Abstract
The effect of long-term aging for 103 to 3.9·104 h at 650 °C on the microstructure, dispersion of secondary phases, and tensile strength properties was studied in a low-nitrogen 10%Cr martensitic steel with 3%Co and 0.008%B additives. Tensile tests of small specimens cut from grip portions of creep tested specimens were carried out at ambient and elevated (650 °C) temperatures. An increase in the yield stress and ultimate tensile strength after 10,000 h of aging was revealed to be associated with the precipitation of V-rich MX carbonitrides, which compensated for the coarsening of the boundary particles and depletion of W and Mo solutes from the ferritic matrix. The effect of changes of the microstructure and dispersion of secondary phases on strengthening of the steel is discussed.
Highlights
High-Cr ferritic/martensitic steels remain the major part of the materials used for components of fossil power plants operable at ultra-supercritical steam parameters [1]
An increase in the yield stress and ultimate tensile strength after 10,000 h of aging was revealed to be associated with the precipitation of V-rich MX carbonitrides, which compensated for the coarsening of the boundary particles and depletion of W and Mo solutes from the ferritic matrix
The main factor providing high creep resistant of these steels is the stability of the tempered martensite lath structure (TMLS), which consists of prior austenite grains, packets, blocks and laths with a high dislocation density in the lath interiors [2]
Summary
High-Cr ferritic/martensitic steels remain the major part of the materials used for components of fossil power plants operable at ultra-supercritical steam parameters [1]. This fact determines further improvement of their creep strength. The main factor providing high creep resistant of these steels is the stability of the tempered martensite lath structure (TMLS), which consists of prior austenite grains, packets, blocks and laths with a high dislocation density in the lath interiors [2]. The 9-11%Cr martensitic steels recently developed in accordance with the new approach by the increasing the B content to approximately 0.01wt% and decreasing the N content to approximately thousandths of a percent, demonstrate high creep resistant at an elevated temperature of 650 C [3,4]. The role of precipitates of secondary phases (M23C6, MX, Laves phase) in the high stability of TMLS under creep condition is the subject of research interest
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