A numerical simulation code is developed to study laser beam propagation and the significance of self-focussing or defocusing effects on a manufacturing process. The solution method includes the thermally stimulated nonlinear optical effects caused by the temperature and intensity dependent index of refraction. The results demonstrate that radial temperature gradient magnitudes of 0.06 to 0.6°C produce significant changes in the focal volume of the beam. The magnitudes of the temperature changes are directly related to differences in the absorption coefficient. The thermal transport of the absorbed radiation is modeled using Fourier heat conduction for long time scales and Hyperbolic heat conduction for short time scales. The focal volume of the beam is significantly altered by the differences between these thermal transport mechanisms. These results were determined using a simulation of a Z-scan measurement technique.