Why???

[jaybee]

I guess easier is a bit subjective. What I mean, cold roll is easier to get a smooth surface on, hot roll tends to tear a bit, and have a somewhat rougher surface, unless you take a smaller cut on the final pass.

I've machined some of that stress relieved bar stock, it's like butta'.

It's the finer grain structure that comes with the extremely high pressures needed to cold roll.

[PeterR]

He told me that a local rolling mill was rerolling railroad rail into structural steel, and that might be the problem.

Railroad wheels apply such localised load on the track that after a while the top of the track becomes work hardened.  I have seen old railway track shatter like glass during loading onto a truck.

If it is rerolled, the processor has to be very careful to cycle it deep enough or the hardened material will just be redistributed throughout the new product making welding difficult and the material subject to cracking under cyclic loads.