DISTRIBUTION OF RESIDUAL STRESS DURING STRAIGHTENING OF COLD DRAWN TUBE
Vjera Novosel Radovic1, Nikol Radovic2, Nada Hrvatov - Crljenica1
1Zeljezara Sisak, Sisak Tube
Mills Ltd, T.C. Tehnieki servisi, KIK, 44103 Sisak,
CROATIA.
2Faculty of Geodesy, University of
Zagreb, Kaciceva 26, 10000 Zagreb, CROATIA.
Keywords: residual stress, cold drawn tube, wall
thickness, bar straightener
In manufacturing links for ball bearings from cold drawn tube, diameter 40.4 x 4.2 mm, made of S4146 steel, changes in the oval lines of the links were observed. In accordance with the results of earlier investigations those changes were attributed to an uneven distribution of residual stress across the surface and the thickness of the tube wall during the process of the manufacture of cold drawn tube 1, 2. In practical manufacturing conditions, it is assumed that the residual stress present can be either removed or uniformly distributed across the tube wall thickness by applying additional annealing and straightening treatments 3. To verify this hypothesis residual stresses across the wall thickness were measured in a finished rolled tube, a link, a finished rolled tube that had been dead annealed at 760oC in a RD - DR 1900/600 type Nassheur furnace in a stream of protective gas (air + max 4 % of H2 + CO mixture) and in a finished rolled tube that had been annealed and straightened on a bar straightener with three pairs of hyperbolic rollers. Samples for analysis were pieces of a tube/ link which were successively removed as 1 mm thick layers and mounted on a specimen holder specifically built for the purpose. Before measurement the sample surface was prepared by grinding with a rough/ fine abrasive paper and by etching. The X - ray diffraction patterns were collected at room temperature by means of the back reflection method and using a Philips counter diffractometer with monochromatized (graphite monochromator) and CoKa radiation. The X - ray powder patterns obtained were analysed visually and by the method of Warren and Averbach.
The results of investigation pointed to a decrease in residual stress in the straigtened tube/ link in comparison with that in the finished rolled tube. The difference in residual stress values between the peripheral and central layers of the straightened tube/ link wall was almost negligible.