Research On Microstructure And Properties Of T/P92 Dissimilar Materials Joints In USC Boilers

The new type T/P92 martensitic steels with excellent resistance of creep, high-temperature corrosion and steam oxidation are being wildly used for superheaters, reheaters and steam pipes. However, due to significant difference between T/P92 and austenitic steels, the applications of T/P92 and austenitic steels such as HR3C, Super304H are currently a bit constrained. In this paper, the in-depth researches on the structure-property relationships of T92/HR3C and T92/Super304H dissimilar materials joints are done. In addition, the affection on mechanical and creep properties of the joint caused by variation of the temperature and stress field across the joint during welding process was also carried out by means of finite element method. The detailed research contents and results are summarized as follows:1. T92/HR3C, T92/Super304H dissimilar materials joints are obtained by gas tungsten arc welding (GTAW) process. For T92/HR3C dissimilar materials joints, Microstructures of T92 base material and its HAZ are both tempered martensite. Average grain size of fine-grained HAZ (FGHAZ) is less than that of T92 base material, and that of coarse-grained HAZ (CGHAZ) is bigger than that of T92 base material. Microstructure of weld metal is coarse dendritic austenite. Microstructures of HR3C base material and it's HAZ are austenite, and average grain size of HR3C HAZ is bigger than that of HR3C base material. For T92/Super304H dissimilar materials joints, Microstructures of T92 base material and its HAZ are both tempered martensite. Average grain size of fine-grained HAZ (FGHAZ) is less than that of T92 base material, and that of coarse-grained HAZ (CGHAZ) is bigger than that of T92 base material. Microstructure of weld metal is coarse dendritic austenite. Microstructures of Super304H base material and it's HAZ are austenite, and average grain size of Super304H HAZ is bigger than that of Super304H base material.2. Electron backscatter diffraction (EBSD) results show that grains in weld metal zone present the oriented feature during the welding process. Grains in weld metal zone grow up respectively from the two interface sides towards the middle of weld metal. However, as for the HAZs of Super304H and T92, the temperature at any time is well distributed and there is no obvious temperature gradient change. This suggests that the orientation distributions of grains in the two HAZs are both random.3. Mechanical properties test results show that both T92/HR3C and T92/Super304H dissimilar materials joints obtained by GTAW process can meet the ASTM standards. Hardness test results suggest that post welding heat treatment (PWHT) can improve the toughness of T92 coarse-grained heat affected zone (CGHAZ). Bending test results show that weld metal has excellent plasticity. The results show that the part of the joints with relatively weak tensile strength is T92 base material, and the decrease of tensile strength in T92 base material is due to PWHT. In addition, impact test results also show that the part of T92/Super304H joint which reveal relatively weak toughness is weld metal, and the weak toughness of weld metal is attributed to its coarse dendritic austenitic structure.4. Creep test results show that with the increase of load stress, the creep rupture time decreases. For stresses(?)140MPa, rupture location is at the T92 base material part. For stresses<140MPa, rupture location is at the T92 coarse-grained HAZ (CGHAZ). When the stress is high ((?)140MPa), the dislocation movement towards the grain boundary is hindered by the precipitates (M23C6, MX) in the grains during the creep process, which results in forming dimples at the interface of precipitate and matrix in these areas. The rupture mode belongs to transcrystalline fracture. When the stress is low (<140MPa), After a long time at 625?, the high energy of grain boundary drives the elements such as Fe, Cr, W, Mo in the grains towards grain boundaries and promotes the growing up of M23C6 particles on the grain boundaries. Furthermore, duo to large size of W and Mo atoms, they are apt to combine with Fe and Cr atoms to form Laves phase on the grain boundaries. Thus, under the effect of long-term low stress, stress concentration is formed at the interface of grown-up precipitates and matrix, which leads to appearance of creep cavities along the grain boundaries. With the time goes on, creep cavities join each other along the grain boundary and lead to intergranular fracture.5. Creep rupture strength is evaluated using double logarithmic plot method, Larson-Miller parameter as well as Manson-Haferd parameter. Service life prediction curves indicate that T92/HR3C dissimilar weld joint could ensure safe service for duration of 105 hours under 30MPa to 40MPa at 625?, Which is in according with the USC steam conditions. 6. Variation of the temperature and stress field across the weld joint during the welding process is carried out by means of finite element analysis software ANSYS. Finite element method (FEM) analysis results reveal that the average temperature value in weld metal zone gradually reduced along the direction where temperature gradient declined and resulted in orientation distribution of weld metal. Stress distribution across the T92/HR3C joint is non-uniform and the largest residual stress is at interface between T92 CGHAZ and weld metal, which results in the failure of T92/HR3C joint under high temperature.