Besides the responses by Lawrence and Al,
Have you double checked the polarity? While arc blow/magnetism is a strong possibility it is also possible the welder was used with innershield or some other electrode that required DCEN whereas you are trying to weld, or should be, with DCEP.
At first glance, both materials should not be an issue for welding as you mentioned.
But check your electrode, welding parameters, shielding gas, polarity, etc and let us know.
Have a Great Day, Brent