Fracture toughness of welded joints of a steam turbine low pressure rotor

In order to ensure safety and reliability of a steam turbine welded rotor, the present investigation focused on evaluation of the crack resistance parameter of the base metal (BM), weld metal (WM), and heat affected zone (HAZ) of a turbine rotor welded joint constituent at both room temperature and 300℃. The property of crack resistance was evaluated in terms of fracture energy J _Ic or plane strain fracture toughness K _{J Ic} . Three-point bend experiments were performed to obtain a J-integral based crack resistance curve ( J–R curve) from a single specimen in accordance with ASTM E1820, from which the fracture toughness was determined. To construct a J–R curve, the corresponding crack extension was calculated by incorporating the crack opening displacement measured by a clip-gage, in a compliance method. From the experimental results, the WM was found to have the lowest K _{J Ic} values whereas the HAZ was found to have the highest K _{J Ic} values at both room and high temperature although the HAZ had little or no crack extension during the experiments. It was also observed that the HAZ samples fractured with crack jumped toward the fusion line. Microstructural root-cause analysis of the abnormal HAZ behavior was performed and discussed.