In the present study, the effects of melt heat treatment (MHT) on melt structure, the structure evolution in the near equilibrium solidification process, the solidification process at low cooling rate and the corresponding solidified microstructure were investigated; the microstructure and creep property were analyzed in K465 alloy with different MHT temperatures and total content of oxygen and nitrogen (gas content); and the rejuvenation effects of recovery heat treatment (RHT) on microstructure and creep property of K465 alloy with different pre-creep time were also considered.
With temperature increasing from 1360℃ to 1550℃, there are two discontinous structural transitions occurred in K465 melt, and structural orders which atoms packing similar with (111) crystal plane of M6C and (100) crystal plane of r'-phase, respectively, are observed to formation in these liquid structural transitions.
Both the solidification process and solidified structure are affected by evolution of melt structure. Under near equilibrium solidification condition, the nucleation mode of K465 alloy transform from the “stabilization mode” to the “saltation mode” with increasing MHT temperature from 1360℃ to 1450℃, and the solidus decreases a lot in sample with a 1550℃ MHT. At a low cooling rate, rising up of MHT temperature increases r-matrix precipitation temperature, decreases MC carbide precipitation temperature, affects the morphologies of both the MC carbide and the dendrite arms around the MC carbide colonies.
When K465 alloy was remelted in industrial vacuum induction furnace, the effects of MHT on the microstructure and creep property are related with gas content. Increase of either gas content or MHT temperature tends MC carbide morphology to transform from script-type to block-type, and gas content do more contribution on MC carbide morphology variation than MHT temperature. In samples with a 1550℃ MHT, the morphology of MC carbide transforms from script-type to block-type with gas content increasing from 18ppm to 40ppm; while the MC carbide morphology keeps to be script-type with rising MHT temperature up to 1900℃ in samples which gas content are less than 18ppm.
The grain size and secondary dendrite arm spacing enlarge with MHT temperature rising, but reduce with gas content increasing. Therefore, the grain size and secondary dendrite arm spacing are decided by which of MHT temperature and gas content is dominator. When gas content is less than 18ppm, the grain size and secondary dendrite arm spacing increase with MHT temperature; when gas content is more than 18ppm, the grain size reduces firstly and increases later, while the secondary dendrite arm spacing decreases with MHT temperature rising.
The degenerated r'-phase in dendrite arms and dislocation net-work in r-channel, which formed in pre-creep process, can be rejuvenated effectively by recovery heat treatment (RHT). But the effects of RHT on r'-phase and dislocation configuration decrease with extension of pre-creep time. Moreover, the transformation from MC carbide to M6C carbide continues in RHT process. Although the RHT can somewhat recovery the creep property of K465 alloy, but it can not rejuvenate the creep property to the level similar to that of alloy with standard heat treatment. Moreover, the RHT carried out at the end of secondary creep stage reduces the creep plasticity dramatically.
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