To estimate the lifetime of welding joints of steam pipes working at high temperatures and high pressures is of great importance for the stability and maximum economic efficiency of steam power plants. Long-term exposure of heat-resistant steels at high temperature leads to deterioration in the mechanical properties because of the changes in the microstructure. Alloying elements and microstructures significantly affect the heat resistance in the welds of low alloy heat resistant steel at high temperature.
In order to illustrate the changing nature of heat resistance, Ri Won Jun, a researcher at the Faculty of Materials Science and Technology, investigated the mechanical properties (ultimate strength, yield strength, elongation, hardness, etc.) of pipe materials and their welding joints with respect to the content of elements in carbides, their morphology and change in the metallic structures. Also, he conducted an experiment by using zirconium instead of vanadium for electrodes to evaluate the microstructure of deposited metal, mechanical properties at room temperature, metallic structures analysis, carbide analysis, scale resistance, short-term creep strength and long-term creep strength.
As a result, he drew the following conclusions.
First, the metallographic study showed that Zr series has less migrant tendency of the grain boundary than V series at high temperatures for a long time. And the comparison of mechanical properties with hybrid carbides showed that Zr series is superior to V series in all indices.
Second, the experiment for oxidation inclination at high temperature showed that the value of scale oxidation of Zr series is 1.4 times higher than that of V series, indicating that Zr series is more stable at high temperature.
Third, the metal deposited by the stick electrode coated with Zr has no ferrite segregation and the primary structure is finer than that with V.
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