Ni合金化球墨铸铁组织和性能研究

Study on microstructure and properties of Ni alloyed ductile cast iron

为获得兼具较高强度和良好低温冲击韧性的球墨铸铁铸件,向球墨铸铁中加入质量分数约0.5%的Ni进行合金化,并对其进行中温奥氏体化(880℃+3h)和低温退火(720℃+4h)处理.采用光学显微镜(OM)、扫描电子显微镜(SEM)对铸态和热处理态试样的显微组织和冲击断口形貌进行分析;利用万能试验机、布氏硬度计和摆锤式冲击试验机等对铸态和热处理态试样进行了室温拉伸、硬度检测、低温冲击等力学性能测试.结果表明:铸态球墨铸铁的微观组织由珠光体、铁素体和球状石墨及少量的渗碳体组成,其强度、硬度偏高,塑性、韧性较差;热处理态试样中的珠光体向铁素体转变后为铁素体和球状石墨,试样强度、硬度有所降低,塑性、韧性得到明显的改善;铸态试样呈现典型的脆性断裂特征,热处理态试样冲击断口处存在少量韧窝,断裂模式以解理断裂为主,伴有少量塑性变形的韧脆混合断裂,且在-40℃冲击功达到12.4J;比较铸态与热处理态的冲击断口形貌可知,试样断裂方式由脆性断裂转变为韧脆混合断裂.

In order to obtain ductile cast iron castings with high strength and good low-temperature impact toughness, alloying element of about 0.5% Ni(in mass fraction) was added into ductile cast iron which was then treated by mid-temperature austenitizing(880 ℃+3 h) and low-temperature annealing(720 ℃+4 h). The microstructures and impact fracture morphology of both as-cast and heat-treated Ni-alloyed ductile cast iron samples were observed and analyzed by means of optical microscopy(OM) and scanning electron microscopy(SEM). The mechanical properties of the as-cast and the heat-treated Ni-alloyed ductile cast iron samples were detected by room temperature tensile, hardness testing, and low temperature impact tests using universal tensile test machine, Brinell hardness tester, and pendulum impact testing machine. Results show that as-cast ductile cast iron was consisted of pearlite, ferrite, nodular graphite, and a small amount of cementite, whose tensile strength and hardness were high, and plasticity and impact toughness were poor. The pearlite in the matrix microstructure transformed into ferrite and its microstructure was consisted of ferrite and nodular graphite after heat treatment, whose tensile strength and hardness declined slightly, and plasticity and impact toughness increased obviously. The analysis results of the fracture morphology reveal that the fracture surface of the as-cast samples exhibited a typical feature of brittle fracture. However, a small amount of dimples appeared on the fracture surface of the samples treated by mid-temperature austenitizing and low-temperature annealing. The impact fracture mode of the samples was a mixed ductile-brittle mode, which had a cleavage fracture accompanying with a small amount of plastic deformation, and the impact toughness values reached 12.4 J at-40 ℃. Compared the impact fracture morphologies of the as-cast and the heat-treated samples, it can be seen that the impact fracture mode changed from the brittle fracture to the ductile-brittle fracture.