Effect Of Niobium On Microstructure And Properties Of Austenitic Heat-resistant Steel For Magnesium Reduction Reactor

At present,more than eighty percent of the world’s primary magnesium are produced in our country by the external thermal silicon thermal method.The magnesium reduction reactor is the most important components in the production of magnesium by silicon thermal process,and is also one of the most important components of the cost of the primary magnesium production.The reactor material is mainly austenitic heat-resistant steel,but the austenitic stainless steel used in the reactor has poor performance at high temperature and is easy to produce creep instability and thermal fatigue cracking,resulting in short service life and high cost of magnesium smelting.Therefore,it is of great significance to develop a new type of austenitic heat resistant steel for reducing the smelting cost and improving the economic benefit.This paper added alloy element Nb on the basis of ZG35Cr26Ni8 NRE,focuses on the influence of niobium content on microstructure,mechanical properties,high temperature oxidation resistance,thermal fatigue resistance and high temperature creep resistance of the heat resistant steel used in magnesium reduction reactor;by using optical microscope,X ray diffraction,scanning electron microscopy and energy dispersive spectrometry explores the mechanism of Nb content on the microstructure and properties,the main results are as follows:The matrix structure of the samples without Nb element is austenite + second phase.The austenite grains are coarse and the grain boundaries are distributed with a large number of chained carbides.There are a large number of skeletal carbides distributed in the crystal,the main types of carbides are M23C6 and M7C3,and M mainly is Cr,Fe and Mn.After the addition of Nb element,the matrix structure of the samples is unchanged,but the austenite grain was refined obviously.And the grain size increases with the increase of Nb content.In addition to M23C6 and M7C3,NbC,NbN and Nb2 CN as the second phase appeared in the structure,and the amount of these types second phase increases with the increase of Nb content.However,when the content of Nb reached 1.5%,all kinds of second phase in the microstructure appeared to grow and stick.Under the condition of 1000℃ and 1200℃,the addition of Nb element is disadvantageous to the oxidation resistance.With the increase of Nb content,the oxidation resistance decreases gradually,and with the increase of temperature,the decrease of antioxidative activity is more obvious.After 1000℃×150 hours high temperature oxidation test,the specimen without addition of Nb element is completely antioxidative,and the specimen with addition of Nb element is antioxidative.After 1200℃×150 hours high temperature oxidation test,the specimen without addition of Nb element is weakly antioxidative,and the specimen with addition of Nb element is not antioxidation.Under the condition that the upper limit temperature of thermal fatigue is 1200℃,after 150 cycles,the thermal fatigue resistance of the samples increased first and then decreased with the increase of Nb content.The thermal fatigue crack of the sample without Nb element was the longest,which was 7.321 mm.The thermal fatigue crack of the sample with 1.0% Nb element was the shortest,which was 5.221 mm.After the thermal fatigue test,the M23C6 type carbides in the specimens were grown and coarsened,while the MC type carbides were dispersed in the matrix.Under the condition of external loading of 20 MPa and temperature of 1000℃,the high temperature creep resistance of the sample in 100 h decreased with the increase of Nb content.At the end of loading,the elongation of the sample without Nb was 0.4365%,which was 0.4925% of the sample when added 0.5% Nb.The elongation of sample added 1.0% Nb was 4.726%,without obvious macroscopic elongation of necking.When the test time was 34 hours,the sample added 1.5% Nb was broken,and the fracture surface shows ductile fracture with obvious necking.Summarized the effect and mechanism of niobium on the microstructure and properties of austenitic heat resistant steel used in magnesium reduction reactor,this paper suggests that the addition of niobium should not exceed 0.5%.