Mechanical properties of T23 steel welded joints without post-weld heat treatment for fossil fired boilers
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Huijun Li Faculty of Engineering and Information Sciences, University of Wollongong, NSW 2522, Australia
Chao Yang and Qing-xu Yang Jiangsu Frontier Electric Technology Co., Ltd., Nanjing 211102, China (Received 19 June 2016; accepted 24 October 2016)
Microstructures of new heat-resistant steel grade T23 welded joint without PWHT and its corresponding mechanical properties including creep were investigated to clarify its premature failure mechanisms in the large water wall panel of the advanced power plant boiler. The results show that the T23 steel GTAW welded joint in a wall thickness of 6.5 mm without PWHT exhibits high tensile strength, good ductility, and sufficient impact toughness, while the hardness of the WM is higher than the maximum permitted value of 350 HV due to the large amount of un-tempered martensite formed during the cooling process of welding. This WM in as-welded condition has higher creep rupture strength but poorer rupture ductility than the tempered BM. Poor rupture ductility taken place in the WM results from inter-granular cracking during creep exposure and is not related to the second hardening because no hardness rise occurs in the fractured WM compared with as-welded condition. The paper does not specifically investigate the effect of service exposure but simulates the failure of WM by a creep test. The main point is that the WM has low creep ductility, especially at a stress concentration.
I. INTRODUCTION
Construction of ultra super critical boilers (USCB) requires materials with higher creep strength at lower possible cost, which has led to the development of low alloy heat-resistant ferritic steels. The new grade steel T23 (7CrWVMoNb9-6) has been developed on the basis of conventional 2.25Cr–1Mo steel (T22) by strengthening the material through alloying with vanadium and niobium act as precipitation hardening elements as well as with tungsten to enhance solid solution hardening.1,2 As a result, the allowable stress of T23 steel at 853 K is approximately 1.8 times higher than that of T22 steel.3 Simultaneously, carbon contents in T23 steel is reduced to below 0.10 wt%, which provides a good weldability. It allows the welding of thin-walled tubes with d , 10 mm without preheating and post-weld heat treatment (PWHT). 4 Membrane waterwalls (MWW) are very large components working at lower temperatures zone of boiler and the final welding is performed on site where the PWHT is difficult to carry out. Furthermore, PWHT is expensive, time consuming, and may cause deformation of the final components. Therefore, the new grade T23 is Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.423
considered as the most promising material for producing the MWW and has been widely used in USC power plants. However, intergranular cracking has occurred in the weld metal zone of the welded joints of T23 steel used for MWW during actual service in several power plants of China since 2008, and the cause is still not clear yet.5,6
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