RT before or after PWHT 1

[Gapinspect]

I am refering to a ASME 1 SSH header. P5A material. I realize that the code does not require Radiography after PWHT but it has become a long standing practice to RT before PWHT (economics) and RT after PWHT(metalurgy). I was wondering if any one could give some insight into what the failure mechanisms that we could see during the PWHT and the likely hood of it. Also if there is any published research on this subject, I would be interested in getting the name(s) of that also. Also the failures after PWHT I would expect to see would be linear and towards the surface of the weld where a mag particle inspection would likely pick it up.

Thanks in advance.

 

[metengr]

Gapinspect;
RT after PWHT should be mandated by the Owner/User or vendor for certain materials. As in your case for P-5A materials and other lower P-No materials groups, I see no benefit from RT after PWHT, provided a surface NDT is performed after PWHT. Why? Well, the main reason is that delayed cracking is the only real damage mechanism for this family of alloys that could ocurr during the initial application of PWHT. Delayed cracking is caused by hydrogen that is entrapped in either the weld or surrounding base metals introduced at some point during the welding process.

If proper preheat requirements are followed and low hydrogen electrodes are used, your chance of having delayed cracking in P-5A materials and for lower P-No materials is very low.

Now, lets talk about P-No 5B materials and the creep strength enhanced ferritic steels, here, RT after PWHT in addition to a surface NDT should be mandated. The reason here is these materials are air hardening and have increased sensitivity to delayed cracking from hydrogen or stress corrosion cracking concerns IF the PWHT is delayed on hardened material after welding. Certain base materials that contain vanadium are prone to reheat cracking during PWHT, and here RT or UT after PWHT should be specified.

 

[jehan17718]

Gapinspect:

Please also review similar discussion at

http://eng-tips.com/viewthread.cfm?qid=140304&page=23

-jehan

 

[Gapinspect]

Metengr, if delayed cracking was the concern and the part is 3.5" thick. Would you expect to see the indications near the surface where a surface (or slightly subsurface) ndt may pick it up?

 

[metengr]

Gapinspect;
Yes, I would expect either subsurface indications opening to the OD surface or OD cracks transverse to the weld.

 

[stanweld]

From experience on P-4, P-5A and P-5B welds, delayed cracking can be either longitudinal or transverse, below surface or at surface (ID or OD). Much depends on the actual preheating/interpass temperature used throughout welding and the application methods. If preheat was appropriate and applied by electric resistance or induction, you should have no problem with delayed cracking. If preheat was locally applied by torch, adequate preheat is suspect and delayed cracking is likely if you wait too long between RT and PWHT or do not wait long enough between weld completion and RT. Dehydrogenation heat treatments, immediately after weld completion or weld interuption are effective methods to preclude delayed cracking.