Strain Induced Martensite in Stainless Steel Strings

In everyday life stainless steel wire is well known, for example as strings for bowed instruments or in dentistry as orthodontic wire, providing corrosion resistance and high strength. It can easily be observed, that the mechanical properties of the final product differ significantly from those of the feedstock. Observations at a microscopic level show, that the wire has a very different and much more complex microstructure than its raw material. During the wire drawing process, part of the crystal grains change due to the deformation from the face-centred cubic, so called austenite, to the body-centred cubic martensite. The significantly changed mechanical and magnetic properties are not only a result of the degree of phase transformation, but also caused by severe deformations in both phases all together producing significant strengthening. Cold drawn steel wire also shows a strong texture, which means that its characteristics in the drawing direction are different from those perpendicular to that. Although the wire`s diameters are less than 1mm, a strong radial gradient in its properties occurs, which can be explained by radial differences of temperature and deformation during the drawing process. The goal of this project is to investigate the strain-induced martensitic transformation during the drawing process by characterising the drawing steps. For this purpose three different batches are studied, which differ in chemical composition around the 18Cr- 8Ni base and the drawing process. These results will be co-related to the properties of the end-product. For this purpose the increase of martensite will be measured quantitatively by x-ray diffraction, the defects in the microstructure will be investigated by means of transmission electron microscopy and the mechanical properties will be determined by tension tests in different stress stages. The comparative investigation of the drawing-steps makes it then possible to have a better understanding of the steel wire`s sensitivity to martensitic transformation. Knowing more about strain induced martensite under the complex multi-axial stresses during wire drawing is not only of scientific interest. The sound of strings, made of stainless steel, is determined by the damping of the complex microstructure of residual austenite, strain induced martensite solute carbon and dislocations, which changes significantly over the radius, as well as the condition of magnetisation. Scientific progress on this field helps to improve the sound of the strings and its reproducibility and will be useful to understand the playing -in conditions of the strings and the in service ageing by means of materials science. Thus the expected results have a musical and cultural dimension as well.

Thin stainless steel wires are commonly used for special applications like strings of bowed instruments. Thereby, defined mechanical damping is a necessity to ensure the playability of the bowed instrument and of course good sound quality, apart from tensile strength and corrosion resistance. During cold drawing a complex microstructure evolves in the 18 8 Cr-Ni steel, which when undeformed, usually exists in the austenitic phase. This complex structure is characterised by a high amount of strain induced martensite, that decreases to the wire`s surface, where the martensite formation is inhibited by higher temperatures that are caused by plastic deformation and friction within the dies. The string is characterised by needle shaped grains with a diameter of 50-100 nm, high dislocation density and a pronounced texture. The Young`s moduli of the samples show a strong dependence on cold deformation correlated to the martensite content that is different in the three batches. Light- and transmission electron microscopy were applied to characterise the feedstock and samples taken out of the drawing process to get a better understanding of how this microstructure with its special characteristics evolves. Synchrotron radiation line profile analysis provides additional information on the microstructure by calculating grain sizes, micro strains and defect probabilities. Measurements of the martensite content and the mechanical properties are done in parallel. The temperature dependent electric resistivity was measured as well as thermo electric power, which means the voltage that builds up between the two junctions of unlike metals, when they are kept at different temperatures. Both phenomena depend strongly on the microstructure and can be used for its characterisation. Above all, a test facility was built with the aim to measure differences in sound of the violin strings, which were less than expected and could be detected only between strings with strong differences in sound and in the elastic modulus. Results in this field have also a practical economic value, because even such a traditional branch as the production of violin strings needs biennially innovative products to maintain the market position.