Effect of solid and annular laser heat sources on thermal cycle and solid phase transformation in rail steel manufactured by laser directed energy deposition

The service performance of U75V rail steel depends entirely on its laser-deposited microstructure, as coarse martensite has a damaging effect on wheel-rail contact behaviors. In this study, the formation mechanism of the microstructure in the rail steel was investigated based on simulated temperature fields in both solid and annular laser heat source models. U75V rail steel with pearlite was first fabricated via laser additive manufacturing with the annular laser beam and had a microstructure superior to that of the commercial U75V rail steel produced using the traditional slow-cooling heat treatment. This overcomes the long-perceived limitation that high-carbon steel can only form martensite through laser additive manufacturing. Active design of the grain size and the microstructure of U75V rail steel were proposed by controlling the cooling rate in two stages of solid phase transformation. The primary stage extended from the melting temperature to Ar1 and the secondary stage extended from Ar1 to the valley temperature. Granular pearlite in the annular laser beam-deposited sample shows considerably finer grains than the tempered martensite of the solid laser beam-deposited counterpart; therefore, the impact toughness of the sample can be increased by approximately 150%. The high cooling rate in the primary stage aids grain refinement in the annular laser beam-deposited sample, while the high valley temperature of the thermal cycle combined with the low cooling rate in the secondary stage leads to the formation of pearlite, instead of twin martensite.