Studyon Microstructure And Mechanical Properties Of A New Austenitic Heat-resistant Steelcontaining Nb

A large amount of dangerous industrial wastes has been produced with the rapid development of the economy and the progress of industrial technology.In order to promote the sustainable development of the economy,these industrial wastes must be handled properly.It has been proposed that these high risk industrial wastes can bemeltedat the high temperature and then poured into a container.After that the container will be seamed anddisposed into a deep geological repository.310 S austenitic heat resistant steel has been chosen as the most promising candidate structural material for the container because of its excellent cold,hot workability and high temperature mechanical properties.However,the strength of 310 S steel at high temperature is limited.In order to improve the high temperature strength of the 310 S steel,Nb has been proposed to be added in 310 S steel to enhance its high temperature strength.Based on this idea,in this paper,experimental steels have beendesigned by Thermo-Calc thermodynamics software andproduced by vacuum induction melting firstly.And then,the microstructure and mechanical properties,as well as the hot deformation properties,of 310 S austenitic heat resistant steel with different Nb content have beeninvestigated.According to the experimental results,the optimal content of Nb has been determined.The main contents and conclusions of this thesis are as follows:(1)Aseries of 310 S austenitic heat resistant steels with different Nb content have beendesigned according to the thermodynamic calculation carried out by Thermo-Calsoftware.And the,a series of experimental austenitic stainless steels with 0 wt.%,0.036 wt.%,0.12 wt.%,0.32 wt.% and 0.54 wt.%Nb Have been produced by vacuum induction furnace.The observation of the microstructure of the experimental steel by using microscope and scanning electron microscope arein good agreement withthe thermodynamic calculation results.(2)The mechanical properties of the new austenitic stainless steels with different content of Nb have been measured at room and elevated temperature after solution heat treatment.It is found that with the increase in Nb content,the strength of the material at 700 oC and 800 oC increases gradually,while the strength of the material at room temperature almost remains as a constant.Additionally,the addition of Nb can slightly reduce the impact toughness of the material at room temperature,although all the samples have good impact toughness.(3)The hot deformation behaviors of the experimental austenitic heat resistant steel withdifferent Nb contentshave been investigated using hot compression test on a Gleeble-3800 Thermal simulation machine at the temperature range of 900~1200 oC and the strain rate of 0.001-1~10s-1.The relationship between the Z parameter and the peak stresshas been established based on hyperbolic sine model,whichillustrates thatwith the increasing of Z value,the peak stress of the alloyand the difficulty of dynamic recrystallization increase.Laasraoui and Jonas models have beenused to describe the high temperature stress vs.strain relationship.It is found that the peak stress,steady state stress and the saturation stress of the material increase with the increasing of the Nb contentat the same strain rate and deformation temperature.At the same time,Nb retards the dynamic recrystallization and increases the hot deformation activation energy of the material.On the base of the dynamic material model and the continuum flow instability criteria,the hot process maps of the experimental austenitic heat resistant steel with different Nb contentshave been established.(4)The mechanism of Nb(C,N)on retardingdynamic recrystallization has been studied using ECCI in the 0.32 wt.%Nb alloy.It is found that the Nb(C,N)precipitates are effective pinning the movement of the dislocation in the hot deformation.