NEW ASPECTS OF THE PLASTOMETRIC TESTS FOR MIDDLE AND HIGH-CARBON STEELS IN THE SUPERPLASTIC CONDITIONS

Searching for the optimum conditions of plastic deformation of some ma terials has to guarantee a possibility of obtaining the final shape of the product without applying additional grinding. This leads to the n ecessity of taking the advantage of the superplasticity effect which a llows to obtain the required mechanical properties and shape of the pr oduct. The superplasticity phenomenon, investigated widely some time a go, is not completely explained. This effect is well observed in the a lloys based on Al, Cu, Sn, V, Ti, Co and Fe. The alloys are divided in to two groups. The first group includes average melting temperature al loys which show good workability in the superplastic conditions. The s econd group includes high melting temperature alloys which are of part icular interest for the technologists. These alloys, however due to hi gh temperatures and long deformation times require thermal-resistant t ools. The strain rate sensitivity parameter m is the main material par ameter which determines the superplasticity. That parameter is a part of the consecutive equation which describes the relationship between t he stress and the strain at high temperatures. The materials with the parameter m exceeding the value of 0.3 are considered to be superplast ic. An attempt of establishing the optimum conditions of plastic defor mation was undertaken using the torsion test. The tests were carried o ut at various temperatures and at various strains.