| In the background of energy saving and emission reducing,China has established over 100 ultra-supercritical units.The first reheat steam temperature of these units is 600?,taking 90%of coal-fired units of the same type globally.To further increase the thermal efficiency and reduce the energy consumption,the National Energy Administration(NEA)approved a paradigmatic project of developing the world 1st 630? ultra-supercritical coal-fired units.In the project,G115(?)martensitic heat-resistant steel was the only material that be used in high-temperature devices,such as boiler header and main steam pipe.Compared with the existing 9?12%Cr martensitic steel,G115 steel obtained a large increase in creep strength by adding 3.0%solid solution element W and 140 ppm gap solid solution element B.However,the specific effect of W and B elements on the microstructure and properties of G115 steel was still unclear,which hindered the revelation of the strengthening mechanism and further improvement of its performance.To further develop the theoretical system of G115 steels,it was needed to investigate the influence of W and B elements on microstructure and property of G115 steels.The present work focused on the effects of W and B on microstructure and mechanical properties in G115 martensitic heat-resistant steels.By adjusting W content,the main element in Laves phase,and B content which affects high temperature thermoplasticity and coarsening rate of carbides,three G115 steels with different W and B contents were made and systematically investigated,respectively.The effect of W and B on microstructure and mechanical properties was interpreted,which provided theoretical instructions for manufacturing of different types of G115 steels in industry.The aging test of G115 steels with different W content at 650? showed that appropriate reduction of W content in steel could reduce the coarsening rate of Laves phase particles and martensitic laths.It was verified by heat-treatment experiments(650?/10000 h+830?/2 h)that the precipitation and coarsening of Laves phase particles were the main reasons for the rapid decrease of toughness in short-term aging.The creep tests of G115 steels with different W content at 650?/120 MPa showed that the Laves phase particles located at the high angle boundaries were the main reason for induction the crack.The proportion of Laves phase particles in the high angle boundaries was an important detection factor for the creep failure of G115 steels.The aging test of G115 steels with different W content and creep experiment under different stresses showed that the precipitates(M23C6,Laves phase and MX)played an important role in maintaining the stability of the microstructure of the material,which increased the strength by pinning dislocations and laths.With the aging and creep time prolonging,the precipitates gradually coarsened,the pinning force from the precipitates decreased,the martensitic lath gradually widened,and the dislocation density also reduced.When the pinning force generated by the precipitates was smaller than 0.64?0.68 MPa,the bundled martensitic lath structure in the G115 steel would transform into the polygonal subgrain structure.The aging test of G115 steels with different B content at 650? showed that appropriate increase of B content would cause B atoms entering M23C6 carbides to replace part of C atoms.Increasing of B content in the steel would reduce the coarsening rate of M23C6 carbides and martensitic laths.The high temperature thermoplasticity experiment of GI 15 steels with different B content showed that when the G115 steel with high B content was stretched at high temperature 1150?1200 ? and high deformation rate,the B atom tended to produce non-equilibrium segregation along the grain boundary,which was the main cause of plasticity decline. |
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