GB1374118A - Method of providing superplastic steel and of producing articles by deformation thereof - Google Patents

Abstract

1374118 Altering physical structure REPUBLIC STEEL CORP 13 Dec 1971 [16 Dec 1970] 57729/71 Heading B3V A non austenitic steel, having a body centred cubic structure is rendered superplastic by rapidly heating it at a rate of at least 10‹ F./sec. to a temperature at which transformation occurs from b.c.c. iron at least partially to gamma iron, holding the steel at the elevated temperature to provide high ductility during an interval of microstructural instability and rapidly deforming the steel during the interval. A plain carbon steel containing less than 1 % C or a ferritic or martensitic stainless steel containing 11-27% Cr and 0-2À5% Ni, is heated by induction at less than 500‹ F./sec. from room temperature or a temperature below any transformation or the recrystallization temperature to a temperature above the A c1 point, peferably a temperature corresponding to a 50% alpha, 50% gamma structure. After holding the steel at this temperature for 5-10 sees., it is worked by coining, drawing, heading, pressing, stretching or upsetting at a strain rate of >1/min., preferably 100-1000/min., to produce an elongation of above 50%, e.g. 100% and 300%. The high ductility in the steel may be increased if it is cold rolled or forged, to produce a 50, 70 or 90% reduction in thickness, before the heating step. After the working step, the superplasticity disappears and the steel may be further worked by repeating the process. In an example, a 0À06% C steel is rapidly heated at 100-200‹ F./sec. to 1375-1550‹ F., preferably 1500‹ F., and a 0À2% C steel is heated to 1350-1450‹ F., preferably 1400‹ F.