The development of ultra-high strength pearlitic steel wires

Abstract

This thesis describes a systematic investigation of the influence of alloying additions on the microstructure and mechanical properties of eutectoid carbon steels. The goal of this work was to develop an alloy which would provide a tensile strength in excess of 2500 MPa in drawn wire. Alloys were fabricated using vacuum induction melting, casting and hot-rol 1 .‘.ng. Specimens of each material were heat treated to produce pearl J. tic structures. M i c r o structures were evaluated using light, scanning electron and transmission electron microscopy. Mechanical properties were determined from hardness and tensile tests. Suitable heat treated rods were subjected to d r awing trials, aid the structures and properties re-determined. In the initial part of the study, the influence of chrom i u m and manganese addition on the properties of a high purity carbon alloy were studied. This work demonstrated the benefits of these small alloying additions on strength, while maintaining adequate ductility. In subsequent work the additional effects of alloying with silicon were studied. Of the alloys investigated, the 2Si-lCr composition achieved a drawn strength in excess of 2500 MPa, This material also exhibited acceptable values in standard shear and torsion tests for commercial wires. Since the strength of steel is strongly dependent on carbon content, the further influence of this variable on properties was examined using the 2Si-lCr base composition. Increasing the carbon content to 0.9% did provide additional strength, but increased the ageing susceptibility during drawing. The work was extended towards commer c i a l compositions by preparing 2Si-lCr alloys using a commercial eutectoid carbonmanganese steel base. Again, in the as-drawn condition, the strength goal was exceeded. However, these materials exhibited inferior ductility compered with the high-purity alloys. This is thought to be due to ageing. A brief study was also conducted on the problem of strain ageing, but no evidence for precipitation of deleterious carbides could be found even using electron microscopy. In summary, small alloying additions are extremely effective in i n c r e a s i n g the s t r e n g t h of p e a r l i t i c wire, w h i l e maintaining adequate ductility. A 2Si-lCr alloy exhibited the best results of the materials studied. This composition has commercial potential, but may require more sophisticated p r o d u c t i o n m e t h o d s to p r o v i d e cleaner, h i g h e r p u r i t y material. Nevertheless, this alloy is capable of providing as-drawn tensile strengths in excess of the 2500 MPa goal