X-ray investigation of solid solution partitioning in 2.25Cr- 1Mo steel after extended elevated temperature service in power station

摘要

Carbide phase transformation and alloying element partitioning in service exposed (about 45 000-160 000 h) 2.25Cr - 1 Mo boiler tubes from a thermal power station were studied using X-ray diffraction and X-ray fluorescence techniques on electrochemically extracted carbide precipitates. The objective of the present investigation was to develop a faster method of health assessment of this steel at elevated temperatures on a routine basis. Various carbide phases were detected with different lengths of service. The molybdenum content in extracted carbide specimens showed a systematic decrease with aging. Replenishment of molybdenum in the matrix may be a useful indicator in life assessment of this steel. When the dissolved molybdenum content in the matrix was less than approx 0.8 wt- percent, the hardness of service exposed specimens was found to decrease. However, carbide precipitates extracted from new specimens subjected to accelerated heat treatment at 600 and 700 deg C showed an increase in molybdenum content with increase in temperature. The X-ray data were supplemented by microscopy, EDAX and hardness measurements.