核主泵口环含硼马氏体不锈钢激光熔覆涂层的组织和硬度研究

Study on microstructure and properties of laser cladding B-containingmartensitic stainless steel coating on front ring of nuclear main pump

核主泵口环服役条件恶劣,口环表面的损伤将影响主泵内部的流场,进而影响主泵的性能,为强化口环表面的性能,本研究采用高速激光熔覆和重力送粉熔覆工艺在核主泵口环表面制备了含硼马氏体不锈钢涂层.利用光学显微镜、扫描电镜和透射电镜研究了不锈钢涂层的显微组织,利用显微硬度机测定了涂层的显微硬度.研究结果表明:含硼马氏体不锈钢涂层在冷却过程中δ-Fe相首先从液相中析出,当温度降低到约1 220℃时,δ-Fe相转变为块状奥氏体,剩余液相共晶转变为层状Cr2B和奥氏体,当温度达到过冷奥氏体的马氏体相变起始温度时,块状奥氏体转变为马氏体,涂层的最终组织为马氏体组织和共晶组织.高速激光熔覆工艺的温度梯度大,晶粒生长速度快,其涂层的晶粒较重力送粉熔覆涂层更细小,枝晶间距仅约为3μm.高速激光熔覆涂层细小的晶粒和更高的位错密度有着更好的强化效果,这使得高速激光熔覆涂层的硬度达到了750 HV,超过了重力送粉熔覆涂层的650 HV.

B-containing martensitic stainless steel coatings were prepared on front ring of nuclear main pump by high speed laser cladding(EHLA)and gravity powder feeding laser cladding.The optical microscopy,scanning electron microscopy and transmission electron microscopy were employed to study the microstructure of the martensitic stainless steel coating.The micro hardness of the coating was measured using micro hardness tester.The results showed that theδ-Fe phase precipitated from the liquid during the early stage.When the temperature reduced to 1220℃,theδ-Fe transformed into bulk austenite.The residual B-rich liquid transformed into eutectic phase,including Cr2B layer and austenite layer.When the temperature reached the martensitic starting temperature,the bulk austenite became the martensitic.Finally,the microstructure of the coating was martensite and eutectic phase.The EHLA coating exhibited a larger temperature gradient and a larger growth rate.The two aspects contributed to a finer microstructure.The dendrite space was only approximately 3μm.The hardness of the EHLA coating reached approximately 650 HV which was higher than the 650 HV of gravity powder feeding laser cladding layer.It should be ascribed to the finer microstructure and higher dislocation density in the EHLA coating.