GB712125A - Process for increasing the strength of unalloyed or alloyed steel bars and tubes - Google Patents

Abstract

712,125. Altering physical structure. SCHWARZ, J. Oct. 8, 1951 [Oct. 13, 1950], No. 23402/51. Classes 83 (2) and 83 (4) A process for increasing the strength of unalloyed or alloyed steel bars and tubes is characterized in that commercial steel sections or the like are alternately slightly cold drawn and cold hammered by means of cheeks fitting to the cross-section of the steel section in one or. more transits. Commercial rolled bar steel is according to the invention, first cold drawn, then coldcompressed by hammering, pressing or transverse rolling and then annealed at about 500 to 700‹ C. to eliminate internal tensions and prevent skin brittleness in order to facilitate cold bending and other deformations, e.g. rolling screw threads, splines, grooves &c. The steel may be annealed after drawing and before cold compression. The process may be repeated until no increase in strength is produced the final strength being about twice the original strength of the bar and the outer layer of the steel bar may be smoothed by a final cold drawing operation. When using steel bar which hardens under heat treatment, e.g. high carbon steel, the bar, after subjecting it to the above process, is finally hardened by heat treatment. This increases the strength by 10-20 Kg/mm<SP>2</SP>. The last cold compression may be repeated several times with the same cheek formation in order to obtain smooth round steel bars and to avoid a final drawing operation. In the first transit of cold compression the diameter of a 10 mm. bar is reduced 5-10 per cent, in the second transit 1 per cent and about 0.02 per cent in the third transit. Tubes are strengthened according to the invention by first annealing and then cold hammering the tube on a mandrel and thereafter externally and internally drawing or burnishing the tube. This process may be repeated several times so that the inner wall of such tubes is more strongly compressed than the outer wall and annular compressive stresses are formed in the direction of the inner wall said stresses being counter-balanced by annular tensile stresses in the outer tube wall making such tubes suitable for conveying fluids under pressure. The mandrel may be withdrawn in the cold compression and drawing so that the tube is compressed into a solid steel rod which may be used as a draw bar. By annealing the outer regions of the cross section of tubes or rods treated as above the tubes or rods may be used as reinforcing irons for concrete and the like or shafts or other structural parts of machines. The strength of normal un-alloyed rolled steels can in this way be increased with a single transit from 40-60 Kg/mm<SP>2</SP> to 80- 100 Kg/mm<SP>2</SP> and with several transits to more than 100 Kg/mm<SP>2</SP>. The invention may be employed on square, hexagonal or other polygonal rods.