Microstructure Control And Mechanical Properties Of CB2 Steel For Ultra-Supercritical Power Generation

As one of the significant measures to achieve the strategic target for energy conservation and emission reduction and ensure the safety of the national energy,our country is heavily committed to developing the ultra-supercritical thermal power plant technology with high efficiency and low pollution in the "Thirteenth Five-Year"development plan.However,the weak performance of the equipment materials largely hinders the advances in the ultra-supercritical technology.Recently,new ferrite heat-resistant steels that involves cobalt and boron constituents represented by CB2 steel have attracted great attention due to the excellent resistance for high temperature creep,and became an ideal alternative to the GX12CrMoWVNbN10-1-1 steel in the complex large-scale castings such as turbine blocks and valve boxes that are widely used under the ultra-supercritical units with a main steam temperature exceeding 625?.At present,the domestically produced CB2 steel castings still have a certain gap with the imported products in both performance and quality.There is a lack of study on the micro structure and mechanical properties of CB2 steel castings in domestic.Particularly,the existence form and action mechanism of cobalt and boron in CB2 steel are still not clear.Therefore,it is unable to offer an effective theoretical guidance for the preparation process of CB2 castings.In this paper,the effects of cobalt and boron on the microstructure and mechanical properties of CB2 steel were systematically studied and the existence form of them was analyzed.The results can be used to direct the optimization of the casting cooling rate and the heat treatment parameters,promoting the mechanical properties of CB2 steel.Meanwhile,the problem of the beneficial effect of solid solution boron and nitrogen on the steel would be canceled by the formation of boron nitride was perfectly resolved.Besides,a novel process that can eliminate the micron-sized boron nitride in CB2 steel and precipitate nano-carbonitride was also proposed.(1)Increasing the casting cooling rate of CB2 steel is contributed to optimize the microstructure and improve the mechanical properties.When the casting cooling rate is increased from 5?8?/min to 50?60?/min,the size of boron nitride in CB2 steel is reduced from 10 ?m to 2 ?m,the width of martensite lath is decreased from 301 nm to 269 nm,the average size of M23C6 is declined from 209 nm to 202 nm,and the content of ? ferrite is lowered from 4%to zero at room temperature.Besides,the tensile strength is increased from 757 MPa to 793 MPa,and the Brinell hardness is raised by HB 248 to HB 252.(2)By adjusting the parameters in the heat treatment process,the micro structure can be optimized and the mechanical properties of CB2 steel can be improved.As the austenitizing temperature increases from 950? to 1200?,the size of M23C6 on the lath boundary and the prior austenite grain boundary decreases and the amount raises.Accordingly,the strength and plasticity of the material suggest a gradual increase.With the rising of the tempering temperature,the size of M23C6 grows and the distribution varies from a single state to a blocking state.The vein-like tempered martensite composed of grain boundary and lath boundary gradually transforms into polygonal ferrite.Moreover,the Brinell hardness and strength drops gradually and the plasticity improves.Overall,a best comprehensive property would be obtained when the austenitizing and tempering temperatures are 1150? and 730?,respectively.(3)The existence of cobalt in CB2 steel suggests a solid solution form.When the cobalt content exceeds 1.5 wt%,the presence of ?-ferrite can be completely suppressed.Continuing increase the cobalt content,the tensile strength of the experimental steel enhances from 765 MPa to 836 MPa.The boron in CB2 steel is almost presented in the form of irregular boron nitride.As the content of boron is added from 0.01 wt%to 0.03 wt%,the size of boron nitride increases from 2 ?m to 8 ?m,and the shape changes from an irregular,sharp-edged block to an onion-like shape accumulated from multiple small pieces.The boundary condition to induce the dissolution of boron nitride is above 1150? with a holding time of at least 1 h.During the dissolution process,the spheroidization of the outer surface is conducted with a simultaneously internal hollowing.Upon the hollow sphere is rapidly formed,the wall of the sphere is thinner and thinner,and fully dissolved at last.(4)The segregation of boron drives the preferential formation of boron nitride in the molten steel and suppresses the precipitation of niobium and vitriol.During the dissolution and heat preservation process,the segregation of boron atoms at the austenite grain boundaries promotes the dissolution of boron nitride in the CB2 steel.The niobium which is inhibited from being precipitated is gradually migrated around the boron nitride and then combined with nitrogen atom by the influence of the crystal distortion energy.Due to the equilibrium concentration of niobium is significantly larger than its equilibrium solute concentration in the austenite temperature range,the free energy of the formation of niobium nitride is lower than that of boron nitride.Therefore,boron nitride in the steel will no longer be precipitated after the dissolution,instead of the precipitation of niobium carbonitride.When tempered at 730?,boron at the grain boundary participates in the precipitation of carbides,and are mainly revealed as boron-containing M23(C,B)6.(5)A new process to eliminate the micro-sized boron nitride and produce nano-carbonitride nitride in CB2 steel is developed by heating the steel above 1200? with a holding time of at least 2 h,followed by a rapid cooling to 900?950? with a holding time of 30?60 min via water cooling to room temperature.By virtue of the new process,the precipitation of 30?40 nm niobium carbonitride is enabled in the matrix.(6)After the boron nitride is dissolved,the niobium atoms in the matrix and the nitrogen atoms that dissolved and released by the boron nitride are presented in a mutually enriched state around the boron nitride when the steel is air-cooled to room temperature.At the ambient temperature,niobium carbonitride suggests nitriding nucleation and growth on ferrite dislocation lines,forming acicular sample precipitation varying from 100 nm to 500 nm.When conducted by a rapid cooling at 1000? with an isothermal time of 1 h,nucleation and growth occur on the austenite dislocation line and a cubic precipitation of 80 nm to 150 nm is conducted.(7)Holding the isothermal time with 90 min,the average size of niobium carbonitride increases from 44 nm to 106 nm,and the shape changes from cube to strip as the temperature increases from 900? to 1000?.At an isothermal temperature of 900?,it is able to precipitate the sample with a smallest size and uniform distribution.When increase the time from 10 min to 120 min,the average size grows from 26 nm to 44 nm.Besides,the precipitated niobium carbonitride reveals a polygonal structure,and the number increases along with the addition of isothermal time.The growth mechanism of the precipitated niobium carbonitride can be classified into two kinds.For the one,one corner of the polygon begins to passivate and eventually forms to be an edge.For the other,owing to the low content of niobium and nitride in the front edge of the growth interface,the demand for simultaneous growth of large growth surfaces cannot be processed,and the center of the large growth surface is protruded and splits into two small growth surfaces.