For the quenched samples, the residual stress near the quenched surface is compressive stress, and the interior residual stress is tensile stress. The typical through-thickness variation of the residual stress is shown in Fig. 8. It can be seen that, the tensile stress and the compressive stress are neutralized at the ‘neutral- stress area’ (residual stress = 0). As the increasing of the distance from the neutral-stress area, both of the tensile stress and the com- pressive stress increase. As shown in Fig. 9, the flux density keeps