The microstructure and properties of Super304H weld joints were developed and studied. The Creq and Nieq composition points of the deposited metal were all located in the austenite region. The quantity and size of the Nb-phase in the deposited metal with 0.28wt.% Nb were lower than tnat in the other two deposited metals. The phases were mainly γ and Nb(C, N). The content of Nb(C, N) phases in the deposited metal with the 0.6 wt.% Nb content was approximately 2.2 times of the deposited metal with the 0.28 wt.% Nb content. The Nb-phases in the deposited metal with 0.28 wt.% Nb content were lower in quantity, weaker in the strengthening effect, of lower yield strength and lower in size than that the deposited metal with the 0.6 wt.% Nb content.
In order to investigate the high temperature rupture property of deposited metal of SUPER304H steel, the high temperature tensile test was carried out, and the microstructure transformation of deposited metal of SUPER304H steel under high temperature persistent stress were studied. Most of the solidification subgrain boundaries dissolve. The effect of the high temperature enduring on the microstructure is not obvious. Temperature and time are the main factors that influence the change of microstructure. Under the action of high temperature stress, the corrosion resistance of austenite decreases significantly due to the occurrence of chromium deficiency. With the persistent stress of 200 MPa, the precipitated phase of deposited metal is Nb (C, N), M 23 C 6 , NbCrN phase, and a certain amount of alpha phase is precipitated in the deposited metal with a persistent stress of 78 MPa. The precipitation of M 23 C 6 phase is the main reason for the decrease of the corrosion resistance, especially the decrease of the corrosion resistance.In order to meet the requirements of energy saving and environmental protection, and realize the high efficiency of thermal power generation, the steam temperature and pressure raise higher requirements for boiler steel. For this, many scholars are constantly exploring ways to improve the performances of heat-resistant steel, and a series of heat resistant steels with good performance and high durability has been developed 1,2 , such as Super304H, T92, TP347HFG, and so on.Super304H steel is widely used in the boiler superheater and reheater 3. The main problems in Super304H steel welding are the hot cracking, corrosion and aging embrittlement 4,5 . The welding joints of Super304H steel have some differences between the weld and heat affected zone compared with the base metal, and these differences make the performance of welded joints different from that of base metal. In terms of precipitation strengthening, the change of size, quantity, shape and distribution of different parts of the welded joints is different, but the mechanism of the effect of these changes on the joint performance is rarely reported 6,7 . The precipitates in the heat affected zone of Super304H steel can precipitate, aggregate and grow, which will lead to the change of material properties 8 . In addition, there will be a softening zone and coarse grain zone in the weld heat affected zone. The mechanism of these changes is not very clear 9 . The welding method of Super304H steel is the gas tungsten arc welding. There is a big difference between the welding material and the base material, and the welding seam can not be treated as the parent material in the welding process 10 . Therefore, there are great differences between the microstructure of the weld and the base metal, such as the type of precipitates, the grain size and the crystal structure. Their influence on the performance is sometimes greater than that of the heat affected zone, which leads to the changes of the weld performance and corrosion. Due to the hi...
Three kinds of electrode with different niobium contents were developed and compared, and the influence of alloying elements on microstructure and mechanical properties was summarized. Strong carbide elements such as Nb, Ti, and V were added to form stable precipitates. The existence of intercrystalline precipitates leads to pinning and zigzag of grain boundaries, hinders the propagation of cracks, and enhances the low temperature strength and impact toughness of the materials. The No. 1 and No. 2 ENiCrMo-6 electrodes meet the requirements of LNG (liquified natural gas) equipment. The tensile strength of the deposited metal reaches 687 MPa, while the average impact energy at −196°C is 131 J. Owing to fluctuations in the stress concentration at the junction of grain boundaries, cracks may easily form. MC carbide can retard the crack propagation. With the increase of Nb and other alloys, the strength and hardness increase gradually, but the plasticity and toughness are retarded to a certain extent.
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