Effect Of Casting Technology And Carbon Content On Microstructure And Thermal Fatigue Properties Of K4648 Alloy

Superalloys are the key materials for aircraft turbine engines,gas turbine,shipbuilding,nuclear industry and other important parts of manufacturing industries.With the rapid development of the aviation industry,materials are needed to improve better performance.Under actual service conditions,fatigue fracture(particularly thermal fatigue)has become one of the main forms of mechanical parts failure,which may cause the loss of personal safety and property.As one model aeroengine is working in the experiment,a crack has initiated and propagated.Because the annular plate sustains hot and cold temperature periodically which generates thermal stress,it may cause thermal fatigue failure.At present,we see little of study on the the thermal fatigue properties of the alloy K4648.Therefore,this subject has not only theoretical value but also the practical significance of production process.In this paper,we can study the effect of casting technology and carbon content on micro structure and thermal fatigue properties of K4648 alloy by adjusting casting parameters(pouring temperature,shell temperature and modeling ways)and improving technology of control carbon.In order to optimize the part design,improve the production process and prolong the life of the alloy,we discuss the mechanism of crack initiation and propagation model,analyzing factors affecting the thermal fatigue properties.The influence of casting parameters and crabon content on the microstructure of K4648 alloy was investigated.The results show that the secondary dendrite arm spacing and average grain size increase with the increasing of pouring temperature.With the increasing of shuttering temperature,the grain size decreases,but the secondary dendrite arm spacing increases.When the content of C ranges from 0.068%to 0.028%,the precipitation amount of carbides reduces and the M23C6 size is finer and more uniform at the grain boundaries.Almost all the primary fatigue cracks at V-notch,then propagates transgranularly.The major positions of cracks propagation are generally at the region of carbides/matrix.At the same time,the oxidation has an important role of accelerating crack initiation and propagation.On the other hand,a few micro-cracks distribute parallel because of shielding effect between neighboring cracks.Then we research the effect of microstructure and carbon content on the thermal fatigue properties of the alloy K4648.The results show that the smooth grain boundary has minimal thermal fatigue damage tolerance.In contrast,the serrated grain boundaries are more resistant for thermal crack.Furthermore,we can improve the propagation resistance of thermal fatigue crack by refining grain structure.Experimental results also indicate that thermal fatigue cracking rate decreases with increasing the maximum heating temperature.After the carbon content is decreased from 0.068%to 0.028%,nucleation rate and expansion rate of thermal fatigue crack decreases,the crack damage decreases,which leads to raising the thermal fatigue property.Modeling of sandbox + cotton and pouring temperature of T0 with shell mould temperature of T1+50℃ shows fine microstructure and excellent thermal fatigue property and it is the optimun.K4648 alloy with C content ranging from 0.03%to 0.05%has fine thermal fatigue performance,and the C content range is able to match requirements of mechanical properties.