The advanced ultra-supercritical (A-USC) power generation system is expected to become the next-generation base-load power station in Japan. Dissimilar weld joints between high-Cr heat-resistant steels and nickel-based alloys with a nickel-based filler metal (Alloy 82) will need to be adopted for this purpose. However, interfacial failure between the steels and weld metal has been observed under high-temperature creep conditions. Fractography and microstructure observations showed the failure initiated in a brittle manner by an oxide notch at the bottom of the U-groove. The fracture then proceeded along the bond line in a ductile manner with shallow dimples, where micro-Vickers hardness tests showed remarkable softening in the steel next to the bond line. In addition, the steel showed a much larger total elongation and reduction of area than the weld metal at low stresses under long-term creep conditions, leading to mismatch deformation at the interface. According to the results, it can be concluded that the interfacial failure between the 9Cr steels and Alloy 82 weld metal is initiated by an oxide notch and promoted by softening and the difference in the plasticity of the steels and weld metal.

Prompt phase transformations make grains in the heat-affected zone (HAZ) smaller during welding of 9% chromium (9Cr) heat-resistant steels leading to premature failure under creep conditions, which is well known as a type IV fracture. Because the type IV fracture shortens the creep lifetime of the steels, suppressing the fracture is an urgent task in the energy industry. The present study shows that boron addition and nitrogen reduction inhibit grain refinement after welding because of a change in the morphology of the precipitate at prior austenite grain boundaries. In conventional 9Cr steel (ASME Gr. 92 steel), a high amount of MX was unable to pin interface migration of the phase transformation and generated fine grains in the HAZ. In the new B-added steels, B-stabilized M23C6 became the dominant precipitate and showed a larger pinning effect of the phase transformation than MX, which resulted in coarse grains in the HAZ. This suggests that designing stabilized M23C6 forms a superior welded microstructure and results in a longer creep lifetime of 9Cr steels.

Modified 9Cr-1Mo steel (ASME Grade 91 steel) is used as a key structural material for boiler components in ultra-supercritical (USC) thermal power plants at approximately 873 K (600 °C). The creep strength of welded joints of this steel decreases as a result of Type IV creep cracking that forms in the heat-affected zone (HAZ) under long-term use at high temperatures. The current article aims to elucidate the damage processes and microstructural degradations that take place in the HAZ of these welded joints. Long-term creep tests for base metal, simulated HAZ, and welded joints were conducted at 823 K, 873 K, and 923 K (550 °C, 600 °C, and 650 °C). Furthermore, creep tests of thick welded joint specimens were interrupted at several time steps at 873 K (600 °C) and 90 MPa, after which the distribution and evolution of creep damage inside the plates were measured quantitatively. It was found that creep voids are initiated in the early stages (0.2 of life) of creep rupture life, which coalesce to form a crack at a later stage (0.8 of life). In a fine-grained HAZ, creep damage is concentrated chiefly in an area approximately 20 pct below the surface of the plate. The experimental creep damage distributions coincide closely with the computed results obtained by damage mechanics analysis using the creep properties of a simulated fine-grained HAZ. Both the concentration of creep strain and the high multiaxial stress conditions in the fine-grained HAZ influence the distribution of Type IV creep damage.

Mod.9Cr–1Mo steel has been used for boiler components in ultra-supercritical (USC) thermal power plants. The creep strength of welded joint of this steel decreases due to the formation of Type IV cracking in heat affected zone (HAZ) at higher temperatures. The present paper aims to clarify the damage processes and mechanisms of the welded joint for Mod.9Cr–1Mo steel. Long-term creep tests of base metal, welded joint and simulated fine- grained HAZ were conducted at 550, 600 and 650 °C. Creep tests using thick plate welded joint specimen were interrupted at several time steps, and evolutions and distributions of creep damages were measured quantitatively using laser microscope. It is found that creep voids initiate at early stage of creep life (0.2 of life), the number of creep voids increases until 0.7 of life, and then voids coalesced into the macro crack at the later stage of life (0.8 of life). Creep damages concentrate mostly at a quarter depths of the plate thickness within the fine-grained HAZ of the present welded joint. The experimental creep damage distributions were compared with the computed results by using the FEM analysis. Both creep strain concentration and high stress triaxiality in fine-grained HAZ of welded joint are considered to accelerate the creep void formation and growth.