Effect Of Co,Cr And C On Microstructure And Mechanical Properties Of Modified HR3C During Long-Term Aging

In this thesis,three types of the modified HR3C austenitic heat-resistant steel were selected:the high-Nb modified HR3C with Co addition,high-Nb modified HR3C with two different Cr contents,and the hot-rolled plates of two modified HR3C with different C content.The mechanical properties of modified HR3C were tested by using the universal testing machine,charpy impact testing machine,creep testing machine,microhardness tester,etc.The structural observations were performed using Optical Microscopy(OM),Scanning Electron Microscopy(SEM),Transmission Electron Microscopy(TEM),Selected Area Electron Diffraction(SAED),X-ray Energy Dispersive Spectrometer(EDS),Electron Backscattered Diffraction(EBSD),and other thermodynamic and kinetic calculations.The effects of Co,Cr and C on the high temperature microstructure stability and phase transformation of the alloy at 650 ? and 700 ?were investigated and the structural-property correlation were drawn.Finally,the optimized HR3C compositions were proposed.The results show that Co plays an important role in inhibiting the precipitation of a phase.The kinetic calculations proved that the addition of Co increased the activation energy of a phase precipitation in the alloy during aging.The a phase precipitation activation energy in modified HR3C without Co is 292.78 kJ·mol-1,and which was increased to 368.80 kJ·mol-1 after Co addition.The content of Cr in the modified HR3C is extremely sensitive to the formation of the? phase.Because the precipitates are hard and brittle they are helpful for increasing the room temperature yield strength.However,the growth and coarsening of the ? phase is very rapid,which deteriorated the thermal stability of the modified HR3C.With the Cr content decreases to a critical value(about 21.5%),the a phase will not be precipitated within grain.With the Cr content decreases,the modified HR3C has higher structural stability(without large size TCP phase)at 700 ?,and no a phase is found at 700 ? till 5000 h,and the second phase particles are dispersed and smaller in size.Moreover,with increasing the C content,the M23C6 precipitates along the grain boundary became continuous with a certain width,and various coarse carbides precipitated in the grain.The carbide along the grain boundary is the main cause of the sharp drop in tensile ductility and impact toughness at room temperature,while the carbide at high temperature reduces the intragranular strength,causing the crack to expand along the grain.In addition,increasing the C content results in a decrease in the creep properties of the modified HR3C at high temperatures and low stresses.According to the influence of three alloying elements of Co,Cr and C on the microstructure and properties of modified HR3C,three modified HR3C optimized alloy compositions were proposed,to obtain a higher volume fraction of nanoprecipitation to strengthen the particles,to avoid the precipitation of harmful phases,and to control the M23C6 at the grain boundary,for fabricating a new austenitic heat-resistant steel with high structural stability at 650?700 ?.