use DC current. Remember, the objective is to introduce a magnetic field that will cancel out the existing field so that you can weld.
Thanks metengr, I have recieved alot of conflicting info on this subject, the contractor has said that the curie temp is as low as 600F-800F which would not be detrimental to the material. The contractor performing the work has struggled with the welding cable approach and proposed the heating remedy. I do not feel that it will work but can not find anything to support the decision to stop them from performing the heating of the material.
DO NOT LET THEM HEAT IT. Unless it is a certified heat treatment with supporting QA and testing. You can screw this up badly.
Contact a local NDT lab that does mag partical testing to come out and demag the pipe.
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Rust never sleeps
Neither should your protection
The Curie temperature is not as low as 600-800 deg F. You have been given wrong information. The Curie temperature for this material is well above 1500 deg F.
QC1971;
I did try this approach with success on 19" OD main steam pipe (3" wall) - have the NDT company use 8 wraps of welders cable over the end of the weld prep, and use as low a current as possible for about 15 minutes.
Thank you all for your input. The contractor is currently looking into bringing in a company with degaussing equipment. I will have them try metengr's suggestion in the meantime.
