| The 316H austenitic stainless steel has excellent high temperature resistance due to the addition of a C content higher than 316.It is particularly suitable for pipeline systems with radioactive liquids,and does not cause brittle failure at low temperatures,eliminating the concerns of impact damage.However,in the production process,a certain amount of ? ferrite will be generated,and its presence in large quantities will endanger the safety of the nuclear power piping system.The steel for nuclear power applications requires that the content of?-ferrite be less than 1%.However,at home and abroad,?-ferrite is present.There are few reports on the content control.This article focuses on 316H austenitic stainless steels which containing ?-ferrite,using CLSM(Confocal Laser Scanning Microscopy),DIL805A/D deformation thermal expansion phase changer,and solution treatment experiments,etc.characterized by scanning electron microscope,electron probe microanalysis,transmission electron microscope,etc,explores methods for controlling ?-ferrite content,and methods for austenite grain size.The main work and innovations are as follows:(1)The solidification mode of 316H austenitic stainless steel with Creq/Nieq=1.355 was studied by CLSM experiment and the solidification mode was found to be FA mode.In the solidification process,delta ferrite precipitates from the liquid phase first,and then a peritectic reaction occurs between the liquid phase and delta ferrite,forming a "gap" on the ferrite.As the cooling rate increases during solidification,the delta ferrite content increases.In the 316H alloy composition range,the appropriate reduction of the Creq/Nieq ratio makes the 316H solidification mode change from the FA mode to the AF mode,which helps to fundamentally reduce the formation of ferrite and control its content.Deformation treatment can significantly promote the decrease of ?-ferrite content in the as-cast microstructure of 316H austenitic stainless steel.(2)Study the effect of different cooling rates on ferrite in 316H austenitic stainless steel.The slower the cooling rates of austenitic stainless steel at 1000?,the smaller the ferrite size.At low cooling rates,carbides nucleate at the ferrite/austenite interface and grow to the interior of the ferrite.At this time,ferrite undergoes eutectoid reaction,and ferrite decomposes into carbide and secondary austenite.Ferrite content decreased.(3)The phase transition behavior of ?-ferrite to austenite in 316H austenitic stainless steel was studied by means of Dictra simulation and solution treatment experiments.It was proved from the aspects of element content and ferrite change trend.Dictra software can simulate the phase transition behavior of ferrite to austenite during the solution treatment of austenite stainless steel,and the Dictra simulation agrees well with the experimental results.For the 316H hot-rolled steel sheet,in the one-stage solution treatment,the higher the solution temperature and the longer the time,the lower the ferrite content.In the two-stage solution treatment,the higher the temperature in the first stage,the lower the temperature in the second stage and the lower the ferrite content.(4)Study the grain growth of austenite in austenitic stainless steel by solution treatment,analyze the change law of grain size of austenite,and find that the austenite grain increases with heating temperature and holding time.The relationship between the average austenite grain size and time is consistent with the power function of n<1.The relationship between the average grain size and the temperature is in accordance with the exponential function,and a comprehensive consideration of the heating temperature and holding time of the Beck is established.The equation can better predict the grain growth of austenite in 316H austenitic stainless steel. |
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