In order to clarify the characteristics and formation mechanism of the reheat cracking in Ti2AlNb weldments,a series of heat treatment conditions were performed to the circular joints welded by electron beam,and then ...In order to clarify the characteristics and formation mechanism of the reheat cracking in Ti2AlNb weldments,a series of heat treatment conditions were performed to the circular joints welded by electron beam,and then the macrostructures and microstructures were investigated using optical microscopy,scanning electron microscopy,X-ray diffractometry,and transmission electron microscopy.The results show that the reheat cracking occurs primarily along the grain boundaries in the weld when the Ti2AlNb circular welded joints are heated up to about 700℃.During the heat treatment,an almost complete transformation of B2→O happens while the temperature goes up through the O single-phase region.Then,O→B2+O phase transformation occurs primarily along the grain boundaries as the weld metal continues to heat up to the B2+O dual-phase region.Under the high tension stress consisting of welding residual stress and phase transformation stress,reheat cracking occurs at the interface between the B2+O dual-phase layer and the O-phase matrix.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
文摘In order to clarify the characteristics and formation mechanism of the reheat cracking in Ti2AlNb weldments,a series of heat treatment conditions were performed to the circular joints welded by electron beam,and then the macrostructures and microstructures were investigated using optical microscopy,scanning electron microscopy,X-ray diffractometry,and transmission electron microscopy.The results show that the reheat cracking occurs primarily along the grain boundaries in the weld when the Ti2AlNb circular welded joints are heated up to about 700℃.During the heat treatment,an almost complete transformation of B2→O happens while the temperature goes up through the O single-phase region.Then,O→B2+O phase transformation occurs primarily along the grain boundaries as the weld metal continues to heat up to the B2+O dual-phase region.Under the high tension stress consisting of welding residual stress and phase transformation stress,reheat cracking occurs at the interface between the B2+O dual-phase layer and the O-phase matrix.
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.
