2.2. Double-stage quenching tests
In accordance with the procedures of single-stage quenching tests, the double-stage quenching tests were conducted with dif- ferent spraying parameters, as shown in Table 2. During the quenching process, when the temperature of the point P1 (see Fig. 1) was found reaching 200 oC, the second quenching stage started immediately. After quenching, the samples were aged at
120 oC for 24 h (T6). The Vickers hardness of the aged samples was tested on a HV-5 machine with a load of 3 kg, and at least
10 measurements were made for each sample to obtain an average value.