The weld joints of pressure vessel and piping which requires stress relieving are normally subjected to acceptance radiography after the stress relieving. If a weld jiont is found OK during rT before SR, what type of defect is expected after SR? Can the RT before SR be considered for acceptance?
It has been decades since I worked with pressure vessel and piping codes, so this is just an opinion. It would be my believe that NDE of any kind before PWHT is for info mainly so that anything found may be repaired before wasting time PWHTing the item, and then having to repair and PWHT again. Things (stress relieving, etc.) can and do change during the PWHT (the reason your are performing the process in the first place) so acceptance should only be after PWHT.
The pressure vessels and piping welds subjected to stress relief DO NOT necessarily require XRay test. That depends on the applicable code and on your personal choice when the code allows to do so. In a carbon steel pressure vessel, for example, you can decide not to XR test the welds, in which case the joint efficiency will be 0.7 according to ASME VIII Div. 1. If the calculated thickness is 3/4 inches or more, PWHT is mandatory.
When the weld is subjected to both, XR test and PWHT, normal practice is to make the XR BEFORE stress relief, so if there's some defect that must be removed you can do it without having to repeat the PWHT.
If you make the XR test AFTER the PWHT and a defect shows up, you'll have to correct the defect and repeat the PWHT.
Giovanni S. Crisi
Sao Paulo - Brazil
sathyanv,
Defects found following PWHT are usually cracks or tears. Areas of porosity, accepted during the first RT sequence, should also be reviewed closely looking for the presence of tails or or linear indications connecting separate pores. The RT techniques used during both RT examinations should be the same to decrease the chances of the second RT examination displaying a discontinuity differently than the first RT examination.
It is not unusual to perform a WFMT or PT on all welding on the process side of a vessel following PWHT. UT following PWHT has also been required by some clients on the long and round seams. Of course when UT is called for after PWHT, an initial UT is performed before PWHT in the event something is not detected during the first RT sequence. I prefer to do the initial UT after the first RT to be able to compare a UT indication with the radiograph.
~thirdeye~
It largely depends on what the code states as pointed out by others.
Process and welder induced defects (lack of fusion, slagh lines, porosity etc) will be there before PWHT.
However, the possible formation of defects as a result of PWHT should also be taken into account. Low alloys steels may result in reheat cracking in the coarse grained heat afftected zone depending mainly on composition and heat input. Recommended heat treatment regimes (soak temp, heating and cooling rates) usually avoid problems but reheat cracking can be associated with areas where grain growth has taken place.
Relaxation of stresses may give rise to small defect extensions of exisitng defects by creep in low alloys steels.
Carbon steels shouldnt be a problem and defects the only metallurgical defect I can think of is hydrogen crackign which will be present after welding.
Commerically its good practice to inspect after welding and before PWHT. Then carry out repairs if necessary, then inspect then PWHT if necessary.
Andy
Andy,
Thanks for explaining the possible defect expected after PWHT. In fact I raising the question with low alloy steel piping in my mind (5Cr 1/2 Mo material)
Since we are the owner user, we have to specify our requirement. Will it be a good practce to do the Rt , then repair if any, repeat Rt and finally do the PWHT with no further NDT
Sathyan V
Apologies for the spelling mistakes in my original post.
A quick look at three European codes (European piping and pressure vessel codes) shows that inspection is always required after PWHT.
A quick look through an old copy of ASME III NB & NC shows that for the 5Cr 1/2Mo material inspection is required after PWHT.
As an example, Class 1 components (NB5120) are required to be examined after PWHT depending on material. The 5Cr1/2Mo is not one of the exempt materials.
The same is true for Class 2 components.
You might want to inspect before PWHT to identify any defects that require repair to save the cost and hassle of repeating the PWHT if the inspection after PWHT reveals defects such as lack of fusion. There is a low risk of introducing defects as a result of PWHT in these materials.
Sathyanv,
It also depend on material. P91 material is more problemmatic. We wait 24 hours after weld out before a pre-pwht x-ray. Then we do a after pwht x-ray. This was a client requirment of their best practice procedure. I'm not saying this is done industry wide. But the client also showed us some pictures of why they do it. These pictures showed cadastrophic failures of P91 material on a number of gas fired power houses that they built around the country. They contributed most of the failures to PWHT problems and not follwing this best practice procedure.
Jim
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