Report on Northwestern's Air-Cooled, Copper Precipitation Hardened, High Strength, Weldable Steel Cast and Hot Rolled at Oregon Steel Mills'Public Deposited
During the past several years an easily weldable, high strength (more than 75 ksi yield), high impact fracture toughness steel (NUCu steel) has been investigated at Northwestern University with bridge applications in mind. For good weldability without pre-heating and post-heating, the carbon content of the steel was kept low and high strength was achieved by copper precipitation hardening. The steel was designed to be air cooled from low temperature hot rolling. Omitting the alloying elements (Cr and Mo), and the quench and temper heat treatment used in other high strength structural steel alloys reduces the cost of the steel, an important requirement for infrastructure applications. Also, chromium is undesirable in steel to be welded because of Cr+6 formation. The steel is a variation of the Navy HSLA 80 and ASTM A710 steels which are usually quenched and tempered. In the NUCu steel 78 Ksi yield was reached in a plate up to 1 inch-thick plate without quenching or aging and the Charpy impact fracture toughness was very high. The initial studies at Northwestern University were conducted on six 100-lb laboratory heats prepared at Inland Steel Company's Research Laboratory by vacuum-induction melting (1). These steel heats were hot-rolled to 1/2-inch-thick plates and air cooled. Mechanical properties and microstructure of these heats were investigated. Based on these results Oregon Steel Mills volunteered to produce a large commercial heat.
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