The rapid coarsening of the M_（23）C_6 carbides has been held responsible for the creep fracture in 9-12Cr martensitic heat resistant steels.A commercial P92 steel was subjected to thermal aging at a high temperature...The rapid coarsening of the M_（23）C_6 carbides has been held responsible for the creep fracture in 9-12Cr martensitic heat resistant steels.A commercial P92 steel was subjected to thermal aging at a high temperature of 800°C to investigate the ripening behavior of the M_（23）C_6 carbides.Scanning electron microscopy（SEM）and transmission electron microscopy（TEM）were employed to characterize the microstructure evolution,especially the ripening process of the M_（23）C_6 carbides.The new concept of the effective mean size,dependent on the critical radius,was introduced to correct the measured mean size and then the Ostwald theory was applied to describe the ripening behavior of the M_（23）C_6 carbides.The ripening of the M_（23）C_6 carbides was revealed to be grain boundary diffusion controlled.展开更多
T23 steel(2.25Cr–1Mo–1.6W–0.24V–0.05Nb,in wt.%)has been widely used as water walls in ultra-supercritical(USC)power plants.However,high reheat cracking susceptibility of T23 steel hazarded the safety of USC power ...T23 steel(2.25Cr–1Mo–1.6W–0.24V–0.05Nb,in wt.%)has been widely used as water walls in ultra-supercritical(USC)power plants.However,high reheat cracking susceptibility of T23 steel hazarded the safety of USC power plants.It is aimed to improve the reheat cracking susceptibility of T23 steel from the perspective of modifying chemical composition.Gleeble-3800 thermal simulator was used to simulate the coarse-grained heat-affected zone(CGHAZ)in T23 and modified T23 steels via thermal simulation of welding,and then evaluate the reheat cracking susceptibility in CGHAZ of T23 and modified T23 steels by the isothermal slow strain rate tensile test.The microstructure was systematically investigated by optical microscopy,scanning electron microscopy and transmission electron microscopy.The results show that the reheat cracking susceptibility of modified T23 steel is extremely improved.Compared with T23 steel,there are fewer M23C6 particles at the grain boundaries in modified T23 steel,and the degree of intergranular weakening is smaller.At the same time,not only MX in the grain interiors but also the solid-solutioned C and W in the matrix decrease in modified T23 steel,leading to the decline of intragranular strengthening.In addition,small grain size is beneficial to the improvement of the reheat cracking susceptibility of modified T23 steel.The decreased number of M23C6 at grain boundaries helps to retard the formation and propagation of reheat cracks because of decreased denuded zones.展开更多
A new ferritic creep resistant steel has been developed by eliminating Nb and adding 1.5 mass % Re to a ferritic steel grade T/P23 with the aim of enhancing its mechanical properties at high temperature.Cast ingots of...A new ferritic creep resistant steel has been developed by eliminating Nb and adding 1.5 mass % Re to a ferritic steel grade T/P23 with the aim of enhancing its mechanical properties at high temperature.Cast ingots of both steels, new grade and ASTM T/P 23, were hot rolled at 900℃ and then submitted to a thermal treatment consisting of solubilization at 1050℃ and tempering at 700℃. Tempered bainitic microstructures obtained contain second phases reinforcing carbide particles, mainly M_6C and M_（23）C_6 at the boundaries of both, prior austenite grains and bainitic ferrite laths, as well as MC within the grains. Mechanical properties at temperatures ranging from 540 to 600℃ were studied by strain-ratechange tests in compression at strain rates between 10^（-7） and 10^（-4）s^（-1）. These tests showed high stress exponents（n ≥ 20） and activation energies（Q ≈ 400 k J/mol） for both alloys, which were associated with a dislocation movement mechanism with a strong interaction between dislocations and precipitates. On the other hand, a creep exponent of 5 was derived for the stress dependence of minimum creep rate from conventional-type creep tests at 600℃. Although this stress exponent is usually related to a dislocation climb controlled creep mechanism, remarkable microstructural degradation observed with increasing creep time makes difficult to elucidate the true deformation mechanism controlling creep.展开更多
文摘The rapid coarsening of the M_（23）C_6 carbides has been held responsible for the creep fracture in 9-12Cr martensitic heat resistant steels.A commercial P92 steel was subjected to thermal aging at a high temperature of 800°C to investigate the ripening behavior of the M_（23）C_6 carbides.Scanning electron microscopy（SEM）and transmission electron microscopy（TEM）were employed to characterize the microstructure evolution,especially the ripening process of the M_（23）C_6 carbides.The new concept of the effective mean size,dependent on the critical radius,was introduced to correct the measured mean size and then the Ostwald theory was applied to describe the ripening behavior of the M_（23）C_6 carbides.The ripening of the M_（23）C_6 carbides was revealed to be grain boundary diffusion controlled.
基金National Key Research and Development Program of China(2016YFC0801901).
文摘T23 steel(2.25Cr–1Mo–1.6W–0.24V–0.05Nb,in wt.%)has been widely used as water walls in ultra-supercritical(USC)power plants.However,high reheat cracking susceptibility of T23 steel hazarded the safety of USC power plants.It is aimed to improve the reheat cracking susceptibility of T23 steel from the perspective of modifying chemical composition.Gleeble-3800 thermal simulator was used to simulate the coarse-grained heat-affected zone(CGHAZ)in T23 and modified T23 steels via thermal simulation of welding,and then evaluate the reheat cracking susceptibility in CGHAZ of T23 and modified T23 steels by the isothermal slow strain rate tensile test.The microstructure was systematically investigated by optical microscopy,scanning electron microscopy and transmission electron microscopy.The results show that the reheat cracking susceptibility of modified T23 steel is extremely improved.Compared with T23 steel,there are fewer M23C6 particles at the grain boundaries in modified T23 steel,and the degree of intergranular weakening is smaller.At the same time,not only MX in the grain interiors but also the solid-solutioned C and W in the matrix decrease in modified T23 steel,leading to the decline of intragranular strengthening.In addition,small grain size is beneficial to the improvement of the reheat cracking susceptibility of modified T23 steel.The decreased number of M23C6 at grain boundaries helps to retard the formation and propagation of reheat cracks because of decreased denuded zones.
基金supported by the Spanish Ministry of Economy and Competitiveness(MINECO)under Grant MAT2012-39124,MAT2015-68919,and MAT2016-80875
文摘A new ferritic creep resistant steel has been developed by eliminating Nb and adding 1.5 mass % Re to a ferritic steel grade T/P23 with the aim of enhancing its mechanical properties at high temperature.Cast ingots of both steels, new grade and ASTM T/P 23, were hot rolled at 900℃ and then submitted to a thermal treatment consisting of solubilization at 1050℃ and tempering at 700℃. Tempered bainitic microstructures obtained contain second phases reinforcing carbide particles, mainly M_6C and M_（23）C_6 at the boundaries of both, prior austenite grains and bainitic ferrite laths, as well as MC within the grains. Mechanical properties at temperatures ranging from 540 to 600℃ were studied by strain-ratechange tests in compression at strain rates between 10^（-7） and 10^（-4）s^（-1）. These tests showed high stress exponents（n ≥ 20） and activation energies（Q ≈ 400 k J/mol） for both alloys, which were associated with a dislocation movement mechanism with a strong interaction between dislocations and precipitates. On the other hand, a creep exponent of 5 was derived for the stress dependence of minimum creep rate from conventional-type creep tests at 600℃. Although this stress exponent is usually related to a dislocation climb controlled creep mechanism, remarkable microstructural degradation observed with increasing creep time makes difficult to elucidate the true deformation mechanism controlling creep.
