Formation mechanism of pearlite during thermal cycling in U75V steel rail repaired by laser directed energy deposition

Abstract

Laser additive manufacturing is used to repair the damaged area of the rail, which is prone to martensite microstructure in the cladding layers, and it seriously affects the mechanical properties and threatens the safety of rail service. Therefore, in order to avoid the formation of martensite microstructure in the cladding layers, U75V steel rail cladding layer process experiment with different layers was carried out and the pearlite microstructure was successfully prepared by considering self-tempering characteristics. Moreover, combined with the finite element method to research the phase transformation behavior of U75V steel rail affected by thermal history, the mechanism of pearlite transformation in the process of laser directed energy deposition (LDED) repairing steel rail was studied. The results indicate that the cooling rate of the solid phase transformation stage is approximately 100–101 °C/s. Two indispensable conditions for pearlite formation in U75V steel repaired by LDED were revealed: the laser thermal accumulation must enable the cladding layers to maintain the valley temperature above the martensite start temperature and the cooling rate of the cladding layers in the solid phase transformation must be lower than the critical cooling rate of pearlite transformation. Based on this theory, pearlite microstructure can be obtained efficiently by means of preheating pretreatment and Co element doped alloying and optimizing the process parameters. Preheating pretreatment is beneficial to increase basic temperature, and Co-doped alloying can change continuous cooling transformation curve of U75V steel rail.