Fig. 3 shows the hardness profile as a function of depth for the baseline annealed, heat hardened and FSP treated samples. For the FSP sample, the hardness measurements were conducted at the center of the processed region. The hardness of the baseline material was about 2.0 GPa and independent of depth as expected since it is in the annealed condition. Heat hardening by water quenching from austenitizing temperature increased the hardness to about 7.0 GPa; although the hardness near the surface was only about 6.2 GPa. In this sample, the hardness was also independent of depth. For steel with 0.4% carbon content, the maximum achievable hardness of martensite by heat treatment only is about 7.0 GPa [17]. Thus, the thermal hardened specimen used in the present study achieved the maximum martensite hardness for the material. FSP treatment produced hardening of the 4140 comparable to the thermal treatment as shown in the surface layer up to a depth of about 4–5 mm as shown in Fig. 3. Beyond the depth of about 5 mm, outside the FSP treated material, the hardness is the same as the baseline material. Thus, FSP can be viewed as a form of case hardening treatment for this steel materials. Some variations were observed in the measured hardness of the process region. The highest hardness of about 7.5 GPa was observed at the surface while values as low as 5.5 GP was observed at the middle of the case layer. This hardness variability in the FSP treated sample is best illustrated in Fig. 4. The photo macrograph of the cross-section of the processed region is shown in Fig. 4a, and the corresponding 2-D measured hardness map is shown in Fig. 4b.