灰铸铁中TiN形态及其形成热力学与生长机理

Morphology and formation thermodynamics and growth mechanism of TiN particle in grey cast iron

为了研究灰铸铁中TiN颗粒形成与长大规律及其组织特征,采用显微组织及热力学计算研究某汽车发动机缸体铸件中TiN颗粒形态、尺寸及分布,并对TiN形成与长大机理进行了分析讨论。结果表明,因回炉料中TiN的遗传性形成的TiN颗粒尺寸相对较大,超过20μm;从铸铁熔体中结晶的TiN颗粒尺寸为4~8μm;从过冷奥氏体中析出的TiN颗粒尺寸相对较小为1~2μm。热力学计算表明,TiN结晶取决于铸铁熔体中[%Ti]·[%N]的溶度积,当铸铁熔体中[Ti]在0.045%附近([%Ti]·[%N]=2.47×10^(-4))时,铸铁熔体过冷到1368℃,即开始结晶TiN。由于TiN与MnS晶体结构相同,点阵常数相近,过冷铸铁熔体中TiN多以MnS为基底非自发成核长大。

In order to clarify the formation and growth mechanism and microstructural characteristics of TiN particles in grey cast iron,morphology,size and distribution of the Ti N particles in an automobile engine cylinder cast were studied by means of optical and SEM observation. The nucleation and growth mechanism of Ti N was analyzed via thermodynamic calculation. The results show that the size of the TiN particles from foundry returns is the largest（ 〉20 μm） due to the hereditary of TiN,while that of the TiN particles crystallized from the cast iron melt is in the middle（ 4-8 μm） and that of the TiN particles precipitated from under-cooled austenite is the smallest（ 1-2 μm）. Thermodynamic calculation shows that crystallization of TiN in the cast iron melt depends on solubility product of[% Ti]·[% N].As the[Ti] content in cast iron melt is around 0. 045 mass%（ [% Ti]·[% N]= 2. 47 × 10^- 4at the time）,TiN begins to form when the iron melt is under-cooled to 1368 ℃. Owing to the same crystal structures and nearly equal lattice constants of TiN and Mn S,TiN particles prefer to form on the Mn S substrate,following the non-heterogeneous nucleation mechanism with a regular quadrangle in shape.