合金元素与蠕化剂含量对蠕墨铸铁凝固行为的影响研究

发布时间：2018-06-06 17:45

本文选题：蠕墨铸铁 + 凝固过程　；参考：《西安工业大学》2017年硕士论文

【摘要】：蠕墨铸铁因其良好的综合性能在高功率密度柴油机缸盖上获得了广泛的应用,然而随着发动机功率的增加,蠕墨铸铁的各项性能指标也必须进一步提升。材料的性能取决于其组织,而组织转变取决于凝固过程,因此就需要更加深入地了解蠕墨铸铁的凝固过程,以求更好地改善其性能。关于蠕墨铸铁的凝固过程,已经有不少学者进行过研究,但是其研究成果绝大多数是定性地描述,鲜有定量的描述。本文以高功率密度柴油机缸盖用蠕墨铸铁为基础,采用液淬的方法获得蠕墨铸铁凝固过程中不同时期的晶体组织状态,结合热分析法分析蠕墨铸铁的组织转变过程,在此基础上研究了合金元素和蠕化剂加入量对蠕墨铸铁凝固过程的影响,总结了蠕墨铸铁的组织转变规律,探索了蠕墨铸铁的蠕化机理。并且利用高分辨率X射线三维扫描成像技术分析了蠕虫状石墨三维形态的转变过程,利用扫描电子显微镜分析了蠕虫状石墨的形核机制,利用DT2000专业金相分析软件总结了蠕虫状石墨的形核和长大规律,研究了蠕虫状石墨形核和结晶的动力学。结果表明:(1)合金元素Cu、Mo、Sn对蠕墨铸铁的凝固曲线特征值、共晶组织转变过程及石墨的长度和含量均没有明显影响。(2)蠕化剂加入量对蠕墨铸铁的共晶组织转变过程影响很大。蠕化剂加入量为0.2%时,共晶奥氏体的生长速度比畸变石墨慢,因此畸变石墨长成片状,且大范围连通,共晶反应结束时片状石墨的连通率为88.87%。蠕化剂加入量为0.4%时,共晶奥氏体的生长速度比畸变石墨快,使畸变石墨不能大范围连通,最终长成蠕虫状。蠕化剂加入量为0.6%时,共晶奥氏体的生长速度更快,导致一些未畸变的球状石墨被完全包裹住而不能畸变,因此蠕化率下降。(3)蠕墨铸铁中的石墨结晶时,其最初形貌是球状。共晶反应开始后由于共晶奥氏体的析出,铁液中的氧、硫含量富集到一定程度,导致和铁液直接接触的球状石墨发生畸变,然后畸变石墨生长连通在一起。由于共晶奥氏体的生长速度比畸变石墨的生长速度快,共晶奥氏体逐渐将畸变石墨包裹住,使畸变石墨只能在小范围内相互连通,最终长成蠕虫状,蠕虫状石墨室温下的连通率为32.5%。(4)蠕虫状石墨的异质核心物质有CeS、MgS、CaS、La_2S_3、FeS、A1203、TiC,石墨异质核心是由其中的几种物质共同组成的,核心尺寸为12.5μm。过共晶蠕墨铸铁中的石墨主要在初生石墨阶段形核,共晶反应初期也有石墨形核,石墨形核速率不断下降,形核数量 N 与时间 t 的关系为 N =-1.1 + 1.1sin[(t+5911.8)π/11929.4]。(5)共晶反应阶段是石墨生长的主要阶段,石墨结晶速率先增长后降低,在共晶反应开始70s时,石墨结晶速率最快,此时石墨长度增长速率为1.33μm/s,石墨含量增长速率为0.18%/s。石墨长度L与时间t的关系为L = 100.54-93.87(1+e~(（t-99.98)/16.98),石墨含量 C与时间 t 的关系为C = 9.02-8.64/(1 + e~((t-101.74)/9.54))。
[Abstract]:Vermicular graphite cast iron has been widely used in the cylinder head of high power density diesel engine because of its good comprehensive properties. However, with the increase of engine power, the performance indexes of vermicular graphite cast iron must be further improved. The properties of the materials depend on their microstructure, while the microstructure transformation depends on the solidification process. Therefore, it is necessary to better understand the solidification process of vermicular graphite cast iron in order to improve its properties. The solidification process of vermicular graphite cast iron has been studied by many scholars, but most of its research results are qualitative description, few quantitative description. Based on vermicular graphite cast iron used in cylinder head of high power density diesel engine, the crystal structure state of vermicular graphite cast iron during solidification process was obtained by liquid quenching method, and the microstructure transformation process of vermicular graphite cast iron was analyzed by means of thermal analysis. On this basis, the effect of alloy elements and vermicularizing agent on the solidification process of vermicular graphite cast iron was studied, the microstructure transformation law of vermicular graphite cast iron was summarized, and the compacted mechanism of vermicular graphite cast iron was explored. The transformation process of vermicular graphite 3D morphology was analyzed by high resolution X-ray 3D scanning imaging, and the nucleation mechanism of vermicular graphite was analyzed by scanning electron microscope (SEM). The nucleation and growth laws of vermicular graphite were summarized by using DT2000, and the kinetics of nucleation and crystallization of vermicular graphite was studied. The results show that the alloy element Cu-Mo-Sn has no obvious effect on the characteristic value of solidification curve, the process of eutectic structure transformation and the length and content of graphite. The addition of vermicularizing agent has great influence on the eutectic transformation process of vermicular graphite cast iron. The growth rate of eutectic austenite is slower than that of distorted graphite when the amount of vermicularizing agent is 0.2 / 0.2.Therefore, the distorted graphite grows into sheet and is connected in a wide range, and the connectivity rate of sheet graphite is 88.87 at the end of eutectic reaction. The growth rate of eutectic austenite is faster than that of distorted graphite when the amount of vermicularizing agent is 0.4, so that the distorted graphite can not be connected in a wide range and eventually grows into a worm. When the amount of vermicularizing agent is 0.6, the growth rate of eutectic austenite is faster, which leads to some undistorted spheroidal graphite being completely enveloped and unable to be distorted. Therefore, the vermicular rate of graphite in vermicular graphite cast iron is decreased. 3) when the graphite in vermicular graphite cast iron crystallizes, its initial morphology is spherical. At the beginning of eutectic reaction, due to the precipitation of eutectic austenite and the enrichment of oxygen and sulfur in molten iron to a certain extent, the nodular graphite in direct contact with molten iron is distorted, and then the distorted graphite grows and connects together. Because the growth rate of eutectic austenite is faster than that of distorted graphite, eutectic austenite is gradually wrapped in distorted graphite, so that the distorted graphite can only be connected to each other in a small range, and eventually grows into a worm. The connectivity of vermicular graphite at room temperature is 32.5 / 4) the heterogeneous core material of vermicular graphite is CeSmg / MgS / CaSU / La _ 2S _ 3 / FES _ (A1203) TiC. The graphite heterostructure core is composed of several kinds of substances and the size of the core is 12.5 渭 m. The nucleation of graphite in hypereutectic vermicular graphite cast iron is mainly in the primary graphite stage, and the nucleation rate of graphite is decreasing at the beginning of eutectic reaction. The relationship between nucleation number N and time t is N ~ (-1.1) 1.1sin [t _ (5911.8) 蟺 / 11929.4]. 5) the phase of eutectic reaction is the main stage of graphite growth. The crystallization rate of graphite increases first and then decreases, and the crystallization rate of graphite is the fastest at the beginning of eutectic reaction for 70s. The graphite length increases at a rate of 1.33 渭 m / s, and the graphite content increases at a rate of 0.18 / s. The relationship between graphite length L and time t is L = 100.54-93.871e1 / 99.98g / r 16.98g / t, and the relationship between graphite content and time t is C = 9.02-8.64 / t / 101.74 / 9.54 / m ~ (-1).
【学位授予单位】：西安工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG250

【相似文献】