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

The heat treatment of the Caribbean steel drum has been found to involve st rain 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 degree s C for 10 min, and either water quenching or air cooling produce similar a geing 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 intera ction, which allows for multiharmonic tuning and the creation of the unique chromatic tones and harmonic overtones that are a characteristic of the va rious instruments. These unique features of note vibrations were illustrate d by comparing dynamic impact hardness profiles with corresponding, static Vickers hardness measurements for actual, tuned notes and the same, corresp onding notes extracted from the drum head, respectively. Elastic-plastic an d plastic-hardness profiles were compared in unique colour maps. Microstruc tural analyses by light metallography and transmission electron microscopy illustrate corresponding dislocation substructures and carbide precipitatio n. Finally, the analysis and comparison of acoustic spectra for specific st eel-drum note zones illustrates their complex, non-linear behaviour, and th e role that deformation-induced defects play in acoustic dispersion and mul tiharmonic signal production. (C) 1999 Kluwer Academic Publishers.