When radiation hardness increases, the half-value thickness (HVT) also increases. Tables 2-2 and 3-2 for steel and lead respectively show this in figures.
This is why in an object with different thicknesses, image contrast diminishes when radiation hardness increases. Figure 7-11 clearly illustrates this.
The left side of a step-wedge is radiographed with 150 kV, the right side with 80 kV. The right side shows the greater contrast between two steps, whereas on the left the contrast range is the biggest.