K441 superalloy is a cast nickel-based superalloy designed by IMR, which is an ideal material for turbine blade. Once, however, its application is limited because of the hot tearing problem encountered during casting.
In order to study the mechanism of hot tearing in K441 superalloy, solidification of K441 superalloy is investigated by water quenching method and the hot tearing surface is observed. It is found that carbides play an important role during the formation of hot tearing. In the last stage of solidification, feeding was prevented by the precipitated large blocky carbides, which causes the formation of microporosities. Cohesion of grain boundaries was weakened by the continuously precipitated carbides, and the hot tearing will be easily formed by the linkage and propagation of the microporosities.
The two abnormal temperature (tan1 and tan2) of K441 superalloy is determined by DSC，which is the key of high temperature treatment of melt. Liquid structure of the K441 superalloy is investigated by high temperature X-ray diffraction and melt spinning (High Rate Solidification of Melt). The result shows that there are Ni3Al-like and carbide clusters in the alloy melt when the temperature is low. With decreasing temperature, the carbide clusters can transform into M6C carbide nucleation centers. With increasing the temperature of the melt treatment, medium-rang ordered structure and the bonding of atoms are destroyed. Therefore, the liquid structure of K441 superalloy is homogenized. M2C carbide is found in the samples obtained by melt spinning and it can change into M6C carbide after solution heat treated at 1220℃ for 2h.
Compared with the conventional process, high temperature treatment of melt obviously improves degree of supercooling during solidification, reduces secondary dendrite arm spacing, enhances cohesion of grain boundary and decreases hot tearing tendency of K441 superalloy. Size of carbides is decreased dramatically, so feeding channel can not be blocked in the last stage of solidification and the hot tearing tendency of K441 superalloy is decreased. Smaller carbides are obtained because nucleation and growth is difficult with the decrease of carbon content. Consequently, hot tearing tendency of K441 superalloy is decreased.
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