Study On Heat Treatment Process And Microstructure And Properties Of Nickel Base Alloy For Nuclear Power Springs

GH4169 is a precipitation-strengthened nickel-basedsuperalloys.Compared with iron-basedsuperalloys and cobalt-basedsuperalloys,it has superior properties in terms of oxidation resistance,corrosion resistance,physical properties,strength,and machinability at high temperatures.Moreover,its material cost is lower than that of cobalt-basedsuperalloys,making it more valuable for research.Therefore,it has been applied to the preparation of springs in nuclear power control rod drive mechanisms and high-temperature components in aerospace engines.This article studies the evolution of microstructure and properties of GH4169 alloy under different heat treatment processes and long-term aging preservation,as well as its strengthening mechanism,and studies its oxidation kinetics behavior at 650℃and 1000℃.The forged GH4169 alloy was solution-treated at 920℃～1060℃,and the alloy grains gradually grew larger from 50.52μm to 90.62μm.The unsolved phase in the alloy gradually dissolved into the matrix with the increase of solution temperature.As the temperature increased,the movement of grain boundaries accelerated,the unsolved phase was dissolved,and its hindering effect on the migration of grain boundaries sharply weakened.Under the joint influence of these factors,the austenite grains grew rapidly,and the hardness of the alloy showed a decreasing trend.When the solution temperature was 980℃,theδphase on the grain boundaries dissolved,and the grain size increased suddenly,leading to a sharp drop in hardness.After holding at 960℃for 0.5h～2h,the changes in grain size and hardness of the alloy were small.Combining the changes in microstructure and properties of the alloy under different solution temperatures and times,960℃×1h was selected as the optimal solution treatment process.After solution treatment,the alloy was in a thermodynamically unbalanced state.With the increase of aging temperature,γ’andγ’phases gradually precipitated from the matrix.As the aging temperature increased,the precipitates grew and coarsened.When aged at 650℃,the alloy did not show overaging because of low temperature.The hardness of the alloy increased with the increase of aging time.When aged at 700℃for 2h～20h,the number of precipitates gradually increased.In the late stage of aging,the precipitates slightly coarsened but not significantly,and the hardness of the alloy showed an upward trend at first and then a downward trend,reaching a peak value of408HV when aged at 700℃for 12h.With the increase of aging temperature,the time for overaging became shorter.When aged at 800℃for 8h,the hardness of the alloy reached the peak.Double-stage aging after solution treatment could precipitate more and finerγ’andγ’phases than single-stage aging,and the precipitates did not coarsen due to the lower temperature of secondary aging during the first stage aging.The precipitates mainly included smallerγ’phases,which played a minor role in strengthening during aging.Therefore,the hardness and strength of the alloy were enhanced to some extent by double-stage aging,while the good plasticity of the alloy was maintained.After being treated at 960℃×1h+720℃×8h+620℃×8h,GH4169 alloy was kept at650℃for long-term aging.The grain size of the alloy did not change significantly during100h～800h of aging.The number ofδphases at the grain boundaries increased,and fineγ’andγ’precipitated inside the grains.After being aged for 800h,the edges of theδphases at the grain boundaries began to form a depletedγ’region,and the boundaries appeared fuzzy and tended to spread.During 1600h～2400h of aging,theδphases at the grain boundaries grew and coarsened,and the precipitates ofγ’’andγ’inside the grains gradually grew larger.When aged for 3200h,theδphases at the grain boundaries formed a significantly coarsened depletedγ’region,and the precipitates ofγ’inside the grains began to transform intoδphases.After being aged for 4000h,a large number ofγ’phases inside the grains transformed intoδphases,and a depletedγ’region appeared around the transformedδphases.The hardness and strength of the GH4169 alloy after long-term aging showed an upward trend at first and then a downward trend with the increase of aging time,while the plasticity continuously decreased.When oxidized at 1000℃for 50h～400h,the main oxidation product of the alloy was Cr₂O₃.In addition to Cr₂O₃,small amounts of Fe₂O₃and Nb₂O₅were found on the outer layer of the oxide by energy dispersive spectroscopy and X-ray diffraction analysis,and Ni Cr₂O₄,Al₂O₃,and Ti O₂were found in the inner layer of the oxide.The oxidation kinetics curve at 1000℃approximated a cubic model.When oxidized at 650℃for 50h～3000h,the main oxidation product of the alloy was Ni Cr₂O₄.In addition to Ni Cr₂O₄,Cr₃O₂and Fe₃O₂were found on the outer layer of the oxide,and a small amount of Al₂O₃was detected by XRD in the inner layer of the oxide.The oxidation kinetics curve at 650℃approximated a square model.