You know how when you quench high carbon steel it becomes *VERY* brittle and can shatter like glass if not tempered to make it tougher? (IF you don't perhaps it would be better to learn the basics before trying to do advanced work! Safer too.)
Well when you weld on HC steel it heats up the area next to the weld hot enough that the cold steel around it can "quench" it leving a very brittle zone in the "Heat Affected Zone, HAZ) that is why you usualy lget failure *next* to the weld instead of at the weld.
So you need to preheat the piece so it doean't quench it---and have a post heat/slow cool down so if it does it auto tempers it.
Temperatures are based on alloys and should be in good welding texts. Welding stores sell templ sticks that show you when the proper preheat has been reached.
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Thomas
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