Microstructural Evolution and Mechanical Properties of CSEF/M P92 Steel Weldments Welded Using Different Filler Composit
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t years, to enhance the efficiency of steam power plants for economic and environmental reasons, the need for creep strength-enhanced ferritic/martensitic (CSEF/M) steels has increased.[1,2] The simplest way to achieve higher efficiency is to increase the inlet pressure and temperature of the steam that passes through the turbines.[3] The primary candidate material for the construction of supercritical coal-fired boilers in the power industries is CSEF/M steels (mainly P91 and P92 steels).[4] The CSEF/M steels offer high creep resistance, excellent thermal conductivity, good toughness, ductility, a low coefficient of thermal expansion, high resistance to stress corrosion cracking and oxidation.[5–7] Microstructural stability at elevated temperatures and good weldability are also lucrative properties exhibited
NITIN SAINI and RAHUL S. MULIK are with the Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee, Roorkee, Uttrakhand, 247667, India. Contact e-mail: [email protected] MANAS MOHAN MAHAPATRA is with the School of Mechanical Sciences, Indian Institute of Technology, Bhubaneswar, Odisha, 751013, India. Manuscript submitted November 21, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
by CSEF/M steels.[8,9] The application of low chromium ferritic alloys [2–3 pct chromium (Cr)] is limited in the boiler up to service temperatures of 500–550 C due to environmental degradation from the fire side.[10] Ferritic steels in the range of 9–12 pct Cr, such as T91, T92, and E911, appear to be capable of being used up to a temperature of 650 C.[11,12] In P92 steel, the primary alloying element is Cr which improves the oxidation and corrosion resistance. The other alloying elements are tungsten (W), vanadium (V), manganese (Mn), molybdenum (Mo), silicon (Si), niobium (Nb), carbon (C), and small amount of boron (B) (up to 0.005 pct).[13] These alloying elements integratively improve the properties at elevated temperature. The P92 steel offers enhanced creep strength that results from the combination of precipitation and solid solution hardening.[14] It also offers better creep strength as compared to any other CSEF/M steels by forming M23(C, B)6 that reduce the coarsening rate of coarse carbide particles.[15] Study of the welded joint behavior is critically important as most of the failures in boiler tubes occur in the weld joints only.[16,17] To increase the range of applications of T/P92 base material, the development of compatible consumables is mandatory. But exceptionally a slightly higher Mn (~ 0.7 pct) and a deliberate Ni addition (~ 0.5 pct) have been advisable to achieve a
satisfactory weld metal strength and Charpy toughness.[18,19] Welding introduces localized changes in the microstructure, which become the life-limiting factor for these steels, In particular, type IV cracking is a phenomenon in which an enhanced rate of creep void formation occurs in the fine-grained heat-affected zone (FGHAZ) or inter-critical heat-affected zone (ICHAZ).[20] During welding of P92 steel, the temper ma
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