I am in a basic stainless welding class, and am having trouble welding the horizontal groove plate position. Because of cost, they are having use mild steel coupons with stainless electrodes. We are using a 1/4" root opening with a mild steel backing plate. I passed the flat position the first try with a bend test on a face coupon and a root coupon. We are using 308L-16 electrodes. I have had three sets of coupons fail the bend test in the horizontal position. The first two broke in the middle of the weld and the first one appeared to have a slag line in it. I was running around 102 amps with an 1/8" electrode. My instructor felt that was a little cool and causing the problem. He suggested running around 113 amps which I did on the third try and felt it burned in much better. However on the bend test the root coupon cracked where it meets the mild steel, very small, hairline. The face coupon broke back in the parent metal and the weld looked very sound. I haven't showed these to my instructor yet, but even he was slightly puzzled at the first two. I tried preheating the metal slightly on the third try to see if it would help. I have felt good about all three welds and was surprised they broke, especially after passing the flat position. I feel I have enough experience to know when I may have a bad weld, but these are really stumping me. I am certified in 6G pipe but just started stainless. My question is should I be using 309 or 309L since I am welding mild steel coupons? I am letting the metal cool reasonably between passes but maybe not enough? Any suggestions would be greatly appreciated.