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A laboratory study was conducted to evaluate the effects of composition and heat treatment on the toughness of ASTM A508 Grade 3 Class 1 material for pressure vessels. Five steels were vacuum induction melted and cast as ingots in the laboratory. These heats included a base steel representing the specification mid-range analysis, a steel containing higher levels of Si, Ni, and Cr (high-side composition) as compared to the base steel, and three steels derived from the high-side composition by adding Al, Al/N, and Nb, respectively. The ingots were rolled to plate, heat treated, and evaluated. Among these steels, the high-side composition with additions of Al and N displays the best strength/toughness combination. For example, a 75 mm-thick plate of this steel has acceptable strength and a reference nil ductility transition temperature (RTNDT) of ≤ -29‡C after austenitizing at 875‡C, air cooling, and tempering at 660‡C for up to 20 hours. Upper-nose temper embrittlement (UNTE) occurs in all these steels. This UNTE is attributed to the precipitation of needle-like Mo-rich carbides during tempering, and is significantly reduced by increasing the cooling rate after austenitizing.
Three thermal models for simulating the heating cycles used for large forgings were developed. They were designed for accuracy, user friendliness, and rapid calculation on a personal computer. The results that are obtained from these models are the temperature profiles that occur within the ingot, forging or roll at various depths from the surface. The values for these temperature versus time curves can be used to examine several features about the heat treatment process. The example presented in this paper showed the effect of heating rate and hold at 704 "C on the surface-to-center temperature difference that occurs in a medium carbon steel ingot.
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