含有Cu、Mo、Sn的高强度蠕墨铸铁的蠕变行为

Creep Behavior of a High Strength Compacted Graphite Cast Iron

研究了一种含有Cu、Mo、Sn的高强度蠕墨铸铁在623～823 K、40～150 MPa的蠕变行为,观察了不同形态的蠕变损伤组织并分析了蠕变变形及断裂机理。当T/Tm>0.5(T为使用温度,Tm为蠕墨铸铁熔点)、载荷大于150 MPa时这种蠕墨铸铁的蠕变变形显著,且变形主要来自基体变形、蠕变空洞的形核长大以及石墨/基体界面的开裂。随着温度的提高和载荷的增加,蠕变变形逐渐由晶界移动转变为晶内变形。在蠕变过程中有两种开裂机制:(I)微裂纹在石墨/基体开裂处形核长大并优先沿铁素体向基体扩展,与邻近石墨/基体开裂连接而逐渐形成主裂纹;(II)晶界处的蠕变空洞形核长大转变成蠕变裂纹。氧原子通过石墨的连通性向组织内部扩散,造成上述两种裂纹表面氧化。由于,石墨、铁素体、珠光体三者性能的差异,石墨/铁素体界面比石墨/珠光体界面更易发生开裂。另外,在773 K、823 K组织中的珠光体分解明显,层片状渗碳体逐渐转变为短棒状,在晶界附近则以颗粒状为主。

The creep behavior of a high strength compacted graphite cast iron(CGI) containing Cu,Mo and Sn under tensile load of 40~150 MPa in air at 623~823 K was investigated, while the creep damage was observed and the relevant mechanism of deformation and fracture during creep test was further analyzed. When the ratio T/Tm>0.5(T represents test temperature, Tm melt point of CGI) and the load is greater than 150 MPa, the creep deformation is significant. The creep deformation consists of matrix deformation, initiation and development of creep cavities at grain boundaries and debondings of the interface graphite/matrix. With the increasing temperature and tensile load, the creep deformation is gradually changing from grain boundary sliding to intragranular deformation. Two kind of cracks were observed in the microstructure of CGI:(1) cracks propagated preferentially in ferrite phase and connected with adjacent debondings of the interface graphite/matrix,(2) microcracks caused by nucleation and growth of creep cavities along grain boundaries. It is worthy to mention that the 3D network of the vermicular graphite in CGI may facilitate the inward diffusion of oxygen atoms throughout the sample of CGI, therewith induces the oxidation of the above mentioned two type cracks. Due to the difference in properties between graphite with ferrite and pearlite respectively, the debonding occurance for the inerface of graphite/ferrite may be easier than that of graphite/pearlite. In addition, pearlite in the microstructure may decompose significantly at 773 K and 823 K for 100 h, as a result, the lamellar cementite should be converted to short rods and granules at grain boundaries.