# Rolling Mill distortion

Despite the well-turned rolls on a mill, rolling metal in cross
directions (especially thin metal) causes it to warp and buckle.
So if you are trying to stretch metal in two directions you must
anneal it between the change in direction. Alan Revere

``````   Despite the well-turned rolls on a mill, rolling metal in
cross directions (especially thin metal) causes it to warp and
buckle. So if you are trying to stretch metal in two directions
you must anneal it between the change in direction.
``````

This is also parly due to the fact that though we think of the
rolls as rigid, they are not quite so. Ever so slightly springy,
even large rolls will deflect just a hair in use, springing back
completely when pressure is gone. The steel is, after all, an
elastic material. Have to be, or they’d shatter. Additionally,
part of what is happening is also that when you roll a sheet of
metal, the metal is free to expand sideways at the edges of the
sheet, while in the middle it can only expand along it’s length.
You can see this if you roll a perfectly square ended piece of
metal. When rolled, you’ll find the ends no longer quite
straight and square, but usually slightly bowed out, since some
metal moved sideways at the edges instead of increasing in
length. This, coupled with the slight elasticiy of the rolls,
ends up meaning that the edges are almost imperceptibly thinner
than the center of the sheet, and most importantly, that the
stresses you’ve built up in the sheet are not uniform across the
sheet. When you cross roll such a stressed sheet, you’re just
asking too much of the process, and the stresses built up now
buckle the sheet. Anealling between changes of direction will
help a lot. One other tip, in rolling sheet, is to remember that
the goal is to produce uniform grain deformation all the way
through the sheet. You want to be sure that you’re taking a
decent “bite” with the mill on each pass, so that deformation
occurs to the center of the thickness of the sheet. And, as you
roll, you may initially find the sheet warping unp and down
instead of staying flat. The initial rolling tends to
concentrate distortion and expansion of the sheet at it’s
surface, leaving a softer core. Subsequent passes then increase
the deformation at the center of the thickness until the stresses
become equalized. What you’ll see in rolling flat stock from
ingots is an initiol tendancy after the first couple passes for
the sheet to “porpoise” up and down along its length. Continue
rolling, however, and these will again straighten out, as
internal stresses in the sheet become equally distributed. As
soon as that occurs, you’ve reached the maximum degree to which
you should reduce that metal between anneals. Now is time to
anneal it before continueing. You should also, however, keep a
vigilant eye on the edges of the sheet as you roll. Some metals
won’t roll all that far before beginning to crack at the edges
of the sheet. If this occurs, stop, anneal gently, and now,
before rolling any more, trim off that cracky edge.

Hope this helps.

Peter Rowe