This paper describes the changes in hardness and microstructure in annealed low carbon steel by jets squirted from high explosives with steel‐lined conical cavities. Hardness measurements and photomicrographs were taken along different radii in a plane normal to the axis of the crater in the steel. Each hardnessversusdistance curve exhibited a series of plateaus along which the hardness remained constant. The plateaus appear to be related to Neumann bands which are present in the steel. It appears that the impact of an element of the jet on the steel initiates a transient compressional stress wave in the body of the steel which results in the hardness plateaus and the increase in the number of twin directions with hardness.