Effects of the Chromium and Molybdenum on Precipitation behavior and Mechanical Properties in SA508 Gr. 4N Low Alloy Steel

Low carbon low alloy steels, applied as nuclear pressure vessels, steam generators and so on, hold a great part of materials for nuclear power plants, and they are very important materials because they determine the safety and the life span of nuclear power plants. In addition, they are used for a long period under extremely severe conditions such as a high pressure, high temperature, neutron irradiation and corrosion, so they require a good combination of strength and toughness, a superior weldability and an good neutron irradiation resistance and so on. SA508 Gr.3 low alloy steel reveals the upper bainite microstructure, which is a fewer tough, so the steel is more difficult to acquire good toughness than to have high strength. And then, if a loss of toughness due to a neutron irradiation during a operation is considered, improving the toughness is very important when a pressure vessel is manufactured. It is known that a higher strength and fracture toughness of low alloy steels could be achieved by increasing the Ni and Cr contents. In this study, we have performed a microstructure observation and mechanical test of SA508 Gr.4N low alloy steel which has higher Ni and Cr contents than SA508 Gr.3 low alloy steel. Based on the microstructure/mechanical property relations obtained from literature research experimental works on SA508 Gr.4N steels, and by observing the precipitation behavior changes with alloying elements (such as Cr, Mo) after a tempering process of the steel with a mechanical test, fundamental information for an alloy design have been discussed