X-Ray after PWHT

[keens]

Are X-Rays of the Welds required after PWHT? I thought that they are required because you need to check the integrity of the welds and the properties. One of our QC guys do not think this is the case. Thanks...

 

[rodofgod]

Hi Keens!
Generally speaking, all qualitative NDT should be performed AFTER PWHT!This is because defects can arise due to the heat treatment or other factors arising from the stress/strain associated with PWHT. However, it is quite common to perform x-ray or UT before PWHT to ensure that the weld is clear of welding defects before another thermal cycle is performed on the weld!

Regards

 

[metengr]

keens;
Nondestructive testing (NDT) requirements, like RT (X-ray) are typically found in the Code of Construction for the vessel. If you are fabricating this vessel to ASME B&PV Code, Section VIII, Div 1 requirements you need to review Part UW for NDT requirements. In most cases, NDT is required based on service conditions, wall thickness and weld joint configuration.

If RT is not specified by Code, you do not need to perform it.

 

[metengr]

keens;
If your question relates to RT after PWHT, this is a different matter from my answer above. The Code implies that RT shall be performed AFTER fabrication of the vessel. Some folks interpret this to mean that after welding, if an RT is shot, this satisfies Code requirement for RT. The Code really does not specifically state or require a post weld treatment RT. It is implied. Other Construction Codes like Power Piping do have specific requirements for post weld RT.

I would follow good engineering practice or judgment and consider after fabrication to mean after post weld heat treatment.

 

[keens]

Thanks for the help metengr and rodofgod.

 

[codeeng]

I don't believe the Code implies RT after PWHT although I've seen this in client specs on occasion. Certainly if this were the case you'd be obligated to RT before as well as after PWHT.

 

[metengr]

For Section VIII applications under Part UW, carefully read UW-1 SCOPE. The words in the scope paragraph state the following….. "The rules in Part UW apply to pressure vessels and parts that are FABRICATED by welding and shall be used in conjunction......"

The key word is fabricated. Heat treatment either pre or post weld is still considered a stage of weld fabrication.

 

[codeeng]

What would happen then in the event of a repair after PWHT?

 

[codeeng]

UW-40(e) also says "PWHT after any repairs". How do you know if have any repairs if RT has not been performed before PWHT? What you are saying in essence is that RT is required before and after PWHT! I don't think so.

 

[CodeRef]

As I have heard before,and agree with the statement, engineers don't idle well..... just follow good engineering practice and defend your decision based on the Code of Construction...or Code of Fabrication as applicable.

Remember the term "construction" would infer design and "fabrication" does not.

Both of you guys make great points and I agree with metengr's position because it makes the most sense. That is why having a proven WPS and competent Welders and Welding Operators will pay off when the dust settles.

Usuually the "Inspector" would help make the final decision for you.

 

[ijzer]

According ASME VIII-1 Code interpretations 98-09 and 89-247R it is not mandarory to perform RT after PWHT except when required by par. UHA-33(b) or(c).
European Codes however require that NDE is performed after PWHT.

 

[codeeng]

Thanks for that clarification ijzer. The British Code PD5500 does actually permit NDE prior to PWHT for thicknesses up to 40mm for carbon and stainless steels.

 

[vesselguy]

Keens,

This is the practice we uses in the Oil and Gas sector. I have found that it is common across the globe.

When PWHT is specified due to either 1. Process reasons such as in sour service, Hydrogen Service, etc., or 2. By Code due to thickness per UCS, all final acceptance NDE shall be performed after final PWHT. The reason is already explained well by Rodofgod above. Code do not require this but this is "standard industry practice" with NO exceptions permitted.

This forces the "good and skilled" fabricators to quote inclusive in their price to perform either RT or UT of welds before PWHT. Purpose is to use the results as the baseline and self-assurance to ensure welds are sound before it goes into the oven. This is how things are done in the oil and gas industries.

 

[codeeng]

vesselguy
I have absolutely no problem with what you are saying although double NDE is certainly not standard industry practice in my part of the world. On the contrary, most fabricators will not include this unless it’s specified up front and you have to pay for it.

 

[qualitypro]

It is always better to do RT before PWHT.The whole idea of doing PWHT is to relieve the internal stresses caused during welding.You do not want to stress the weld joint twice in case there is a RT repair after performing PWHT.You could end up softening the metal so much that it may lose some of its mechanical properties like hardness by re-PWHT. Stress relieving cracks after PWHT especially in Cr-Mo steels sometimes do occur if the rate of heating or cooling is not controlled or if proper preheat and postheat operations are not followed during welding.
Although many codes do not specify RT after PWHT, many client specifications call for a surface detection method like MT or PT to be applied after PWHT.Otherwise RT could become a very expensive affair.

 

[PAN]

keens,
I used to receive the comment that PWHT may be a cause of "surface crack". MT or PT after PWHT as qualitypro's comment is recommended instead of RT/TOFD after PWHT.

 

[JStephen]

"The rules in Part UW apply to pressure vessels and parts that are FABRICATED by welding and shall be used in conjunction......"

The key word is fabricated. Heat treatment either pre or post weld is still considered a stage of weld fabrication."

I think this is a fallacy in argument- if "fabricated" here includes heat treatment, then it isn't "fabricated by welding"- it's "fabricated by welding and heat treating". IE, I think that interpretation puts more meaning in the term than is intended, which makes the phrase disagree with itself.

Maybe some of you guys can clue me in here- but I was under the impression that the main flaws that might be worsened or caused by stress relief would be cracks- but that xray is primarily aimed at other types of flaws.

 

[SJones]

I thought that RT was aimed at all types of imperfections - that's why there are acceptance criteria for them. It's just that probability of detection for cracks in certain orientations with respect to the beam can be worryingly low.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[weldtek]

ASME Code does not require RT after PWHT,with the exceptions noted in UHA 33 (b) & (c). Alsosee interpretations VIII-1-98-09 & VIII-1-98-09R

 

[MetalsInc]

The use of RT to find cracks is troubling. It's extremely difficult. Porosity, slag inclusions, and other 3D flaws are easily findable with a well trained RT crew. I've had conversations with the local Chief Inspector and he concedes that RT is not as useful as people give it credit for. UT is much better at finding cracks - and cracks are more serious than porosity (generally). But too many people are afraid of UT because 1. it takes a skilled operator and 2. it leaves no permanent record.