Mr. Lawrence, thank you for your response .
I'll answer your doubts
1. There was a replacement of shielding gas used in my company, many years back(long before when I joined). Maybe at that time they had only E71 T1-C electrode in stock and they retested it with the mixed gas , since gas composition is an essential variable. Tests were satisfactory & WPS was established.I guess, even some clients also accepted it , at that time. Later somebody questioned using C electrode with mixed gas.Then all the WPSs were changed with M electrode. One 6G WPS was not changed, since there was not tubular jobs at that time. Now we got some orders of tubular joints & only available WPS is the above mentioned one with C electrode , mixed gas & the electrode presently in stock is M type. While reviewing this old WPS, this doubt came into mind & I raised the question in this forum. Anyway my doubt is cleared.
2. I dont know exactly the benefits of this composition(it was established long before I joined). But from literatures I've read, Ar will give a smooth arc , Co2 will enhance productivity & O2 will give visually appealing welds. When using 100 % CO2, productivity will be high, but spatter problem arises & weld profile also looks a little odd. Mixture of Ar & Co2 is fine, but a little bit O2 gives good looking welds.
While talking about prequalification, yes it it is possible to write a prequalified WPS. But most of our client specifications, stresses on the WPS produced by testing as per clause 4.
I hope I've made myself clear. Expecting support in future also.
Thank you.