Effect Of Aluminum On Microstructure And High Temperature Properties Of ZG40Cr20Ni20 Austenitic Heat-resistant Steel

HK40（ZG40Cr20Ni20）heat-resistant cast steel has good high-temperature oxidation resistance and mechanical properties and is widely used in key components such as pulverized coal burner nozzle,ethylene cracking pipe,converter pipe,radiation pipe,etc.Cr is the main high-temperature antioxidant element in HK40 steel.However,with the increase of temperature,Cr₂O₃ film in HK40 can not meet the requirements gradually.In this paper,Al with better oxidation resistance is introduced into HK40 heat-resistant cast steel,and Cr content is reduced to 20%to obtain austenite matrix.A new type of heat-resistant cast steel ZG40Cr20Ni20Alx is obtained.This paper focuses on the effect of Al content on the structure and properties of ZG40Cr20Ni20 heat-resistant cast steel.In order to improve its high-temperature mechanical properties,0.7%Nb was added to the heat-resistant steel.The results show that the microstructure of ZG40Cr20Ni20 is composed of austenite matrix and eutectic carbide（Nb C and M₇C₃）.The austenite grain is relatively coarse,and a small amount of Al is mainly dissolved in the austenite matrix.With the increase of Al content,the matrix began to precipitate fine particles of the B2-Ni Al phase and ferrite phase,so the strength of the alloy increased gradually and the elongation decreased.After oxidation in air at 1100℃for 480 h,the oxidation resistance of heat-resistant cast steel is improved obviously after adding Al.The oxidation layer of heat-resistant steel without Al is mainly composed of outer Fe oxide and inner Fe,Cr and Ni mixed oxide,and the oxidation layer is seriously damaged.The oxidation layer of heat-resistant steel containing 1.76%Al and 3.45%Al is mainly composed of inner chromium oxide and outer spinel oxide（Fe Cr₂O₄）.Low porosity Fe Cr₂O₄attached to Cr₂O₃ near the substrate prevents chromium oxide volatilization.The presence of Al₂O₃ in the oxide skin of 3.45%Al cast steel can be detected to fill the gap of spinel and improve the oxidation resistance of the alloy.The oxide layer of5.34%Al heat-resistant steel is mainly composed of dense Al₂O₃.The oxidation resistance of heat-resistant cast steel increases with the increase of Al element after being oxidized in 900℃steam for 240 h.The oxidation layer of heat-resistant steel without Al and with 1.76%Al is mainly composed of external Fe oxide film,Ni Fe₂O₄ oxide film with spinel structure in the middle,and internal Cr₂O₃oxide film.ZG40Cr20Ni20Alx oxide film with 3.45%and 5.34%Al is mainly composed of Al₂O₃.The creep resistance of ZG40Cr20Ni20Alx increased first and then decreased with the increase of Al content under the condition of 20MPa load and 1000℃.The precipitates of heat-resistant steel without Al and with 1.76%Al are composed of a large number of fine dispersed second phase M₂₃C₆ and skeleton Nb C precipitated along the grain boundary.As the Al content increases to 3.45%,the amount of M₂₃C₆inside the heat-resistant steel crystal decreases,the size of M₂₃C₆ increases,and M₂₃C₆ is dispersed in the matrix,resulting in a reduction in resistance to dislocation motion and grain boundary slip.The secondary carbides near the grain boundary aggregate towards the grain boundary,and the precipitation tendency of the brittle phase increase.Besides,the B2-Ni Al phase is detected.With the further increase of Al content to 5.34%,it is difficult to work harden the cast steel due to a large amount of ferrite precipitated in the matrix,which leads to the sharp decrease of creep resistance.