Molecular dynamics simulations have been employed to explore the response of crystalline Ar systems with and without a free surface to a gradual temperature rise. The surface-free crystalline bulk undergoes a homogeneous melting process at the limit of superheating, whereas the semicrystal terminating with a free plane surface melts with a heterogeneous mechanism at a temperature corresponding to the equilibrium melting point. Numerical findings suggest that the gradual disordering of the crystalline lattice as well as the homogeneous and heterogeneous melting processes are mediated by atoms with defective coordination. Their concentration in the regions close to the semicrystal surface at the equilibrium melting point is found to be approximately the same as in the surface-free bulk at the limit of superheating.