Case hardening is widely applied to many critical parts in the automotive industry. Determining the hardened case depth is presently a time-consuming destructive process which would greatly benefit from an efficient nondestructive technique. It has been shown that ultrasonic surface-wave measurements are a candidate solution to the problem. However, their implementation and acceptance by industry has not taken place due to poor reproducibility and limited accuracy. We have developed an ultrasonic surface-microscopy technique based on a custom-designed broadband, lensless, line-focus PVDF transducer. This technique can be used to accurately measure leaky surface waves of a solid submerged in water, and has been demonstrated in many materials characterization applications. In this paper we show how the techniques can be used to determine hardened case depth in plasma-nitrided alloy steels. By operating in a time-resolved echo mode with the transducer defocused relative to the sample surface, a set of waveforms are obtained as a function of incremental changes in the defocus distance. The group velocity of the leaky surface wave can be accurately determined from the slope of path length versus echo arrival time. By straightforward manipulation of the data and subsequent signal processing, the phase velocity and other leaky-surface-wave information may be obtained. The hardened case depth can then be evaluated from these results.