Bouchard P.J.

Francis, JA

Paddea S

Paradowska AM

Shibli IA

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Residual stresses in as-welded P91 steel pipeIntroductionUK power generation plants are following a worldwide trend inthe electricity generating industry towards operating at highertemperatures, which leads to increases in efficiency in energyproduction and reductions in emissions of greenhouse gases. Aspart of this trend, the development and assessment of newsteel-based materials, and suitable welding processes, is vital.When compared to widely available steels such as P22(2.25Cr-1Mo), P91 steel offers superior creep (high temperature)strength. However, experience has shown that the weldments inthis steel are particularly prone to premature creep failure, dueto a localised form of cracking in the heat-affected zone (HAZ),which is referred to as Type IV cracking.Type IV crackingDuring welding, the base metal adjacent to the fusion zoneexperiences complex changes in temperature and stress, whichresult in a gradient in microstructure across the HAZ, wherebydifferent zones are generated with each being defined by thepeak temperature experienced. Type IV cracking occurs in thefine-grained and/or intercritical HAZ.S. Paddea1, J.A. Francis1, A. Paradowska2,  P.J. Bouchard1 and I.A. Shibli31Materials Engineering, The Open University, Milton Keynes, U.K., 2ISIS Facility, Rutherford Appleton Laboratory, Didcot, Oxfordshire, U.K.3European Technology Development Ltd., Leatherhead, Surrey, U.K.Neutron diffractionNeutron diffraction measurements were made using theENGIN-X instrument at the ISIS facility (Rutherford AppletonLaboratory, UK). The instrument can be controlled by aunique virtual laboratory strain scanning software (SScanSS),which employs serial robotic kinematics to bring a specifiedsample point to the instrument focus, and aligns the specifiedsample direction(s) along the instrument scattering vector(s)[Nuc Inst Meth A571 (2007) 709].SSCANSS Simulation in progress Experimental set-up of pipe at ENGIN-XStress-free lattice parameterPrecipitation and dissolution of second phases during weldingcan bring about large changes in lattice spacing.Consequently, a comb specimen for the weld, and a pin forProject aimIn this project, we aim to assess the significance of residualstresses and material constraint in determining type-IV-limitedcreep performance in welds.Welded test sampleThe P91 pipe having an outer diameter of 324 mm and a 25.4mm wall thickness was welded using TIG for the root runs andSAW for filling passes. A slot (60 mm x 60 mm) was machinedinto the component to facilitate neutron diffraction stressmeasurements.the parent metal, were extracted from the weldment and aseries of strain-free lattice spacing (d0) measurements weremade in the weld metal, HAZ & parent metal.Reference samples: pin and comb specimen to measure position dependent doResultsHigh tensile residual stresses were found in the hoopdirection with peak stresses of about 600 MPa located in theHAZ. There was also evidence of high hydrostatic tension inthe same region which is known to facilitate growth andcoalescence of creep cavities.Type IV CrackingTemperature and microstructure distribution across the weld and example of Type IV cracking in P91 weldmentType IV cracking

Residual stresses in as-welded P91 steel pipe

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