Effect Of Mo On Microstructure And Properties Of Austenitic Heat-resistant Steel For Magnesium Reduction Tank

China has the largest magnesium resource andthe output of raw magnesium accounts for 80%of the total output in the world.In China,more than 98%of the raw magnesium is produced by silicon thermal process,and the reduction tank is an important part in the process of producing magnesium ingot by silicon thermal process.The main material of the tank is austenitic heat-resistant steel,which is short life and consumable.The reduction tank works for a long time under the condition of1150¹200°C and vacuuming.The working condition is bad.Oxidation peeling,fatigue cracking and creep instability will occur during service,which will seriously affect the economic benefits of the magnesium industry.Therefore,the research and development of excellent performance of austenitic heat-resistant steel is of great significance.On the basis of ZG35Cr26Ni8NRE,Mo was added to optimize the composition.The effects of Mo on microstructure,conventional mechanical properties,thermal fatigue resistance,high temperature oxidation resistance and high temperature creep resistance of austenitic heat-resistant steel in reduction tank were analyzed by OM,SEM,EDS and XRD.The results of the study are as follows:The microstructure of the heat-resistant steel without Mo is austenite,and its grain size is larger.The second phase of M₂₃C₆,M₇C₃,Cr₂Fe₁₄C and WC were mainly distributed in the grain boundary,and a littlewas distributed in the crystal.After added Mo,the matrix structure remained austenite,but the grain refined and formed granular Mo₂C.The two increased with the increased of Mo content,while M₂₃C₆ and other carbides grow with Mo content increased.With the increased of Mo content,the thermal fatigue properties of heat-resistant steel increased first and then decreased with the increased of Mo content for 80 times fatigue cycle at 1200℃.The crack length of the heat-resistant steel without Mo is9.712mm.The heat resistant steel with 0.49%Mo appeared transverse crack,and the carbide dispersion was distributed,and the crack is the shortest 5.619mm.The M₂₃C₆phase of heat-resistant steel containing 0.73%and 0.95%Mo is coarse,and the brittleσ（Ni-Cr-Mo）phase was precipitated,and the added 0.95%Mo has the longest crack is 9.923mm.At 1200℃,the oxidation weight gain of heat treated steel increased with time,and the oxidation rate gradually decreased,and the oxidation rate first decreased and then increased with the increased of Mo content.The oxide film of the heat-resistant steel without added Mo is spinel structure,and the oxidation 60h forms a more complete oxide film,and the oxide film began to peel after 60h.After oxided80h,the oxide film of heat-resistant steel containing 0.24%Mo is complete,but there were cracks and oxidation holes.Oxidation of 20h containing 0.49%Mo heat resistant steel forms a continuous,dense and complete oxide film.After 80h it was still relatively intact.After 100h,the oxide film thickness was about 60μm,which has good bonding with the substrate.The oxide film of heat-resistant steel which containing 0.73%and 0.95%Mo formed earlier,but the peeling earlier and incomplete.The oxide film without Mo contains Cr-O,Cr_1.3Fe_0.7O₃,[Cr,Fe]₂O₃,spinel,SiO₂,Fe₃SiO₄,Ni₆MnO₄ and so on.The oxide film containing Mo also generated MnO₂ and MnMoO₄.The five groups of samples all belong to the antioxidant grade,of which the oxidation rate of oxidized 100h containing 0.49%Mo is 0.4515g/（m²·h）,and the oxidation resistance of high temperature is the best.The creep test of 100h under the loading of 16MPa at 1000℃was carried out.With the increased of Mo content,the creep resistance of austenitic heat-resistant steel increased first and then decreased.The creep resistance of 0.24%Mo is the best,and the minimum elongation is 0.03%.The creep resistance of 0.95%Mo is the worst and the elongation is 0.13%.The effect of Mo on creep is solid solution strengthening,stabilizing carbide,forming high temperature stable Mo₂C and Mo₂N,and improving the creep resistance of the material.The analysis of the microstructure and properties of austenitic heat-resistant steel for magnesium reduction tank by added Mo.It was suggested that the optimum addition of Mo in austenitic heat resistant steel is 0.24%⁰.49%.