Microstructure And Mechanical Properties Of A Si Enhanced 9Cr Martensitic Heat Resistant Steel

Aiming at the requirements of lead-bismuth corrosion-resistant,radiation-resistant,and high temperature-resistant structural materials for lead-cooled fast reactor heat exchange tubes,the author’s group developed a Si enhanced 9Cr martensitic heat resistant steel(denoted as H-Si steel).The H-Si steel was designed on the basis of P91 steel by mainly increasing Si content and slightly adjusting other alloy compositions.The previous results showed that the H-Si steel has good resistance to liquid lead-bismuth,and its room temperature and high temperature tensile properties,stress rupture and fatigue properties can meet the design requirements.As an alternative structural material that may be applied to lead-cooled fast reactor,the H-Si steel will also face long-term service conditions at high temperature.Therefore,it is necessary to study the microstructure stability and mechanical properties of the newly developed H-Si steel during long-term aging at high temperature.In this paper,the effects of Si and high temperature(550℃)aging on the microstructure and mechanical properties of H-Si steel were investigated by OM,SEM,TEM,tensile and impact test and so on.The main research works and conclusions are as follows:By comparing and analyzing the changes of microstructure and mechanical properties of as cast,homogenized,tempered and aged two experimental steels(P91 and H-Si),the effects of Si reinforcement on microstructure and mechanical properties were studied.The results show that the increase of Si content leads to 3.9%δ ferrite(volume fraction)in as cast H-Si steel,but which can be eliminated completely by homogenization heat treatment.δ ferrite will not precipitate again during subsequent hot working,normalizing,tempering and aging.Meanwhile,after Si reinforcement the H-Si steel has the same matrix structure as P91,which is a typical tempered martensite with a large amount of M23C6 and a small amount of NbC.The difference is that there appears a Si enriched layer around M23C6 after tempering due to the high Si content in the H-Si steel,which can suppress the rapid growth of M23C6 and maintain a smaller M23C6 size than that in P91 steel.During aging at 550℃,this Si enriched layer can also slow down the growth of M23C6 and promote the nucleation and growth of Laves phase,which induces smaller M23C6 and larger and more Laves phase in aged H-Si steel than that in aged P91 steel.Due to higher Si content and smaller M23C6,the tempered H-Si steel exhibits obviously higher strength than P91 steel and the strength of aged H-Si steel further increases because of the precipitation of Laves phase.Both tempered alloy steels have similar impact energy(about 210 J)and exhibit ductile fracture.After aging treatment,the impact energy of two alloy steels decreased due to the precipitation of Laves phase.However,due to the larger size of Laves phase in the aged H-Si steel,a certain proportion of cleavage zone appeared in the impact fracture,which induces the impact energy of the aged H-Si steel decreased more obviously than that of aged P91 steel.Aging experiments at 550℃ for different time were carried out to study the microstructure stability and mechanical properties change as for H-Si steel.The results show that the prior austenite grain size,fraction of high/low angle boundary,effective grain size,lath width and dislocation density of H-Si steel have little change during aging,which indicates that the H-Si steel has better microstructure stability.In contrast,the precipitates including M23C6 and Laves phase show obvious changes during aging.The M23C6 of H-Si steel gradually coarsens with increasing aging time and the morphology of M23C6 on grain boundary tends to be chain-shaped state from discrete state,and then gradually becomes discrete state again.As compared with P91 steel,the Laves phase will precipitate quickly adjacent to M23C6 due to a Si-rich layer around M23C6 in H-Si steel and coarsens obviously before 3000 h.With further increasing aging time to 10,000 h,the size of Laves phase tends to be stable(about 136 nm).The mechanical properties test results show that the yield strength and elongation of H-Si steel have little change with increasing aging time.However,the impact energy of H-Si steel drops obviously because of the precipitation and quick coarsening of Laves phase and the impact fracture contains a certain proportion of cleavage zone.When the aging time is over 3000 h,the H-Si steel exhibits a relatively stable impact energy(about 113 J)due to the relatively stable size and volume fraction of Laves phase.