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 A_r1 and the secondary stage extended from A_r1 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.

中文翻译：

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固态和环形激光热源对激光定向能量沉积制造的钢轨中热循环和固相转变的影响

U75V钢轨的使用性能完全取决于其激光沉积的显微组织，因为粗马氏体会对轮轨接触行为产生破坏作用。在这项研究中，基于固态和环形激光热源模型中的模拟温度场，研究了钢轨中微观组织的形成机理。带有珠光体的U75V钢轨钢是首先通过环形激光束的激光增材制造工艺制造的，其微观结构优于使用传统的慢冷热处理工艺生产的商用U75V钢轨钢。这克服了长期以来人们所认识到的限制，即高碳钢只能通过激光增材制造来形成马氏体。通过控制固相转变两个阶段的冷却速率，提出了U75V钢的晶粒尺寸和微观组织的主动设计。初级阶段从熔化温度扩展到A _{r 1}和第二级从A _{r 1}延伸至谷底温度。环形激光束沉积样品中的粒状珠光体显示出比固态激光束沉积对应物的回火马氏体细得多的晶粒。因此，样品的冲击韧性可以提高约150％。初级阶段的高冷却速率有助于环形激光束沉积样品的晶粒细化，而热循环的高谷温度和次级阶段的低冷却速率导致珠光体的形成，而不是双马氏体的形成。