Preheat And PWHT For Welder Qualification

In this article, we shall explore the matter of preheat and PWHT for welder qualification as per ASME Section IX. Is it necessary to do preheat and/or PWHT? Can a WPS qualified with preheat be used for preparing a welder qualification coupon without preheat? Likewise, can a WPS qualified with PWHT be used for preparing a welder qualification coupon without PWHT?

pwht for welder qualification

Keep reading to understand the ASME Section IX’s intent on this question.

Is PWHT Mandatory For Welder Qualification?

  • In my shop, we have jobs that almost always undergo post weld heat treatment after the welding. Should the coupon welded by the welder for his performance qualification undergo PWHT as well?

It makes logical sense. Some weldments attain their desired properties only after heat treatment. So, if you subject a coupon to mechanical tests without giving it that heat treatment, and the tests do not pass, you cannot be certain about whether it was the welder’s skill that impacted the test result, or something else?

So, thinking logically, you should give PWHT to your welder qualification coupon.

Also, QW 301.2 requires that only a qualified WPS (or SWPS) be used to weld a performance qualification coupon. And PWHT is an essential variable for Procedure Qualifications. So, if the WPS that you use for qualifying your welder requires it, it follows that you must do PWHT for your coupon too.

However, there is an exception given in QW 301.2 that permits us to forego PWHT for welder qualification test.

The wording in QW 301.2 goes something like this: Test coupon shall be welded in accordance with a qualified WPS or SWPS, “except that when performance qualification is done in accordance with a WPS or SWPS that requires a preheat or postweld heat treatment, these may be omitted.”

The wording used here is “may be omitted”, which means that it is choice in the hands of welding engineer.

If the base material chosen for the test coupon is a high alloyed metal such as P91 or P92, it would be unwise to leave it without PWHT. However, if the coupon is made up of carbon steel of 15 mm thickness, then it is not necessary to do PWHT of the test coupon, even though the WPS used for welding the coupon might have been qualified with PWHT.

Leaving the coupon without PWHT would not violate the Code.

In my shop, like I said – almost all production jobs undergo PWHT. Hence the available WPS is with PWHT as well. However, for welder qualification we do not subject the coupons to PWHT. Post welding, we send them straight for RT. We don’t see any point in doing PWHT.

PWHT is not going to change the soundness of performance qualification coupon in anyway (proven through experience). The authorized inspector agrees with this.

Note: If you were qualifying the welder through mechanical tests, and welding has been done in accordance with a WPS that requires PWHT, I would recommend subjecting the coupon to PWHT too!

  • A welder was qualified without subjecting his performance qualification coupon to PWHT. Can that welder be deployed on a job that requires PWHT?

Well, Section IX does not restrict you from doing so, in any manner. For welder qualifications, PWHT is not an essential variable. So, it does not matter whether the welder’s qualification coupon had PWHT or not, as long as it passed all tests specified in QW 304 or QW 305.

If you want to avoid the hassle of a PWHT for a welder qualification, choose a carbon steel material as the base metal (this qualifies welder to weld on all other steels), and keep the thickness of coupon to no more than 15 mm (this qualifies the welder for depositing unlimited weld thickness).

Of course, this is true only for steels. Similar sound engineering judgment would need to be applied for other materials.

Is Preheat Mandatory For Welder Qualification?

Welding with preheat requires skill. So one would think that if your production joint is to be welded with preheat, the welder qualifications coupon should be too.

Also, Section IX requires that the welding of performance qualification coupon should be done with a qualified WPS. So, if you only have a WPS qualified with preheat, it follows that the coupon should be welded with preheat too, since preheat is an essential variable under QW 250.

However, as discussed for PWHT above, Section IX exempts welder qualification coupons from preheat, even if the applicable WPS has been qualified with preheat. As discussed in above paragraphs, this exemption has been given in QW 301.2.

So, regardless of whether the WPS available with you has preheat or not, you can choose to not specify preheat for a welder qualification coupon. But this decision must be based on sound judgment. For example, a carbon steel would be alright if welded without preheat, while an alloyed steel like P91 would not.

  • Can a welder qualified without preheat weld on production jobs requiring preheat?

In the eyes of Section IX, preheat is not an essential variable for welder qualifications. A welder qualified on a coupon without preheat (with a qualified WPS, of course) is equally qualified to weld on a job that requires preheat, as a welder that was qualified on a coupon with preheat. And vice-versa.

So, this was all about the question of preheat and PWHT for preparing welder qualification test coupons. Please do leave your thoughts in the comments section below.

See More:

Welder Qualification For Corrosion Resistant Overlay

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