Mohamed

I don't think as many people look at the "metallurgy" section as some of the other sections.

You may want to repost your question in the D1.1 Section or in the Technical section to get more responses.

That said.. Welders should obey the WPS in every case, so if you wish to change your procedure you should generate a new WPS. 

However, It may not be as simple as just generating another WPS.  Different codes have some restrictions along these lines so you will need to provide us more data to get a usable answer.  Let us know what code you are using a prequalified procedure or if your procedures must be qualified by testing.

Agree with Lawrence.

You can use 7018 for the root, as long as your qualified procedure and the code allow it.

What you do not want is welders making the decision to weld it up with whatever they fell is best.  Welders with experience may be able to make good judgement calls on rod selection, but what is important is adherence to the code and WPS.

This would get more comments in a more relevent section of the forum, BTW.

Justin