Life assessment from weld microstructures at high temperatures

Sammanfattning: Most pressure vessels in fossil power plant are designed forcreep. After operation of the order of the design lifetime thematerial may be degraded and further operation is limited.Methods which can prolong the lifetime and assess thecomponents is therefore demanded. In low alloy steel theexpired life is related to the formation and development ofcreep cavities. Such damage is often located to welds in plantcomponents. A weld consists of various microstructures and theyrepresent different creep behaviours. The role of themicrostructure is fundamental to determine the performance ofthe weld. The present thesis covers creep behaviour includingdamage development in creep tests of welds, comparisons toplant observations and analysis of stress distributions inwelds.A number of circumferential pipe welds with controlledmicrostructures were fabricated, systematically creep testedand microstructurally investigated. The welding parameters andpost weld heat treatment (PWHT) varied the microstructures.Possibilities to omit PWHT is of practical importance for lifeextension purposes. Methods for grain refinement of weldmicrostructures which may compensate for possible beneficialeffects of the PWHT such as improved creep ductility andtoughness are examined. The test conditions where the damagedevelopment is similar to welded components are revealed. Forthis purpose also conventionally welded and post service weldswere tested and compared to studies of retired components aswell as non destructive monitor of the damage by use of thereplica technique. The thesis also focuses on differences instress distributions between laboratory specimens of welds andof those in plant. Using creep test data the influence ofmultiaxial stress distributions in uniaxial cross weldspecimens is examined. This model is applied in a continuumdamage mechanics approach to predict damage development andcreep cracks in circumferential pipe welds.Keywords:Low alloy steel, creep, cavity,microstructure, weld, post weld heat treatment,service-exposed,stress distribution.