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Ultrasonic testing (UT) is a widely used nondestructive testing technique. The research presented here aims to relate UT signal features to the hardness of welded mild steel material that has undergone postweld heat treatment. The weld specimens were heat treated at different temperatures to emulate different levels of postweld heat treatments in the study. Using destructive methods, the hardness of all the specimens, a mechanical property, is computed. The change in the attenuation of UT signals is related to the change in the hardness. Possible classification of the UT signals acquired from weld areas in terms of different levels of postweld heat treatment is also explored using the continuous wavelet transform (CWT) technique. The CWT technique has been applied to the raw UT signals to compute the wavelet coefficients. Different candidate wavelets are investigated to decide which wavelet offers the best discriminatory strength between UT signals acquired from the samples with postweld heat treatment of different levels. The resulting empirical relationships between hardness and UT signal features are reported in this research work. These relationships will enable the inspectors to predict the hardness of the weld specimen under test just through in situ UT.