| Demands on energy saving and environmental protection have been driven the improvement of production efficiency of modern electric power plant, and the last 20 years has seen the rapid development of ultra supercritical (USC) power station unit which is recognized as one of the most matured clean-coal electric power generating technologies. The rapid development of USC power station unit benefits firstly from the research and application of new heat resistance steels with superior properties. It is well documented that Super304H steel has gained wide applications as superheater/reheater in USC unit in China. However, there is still a lack of comprehensive investigation on Super304H steel and its welded joint in our country. As a result, it is of importance to study the welded joint, especially the influence of high temperature aging on its microstructure and properties. It is also of practical significance to conduct these investigations to support the safety running, commercial application, supervising and customizing of this steel in our country.In the present paper, comprehensive characterization of the welded joint was performed by using optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, transmission microscopy, electron probe microscopy, X-ray diffractometer, room-temperature test, high temperature creep test and electrochemical potentiokinetic reactivation test. Objections of this investigation include microstructure characterization and its effect on properties of Super304H steel and its welded joint, microstructure evolution of Super304H steel and its welded joint during high temperature aging, as well as precipitates evolution during welding and aging treatment. Besides, it is also studied systematically in the present paper that the influence of the above-mentioned factors on room temperature mechanical properties, high temperature creep strength, intergranular corrosion behaviors and the degrading mechanism of the welded joint. It is expected that this investigation can be used as a technical support for supervising the safety running of the welded joint, as well as to provide technical data for the customization of this steel in our country.The as-received Super304H steel shows microstructure of y matrix made up of fine and evenly distributed y grains with precipitates. Precipitates consist of Nb(C, N) and Cu-rich phase. Nb(C, N) shows morphology of finely dispersed small particles and relatively large block and stripe which distribute along certain direction. Microstructure of the as-welded joint of Super304H steel consists of y phase and precipitates. Weld metal shows typical cellular dendrite morphology and the HAZ adjacent to the fusing zone demonstrates obvious grain growth. Precipitates in the welded joint mainly comprises Cu-rich phase and Nb(C, N) which distributes in the dendritic grain boundaries with particulate and worm-like morphology.After being high temperature treatment with high temperature of 1350℃, matrix of Super304H steel still consists of y phase. When the treatment temperature is below 1100℃,γgrain grows slowly in contrast to its rapid growth at temperature higher than1150℃. During high temperature treatment, precipitates presented include Nb(C, N), Cu-rich phase, M23C6 and NbCrN, which show different precipitating and dissolution behaviors, morphology, distribution and amount with the changing of treatment temperature. And these also subsequently cause the corresponding changes of impact toughness and hardness of Super304H steel. As a result, it is suggested that long-term serving temperature should be below 700℃for Super304H steel and in addition to refining the weld metal microstructure through improving metallurgical conditions the time experienced above 1150℃should be controlled in order to improve its oxidation resistance to high temperature steam.After being aged from 600℃to 700℃, microstructure of Super304H steel and the welded joint consists of y phase and precipitates. The matrix is basically absent of any noticeable microstructure changing. Precipitates are mainly made up of Nb(C, N), copper-rich phase and M23C6. Precipitating behavior varies with the changing of aging condition. It is found that the shape of curve showing relative amount of Nb(C, N) and M23C6 is related to their content in the initial microstructure. M23C6 mainly distributed along grain boundaries and its morphology changes from thin stripe/particle to chain-like (block or stripe) and isolated particles. Compared with time, temperature has a more profound influence on the precipitating behavior of M23C6.Under the experimental condition creep strength of the welded joint is higher than that of the base metal. Creep fracture occurs in the area of the base metal far from the fusing line. Furthermore, it is found that creep cavity nucleates near the relatively large Nb(C, N) block and stripe, and with the increasing of aging time, cavity also forms near M23C6 particles on grain boundaries.After being aged at high temperature from 600℃to 700℃, obvious aging brittleness occurs for the welded joint, which is attributed to the precipitation of M23C6 along grain boundaries. Besides, it is found that the amount of precipitates is a determinate factor. When aged at the serving temperature, impact energy of the weld metal is only 13J, which is much less than these of the base metal and HAZ. Therefore, welding consumable used for the Super304H steel needs further evaluation and improvement.Both the Super304H steel in as-received condition and the as-welded welded joint possess superior intergranular corrosion resistance but sensitization occurs after being aged at high temperature from 600℃to 700℃. Susceptibility of intergranular corrosion of Super304H steel and its welded joint during aging treatment is in accordance with the amount, morphology and distribution of M23C6. Intergranular corrosion of Super304H steel and its welded joint is attributed to the precipitation of M23C6 along grain boundaries and is determined by the amount of precipitates. Intergranular corrosion induced by Cr depletion was caused by precipitating of M23C6 along grain boundaries, and it is also found that the precipitating of M23C6 inside grains also leads to corrosion near the Cr-depleted area. Therefore, supervising of Super304H steel and the welded joint should be enhanced which was normally serviced in the boiler in highly corrosive medium.
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