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

Abstract

Carbide phase transformation and alloying element partitioning in service exposed (about 45 000 -160 000 h) 2 . 25Cr -1Mo boiler tubes from a thermal power station were studied using X-ray diffraction and X-ray uorescence 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 thañ 0 . 8 wt-%, 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°C showed an increase in molybdenum content with increase in temperature. The X-ray data were supplemented by microscopy, EDAX and hardness measurements.MST/5687

Coarsening of nano sized carbide particles in 2.25Cr–1Mo power plant steel after extended service

“…With extended service, carbides present in the microstructure transform towards equilibrium. A detailed XRD, TEM and EDAX investigation on carbide transformations [5] and solid solution partitioning [6] of 2.25Cr-1Mo power plant specimens were reported earlier. M 23 C 6 is the final equilibrium precipitate in this steel and hence variation of size of this phase is investigated here in detail.…”

“…The specimens were in service at~5 40 jC in a well-maintained power station. However, manufacturer's data to ascertain initial thermal history of the specimens is not available which is discussed elsewhere also [6]. Coupons weighing 2 -3 g were made from the specimens for electrochemical extraction.…”

“…X-ray investigation of carbide phase transformation [5] and partitioning of alloying elements [6] in service aged 2.25Cr -1Mo boiler tubes were reported elsewhere. The coarsening characteristics of fine carbide particles in a boiler metal are of prime importance as the coarsening kinetics and degradation of the metal have close relation with service conditions.…”

X-ray analysis of nano-carbide transformations for actual life assessment of service-exposed 2.25Cr–1Mo boiler tubes in a thermal power station

“…In this study, it was seen that Cr and Fe concentration was increasing with service and that of Mn remained constant. Mo concentration showed a decreasing trend with ageing [8].…”

“…Detailed X-ray and electron microscopy investigation of carbide phase transformation [7] and partitioning of alloying elements [8] in service-aged 2.25Cr -1Mo boiler tubes was the subject of our earlier studies. However, a time-temperature dependent parameter to assess the thermal history of this steel is not available.…”