Effect Of Nitrogen Content On Microsturctures And Properties Of 0Cr16Ni5Mo Martensitic Stainless Steel

0Cr16Ni5Mo martensitic stainless steel has been widely used in water conservancy machinery,aerospace structures and other areas because of its high strength,toughness and corrosion resistance.The high chromium content of the steel makes the structure often contain a certain amount of δ ferrite,which will cause fracture during forging.In order to inhibite the content of δ ferrite,the strong austenite forming element nitrogen was added to the the tested steel,and the steels with nitrogen content of 0.008%,0.036%and 0.051%were obtained.The microstructures and properties of test steels were investigated by means of Thermo-Calc、SEM、TEM、XRD and so on.The main conclusions are as follows:(1)The phase transformation temperature was measured,the result shows that adding nitrogen content makes a reduction on austenite transformation temperature and the martensitic transformation temperature.The quenching treatment of the tested steel was also carried out at 850℃ to 1050℃,with the increase of the quenching temperature,the grain size of the tested steel grows.And nitrogen element can effectively suppresses the grain size of the test steel at high temperature.The 8 ferrite area in the tested steels was counted,it was found that the percentage of 8 ferrite area decreased with the increase of nitrogen content.The tensile strength of the material increases with the increase of the N content,which has little effect on the impact toughness of the tempering temperature above 525 ℃,and the test steels are mainly ductile fracture.(2)After normal heat treatment,it was found that there were carbides around 8 ferrite in experimental steels with nitrogen content,which were dissolved in the matrix at the tempering temperature higher than 525 ℃.The impact value various after tempered at 450 ℃,the main reason is that(Cr,Mo)2C carbide exists in the experimental steel with 0.036%nitrogen content,which will cause Cr segregation,that reduce the impact toughness of the steel.The type of the second phase of the tested steels was analyzed,compared with the common without nitrogen steel,besides the carbide the second phase contains the nitride in the experimental steels with nitrogen content.(3)In order to observe the microstructural evolution under the test heat treatment system,the hightemperature metallographic was carried out.It was found that there were segregation in the expterimental steels,and it was especially serious in 0.036%nitrogen containing steel.The volume fraction and morphology of the retained austenite in the quenched and tempered samples of the tested steels were observed.It was found that the retained austenite fraction increased with the increase of the nitrogen content.And the retained austenite morphology in the test steel was mainly film and lump.(4)The experimental steel is subjected to the isothermal compression experiment with deformation temperature of 800～1100℃,deformation temperature of 0.01～10s-1 and true strain of 0.9.It is found that under the same thermal deformation condition,the increase of nitrogen content makes the true flow stress increased.The constitutive equation of the test steel is modeled,and the constitutive equation of the three test steels is obtained by the optimized hyperbolic sine model.The hot working process parameters of the tested steels were determined by the drawing of the hot drawing of the steels.It was found that the increase of nitrogen content was more stringent to the hot working process.