PWHT AND NDT

To find out how the heat treatment will affect the material you need to consult a heat treatment specialist as it depends on what material you work with and the welding process used.

The question as for in which order it needs doing is in my opinion not applicable with regards to being better or not, it is down to your local regulations for pressure vessels. In that case it always depends on application.

Unfortunately you are correct in saying that if a fault is detected after the heat treatment, the heat treatment has to be done again if specified. You are also correct in saying that the NDT needs doing again but , depending on specifications, only the affected area needs re-checking.

Post weld heat treat can open up indications that might not show before, I would suggest that at least for stainless material that you PWHT and then NDT..

PWHT must be carried out after full fabrication, full inspection & NDT and passed hydrostatic test as cleared by all codes and standards like ASME. But sometimes we need to check for any malfunction due to PWHT such as changing of hardness or may be proceeding a UT for detecting any cracks due to PWHT.

After PWHT, if there is any repair requiring any welding and/or heating (above the critical limits) to the pressure part(s) (other than those nonpressure parts), the PWHT have to be redone, or may be a partial PWHT for that part is carried out.

Dear Abdel, PWHT is one of the fabrication stages so, intrinsically, it can't be performed after full fabrication.

Hydrostatic test is usually the last test upon manufacturing completion. (In some cases we have specified some NDT after hydrotest)

The election between PWHT + NDT or NDT+PWHT+NDT is a decision to take according to the criticality of service, the specific experience of welding workshop, time schedule, costs, etc.

Either of them are acceptable from the point of view of codes.

One thing to take into account and not mentioned is that when you make NDT after PWHT (in any of both options) and some non acceptable indications appear that imply weld repairing and further new PWHT, this could invalidate the Welding Procedure Qualification if the time of simulated PWHT doesn't cover the real time. So I recommend to take into account this for welding qualification.

Furthermore, this is an example of what ASME Code Section VIII, Division I says:

Paragraph UW-40 (e):

Quote:

"Post weld heat treatment, when required, shall be done before the hydrostatic tests and after any weded repairs except as permitted by UCS-56(f).

A preliminary hydrostatic test to reveal leaks prior to postweld heat treatment is permissible"

Unquote

Best regards

Yes Kwetz, you are right,

It is logic to release all the residual stresses from the material before high stressing them in hydrostatic test. But as you said and derived by ASME UG-40(e) a preliminary hydrostatic test to reveal leaks prior to PWHT is permissible to sure that the full hydrotest shall be passed without any repairing or welding work, otherwise PWHT may be redone. Thanks Kwetz, and I vote for you.

With due respect to all the previous posters, I think the proper acronym (and term) is "NDE" (Non-Distructive Examination) not "NDT" (Non-Dustructive Testing).

The main difference, as I was taught, is that to Examine is to not destroy but to Test is to destroy. Therefore, you cannot have "Testing" that is Non-Destructive.

Here is a good link for "NDE" information.

http://www.ndt.net/article/0698/hayes/hayes.htm

I was learned that the terms of ndt, nde, ndi are different but people still usually use the term of ndt.

From many years this term has been subject of controversial. But... Have you take a look on the three characters that follow the WWW. in your link?

Anyway I agree with you. Even if look at the last edition of ASM Handbook, the volume 17, which deals with those matters have change its old name by "Non Destructive Evaluation and Quality Control"

Too many specialists answered very well, some of their principles I dont know very well. but I can say I can offer NDT/NDE equipment for your test or inspection. like RT,UT,EDDY etc. if you need.

I'll respond to your secondary question, "if we go for 2nd option and find repair in the weld by doing NDT , after repair we have to do again pwht then how it will affect the material?"

It will effect the material, but whether it does so detrimentally needs to be determined. In ASME Section VIII / IX work, welding procedures have to be qualified with or without the presence of any PWHT and the hold time in the qualification has to be at least as long as the part will actually be held at temperature.

Example: if the part will be heat treated for 1½ hours at 1100ºF (carbon steel), then the WPS needs to be heat treated for at least that, so 2 hours would be sufficient. However, if you need to rePWHT, your part has now been heat treated for 3 hours and you have exceeded the qualification range of the WPS. It is wiser to have the WPS qualified for 3~4 cycles of heat treatment (6 hours). This will cover you in rePWHT situations. Part of the WPS qualification is mechanical testing of the base material, so that answers beforehand the "will it affect my material" question.

I think it is also wiser to NDE - PWHT - NDE. The NDE afterward is usually the Code mandatory test. The NDE before is cheap insurance - saving time, $ for PHWT, and embarrassment.