| With the emphasis on healthy living and sustainable development,people hope to create more energy-saving and environmentally-friendly materials to reduce environmental damage and meet the needs of economic development.Ferritic stainless steel has good thermal stability,thermal conductivity,corrosion resistance,oxidation resistance and other properties.Reducing the use of Ni makes ferritic stainless steel have good economic and environmental benefits.17CrNbCu steel is a new type of stainless steel which can be used in automobile exhaust system and other fields.In order to evaluate the performance of materials at high temperature,this paper focuses on the mechanical properties of 17CrNbCu steel at high temperature.The 17CrNbCu steel hot-rolled plates were heat-treated at 1100°C for 30 minutes and quenched in water to produce homogeneous structure.The deformation behaviors of 17CrNbCu steel after solution treatment were studied by high-temperature tensile and creep experiments,and the evolution of microstructure was analyzed by OM,SEM,TEM,EDS and other characterization methods.The main conclusions are as follows:In the range of 600°C-900°C,with the increase of deformation temperature,the yield strength,tensile strength and uniform elongation of 17CrNbCu steel decreased,while the fracture elongation increased.This was related to the enhancement of dynamic recovery,atom diffusion and grain boundary migration at high temperature.In the range of 8.33×10-55 s-1-1.67×10-3 s-1,the yield strength,tensile strength,uniform elongation and fracture elongation of 17CrNbCu steel increased with the increase of strain rate.Due to the longer test time under lower strain rate,the size of Nb-rich and Cu-rich precipitates increased,which leaded to the decrease of the fracture elongation of 17CrNbCu steel under lower strain rate.The tensile stress exponent of 17CrNbCu steel at 700?C and 850?C were 11.3 and 7 respectively,and the deformation activation energy was 784.8 kJ mol-1.The mechanism of high temperature tensile deformation was the interaction of precipitation strengthening and dislocation climbing.At 700°C-850°C,15 MPa-90 MPa,the creep curves of 17CrNbCu steel showed typical three-stage creep characteristics.With the decrease of applied stress or temperature,the minimum creep rate decreased and the creep life increased.The creep stress exponent of 17CrNbCu steel were 6-9.7 and apparent activation energy was about 555.9 kJ mol-1 at 700°C-850°C.The high stress exponent and activation energy indicated that the deformation mechanism of creep was the interaction of power-law dislocation creep mechanism and precipitation strengthening mechanism.According to the experimental data,the Larson-Miller equation was established:log=-1.25838×10-4LM+5.13416,which can be used to predict creep life of17CrNbCu steel.17CrNbCu steel produced Cu-rich phase,Laves phase Fe2Nb,carbide NbC and Fe3Nb3C during creep.Precipitate free zones?PFZs?formed around the grain boundaries due to the diffusion of second-phase forming elements near the grain boundaries.As the creep time increased,the size of the precipitates and the PFZs area increased continuously.The applied stress promoted nucleation and growth of precipitates,which was attributed to the stress promoting dislocations movement and increasing the number of dislocations;With the increase of creep temperature,the dynamic recovery was strengthened and the critical nucleation work was increased,which inhibited the nucleation of precipitates.As the increase of temperature was conducive to the diffusion of elements and dislocations movement,it promoted coarsening of the precipitates.The Orowan mechanism played the main strengthening role.The growth of the precipitates reduced the precipitation strengthening effect.The high temperature tensile and creep fracture forms of 17CrNbCu steel were ductile fracture.The microvoids were prone to generate and expand around the precipitates to form cracks,which eventually leads to 17CrNbCu steel fracture. |
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