Effect of Pulse Electric Current Stimulation on the Microstructure, Mechanical Properties and Thermal Fatigue Behavior of Cast-hot-working Die Steel

摘要

The microstructure variations in the heat affected zone (HAZ) of cast hot working die (CHWD) steels initiated by pulse electric current stimulation were investigated by the transmission electron microscopy (TEM) observation. Under the function of pulse electric current stimulation, some carbide particles segregating on the boundaries of neighboring lathing martensite in the original steel, were diffused as solid solution phases to the inner of martensite matrix. Compared with carbide agglomeration precipitating with irregular shape in the fatigued steel after 500 thermal fatigue cycles, the globularity of carbide was enhanced by degree and the interface between carbide and matrix was much improved in the corresponding electrostimulated steel. The X-ray diffraction analysis showed that the dislocation density was obviously increased in the HAZ when the application of pulse electric current through the original steel, especially for the stimulated specimen after thermal fatigue for 500 cycles. The reason was regarded as that the dislocation motion and multiplication were enhanced by the electro-dislocation interaction, Joule heating effect and thermal compressive stress under the function of pulse electric current stimulation. By the coexisting action of strengthening mechanisms by refining grain, increasing dislocation density and dispersively distributing nanosized carbide particles, the ultra tensile strength (UTS) of the HAZ was improved from 1 738 MPa in original state to 2 420 MPa by the stimulation of pulse electric current. Especially for the stimulated steel after thermal fatigue for 500 cycles, the UTS even reached 2 687 MPa. Pulse electric current stimulation played a strengthening role on CHWD steel material and inevitably improved the thermal fatigue behavior and prolonged the service lifetime of die material.