Hardening behaviour of ultra high strength pearlitic steels: An experimental study

Pearlitic steel wire products are used in many different applications due to their superior strength and performance in comparison to other low carbon steels and metallic alloys. The applications include bridge construction, rail tracks, reinforced tyres, guitar strings etc. These wires are drawn successively through different diameters to be used in different applications. Many different computational models have been created to understand the plastic behaviour of these steels in different deformation mechanisms, but the experimental characterisation of their material parameters with significant plastic anisotropy remains a challenge. In the past quarter century, simulation techniques such as the finite element and finite volume methods have been gaining prominence in the sector, providing unique insights into the complex deformation characteristics of high strength pearlitic steels under the forming processes, as well as helping improve the design of the products and processes. Despite their success, such simulation techniques are uncertain in the choice of material property input data and simplified constitutive relations. Standard material characterisation still centres on simple uniaxial tension, compression and torsion tests, whereas during rolling, drawing and other forming operations, the materials can routinely experience strains of greater than 200%, so accurate large strain experimental behaviour under different deformation mechanisms developed due to deformation processes such as tension, torsion and compression, is significant in calibrating simulation models. The results of the research include the large strain experimental characterisation of pearlitic steel wires under different deformation processes (tension, torsion and compression) drawn through six consecutive passes, using a linear and power law relations, up to 200% strains. The basic mechanical properties observed under each type of deformation process is inline with the standard steel values. The tension and compression large strain behaviour is studied using Digital Image Correlation technique which is a non-contact type strain measurement approach. Scanning Electron Microscopy (SEM) is used to understand and classify the deformation of the cementite and ferrite lamellas of consecutively drawn wires up to sixth pass. The cumulative effect of drawing and deformation has been studied for the each type of deformation considered. The orientation of the ferrite and cementite lamellas linked with a particular type of deformation has been studied for the consecutive drawing passes, justifying the results obtained using hardness tests. The average radial hardness seems to follow a linear trend with respect to the tension, compression and torsion true stresses. The main objective of this thesis is to quantify the mechanical behaviour of cold drawn pearlitic steel wires undergoing different deformation mechanisms under the specific deformation processes; tension, torsion and compression, to calibrate the existing plasticity models for better precision. This work experimentally investigates the plastic behaviour of pearlitic steel wires up to 200% strains and its effect on wire diameters drawn through six consecutive passes under three different types of deformation processes mentioned above. It also shows the evolution of the microstructure with respect to the drawing pass and deformations. The variation in hardness with respect to strength has also been characterised for the consecutive drawing passes. The main focus of this research is to understand and quantify the behaviour of the material properties of cold drawn pearlitic steel wires, drawn by the basic process of metal transformation, influencing their mechanical properties through different heat treatment processes to produce technologically advanced and better quality products. The data from this research can be used to calibrate the existing plasticity models in future.