I' m no expert on AR steel, but a few yrs back we were welding AR 360 plate onto cat plow shanks ( used to plow in telephone & power cable & gas line ) If I remember correctly the preheat was about 3-400 F and we used Arctec 223 rod ( also works well on cast steels ). There is also good steel/welding info at
http://www.key-to-steel.com/default.aspx?ID=Articles Look at # 11 Welding - Welding of special steels . They are recommending 400 F preheat and a low/hy rod. Have a good day - WS