I agree with PSE. Welders are famous for either slowing down or speeding up when they know you are there and what you are doing. I have found it helpfull to hideout while performing the studies.
A good time study should include every aspect of the job performed. For instance if I were conductiong a time study in a weld cell I would record as follows:
picked up stinger...25sec
cleaned nozzle......30sec
begin weld..........35sec
end weld............122sec
raised hood.........127sec
and...
I have welded this steel in the past with good success. It required a pre-heat to help prevent cracking. Welding was performed using GTAW but I don't remember the filler metal type. I will check to see what it was and get back to you. I have a WPS for this somewhere.