IMHO your not reducing cross section enough - get as hot as you can and hit as hard as you can only once and then re-heat - small hammer blows do not allow enough time for the steel to relax (stress relieve) leading to an "allegator" in extreme cases.
BTW - You can check temperature with magnet - Springs are usually 6150 steel and start to go nonmagnetic ~1360ºF fully nonmagnetic 1450ºF .
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