The surface residual stress magnitudes in the quenched samples were measured using the hole drilling method, as detailed in ASTM: E837-08e2. As illustrated in Fig. 2(a), when a small hole (both diameter and depth are 2 mm) is drilled into the test surface, the relaxed strains can be measured by the pre-installed electrical strain gages in the form of a rosette around the site of the hole. The layer-removal method was used to determine through-thickness residual stress distributions in the quenched samples. As shown in Fig. 2(b), when a 2 mm thick layer was electro-discharge machined from the surface, the residual stresses releases a force and moment acting on the remaining piece, the strain changes during these layer-removal operations can be recorded by the strain gages on the remaining piece. Then the stresses in the layer removed and the change in stresses of the remaining piece can be calculated from force and moment equilibrium, the linear strain change assumption, the strain rosette readings, and the stress–strain properties of the material. The detailed principles and calculation procedures of the two measuring methods have been introduced in several references [18–21]. The thicknesses of the quenched samples, which were used for measuring the residual stress, are
25 mm, 50 mm, 75 mm, 100 mm, and 150 mm.