Fig. 1 Schematic diagram of the pulsed electric current generator

Fig. 2 Microstructures of the original GCr15 bearing steel: a optical microscopy, b SEM

Fig. 3 Microstructures of GCr15 bearing steel treated by pulsed electric current at different frequencies: a 10 Hz at 720 °C; b 1000 Hz at 720 °C; c 10 Hz at 740 °C; d 1000 Hz at 740 °C; e 10 Hz at 760 °C; f 1000 Hz at 760 °C; g 10 Hz at 780 °C; h 1000 Hz at 780 °C

Fig. 4 Microstructures of GCr15 bearing steel treated by pulsed electric current at different duty cycle and current density: a-d 720 °C, 740 °C, 760 °C and 780 °C, respectively, for 40 A/mm2 and 5% duty cycle; e-h 720 °C, 740 °C, 760 °C and 780 °C, respectively, for 32 A/mm2 and 10% duty cycle; i-l 720 °C, 740 °C, 760 °C and 780 °C, respectively, for 20 A/mm2 and 40% duty cycle

Fig. 5 Carbide proportion gained by Image J after pulsed electric current treatment of different parameters: a1-a4 720 °C, 740 °C, 760 °C and 780 °C, respectively, for E27-5-10; b1-b4 correspond to 720 °C, 740 °C, 760 °C and 780 °C, respectively, for E27-5-1000; c1-c4 correspond to 720 °C, 740 °C, 760 °C and 780 °C, respectively, for E40-5-200; d1-d4 correspond to 720 °C, 740 °C, 760 °C and 780 °C, respectively, for E32-10-200; e1-e4 correspond to 720 °C, 740 °C, 760 °C and 780 °C, respectively, for E20-40-200

Fig. 6 Statistics of carbide area proportion under different treatment conditions

Fig. 7 Diagram of pulsed electric current waveform with different duty cycle when the processing temperature remains consistent

Fig. 8 Schematic diagram of the relationship between atom migration rate and action time

Fig. 9 Structure used herein for modelling the area percentage of carbide (APC) based on numerous input parameters

Fig. 10 a Adaptation value changes with the number of particles and the iteration time; b locally enlarged image corresponding to green dotted box in a

Fig. 11 Predicted results of the training and validation (testing) set in relation to the experimental area percentage of carbide data

Fig. 12 a Influence of input variables on the dissolution effect of carbide, and b relationship between partial dependence and input variables

Fig. 13 Microstructures of GCr15 bearing steel treated by pulsed electric current of a 60 min, and b 90 min; c microstructure by conventional spheroidizing annealing; d schematic diagram of spheroidizing annealing by pulsed electric current