Residual stresses in spring steel round rods subjected to two separate manufacturing processes

Abstract

The residual stresses remaining after each individual production phase in two separate manufacturing processes involving production of round rods from .4830 (BS 735 A 50, AISI 6150) are discussed. Thefirst manufacturing process comprises annealing, straightening, grinding stage 1, and grinding stage 2, and the second consists of annealing, straightening, grinding stage 1, and roll milling. The aim of the investigation was the assessment of both manufacturing processes with regard to the residual stress distribution produced. The measurements were carried out via electrochemical removal and on line measurement of strain using strain gauges. The specimens for the measurements were randomly selected from the normal production process. This caused considerable spread, and an improved representation of the results obtained was achieved via a statistical treatment. A characteristic feature of both production processes is the occurrence of compressive residual stresses in the surface and in the subsurface layers to a depth of 0·5 mm. Roll milling, in the second manufacturing process, is carried out between two roll mills with displaced axes and the residual stress standard deviation is increased because of the spiral movement of the spring steel rod between the two roll mills, and the random specimen location along the round rod. The first production process proved to be more desirable because it provides smaller scatter of residual stress values along the rods.MST/1871 List of symbolsa length of electrochemically polished area ak orthogonal calculation coefficient A fracture strain b specimen width d diameter of steel rod Gk(i) kth grade polynomial value in ith segment h specimen thickness segment number j specimen number J number of specimens (measurements in specific depth segment) n number of depth segments (25 in present example) Pk(X) orthogonal polynomial of kth grade S(i, j) residual stress in ith segment of jth specimen Sm(i) residual stress mean value in ith depth segment t time T temperature Xl. .. n points in point system 13k orthogonal polynomial calculation coefficient (J eH yield strength (Jj estimated standard deviation in ith point (Jm average standard deviation for group of specimens (J pm (k) a verage difference between kth grade polynomial and mean value Sm (i) for all 25 segments (J UTS ultimate tensile strength