Influencia de la Microestructura sobre las Propiedades Mecánicas en Varillas de Acero.

Abstract

With the purpose of determining the micro-structures with optimun mechanical properties, the Influence of the Micro-Structure on the Mechanical Properties in Steel Rods has been studied. Phase transformations were performed by using different cooling speeds, and then, both the mechanical and technological properties (metallographic, mechanical, and technological tests) were characterized in corrugated rods of 12 and 14 mm diameter provided by NOVACERO S. A. The corrugated rod test-tubes were put into a heating process up to a temperature of 900ºC followed by a cooling one up to 400ºC. The cooling speeds to which the samples were exposed were four and gave as a result percentage variations of present phases in the different samples. Both the mechanical and technological tests showed that the test-tubes cooled under speeds of 2,9762ºC/s for 12 mm-diameter rods, and 2,6316ºC/s for 14 mm-diameter rods showed the best stress mechanical properties. However, these also produce the decrease of the impact resistance. The reason is that the micro-structures found to the cooling speeds above mentioned in the sub-surface stratum (zone 2) were medium pearlite and Widmanstatten ferrite, primary and secondary side plates. This stratum shows an important variations with respect to the nucleus (zone 3) into which gross pearlite and allotriomorphs ferrite were found. On the other side, the results of the bending test were completely satisfactory and superficial fissures were not found. The results got show that on cooling the rods in the variation of the speeds studied, increases of the stress mechanical properties can be obtained in about 10,96% at the cost of the reduction of the impact resistance in about 8,68% in comparison with the samples of corrugated rod in supply conditions. However, the decrease of the impact resistance will not affect meaningfully on the performance of the rod in the bending standardized test.