Effect Of Second-Phase Particles On Mechanical And Oxidation Properties Of High-Mn Heat Resistant Steel

Austenitic heat-resistant steels have good high-temperature oxidation and mechanical properties,and are widely used in the production of boilers,internal combustion engines,heat treatment equipment and other components;traditional austenitic heat-resistant steels have a high content of nickel elements,resulting in high material costs and limiting their applications;substituting manganese for nickel is one of the important directions for the sustainable development of austenitic heat-resistant steels.In this paper,three high-manganese austenitic heat-resistant steels containing（Cr,Fe）₂B,Ti C and Ti B₂particles,respectively,were prepared by endogenous precipitation based on ZG30Cr19Mn12Ni4 austenitic heat-resistant steel;the effects of（Cr,Fe）₂B,Ti C and Ti B₂second phase particles on high manganese heat-resistant steels were investigated by tissue observation,physical phase analysis,performance testing and oxidation weight gain.The effects of（Cr,Fe）₂B,Ti C and Ti B₂second-phase particles on the microstructure,room-temperature mechanical properties,high-temperature mechanical properties and oxidation resistance of high-manganese austenitic heat-resistant steels were investigated by means of tissue observation,property analysis and oxidation weight gain.The results of the study showed that:The microstructure of the matrix steel（C1 steel）consists of austenite and ferrite,while the microstructure of the particle-reinforced steel consists of austenite,ferrite,and second-phase particles,where the second-phase particles are（Cr,Fe）₂B or Ti C or Ti B₂.compared with the matrix steel,the grain size of the particle-reinforced steel is significantly finer.At room temperature,the second phase particles can effectively improve the tensile strength of the material,where the tensile strength of the high manganese steel containing（Cr,Fe）₂B particles reached 909 MPa.At 800°C,the tensile strength of the high manganese steel containing Ti C and Ti B₂particles increased,while the tensile strength of the high manganese steel containing（Cr,Fe）₂B particles decreased by 16.3%relative to that of the base steel,and the elongation of variation ranged from 43.9%to 50.5%.The yield and tensile strengths of the particle-reinforced steels increased at 875°C.The best results were obtained for the high-manganese steels containing Ti C particles,where the yield and tensile strengths increased by 32.3%and 21.5%,respectively,compared with the base steel.However,the elongation of the high manganese steel containing（Cr,Fe）₂B and Ti C particles was lower than that of the Ti B₂particles and the base steel.A new type of austenitic heat-resistant steel was obtained by improving its composition based on C1 steel,whose microstructure consists of austenite,carbide（Cr,Fe）₂₃C₆.After introducing different volume fractions of（Cr,Fe）₂B particles,the microstructure consists of austenite,（Cr,Fe）₂₃C₆and（Cr,Fe）₂B phases.The strength of the material increases with increasing volume fraction,and the toughness gradually decreases.When the B content was 0.4wt%,the yield strength and tensile strength increased by 27.2%and 2.2%,respectively,compared to the base steel.The introduction of second phase particles can significantly improve the oxidation resistance of the material.The oxidation resistance of（Cr,Fe）₂B particle containing steel,Ti B₂particle steel,Ti C particle steel,and matrix steel in descending order of strength,where the oxidation rate constant of high manganese steel containing（Cr,Fe）₂B particles is reduced by 30times compared to that of matrix steel.The main reasons for the excellent oxidation resistance of particle-reinforced steels are:（1）the introduction of second-phase particles changes the internal oxidation phenomenon,while promoting the generation of Si oxide at the oxide film/substrate,improving the adhesion of the oxide film and reducing the risk of cracking of the oxide film;（2）the preferential oxidation of second-phase particles,containing Ti particles will preferentially oxidize to Ti O₂,which acts as a Cr₂O₃nucleation site,and Ti O₂oxide particles play a pegging role on the oxide skin,which is not conducive to shedding;in addition,（Cr,Fe）₂B particles can effectively inhibit the diffusion of Mn elements,hindering the transformation of austenite to ferrite;（3）The second phase particles can effectively reduce the grain size.During the oxidation process,the smaller effective grain size can improve the diffusion flux of metal elements and promote the uniform growth of the oxide film,which in turn improves the antioxidant performance.