球化孕育处理工艺对下贝氏体铸铁组织特性及力学性能的影响

Influence of Nodularizing and Inoculation Treatment on the Microstructure Characteristic and Mechanical Property of Lower Bainite Cast Iron

采用真空中频感应炉,通过改变球化孕育处理工艺获得具有不同形态石墨的铸造试样,将铸件加热到860℃保温2 h,使用20℃的硝酸钠、亚硝酸钠饱和溶液将其连续冷却至室温,并在250℃低温回火2 h后空冷,获得具有不同形态石墨的下贝氏体基体组织。研究了球化孕育处理工艺对石墨球化效果的影响,以及石墨形态对热处理显微组织和力学性能的影响。结果表明:球化处理温度及时间对铸铁中的镁吸收率有显著影响,超过1 500℃时,镁烧损加剧,当球化处理温度为1 450℃,镁吸收率达到64.2%;孕育剂添加工艺也是影响铸铁中石墨的形态和分布的重要因素,当一次孕育和二次孕育添加量分别为0.8%和1.2%时,石墨的形态以球状为主,尺寸细小且分布均匀,石墨的球化率为93%,球化级别达到2级,尺寸级别为9级;另外,均匀分布的球状石墨有利于提高热处理后下贝氏体铸铁的综合力学性能。

Casting samples contained different forms of graphite are obtained by different nodularizing and inoculation treatments in vacuum mid-frequency induction furnace. The lower bainite cast irons are prepared after continuous cooling in nitrate and nitrite mixed media and tempering at 250 ℃ for 2 h. The influence of nodularizing and inoculation treatment on spheroidization effect is confirmed. The relationships between microstructure characteristic, mechanical property and graphite forms are investigated. Results show that ： The temperature and time of spheroidizing treatment have a significant effect on the absorptivity of Mg, the evaporation loss of Mg is intensified as the temperature exceeds 1 500 ℃, and the absorptivity of Mg reaches up to 64.2% as spheroidizing treatment temperature of 1 450 ℃ is used; Meanwhile, there is also obvious influence of inoculation treatment on the form and distribution of graphite, as the addition of post inoculation increases up to 1.2%, the graphite is spheroidized adequately. The spheroidization rate of graphite is 93%, and the grain size reaches up to grade 9; In addition, the comprehensive mechanical property of lower bainite cast iron after heat treatment is improved for the small size and uniform distribution of spheroidal graphite.