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Castings used in nuclear power stations are subject to extremely high quality specifications. The example of cast steel housings with a unit weight of 1.7 metric tonnes made of a steel similar to ASTM A296, Grade CF-8C, is used to show that by consistent application of foundry practice castings can be produced that meet the highest requirements.
In castings a basic distinction is made between systematic and statistical flaws: Systematic flaws are defined as those occurring at one and the same point in all castings made from one and the same pattern, such as shrinkage and cracks. These flaws are predictable and, therefore, avoidable through proper preparation of casting technique and exact application of good foundry practice. Statistical flaws are defined as flaws distributed at random over the casting, such as gas holes and nonmetallic inclusions. While it is true that these flaws are not predictable, they can be influenced to a considerable degree through well-directed measures in designing the gating technique. Such measures are described.
The effectiveness of the preceding measures is shown on the basis of the extent of repair to the trial castings and the influence of corrective actions on the extent of repair.
German rules prescribe a notch impact strength of not less than 80 J as an average of three specimens of austenitic steel grades, where the notch impact strength of the individual specimen should not fall below 67 J. It is shown that the carbon content must be kept as low as possible and a niobium/carbon ratio of approximately 8:10 must be attempted. These aspects were taken into consideration when establishing the melting analysis. The tensile properties and the notch impact strength are given for wall thicknesses of 60, 100, and 300 mm. Investigations on construction welds confirm the influence of the niobium/carbon ratio on the notch impact strength values of weld deposits.
Castings meet high safety requirements equally as well as components made by other production processes, with advantages such as freedom of design, optimum flow design, and direction-independent material properties.
stabilized chromium-nickel cast steel, section thickness, casting and gating technique, repair welds, mechanical properties, influence of carbon, niobium/carbon ratio, impact strength
Stahlwerke Bochum AG, Bochum,