CREEP STRENGTH BEHAVIOR OF BORON ADDED P91 STEEL AND ITS WELD IN THE TEMPERATURE RANGE OF 600–650  C J.Swaminathan 1 C.R. Das 2 Jayashree Baral 3 , C. Phaniraj 2 R.N. Ghosh 3 S.K. Albert 2 , and A.K.Bhaduri 2 , 1 National Metallurgical Laboratory, Jamshedpur -831007,INDIA 2 Indira Gandhi Centre for Atomic Research, Kalpakkam -603102 INDIA 3 Indian Institute of Technology, Kharagpur-721302,India Keywords: Type IV cracking, P91 steel, Boron addition, creep Abstract One of the promising ways for mitigation of Type IV cracking – a failure by cracking at the intercritical /fine grained heat affected zone, a life limiting problem in advanced 9–12 Cr ferritic steel weld like that of P91 is through modification of alloy composition by addition of boron. Addition of boron was observed to improve the microstructure at the weld zone and hence the creep strength. In the present work, boron (100 ppm with controlled nitrogen) added P91 steel after normalizing at 1050C and 1150C and tempered at 760C were studied for the creep behavior in the base metal and welded condition in the temperature range of 600–650C. Creep strength was characterized in terms of stress and temperature dependence of creep rate and rupture time. Weld creep life was reduced compared to the base metal with rupture occurring at the ICHAZ (Type IV crack). However at longer time (at lower stress levels) exposure creep crack moves from weld metal to HAZ (Type II crack). Rupture life was found to superior for the base and weld in the boron containing steel when higher normalizing temperature is used. Estimation of 10 5 h was attempted based on short term rupture data available and weld strength factors were calculated. Observed values are better for P91BH condition than the values for P91BLcondition as well as those available for P91 in open literature Introduction The advent of ultra super critical plants has resulted in development of 9–12% Cr ferritic– martensitic steels with better resistance to high temperature creep and corrosion in addition to thermal conductivity and lower cost than austenitic stainless steels [1–3]. Modified 9Cr–1Mo (P91) steel derives higher creep strength from the presence of stable carbides and carbo-nitrides which provide resistance to motion of free dislocations [3]. However, premature failures of weldments have been reported [4] in the intercritical/ fine grain regions of heat affected zone (HAZ) which is popularly known as Type IV cracking, and has been commonly attributed to the lower creep strength of these regions when compared to base metal. There are studies that show [5–7] that the addition of boron improves resistance to creep and type IV cracking in ferritic steels. The observed beneficial effect was attributed [6] to the microstructural changes like uniform prior austenitic grain size in HAZ and stabilization of precipitates which delays the substructure development. Das and co–workers [8–11] have observed that 100 ppm boron containing P91 steel (P91B) exhibited lower minimum creep rates and enhanced creep–rupture lives compared to boron free P91 steel at 600–650  C for normalized (1050  C/1h) and tempered (760  C/3h) condition. The Energy Materials 2014 The Chinese Society for Metals (CSM) and The Minerals, Metals & Materials Society (TMS) TMS, 2014 111