QW-403.6 Basis

[viper3982]

All,

I am currently going though some issues with an impact tested weld procedure that also needs PED sign offs P1G1-P1G1 1.5" coupon GTAW root with SAW fill/cap.

I completely understand that I need to use QW-403.6 and that it only qualifies me for a min weld thickness of 5/8. It's pretty straight forward on WHAT the requirement is...

The question: Why? This question is merely to glean some understanding of where this reason comes from. (I like knowledge.) There are many code clarifications but they, of course, only really give a 'yes' or 'no' to specific shops' coupon questions. I am trying to understand the reason for only allowing a min of 5/8. You can use combination processes and get various thicknesses out of it: EG: if it WASN'T impact tested we could qualify GTAW and SAW at descent ranges independently. Why would impact testing prevent this ability? I know how charpy testing works; but in my mind it's not too different to lob off a chunk of a coupon and pull it or bend test it than to whack it with a charpy block.

Part 2: continuing on with QW-403.6: if you take it above the critical range then this variable does not apply. Why would turning 516-70 material into austenite change the thickness of the qualified level? If it passes impact testing: why would the change from ferrite to austenite?

I know this is getting into some deeper thoughts than a simple code-based answer...

weldstan, SnTman, metengr, Et al.: Edjumacate me, please.

/regards

 

[DekDee]

Viper,
I may be totally misunderstanding your post but this is my opinion based on Q1.
"EG: if it WASN'T impact tested we could qualify GTAW and SAW at descent ranges independently. Why would impact testing prevent this ability?"
If you choose to impact test a weld coupon it does not limit you to QW 403.6 requirements - that only comes in to play if the construction code requires impact testing.

QG-105.3 Supplementary Essential Variables
When procedure qualification does not require the addition of toughness testing, supplementary essential variables are not applicable

If no impact testing required by the construction code you use the requirements of QW 451.1,
Many companies write a dual WPS with thicknesses based on the two clauses noted above - one for when impacts are required and one without.
Others write one WPS for each scenario.

As for the 5/8"minimum - no idea. When I get a spare moment I will have a look for you.
Cheers,
Shane

 

[weldstan]

The 5/8" requirement came about in the 1974 Edition of ASME IX and was largely instituted by the major changes to ASME III in the 1974 Edition. One hopes that the mandate was based on the effect of greater cooling rates on 5/8" and greater thickness and the slower cooling rates for thickness < 5/8".