The Influence of Steel Composition on the Strain Ageing Response of High Strength Pearlitic Wire.

The aim of the EngD was to identify if cost effective compositional adjustments can be made to a commercial high carbon steel to retard or even eliminate the strain ageing reaction observed in cold drawn high strength wire. This was tested via the production of a set of experimental steels, with systematically varied compositions, identified after a literature review, discussions with British Steel on previous work/experience and thermodynamic modelling of the cementite stability. The experimental steels were produced in a vacuum induction melt furnace to ensure precise control over composition, forged and hot rolled to rod. The rod samples were then heat treated to the appropriate starting microstructure (fully pearlitic) and drawn to wire. The set of experimental steels were artificially aged and tested alongside a commercially produced steel wire for comparison. A selection of available global and localised experimental techniques was used to characterise the strain ageing response: DSC, Torsion and Tensile Testing, Magnetic Sensors, (S)TEM and APT.

Generally, high variation was observed during testing. The heterogeneous pearlitic microstructure following severe plastic deformation and the subsequent strain ageing response leads to localised regions of enhanced cementite dissolution, which leads to variation of mechanical properties during testing. Differential scanning calorimetry measurements suggested a nickel addition may partially delay the initial stage of strain ageing. Magnetic sensor measurements showed clear differences between steel compositions, although the differences cannot be exclusively attributed to the strain ageing response, as the redistribution of alloying elements or carbide precipitation is likely contributing.

Chemical analysis of carbon resulted a great deal of measurement error, which made accurate determination of carbon concentration challenging. Measured differences between steel compositions in the carbon concentration of cementite were attributed to error and experimental variation. Significantly more chemical analysis data is required to ascertain if alloying elements can improve cementite stability with strain ageing. However, energy dispersive x-ray spectroscopy suggested carbon may be segregating to dislocations, which has also been reported in the literature. Thus, suggesting a dislocation based mechanism of cementite dissolution may be active.

The effects of silicon, nickel, cobalt and vanadium on the strain ageing kinetics require further characterisation over a wider range of drawing and ageing conditions to determine their suitability for commercial use. However, when considering the improved ductility of a nickel containing steel shown in section 3.1 - British Steel R&D Study - Influence of Nickel on Pearlite Stability and the potentially delayed initial stage of strain ageing, as observed by DSC observations (Figure 6:1), a nickel addition may in fact have a minor beneficial effect on torsional ductility with strain ageing. Reducing manganese content as much as reasonably achievable is also recommended. Therefore, alternative alloying additions may be required to meet tensile strength specifications. Silicon may be a suitable alternative. Vanadium is also effective at increasing tensile strength and may be suitable. Vanadium carbides or nitrides may provide alternative strengthening following the dissolution of cementite, or where amorphous cementite is present following severe wire drawing.