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The objective of this program is to determine the applicability of high-strength steels and alloys to nuclear reactor pressure vessels and similar critical applications. Materials under investigation include % 12Ni -5Cr -3Mo maraging steel and precipitationhardening stainless steel, PH13-8Mo. The nickel-base alloy, Inconel alloy 718, is also being investigated together with a quenched and tempered alloy steel, H P 9Ni -4 c 0 -0.2C.All alloys studied in this program can be processed to far greater tensile and yield strength levels than the current alloys which are ASME Boiler and Pressure Vessel Codeapproved and in current use. If it can be established that these alloys meet strength criteria and other significant requirements for pressure vessel applications, their eventual use may achieve one o r more of the following:1. Greater pressure vessel reliability and safety. 2. Reduced pressure vessel weight and section size. 3. Improved reactor performance resulting from higher permissible pressures and/orThe type of materials being studied will necessitate greater unit costs for materials and processing, but it is believed that overall reactor economics may disclose an acceptable cost balance, particularly in view of such intangibles as greater safety o r equal safety with less weight. The experimental approach being followed encompasses two phases. The first phase consists of selecting and procuring candidate materials in the form of approximately 2.54-cmthick plate, and evaluating pertinent properties, response to and radiation resistance. The second phase involves procurement of-heavier plate ( 2 10 cm thick) to determine size effects on properties, response to heat treatment, and weldability. Ultimately one o r more materials are expected to emerge as candidates for high-strength pressure vessels. For the most part all alloys currently under study are well into the first phase. The 12-5-3 maraging steel is entering the second phase. The 12-5-3 maraging steel is a heat-treatable alloy capable of high strength with excellent toughness. Heat treatment consists of.-solution annealing, air cooling or quenching to form martensite, and aging to effect precipitation strengthening. Because of the low carbon level (0.03C max, Table 7.1) the alloy is relatively soft (approximately RC 30) and is work-*Project leader. +principal investigator.

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