metallurgy 101. you effectively air quenched the steel when you arc welded it. to soften it up again, you need to heat it up to about 800°F (like YJ said, a decent torch can easily achieve this) and then let it cool slowly. if it's a small enough piece, get two buckets and fill one of them with dry sand and the other with a layer of sand on the bottom a couple inches thick. make sure there's enough room for the part you're softening. then, heat the part with the torch as hot as you can or until cherry red, whichever comes first depending on the torch you're using. then, quickly put the part in the bucket with the sand layer (using tongs of course) and have someone pour sand from the other bucket on top of it to complete cover it. let it sit for a couple of hours or even better, overnight. it should soften it back up. this is called an annealing process. the sand acts as an insulator keeping the part hot much longer. we did this in HS when we made cold chisels and punches during our forging projects. if you want to harden the steel again before finalizing it, shine up a small area of the part and then heat it to cherry red again. using heavy gloves and some emery cloth (several layers think) sand the open spot to expose the steel to air again. watch the steel carefully and as it cools, you'll see it change color. when it reaches a straw yellow color, quench it in a bucket of water until cool. depending on thickness of the part and other factors, you'll have a part with varyin degrees of hardness, but for the most part it will be close to the hardness of tool steel. lol. probably more info then you ever wanted, but it's good to learn something new. HTH
RIP: '88 YJ 2.5L Ax-5 NP231
Posi-Loked. Herculined. Optima yellow top. 1" Shackle, 2" BDS. Cragar 397's Aussie front.
92 YJ 4.0L Ax-15 231
5" springs, 1" shackle 31's or 35's depending on my mood