Not sure about that axle and a steel axle could be heated and bent but depending on the temper it may be weakened somewhat, whether or not enough to cause a problem would just be a guess. I remember a little incident in KY some years back where an alignment shop got in a lot of hot water when a guy sued them over his Ford truck I-beam and then about everyone else that had been to that shop jumped on the wagon and made him pay for damages. The guy who sued did so after the axle broke where it had been heated but that is the only one I have heard about breaking, the others just had problems maintaining alignment. The words "do not weld or heat" are cast right into the Ford axle in plain sight but people still do it anyway. What it is made of means everything and as I said already a Malleable iron axle would be ruined by simply heating it because of the structural changes that would occur (this is why malleable iron can not be welded even though it may SEEM to weld just fine), a steel axle would fare much better but it is still a really bad idea especially when the means to do it right are readily available.
Just a note on identifying malleable iron from steel, use a grinder and identify from the sparks because some high quality malleable iron castings will cut pretty good with a torch and if you just nick it a little in an attempt to tell if it is steel it could be misidentified and I have seen this happen.