High Temperature Strengthening Mechanism And Thermal Stability Of Laves Phase And M₂₃C₆ In Ferritic Steel

Ferritic heat-resistant steels are widely used in ultra-supercritical thermal power plants for their excellent high-temperature oxidation resistance,corrosion resistance and creep resistance,as well as in other critical components of high-temperature equipment,such as steam pipes and high-temperature super（re）heaters.Ferritic heat-resistant steels are subject to"tissue degradation"during service,which leads to a reduction in the high-temperature strength and life of the heat-resistant steel.In this thesis,two types of ferritic-based steels,Laves phase strengthened and M₂₃C₆strengthened,were designed and prepared,and their high-temperature creep strengthening mechanisms and thermal stability were comparatively studied to provide new ideas for two-phase strengthening of heat-resistant steels.The main research contents and conclusions are as follows:（1）Two Laves phase reinforced alloys were designed and prepared:alloy S1（Fe₂Nb type+ferrite）and alloy S2（Fe₂W type+ferrite）,and the high-temperature creep properties of the two alloys and the high-temperature creep strengthening mechanisms of the Laves phases of different compositions in the ferrite matrix were revealed by high-temperature creep tests,and it was concluded that the creep strength of alloy S1 was better than that of alloy S2;the true stress index of both alloys at650°C is 5,and the true creep activation energies are 372.47 and 365.82 k J/mol,respectively;the dominant high-temperature creep mechanism is dislocation climbing.（2）By studying the microstructure of two different Laves phase strengthened heat-resistant steels,it can be concluded that:the precipitation distribution of the two different Laves phases are mainly concentrated on the grain boundaries,with a small amount of intracrystalline distribution;the grain size of alloy S1 is smaller than that of alloy S2;after high temperature creep,the dislocation density in alloy S1 is much higher than that in S2;a large number of dislocation lines in alloy S1 are entangled together to form a dislocation wall,while most of the dislocation lines in alloy S2 are straight and much less dense.（3）By analyzing the structural characteristics of the interfaces between the precipitated Laves phases（Fe₂Nb and Fe₂W types）and the ferrite matrix after high temperature creep of the two alloys,it can be concluded that the atomic interface mismatch between（011）_α-Feand（033 1）_Fe2Nband（011 1）_Fe2Win alloy S1 and alloy S2are 1.6%and 1.9%,respectively;the Fe₂Nb type Laves phase in may be more suitable as a second phase strengthening phase in the ferrite matrix.（4）A M₂₃C₆-reinforced alloy was designed and prepared,and based on the tissue analysis,it was concluded that M₂₃C₆is mainly distributed at the grain boundaries.The dislocation density in the alloy is high and dislocation pinning exists.the orientation relationship between M₂₃C₆and the matrix phaseα-Fe is:[001]_α-Fe//[011]_M23C6,（020）_α-Fe//（711）_M23C6;the atomic interface mismatch between（020）_α-Feand（711）_M23C6is 20%.（5）By comparing and analyzing the tissue characteristics and interfacial properties with the matrix of ferrite based steel strengthened by Laves phase and M₂₃C₆,it is concluded that:in ferrite base,the strengthening effect and thermal stability of Laves phase are better than M₂₃C₆;Fe2Nb type Laves phase is more suitable as the strengthening phase of ferrite based steel.