In the heating process, the tempera

In the heating process, the temperature of the HZ exceeded the Ac3 in vary degrees, thus converting almost all austenite into martensite with sufficiently cooling rate. For the TZ, where the temperature was only between the Ac1 and Ac3 due to heat conduction, the microstructure was mainly composed of martensite + ferrite after LTH process because only a part of austenite transformed into martensite, so that the hardness was lower than the HZ. Due to the lower temperature (< Ac1) of the AZ, the original microstructure was preserved, which led to the slightly changing in hardness. As expected, the depth of laser transformation hardened layer of the sample with 1.8 kW 3 mm/s (770 μm) was deeper than the sample with 1.7 kW 3 mm/s (650 μm)and the sample with 1.8 kW 4 mm/s (530 μm). Accordingly, the hardnessof the former (789.7 HV) was higher than those of the two latter(693.6 HV and 686.6 HV, respectively). This phenomenon can be attributedto the facts that: (1) the solid solution strengthening mentionedabove increased the hardness; (2) a larger number of PM made thehardness of the sample with 1.8 kW 3 mm/s was higher than other