In this article, we shall discuss how to address ‘backing’ on a WPS. There may be several equally acceptable ways of addressing it. In this article, I have offered my two cents on how to address this variable optimally.
Please note that this article has been written keeping ASME Section IX in mind. The discussion herein might not suit the context of other codes.
Backing is a non-essential variable for procedure qualification for almost all processes, except laser beam welding and electro-slag welding.
Since it is a non-essential variable, I am sometimes tempted to write ‘anything’ to address this variable in the WPS. Or, I could write – ‘with or without backing’. Or I could write – ‘both with and without’. Would these be acceptable ways of addressing this variable?
No, these would not be acceptable ways of addressing this variable.
But, why not? Since it is a non-essential variable, neither addition nor deletion of backing would require requalification. Why can’t I address this variable using one of the responses mentioned above?
Here is why I cannot.
See, the WPS is for giving direction and guidance to the welder. Words like ‘anything’, or ‘both with and without backing’ do not serve to provide any meaningful guidance to the welder (the Section IX Committee’s intent to this effect appears in the interpretation IX-82-02). On the other hand, such instructions may confuse him.
Let us see what might be the right way of addressing this variable.
Say, your WPS is a single-process WPS, with SMAW process. If the production joint has a backing, the WPS should show ‘with backing’. If the joint is going to be back grinded and welded from back side, then also the WPS should show ‘with backing’.
A GTAW weld can be generally made with or without backing. So, an ‘optional’ would be alright.
Say, the WPS is a multi-process WPS, with GTAW at root and SMAW for the fill passes. In such WPSs, the fill passes would always have ‘backing’ underneath them. So, it is not mandatory to address backing for SMAW separately. Because, in such a WPS – backing for SMAW is implied. Committee’s affirmation to this effect appears in the Interpretation IX-95-01.
However, sometimes the manufacturer may want to use only the SMAW process on the WPS (doing this is expressly permitted in QW-200.4). In such instance the SMAW would not have backing beneath it. The WPS would then be found lacking in addressing this variable.
So, to account for all such scenarios, the welding engineer can write – ‘optional for GTAW, required for SMAW’. This would be a better and more complete way to address this variable.
Here is another situation that may occur with regularity with any welding engineer.
A WPS shows with backing. Can this WPS be used to weld a joint having no-backing. Or, would it be necessary to amend this WPS to show no-backing.
The answer to this is that the WPS must be revised. Or a new one can be written. A written instruction on the WPS cannot be violated.
However, since backing is a non-essential variable, a new WPS can be written showing the correct configuration. Doing this would not require requalification of the procedure.
Guidance to this effect appears in the interpretation with record number 15-887.
Notably, for SMAW, GMAW and SAW processes – deletion of backing is a non-essential variable. For GTAW and PAW – addition of backing is a non-essential variable. Now, I do not know why it is addressed differently in this way. Since it is non-essential, why could they not simply say – addition or deletion of backing is a non-essential variable?
That is food for thought.
So that is all I had to offer on this subject. Please share your thoughts in the comments section below.