Clearly, case depth Awill fail first at
the case-core interface. The applied
stress curve exceeds the strength curve
at the case-core interface. However,
case B is able to take full advantage of
the 52 HRC surface hardness. The applied
stress curve just touches the
strength curve at the surface and at the
case core interface. Thus, it may fail at
the surface or at the case-core interface.
Hardening deeper than case B in this
situation will do no good because it
will fail from the surface even if the
strength curve is shifted farther to the
right. This is the optimum case depth.
In addition, by hardening too deep, the
residual surface compressive stress
may be reduced.