Preparation And Corrosion Evaluation Of High Nitrogen 9Cr Martensitic Heat Resistant Steels

Excellent corrosion resistance and high temperature oxidation resistance are the important foundation to ensure the long-term stable service of new heat-resistant steel under high temperature and high pressure.Based on the recent studies of 9Cr martensitic heat-resistant steel,four novel high N heat-resistant steels were designed and prepared: experimental steel A(0.15 wt.% N),experimental steel B(0.25 wt.% N),experimental steel C(0.27 wt.% N)and experimental steel D(0.30 wt.% N)(Among them,the experimental steel D-1 has no δ Ferrite;The experimental steel D-2 contains14.95 vol.% δ ferrite)were heat treated by different processes.OM,XRD,AFM,SEM-EDS,Raman spectrum characterization,electrochemical test,immersion weight loss test and oxidation weight gain test were used to study the effects of δ ferrite content and N content on the corrosion resistance and high temperature oxidation resistance of high nitrogen martensitic heat resistant steel.The oxidation resistance grades of the new high nitrogen martensitic heat resistant steel at different temperatures(650℃,750℃ and 850℃)were evaluated to provide theoretical support for the subsequent industrialization.The main conclusions of this paper are as follows:There is no significant effect on the corrosion resistance of high nitrogen martensitic heat resistant steel with different δ ferrite content,which is due to the uniform distribution of alloying elements Ni,Cr,Mo and Mn in martensite and δferrite.Different δ ferrite content has no significant difference on the oxidation resistance of high nitrogen martensitic heat resistant steel at 650℃ and 750℃.However,the higher δ ferrite content has a weak adverse effect on the oxidation resistance at 850℃,which is due to the lower oxidation resistance of δ ferrite at higher temperature than that of martensite with a small amount of N.The results of polarization curve test,electrochemical impedance spectroscopy test and immersion test of high nitrogen martensitic heat resistant steel with different nitrogen content in 3.5 wt.% Na Cl solution show that the corrosion resistance of high nitrogen martensitic heat resistant steel increases with the increase of nitrogen content(0.15 wt.% to 0.27 wt.%);When the nitrogen content exceeds 0.27 wt.%(0.30 wt.%N),the corrosion resistance of high nitrogen martensitic heat resistant steel will be reduced.The high temperature oxidation resistance of high nitrogen martensitic heat resistant steel increases with the increase of N content,which is due to the consumption of Cr,V and other elements in the matrix due to the formation of nitride.In the meantime,the uniform formation of Cr,Fe rich lamellar oxides is promoted,and the nucleation and growth of granular oxides rich in Cr,V,W and Mn,which are not conducive to the formation of compact oxide film,are inhibited.In addition,four kinds of high nitrogen martensitic heat-resistant steels belong to complete oxidation resistance at 650℃ and 750℃,which is better than commercial heat-resistant steel CB2 at 650℃ and similar to CB2 at 750℃.Experimental steel D-1belongs to complete oxidation resistance at 850℃,which is similar to CB2 at 850℃.