Type IV Creep Damage Behavior in Gr.91 Steel Welded Joints |
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Authors: | Hiromichi Hongo Masaaki Tabuchi Takashi Watanabe |
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Affiliation: | (1) Materials Information Station, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan;(2) Creep Resistant Materials Group, Materials Reliability Unit, National Institute for Materials Science, Tsukuba, Japan; |
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Abstract: | 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. |
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