| In China, P92 steel with good creep resistance properties, high temperature strength, low expansion coefficient, good weldability and workability has been widely used for important pressure parts in ultra-supercritical unit such as main steam pipe and high-temperature reheat steam pipe. P92 weld metal and base metal serviced after 50000 hours and P92 weld metal during aging were taken as research subjects. Evolution of microstructure and mechanical properties during aging and influence of evolution of microstructure on mechanical properties were researched by means of optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), universal testing machine, impact testing machine and Brinell hardness tester, respectively..Serviced after 50000 hours, P92 base metal still kept tempered martensite lath morphology. Recovery of martensite lath hasn’t been observed. The size and amount of precipitated phase along prior austenite grain boundary and packet boundary increased during service. After serving, strength and ductility of P92 base metal were still good, but toughness decreased obviously. The impact energy of the base metal reduced, that meant embrittlement occurred.P92 weld metal austenite grains were composed of martensite packet with different orientation after serving 50000 hours. Due to the lower critical nucleation energy in the grain boundary, Laves phase and M23C6 phase distuributed along grain boundary and packet boundary. P92 welding joint that served after 50000 hours remained good strength, but ductility and toughness deteriorated. Serious embrittlement occurred in welding joint, the average impact energy less than 20J. A number of worm-like micro-cracks were found in weld metal after serving, and some small size micro-cracks gathered in local area. The maximum size of the micro-cracks is not more than 1mm, 0.1mm or less in width.The grain size of each region was difference in heat affected zone due to weld thermal cycle. Delta ferrite was found in the heat affected zone near the fusion zone. Precipitated phase appeared rapidly in the matrix after post weld heat treatment. M23C6 phase distributed along prior austenite grain boundary and packet boundary, and MX phase dispersively distributed in the interior of the martensite lath. Post weld heat treatment improved toughness of weld metal and reduced hardness. Optical microstructure of weld metal didn’t change significantly druing aging at 650℃. Coarsening and growth of precipitated phase induced the reduction of emelent solutioned in the matrix, which caused the change of weld metal lattice parameter. Laves phase precipitated adjacent to M23C6 phase during aging. Aging treatment at 650℃ greatly influenced on the toughness of weld metal, with impact energy fell to 26J from 45J after aging 5000 hours. Aging treatment had little effect on the hardness, with hadrness of weld metal fluctuated within the range of 210-230 HBW.TEM results showed there were three precipitated phase in aging and service P92 weld metal, M23C6 phase, MX phase and Laves phase, respectively. The size of Laves phase increased continuously during aging at 650℃, approached to 400nm after 5000 hours. Ostwald ripening of Laves phase occurred after aging 1000 hours, and coarsening rate k was 6.69×10-28m3/s. Laves phase coarsened along grain boundary during aging, which reduced precipitation strength. Network distribution of Laves phase along grain boundary induced crack extended along grain boundary, and increased crack nucleation position.The above point led to the reduction of toughness during aging. |
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