Radiographic Testing or PWHT which comes first?

[CaracasEC]

HI guys, there is a question on our team today which concern on which comes first for a welding joints requiring PWHT. Is it needed first to conduct radiography then PWHT or finish first PWHT then conduct radiography...Hope to your from you guys and thanks in advance...

 

[racookpe1978]

The question has come up before.

It is extremely difficult to take xrays of very hot pipe.

It is important (best for the metal) to go straight from the pre-heat temperature to the welding temperature to the post-weld-heat-treatment temperature without cooling down.

If you find a flaw in the x-ray (after PWHT) it is very time-consuming and expensive to re-apply all the heat treat pads and insulation, re-heat everything, re-weld the flaw, re-PWHT, and re-x-ray.

There are rare occasions when a PWHT causes cracks not present before the PWHT.

Most reg's call for the final x-ray after the PWHT for tha reason. If you suspect a problem (for example, you have a very thick-walled material that is difficult to weld), work with your NDE/x-ray company to x-ray preliminary before the PWHT and make therepair, then re-xray the joint.

 

[eyec]

the xray is the final determination of the weld integrity. if you do anything to the weld, PWHT or otherwise, you no longer have that determination.

if you really want to ensure not having to xray twice then you may look at doing dye penetrant or mag particle testing after each pass, including the back side of the weld if you can get to it.

 

[XL83NL]

From what Ive heard/learned during a course for IWT, X-raying 72 (or 48) hrs after any heat treatment (which thus includes the welding itself)
is 'better'/recommended, since (hydrogen) cracking may occur in that period due hydrogen relief out of the weld.

This may refer to the 'rare occasions' racookpe1978 mentions.

 

[Duwe6]

"Most reg's call for the final x-ray after the PWHT for tha reason"

ASME leaves the sequence up to the customer. There are very good arguements for both ways. However, I find that the "take the preheat/interpass temperature directly to PWHT temp" to be the most compelling.

If you cannot bring yourself to trust your welding crew, and feel the need to RT first, perform a hydrogen bake-out of 600°F and 1 hour per inch of thickness. Without the bake-out, it is not uncommon for 5-Chrome and 9Cr carbon steels to crack when approaching room temp.

 

[CaracasEC]

Thank you very much guys for these informative experience you shared...

 

[calliste]

For B31.1 power piping side, it refers to engineering choice. However, you should apply RT examination after PWHT for P3, P4, P5A P5B and 15E materials if there is no engineering directive to apply RT before PWHT from engineering.

 

[CaracasEC]

thanks calliste...