The development of ultra-high strength pearlitic steel wires
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Date
2015-07-28
Authors
Itay, Tamar Judith
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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
Description
A dissertation s u b m i t t e d to the Faculty of Engineering,
University of the Witwatersrand, Johannesburg, in fulfilment
of the requirements for the Degree of Doctor of Philosophy.
Department of Metallurgy
University of the Witwatersrand
November 1985