Influence of heat input and post-weld heat treatment on boiler steel P91 (9Cr–1Mo–V–Nb) weld joints Part 1 – Microstructure M. Abd El-Rahman Abd El-Salam, I. El-Mahallawi* and M. R. El-Koussy Steel P91 is known for its excellent high temperature properties. The achievement of optimum weld metal properties for steel P91 within the course of its extensive applications in power plants has however often caused concern. In the present work, three thick pipes of P91 steel were welded using three different levels of heat inputs within the range of 1?15–3?5 kJ mm 21 .A circumferentially multipass butt welded P91 steel pipe, typically used for high temperature applications in power plants was selected for this investigation. The achievements of optimum weld metal properties, which are closely linked to microstructure, are known to cause concern in such weldments. Two types of heat treatments were employed, subcritical post-weld heat treatment and normalising/tempering treatment. The microstructure was evaluated by optical, scanning electron microscope, X-ray diffraction and magnetic permeability. The results have shown a great influence of heat input and heat treatment on the microstructure. Martensite and ferrite were the main structures obtained. Bainite and d-ferrite have been observed in the weld metal, heat affected zone and weld metal for all heat input/treatment conditions. The volume fraction of bainite and d-ferrite increased with increase in heat input till a critical value slightly lower than 1?15 kJ mm 21 ; then decreased with the increase in heat input. The normalising/ tempering treatment resulted in a decrease in the volume fraction of d-ferrite and bainite compared to the subcritical post-weld treatment which is conventionally used. This explains the enhancement in the toughness and creep properties of the steel presented in the second paper. 1 Keywords: Microstructures, d-ferrite, Modified 9Cr–1Mo steel, Welding, Heat treatments, Heat input Introduction The need to reduce the fuel cost as well as environmental pollution from fossil fuels by significantly decreasing carbon dioxide emissions from power generation plants has led to efforts to increase the thermal efficiency of power plants. 1 The increase in the thermal efficiency of fossil fuel fired steam power plant that can be achieved by increasing the steam temperature and pressure; has provided the incentive for the development of heat resistant steels with excellent creep properties as well as superior oxidation and corrosion resistance properties. 2 In the last two decades and to face up these require- ments; several new Cr–Mo and 9–12% chromium steels were developed ranging from P11 (1Cr–0?5Mo) to P122 (12Cr–1Mo). 1,3 Modified 9Cr–1Mo steel tubes with high strength at elevated temperatures and oxidation resistance (ASME SA213 T91, abbreviated SA335 P91 or super 9Cr steel) are frequently used in high temperature boilers for high efficiency power generation. 4 Hence, the welding char- acteristics of P91 constitute an important criterion for its selection; it has been shown that P91 can be welded satisfactorily by many processes including manual metal arc, submerged arc, and gas tungsten arc welding. 5 Post- weld heat treatment (PWHT) is necessary for tempering the martensite formed during welding, and many investigations have highlighted the need for optimising the PWHT temperature and time as well as filler material composition. 5,6 On the other hand, due to the heat input during welding, not only different micro- structures are obtained within the weld, but also, microstructural changes take place in a small area besides the fusion line. If the A 1 temperature (the lower transformation temperature at which ferrite is partly transformed into austenite)is exceeded in an area, phase transformations lead to changes in the microstructure of Department of Metallurgy, Cairo University/Faculty of Engineering, University Street, University Square, El-Giza, Egypt *Corresponding author, email saiman@eng.cu.edu.eg ß 2013 IHTSE Partnership Published by Maney on behalf of the Partnership DOI 10.1179/1749514813Z.00000000050 International Heat Treatment and Surface Engineering 2013 VOL 7 NO 1 23