Purging During Welding of SS & MS

Why not ask your welder?

welder usually do not know the technicall reason for this my friend

Good explanation here: http://www.tubenet.org.uk/technical/whypurge.html

"When welding stainless steel, titanium and other corrosion-resistant materials, creating a perfect weld environment is of the utmost importance, because the desired corrosion resistance depends on it. During welding, the heated weld seam is exposed to air and oxidizes if it is not protected. This oxidation must either be prevented or treated later. Successful welding depends in part on a combination of proper purging equipment and techniques, and these are the focus of this article."

Article includes photo showing effects of different purge methods.

Thats where myy question arises i know why the purging is required in SS.

But im not understanding y do we not required during MS welding as there also oxidation takes place on the otherside hence there must be a sugaring effect on it. hence the question

Notice the qualifier, "and other corrosion-resistant materials"? Common "mild steel" is not corrosion resistant.

There is no reason you can't back-purge when welding it, it's just not necessary.