Materials science and metallurgy of the Caribbean steel drum Part II Heat treatment, microstructures, hardness profiles and tuning effects

Abstract

The heat treatment of the Caribbean steel drum has been found to involve strain ageing and is especially prominent in drum steels containing 0.03–0.04 wt% C. The optimum strain-ageing conditions appear to be about 350 °C for 10 min, and either water quenching or air cooling produce similar ageing effects (hardness increases) ranging from about 5 to 20%. The strain ageing combined with the strain hardening applied to the drum-head sinking and note-fabrication processes, produces a requisite elastic–plastic interaction, which allows for multiharmonic tuning and the creation of the unique chromatic tones and harmonic overtones that are a characteristic of the various instruments. These unique features of note vibrations were illustrated by comparing dynamic impact hardness profiles with corresponding, static Vickers hardness measurements for actual, tuned notes and the same, corresponding notes extracted from the drum head, respectively. Elastic–plastic and plastic-hardness profiles were compared in unique colour maps. Microstructural analyses by light metallography and transmission electron microscopy illustrate corresponding dislocation substructures and carbide precipitation. Finally, the analysis and comparison of acoustic spectra for specific steel-drum note zones illustrates their complex, non-linear behaviour, and the role that deformation-induced defects play in acoustic dispersion and multiharmonic signal production.