Severe plastic deformation of metals

A method of treating a metal billet to change its mechanical and/or physical properties by reducing grain size, the method involving feeding the billet through a first passage and into a second passage inclined to the first passage characterised by deforming the billet by repeatedly loading and unloading it using at list one reciprocating die at a junction between the first and second passages in order to process the whole length of the billet, wherein the reciprocating die defines part of the second passage and the first and second passages constrain the billet in a direction perpendicular to a flow plane of the billet, thereby to cause plane strain of the billet.