Influnence Of P Addition On Microstructure And Propertiesof Cast Alloy K984G

GH984 is a resistant Fe-Ni based wrought superalloy. It has excellent rupture stress strength, oxidation resistance, organizational stability and processing performance. A new type Ni-Fe-based superalloy K984 G is developed on the basis of superalloy GH984. The alloy will be used as steam turbine and other large cylinder and valve body castings superalloy candidate materials in 700 oC advance ultra-supercritical(A-USC) power plants. The influence of P on the microstructure(including segregation, grain size, grain boundary character,etc.) and mechanical properties of the organization is not clear. To confirm the influence of P element in the new type Ni-Fe-based casting superalloy, the microstructure and mechanical properties of the new Ni-Fe-based superalloy by adding P into GH984 are studied. These may provide experimental evidence and theoretical basis for component optimization and elements designed.The main results of this paper are as follows:The role of P element on organizational characteristics and mechanical properties of steam turbine cylinder and valve body castings which is uesd in a 700 oC advanced ultra-supercritical(A-USC) power plants is studied. The results show that: the P element adds dendrite structure and promotes the degree of element segregation. After standard heat treatment, the main phase in the alloy is γ’ precipitated, MC, M23C6 and Ti(C,N). After adding P, the type of precipitation phase remains, while the M23C6 along the grain boundary increases significantly. Tensile strength of the alloy have no change. However, stress rupture life and plasticity at 700oC/400 MPa reduce significantly, which is attributed to the dendrite coarsening and segregation promotion which reduce strength between dendrite and grain boundaries.The oxidation process of alloy K984G-1 and alloy K984G-2 at 800oC、850oC and 900 oC mainly consists of two parts: The first part of the oxidation kinetic law deviates from the parabolic law, which may be related to high levels of Cr and its rapid diffusion in the initial oxidation stage. In this stage, the oxidation behavior is controlled by the diffusion of Cr3+ through the film of Cr2O3; the second part of oxidation kinetic law is more in line with the cube law. After adding P, the oxidation rate increases significantly, this is due to that P is an element which promotes the segregation of Cr to the dendrite. And then the distribution of the main elements in the surface oxide film is more inhomogeneous and the oxidation weight gains significantly. Furthermore, after adding P, the oxidation activation energy increase which reduce oxidation resistance.After aging at 650oC、700oC、725oC and 750 oC for 500h、1000h、3000h and 5000 h, respectively. There is no significant difference between alloy K984G-1 and alloy K984G-2. The main precipitates in the alloy are γ ’phase, Ti(C,N) carbide, MC, M23C6,debris type M23C6 and needle-like harmful σ phase.