Study On Microstructure And Property Of The Welding Joint Of T92 Steel Tube

At present,power station unit with large capacity and high steam parameters is an effective way to increase thermal efficiency of the fuel and reduce the release of CO₂ emission,but increased operating parameters,especially the steam temperature, presents a higher demand for the heat resistance steels applied for power station boiler. When used for the high temperature components of superheater and reheater,the austenitic steels such as TP304 and TP347 can provide high creep strength at elevated temperature,but they do not meet the demands of power station unit for operating in safety and high efficiency status well because of the low thermal conductivity coefficient,high stress corrosion sensitivity and high thermal expansion coefficient. Therefore,it is one of key technologies to develop the new heat resistance steels for manufacturing high efficiency and clean power station equipmentsBased on T91 steel(9Cr1Mo-VNb),the newest generation of ferritic heat resistance steel,T92 steel(9Cr0.5Mo1.8W-VNb),was developed through decreasing Mo to 0.5 percent and increasing W to 1.8 percent as well as being manufactured with thermal mechanical control process(TMCP).It is the promising candidate for the superheater and reheater with steam temperature of 580～600℃and metal wall temperature of 600-620℃in new generation of supercritical and ultra supercritical power station boiler.Based on partial statistics,the number of USC unit under or prepared for construction is near to 50 all over our country.These units cover two levels of 600MW and 1000MW with the main steam inlet pressure of 25～26.5MPa and the temperature of the main steam & reheating steam of 600/600℃,whose pressure and temperature are higher than their counterparts in Japan and Europe respectively. There is no similar unit can be compared in the word when we select the materials for piping and tubing.Meanwhile,study on T92 steel and its welding in our country lags behind that in USA,Europe and Japan further,the technical datas mainly depend on the technical literatures and the supplier of steel and welding material,which to a great extent restricts the designing,manufacturing and fabricating of power unit as well as technical- supervising in our country.The welding and operating practice of 9-12Cr%ferritic steel indicates that bad toughness in welding joint is the predominant difficulty when they were welded. Because T92 steel has been developed only few years,in order to meet the urgent desire for constructing USC unit,at present study on T92 welding mainly focuses on engineering application fields.Study on welding materials and welding process also aim only at reaching the lowest toughness index specified by ASME standard.There is no comprehensive theory to explain the worse toughness in T92 welding joint although it is very important to clearly definite the brittle mechanism of T92 welding joint for developing welding materials and formulating welding process.The paper made a systematic study on the fundamental microstructure of T92 base metal,properties & microstmctures in HAZ and WM,aiming at finding out the reasons for brittlement in T92 welding joint and giving theoretical basis for formulating welding process as well as giving technical supports for manufacturing, fabricating and in safety running of USC unit in our country.It is beneficial to clear the microstructure of base metal for comprehending accurately the changing rule of microstructure and property of HAZ under welding thermal cycles.The results of this paper indicate that microstructure of experimental T92 steel comprises lath-like tampered martensite,a small amount ofα'-phase,long and slice-likeε-martensite,dispersed carbides(M₂₃C₆)and carbonitrides(MX). Tempered martensite lath is Fe-Cr solid solution with the cubic lattice,element W and Mo function as solid solution strengthening elements.There are two shapes of precipitates between and inside martensite laths,one is short stick-like M₂₃C₆ carbide rich in Fe and Cr,the other is dot-like MX carbonitride rich in V and Nb with very small amount.The thin slice phase between martensite laths is s-martensite with the hexagonal lattice whose lattice parameters isα=2.5588 A(1A=0.1 nm)and c=4.1237 A and sub-structure is twin crystal.By means of welding thermal simulation,the paper made studies on the influences of welding energies(E)on the microstructure and toughness of the coarse grain zone in HAZ,the microstructures and toughness of different zone in HAZ under the same welding energy,local brittlement zone derived from the second thermal cycle and the evolution of HAZ microstructure under the welding thermal cycle. The results show that the peak temperature(T_max)of thermal recycle curve influences microstructure and property of HAZ seriously.At the t_8/5of 10s when the peak temperature(T_max)is lower than A_C1,the main transformation is that martensites merge into ferritic phase,accompanied with the micro-hardness and impact toughness near to base metal.When T_maxexceeds A_C1,with the peak temperature increasing M-A appears in HAZ and micro-hardness increases and impact toughness decreases with E= 38J at T_maxof 1350℃.Therefore the core study on HAZ should be concentrated on the coarse zone of HAZ with the lowest impact energy.The t_8/5can influence microstructure and property of HAZ greatly.With the t_8/5 increasing the micro-hardness of HAZ at 1350℃rises at the beginning and then decreases,whereas the impact toughness presents reverse trend.With different t_8/5the distribution and shape of M-A in HAZ of 1350℃is different,and the distribution and shape of M-A greatly influence the impact toughness of HAZ.When the t_8/5is 7s,the impact energy is 50J with dispersed M-A.When the t_8/5is 40s chain-like M-A lowers the impact toughness of HAZ seriously.The local brittlement zone appears in the coarse grain zone of HAZ when the second thermal cycle is performed to the coarse grain zone with T_maxnear to A_C1of T92 steel.Post weld heat treatment(PWHT)can raise the toughness of HAZ dramatically. By PWHT of 760℃×60mins impact energy of HAZ rises greatly with M-A transforming toα-phase and M₂₃C₆ carbide.More continuous M₂₃C₆ carbides result in lower toughness in coarse zone(T_max=1350℃)than other zones in HAZ.Worsened impact toughness in weld metal(WM),especially in weld metal by shield metal arc welding(SMAW)is a general problem when welding ferritic heat resistance steel manufactured by TCMP process.Based on transmission electron microscope(TEM)the relationship between microstructure and impact toughness in weld metal obtained with different welding methods and PWHT parameters was studied.It is concluded that the impact toughness of as-welded WM is much lower than BM's.The impact toughness of WM by TIG is higher than that by SMAW under the same welding heat input and PWHT parameter.It is mainly M-A in WM by SMAW to result in its impact toughness very low.With welding heat input reduced amount of M-A decreases and its distribution status changes as well,consequently the impact toughness of WM improves.PWHT can improve the toughness of WM dramatically.M-A decomposes intoα'-phase and M₂₃C₆ carbide during PWHT,which result in WM's toughness increasing.After PWHT of 760℃×x60min the impact toughness of WM by TIG is near to BM's,and will rise further with holding time prolonged.In WM of SMAW M-A decomposes into Lath-like carbide distributed between bainite-ferrit boundaries with high temper resistance.After PWHT of 760℃×120min impact toughness of WM rises obviously,but chain-like carbide make impact toughness of WM by SMAW still much lower than that of BM.In order to meet the urgent desire for welding procedure in engineering,two kinds of welding methods of TIG and TIG for root weld pass plus SMAW for the other weld passes were employed in welding procedure qualification for two specifications of T92 steel tubes ofφ48.2×7.5mm andφ50.8×8.8mm.The mechanical properties and microstructures of welding joints reached the basic requirements regulated by "Welding procedure specification DL/T868-2004".The welding process with consideration of specific conditions in construction field was successfully applied to weld 1662 joints of T92 steel of 1000MW USC power unit.