Demonstration of Improved Radiation Embrittlement Resistance of A533B Steel Through Control of Selected Residual Elements
Authors: Hawthorne JR 
Published: March 2009

A successful demonstration test of metallurgically controlled radiation embrittlement sensitivity has been conducted with a large commercial melt of A533B steel. The 30-ton melt represents a scaleup from 300-lb laboratory melts which provided the first evidence of the potential for very low sensitivity to radiation embrittlement at power reactor pressure vessel service temperatures (∼550 F, 288 C). The commercial scale demonstration test was sponsored by the U.S. Atomic Energy Commission (AEC), Division of Reactor Development and Technology (DRDT), Fuels and Materials Branch, and depicts the “composition specification approach” to the development of optimum radiation resistance in structural steels.

The primary objective of special melt specifications and melt planning was the reduction of copper and phosphorus contents to the lowest possible level. Restrictions were also imposed on the content of other residual impurity elements with known or suspected influences on radiation embrittlement resistance. For a broad experimental analysis, the melt was split to provide material representing the primary melt analysis (0.03 percent copper) and a melt modification (0.13 percent copper). Plates representing each analysis were also split and sections individually heat-treated to Class 1 or Class 2 strength conditions. All procedures used were standard mill practices.

Radiation assessments showed the primary melt analysis to have very low sensitivity to radiation embrittlement at 550 F (288 C), thereby validating the composition specification approach for future melts. Transition temperature increases measured independently by Charpy V (Cv) and dynamic tear (DT) test methods were 70 F (39 C) or less for fluences up to 3.1×1019 n/cm2, E &gt; 1 MeV. Based upon DT results, the nil ductility transition (NDT) temperature of the Class 1 plate remained below 75 F (24 C) after 550 F (288 C) irradiation.

Results for the 0.13 percent copper melt modification provided direct confirmation of the primary, highly detrimental influence of copper content on radiation embrittlement resistance. The Cv 30 ft∙lb transition temperature increases for the Class 1 and Class 2 plates were 140 and 125 F (78 and 69 C), respectively, for a fluence of 2.8×1019 n/cm2, E &gt; 1 MeV. The enhancement of radiation sensitivity by copper content appeared independent of the strength class.

Postirradiation DT characteristics of the primary melt analysis (Class 1 plate) were indicative of excellent fracture resistance at shelf level temperatures.

The A533B scaleup demonstration test fully supports the principles for control of radiation embrittlement sensitivity developed in laboratory research. A summary of 550 F (288 C) radiation data for the ASTM A302B reference plate and A533 standard production plate and weld metals places the results for the special melt in full perspective. The radiation embrittlement sensitivity of the primary melt analysis is shown to be only one-third that of the reference plate and significantly less than that of A533 production materials.