Abstract
Creep rupture tests of 9Cr-3W-3Co steel were conducted in the range of 120 to 200 MPa at 650 °C. The influence of stress on microstructure evolution was investigated in detail. In the high stress regime, a large density of dislocation was generated and induced precipitation of fine and dispersive particles. However, at lower stresses, a transformation from martensite laths to large size subgrains and a coarsening of precipitates took place due to significant recovery and loss of pinning effect during long term exposure. Thermodynamic results revealed decreasing tungsten content effectively retarded the coarsening behavior of M23C6 and Laves phase, hence further improvement of creep rupture time was achieved experimentally.
Highlights
In the ultra-supercritical (USC) coal-fired power plants, 9–12% Cr heat resistant steels are widely used as structural materials for boilers, main steam pipes and tubes [1]
The substructure of these steels consists of prior austenite grains (PAGs), tempered martensite laths, high-density dislocations and dispersive second-phase particles
MX particles are mainly formed within grains during tempering, while M23 C6 and Laves are mostly precipitated along grain or subgrain boundaries during subsequent aging or creep
Summary
In the ultra-supercritical (USC) coal-fired power plants, 9–12% Cr heat resistant steels are widely used as structural materials for boilers, main steam pipes and tubes [1]. 9–12% Cr steels is due to the high microstructure stability at elevated temperatures [2]. The substructure of these steels consists of prior austenite grains (PAGs), tempered martensite laths, high-density dislocations and dispersive second-phase particles. In the case of no transformation to Z phase, MX particles exhibit great coarsening-resistance and the pinning effect on dislocations by MX could be well maintained during high temperature exposure. At the initial stage of creep, fine and dispersive M23 C6 and Laves phase particles contribute high creep-resistance by exerting great Zener drag force on subgrain boundaries, whereas the high growing rate of these precipitates degrades this effect gradually [5,6,7]. Recent works [8,9,10] suggest that the coarsening behavior of
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