Nonuniformity of the ductility of 15X2HMΦA low-alloy steel

Abstract

Irradiation of low-alloy steel used for the manufacture of nuclear-reactor housings poses the risk of reducing its plasticity and shifting the transition from ductile failure to brittle failure to markedly higher temperatures [1, 2]. The embrittlement of reactor housings will largely depend on the content of phosphorus, copper, and nickel in the steel [3‐5]. The resistance to embrittlement is specified by the structure and hence the grade composition of the steel. It depends on factors such as the position of the nonmetallic inclusions, small impurities, and structural inhomogeneity. These factors have not been adequately studied for steel used in reactor housings [6]. Estimation of their contribution to failure may provide useful information for the verification of models and refinement of mechanisms of radiation embrittlement; the prediction of the remaining operational life of reactor housings; identification of effective reduction conditions for housings with short radiation life; and improvement in the production technology for such materials [5, 7]. The housings of the water‐water reactors installed at most nuclear power plants are generally made from 15X2M iA steel (VVER-440 reactors) and 15X2HM iA steel (second-generation VVER-1000 reactors) [5]. In the present work, we investigate the ductility of 15X2HM iA steel and the weld-seam metal for the given technology; the composition of these metals is summarized in the table. After standard heat treatment, some of the Charpy samples (from the basic metal and the weld seam) are subjected to additional (reducing [5]) annealing, in the temperature range corresponding to the development of reversible tempering brittleness. Impact tests (with a Roell Amsler RKP-450) pile driver in the range 20‐100 ° C are accompanied by ductile failure both after initial heat treatment and after reductive annealing. However, the spread of the work of destruction is relatively large: 178‐240 J for the basic metal; and 86‐159 J for the weld seam. Such nonuniformity of the ductility is usually a consequence not only of an increased content of nonmetallic inclusions but also of their wide dimensional range and nonuniform distribution within the metal [6].