Weldability and Post Weld Heat Treatment of T23 Boiler Grade Steel | Heat Treating

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T23 is a low alloy ferritic steel used for boiler super heater and re-heater tubes and water panels in complex fuelled power plants as they exhibit superior high temperature strength and creep behaviour compared to other low alloy ferritic steels. T23 steel is in bainitic structure which holds greater toughness than tempered martensite and also they possess increased resistance to thermal fatigue. With the help of T23 steel, the weight and piping component of the boiler can be reduced. The microstructure of the weldment without proper Post Weld Heat Treatment (PWHT) was not in equilibrium state which led to the secondary hardening during subsequent exposure at high temperature. So there occurred a significant increase in hardness due to non-tempered weld joints and decrease in toughness. Therefore, Post Weld Heat Treatment for T23 steel is necessary. Using Tungsten Inert Gas (TIG) process, the T23 steel was welded under suitable welding parameters (welding current, voltage and shielding gases). A study was done to know the effect of Post Weld Heat Treatment on T23 steel and also the influence of welding parameters. Hardness of T23 steel was measured. The heat treated specimen is examined through Optical microscope and Scanning Electron Microscope. Finally it was noticed that the Post Weld Heat Treatment has enhanced the mechanical properties as well as the metallurgical arrangement of the T23 steel.
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  International Journal of Advanced Engineering Research and Science (IJAERS) [Vol-4, Issue-2, Feb- 2017] https://dx.doi.org/10.22161/ijaers.4.2.33 ISSN: 2349-6495(P) | 2456-1908(O) www.ijaers.com Page  | 168   Weldability and Post Weld Heat Treatment of T23 Boiler Grade Steel M.Mohandas Pravin kumar, M.Deepak Kumar, V.Karthik    Department of Metallurgical Engineering, Government College of Engineering, Salem, India Abstract   —  T23 is a low alloy ferritic steel used for boiler  super heater and re-heater tubes and water panels in complex fuelled power plants as they exhibit superior high temperature strength and creep behaviour compared to other low alloy ferritic steels. T23 steel is in bainitic  structure which holds greater toughness than tempered martensite and also they possess increased resistance to thermal fatigue. With the help of T23 steel, the weight and  piping component of the boiler can be reduced. The microstructure of the weldment without proper Post Weld  Heat Treatment (PWHT) was not in equilibrium state which led to the secondary hardening during subsequent exposure at high temperature. So there occurred a  significant increase in hardness due to non-tempered weld  joints and decrease in toughness. Therefore, Post Weld  Heat Treatment for T23 steel is necessary. Using Tungsten  Inert Gas (TIG) process, the T23 steel was welded under  suitable welding parameters (welding current, voltage and  shielding gases). A study was done to know the effect of  Post Weld Heat Treatment on T23 steel and also the influence of welding parameters. Hardness of T23 steel was measured. The heat treated specimen is examined through Optical microscope and Scanning Electron  Microscope. Finally it was noticed that the Post Weld  Heat Treatment has enhanced the mechanical properties as well as the metallurgical arrangement of the T23 steel.  Keywords   —   T23steel, TIG process, Alloying elements, Post weld Heat Treatment, SEM analysis.  I.   INTRODUCTION The aim of increasing the power plant efficiency through raising the steam pressure and temperature and also to  provide energy at lowest possible cost seems to be a challenge for the material and plant layout. Thus, the increase in steam parameters acquires the new requirements to the ferritic steel used in the boiler. The most recent requirement in power plant is increased creep rupture strength, which led to the introduction of low-alloy Cr steel in bainitic phase. The low chromium steel grade T23 is the fine material for the components of ultra-supercritical power plant and also acts as the  potential replacement material for ASTM T22 which was in older stages for the power plant. The T23 steel was derived from T22 steel by the suitable addition of alloying elements like Tungsten(W), Vanadium(V), Niobium(Nb) and limiting the Carbon(C) and Molybdenum(Mo) content. This enables the steel to enhance the creep resistance and easier weldability. So, it is suited for water wall panels in ultra-super critical  boilers. They are also used for super heater, reheaters of conventional power plant boilers. The material is capable of being used and exhibits properties of creep resistance and toughness without PWHT in case of thin walled tubes. This paper sets out to know the effect of PWHT for T23  boiler grade steel. In order to investigate the effects, the microstructure using Optical Microscope and the SEM images are plotted. The behaviour of the material at various temperatures is processed for better understanding the nature and responses of the material at the particular temperature. 1.1 COMPOSITION AND PROPERTIES OF NEW STEEL GRADES To enhance the creep-rupture strength, the elements V, Nb and N were added to from stable particles. Also, W was added and simultaneously the Mo content was reduced, to form T23 grade steel. The slow cooling rate of T23 results in tempered bainitic and martensite structure. Due to low carbon content the hardness value is limited to about 350HV 10 . The addition of boron retards the transformation into ferrite. Due to low carbon content it provides a good weldability. It avoids PWHT for thin products. Addition of V, Nb combined with C and N forms carbides, nitrides and carbo-nitrides of MX type and cause a fine precipitation in the matrix, increasing the strength of the new grade material. Further addition of tungsten improved the creep resistance. According to ASTM standard A213 standard specification for seamless ferritic and austenitic alloy steel  boiler, super heater and heat-exchanges tubes: Table: 1.1 Composition Weight Percentage Carbon 0.04-0.10 Manganese 0.10-0.60 Phosphorous (max) 0.030 Sulphur (max) 0.010 Chromium 1.90-2.60  International Journal of Advanced Engineering Research and Science (IJAERS) [Vol-4, Issue-2, Feb- 2017] https://dx.doi.org/10.22161/ijaers.4.2.33 ISSN: 2349-6495(P) | 2456-1908(O) www.ijaers.com Page  | 169   Molybdenum 0.05-0.30 Vanadium 0.20-0.30 Tungsten 1.45-1.75 Columbium 0.02-0.08 Boron 0.0005-0.006  Nitrogen 0.030 Aluminium (max) 0.030 The ferritic alloy steel can be reheated for heat treatment. Heat treatment shall be carried out separately and in addition to heating for hot forming. 1.2 EFFECTS OF ALLOYING ELEMENTS a) Chromium: It enables the better corrosion resistant property. It increases the scaling resistance and also increases tensile strength of the material.  b) Manganese: It improves hot working properties. Up to 2% there is no effect on strength, ductility and toughness and above 2% there is increased yield and tensile strength. It also stabilizes austenitic structure. c) Molybdenum: It increases the creep resistance of the material when subjected to higher temperature. d) Silicon: It increases scaling resistance. e) Sulphur and Phosphorous: It increases the machinability and it increases the ductility of the material. f) Titanium and Columbium: It prevents intergranular corrosion by stabilizing the carbon and also it produces the fine grain size. 1.3 OXIDATION BEHAVIOR OF T23 STEEL Internal steam oxidation is the cause of various problems in power plants, such as the formation of oxide layers reducing the tube thickness and increasing the stress and increase in metal temperature during operating II.   WELDING PARAMETERS FOR T23 BOILER GRADE STEEL The welding employed is Gas Tungsten Arc Welding Process (GTAW) which is done manually. Groove weld with butt joint angle of 167° and root spacing of 2mm is maintained uniformly throughout the process. 2CrWV type solid filler metal is used in this process. Table 2 illustrates the composition of filler metal. The diameter of the filler metal is 2.4mm.Table 3 shows the electrical characteristics and Table 4 shows the composition percentage of shielding gas. The GTAW filler metal allows the welding of thin wall tubes without PWHT. Here the thickness of the material is above 10mm and also the investigation of hardness values in the weld metal is below the hardness number as per ASTM E384 for the material so PWHT of 720°C-760°C is carried out. Table.2.1 Sample Impact Sample Impact Ref No. Energy (J) Ref No. Energy (J) J1 36, 39, 39 J4 31, 30, 27 J2 28, 28, 20 J5 19, 18, 14 J3 37, 35, 36 J6 14, 14, 16 Req. 14J Req. 14J (Average) (Average) III.   TESTING The low alloy bainitic steel T23 is welded with the above welding parameters. The test begun by calculating the tensile strength of the parent metal followed by hardness test, carried out by Vicker’s Hardness Scale. The samples were prepared for examination by means of both Optical microscope and Scanning Electron Microscope (SEM) studies were carried out on the effect of PWHT in the specimen. 3.1 EXPERIMENTAL RESULTS 3.1.1 HARDNESS MEASURMENT. Test Method  –   ASTM E384 Table.3.1 Hardness  –   HV  10 PM HAZ Weld HAZ PM J1 147/149 156/160 167/170 156/158 147/151 J2 176/176 228/232 224/224 228/232 176/176 J3 148/148 147/147 183/181 165/167 148/148 J4 183/181 240/240 228/232 232/230 183/181 J5 183/181 279/274 268/268 279/285 183/181 J6 181/183 274/279 274/274 274/274 181/183 3.2 CHARPY IMPACT TEST  : Test Method - ASTM A370 Longitudinal impact samples of size 2.5 x 10.0 x 55.0 mm, three each from weld region were taken, tested at +20°C for checking the toughness of material and the values are as below. 3.3 MICROSTRUCTURE The samples were polished with simple hand belt grinding  placed with abrasive paper (240,300,400 and 600 grit) on  International Journal of Advanced Engineering Research and Science (IJAERS) [Vol-4, Issue-2, Feb- 2017] https://dx.doi.org/10.22161/ijaers.4.2.33 ISSN: 2349-6495(P) | 2456-1908(O) www.ijaers.com Page  | 170   the top. The specimen is etched with 5% Nital at a time of 30 seconds. Micro examination of the weld joints show a microstructure of bainite in weld, HAZ and parent metal of all samples. Refer Photos 1 to 18. POST WELD HEAT TREATMENT IN ALL ZONES  Fig.1:HAZ+Weld 100X  Fig.2:Parent Metal 100X  Fig 3:Weld 100X  Fig 4:Parent Metal 100X  Fig 5:Weld 100X  Fig. 6:HAZ+Weld 100X  Fig 7:HAZ+Weld 100X  International Journal of Advanced Engineering Research and Science (IJAERS) [Vol-4, Issue-2, Feb- 2017] https://dx.doi.org/10.22161/ijaers.4.2.33 ISSN: 2349-6495(P) | 2456-1908(O) www.ijaers.com Page  | 171    Fig 8:Parent Metal 100X  Fig.9:Weld 100X 3.4 SEM ANALYSIS  Fig.10: WELD METAL ZONE AT 1500X From SEM analysis it was found that the material is free from defect and inclusion. It is also seen that, after PWHT the bainite has uniformly distributed which increases the mechanical and metallurgical features of the T23 material. IV.   OBSERVATION From the hardness test of T23 sample is increased by the effect of Post Weld Heat Treatment (PWHT) and the energy absorption rate of the sample is increased enormously. In Microscopic image the grain arrangement after PWHT seems to be fine grain structure. In the Scanning Electron Microscope it seems that the sample is free from defect and inclusion and also the effect of PWHT increased the material properties. V.   CONCLUSION From the results of tensile test, hardness test, impact test, microstructure and SEM analysis the actual state and the effect of Post Weld Heat Treatment is sorted out. Post Weld Heat Treatment for the material thickness greater than 13mm is necessary to enhance the properties of the T23 steel used in water panels of Boiler. Post Weld Heat Treatment of ferritic steel improves resistance to brittle fracture also it improves the toughness and relieves the residual stresses developed in it. REFERENCES [1]    N.JEYAPRAKASH, R.PRABHAKARAN, G.MURUGESAN, Weldability of T23 Steel using TIG process. (2014) PP.01-04. [2]   CARLOSROBERTOXAVIER, HORACIOGUIMARAESDELGADO JUNIOR, JOSE ADILSON DE CASTRO, Numerical Evaluation of the Weldability of the Low Alloy Ferritic Steels T/P23 and T/P24.(2011) [3]   J.C.VAILLANT, B.VANDENBERGHE, B.HAHN, H.HEUSER, C.JOCHUM, T/P23, 24, 91 and 92:  New grades for advanced coal-fired power plants-Properties and experience.(2008) PP.38-46. [4]   W.BENDICK, J.GABREL, B.HAHN, B.VANDENBERGHE on New Low Alloy Heat Resistant Ferritic Steels T/P23 and T/P24 for power  plant application in International Journal of Pressure Vessels and Piping 84(2007)13-20. [5]   MOHYLA.O and KOUKAL.J presented paper on Influence of PWHT on Secondary Hardening of CrMoV welded joints,(2004) vol.53 Pg.374-378. [6]   K. KUCHAROVA, V. SKLENICKA, M.KVAPILOVA and M.SVOBODA, Creep and Microstructural process in a Low Alloy(2000) 2.25%Cr1.6%W steel.
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