Investigation On Heat Treatment,microstructure And Properties Of 9Ni Steel For Low Temperature Impeller

Hot rolled 9Ni steel plate has been widely used in cryogenic industry for its excellent low-temperature toughness.In order to expand its application in fan impeller used in low temperature,the microstructure and mechanical properties of 9Ni steel forgings after different heat treatment,especially the thermal stability and deformation induced phase transformation of reversed austenite,have been investigated.The main content and conclusions of this work were:1.The phase constitution and transformation behavior of 9 Ni steel.The equilibrium phase constitution and the influence of main alloy elements have been analyzed by the thermodynamic calculation software.It is found that the 9Ni steel transforms to austenite rather than delta ferrite after solidification.And the C and Ni contents cannot change the phase constitution of the materials.In addition,the equilibrium phase transition point and the continuous cooling transformation phase diagram of 9Ni steel are measured by the method of thermal expansion.The microstructure of 9Ni steel after quenching with different cooling rates were observed by OM and SEM.2.The heat treatment stability of reversed austenite of 9 Ni steel.The influences of heat treatment process parameters on microstructure and mechanical properties were studied.According to the experimental results,an optimized heat treatment technology including 790±10? water cooling + 670±10? furnace cooling + 570±10? water cooling has been designed.After heat treatment,an excellent toughness at ?196? combinating with a good strength and ductility of 9Ni steel has been achieved.The stability of austenite after different heat treatment was studied by cryogenic experiments.It was found that little austenitic obtained by quenching + lamellarizing + tempering heat treatment changed to martensite at ?150?.However,the austenite obtained by quenching + tempering transformed to martensite significantly during the cryogenic treatment.3.In situ synchrotron x-ray diffraction investigation on the tensile properties of 9Ni steel.In order to clarify the relationship between the strength and the reversed austenite in the 9Ni steel after different temperature tempering,a systematic investigation on the amount of reversed austenite,deformation induced phase transformation(DIPT)of reversed austenite and its influence on the mechanical properties of 9Ni steel has been carried out by dilatometer,XRD,in situ synchrotron high-energy X-ray diffraction and TEM.The experimental results indicated that the amount of reversed austenite showed a parabolic trend with increase in tempering temperature.And the DIPT of reversed austenite occurred after the material yielding during the uniaxial tension test.