Heating stainless steel does not make it harder! SS does not behave like plane steel, apart from the fact that steel cannot be hardened by heating, just softened. Steel can be hardened by deformation or by heating AND quenching it properly. Finally, you cannot change the chemical makeup of steel or SS enough to change corrosion properties without going to very specific processes, let alone with a garage torch. This in the context of this forum,
I don't know the alloy of the tubing and I don't know what was used to heat it or how hot it got. It can happen. Google "intergranular corrosion" and "carbide precipitation"
Any tutorial on stainless brake lines stress using a cutter that does not make the end of the tube hot.
Regardless, the answer to the question of flaring is above in several places from folks who are at the top of the game.
