Effect Of Ni,Mo,Cr And RE On Microstructure And Properties Of SA351-CK20 Austenitic Heat Resistant Steel

With the increasing demand for electricity in China and the need for energy conservation and emission reduction,the development of ultra-supercritical technology with higher power generation efficiency has become the main trend of future power generation.Austenitic heat-resistant steel is widely used in pressure components such as superheaters and reheaters of ultra-supercritical units due to its good comprehensive mechanical properties,excellent high-temperature oxidation resistance,and corrosion resistance.With the further development of ultra-supercritical technology,higher steam parameters put forward higher requirements for the performance of the materials used.In this paper,SA351-CK20 austenitic heat-resistant steel is taken as the research object.The effects of Ni,Mo,and Cr on the microstructure and properties of SA351-CK20 austenitic heat-resistant steel are studied.The best Ni,Mo,and Cr content is selected by comparing the mechanical properties and high-temperature oxidation resistance.On this basis,RE is added for microalloying to study the effect of RE on the microstructure and properties of SA351-CK20 austenitic heat-resistant steel.Firstly,the effects of different Ni,Mo,and Cr contents on the microstructure and mechanical properties of SA351-CK20 austenitic heat-resistant steel were studied.The results show that with the increase of Ni content,the tensile strength,yield strength and elongation of the tested steel increase first and then decrease,the hardness increases and the impact energy decreases.With the increase of Mo content,the tensile strength,yield strength,elongation,and hardness of the test steel increase first and then decrease,and the impact energy decreases continuously.With the increase of Cr content,the austenite phase region gradually shrinks,andσphase and carbide M₂₃C₆appear in the microstructure.These hard and brittle phases lead to the increase of tensile strength,yield strength,and hardness of the test steel,the increase of elongation,and the decrease of impact energy.With the increase of Cr content,the oxidation weight gain per unit area of the tested steel decreases gradually,and the density and integrity of the oxide film increase continuously.Overall,when the content of Ni is20%,Mo is 0.4%,and Cr is 25%,the comprehensive performance of SA351-CK20austenitic heat-resistant steel is the best.Secondly,the effect of RE microalloying on the microstructure and properties of heat-resistant steel was studied based on the optimum composition.The results show that the addition of RE can refine the austenite grain size.With the increase of RE content,the austenite grain size of the test steel decreases.When the RE content is0.15%,the minimum grain size is 30～100μm.With the increase of RE content,the tensile strength,yield strength,and hardness of the test steel at room temperature and700℃increase first and then decrease,the elongation has no obvious change,and the impact energy is greatly improved.When the RE content is 0.15%,the comprehensive mechanical properties are the best.At this time,the tensile strength at room temperature is 545 MPa,the yield strength is 236 MPa,the elongation is 49.6%,the hardness value is 175 HBW,and the impact energy is 300 J,the tensile strength at700°C is 262 MPa,the yield strength is 133 MPa,and the elongation is 43.2%.The addition of RE can significantly improve the high-temperature oxidation resistance.With the increase of RE content,the oxidation weight gain per unit area of the test steel decreases,the thickness of the oxide film decreases,the spalling resistance increases,and the oxide particle size decreases.When the RE content is 0.2%,the high-temperature oxidation resistance is the best.The addition of RE can inhibit the precipitation of carbide M₂₃C₆ on the grain boundary during aging,reduce the aging embrittlement trend of the alloy,and enhance the thermal stability of SA351-CK20austenitic heat-resistant steel.With the increase in RE content,the number of M₂₃C₆precipitates decreased significantly.The impact energy of 0%～0.2%RE aged for 500hours was 32 J,58 J,83 J,99 J,and 80 J,respectively,which decreased by 79%,73.6%,70.6%,67%,and 71.3%,respectively,compared with that of 0 h aging.On the whole,the optimum addition amount of RE is 0.15%.