A Comparison of Steels for Nuclear Bolting Applications

目前，螺栓由SA 540钢生产，其成分与常见的AISI 4340钢非常相似。先前对SA 540的研究表明，其成分对于用于非常大的螺栓并没有很好地优化-结果表明，当油淬火时，不太可能在一致的基础上实现向100%马氏体的全厚度转变。如果加工过程完美，则该等级能够实现强度和韧性的出色组合;然而，该等级对加工过程中的变化并不稳健。部件尺寸处于组合物的可硬化性的极限。因此，整个截面的属性是不均匀的。此外，已经证明，冷却速率在临界值附近的微小差异可以导致显微组织演变的显著变化，这反过来又导致硬度值的显著分散。该项目将寻求将SA 540与另一种等级的钢A723进行比较，A723在厚截面中可能更具可硬化性。它将涉及对SA 540和A723应用工业热处理工艺，并监测微观结构和性能的演变。将使用许多技术，包括硬度计、电子显微镜（扫描和透射）和机械测试（可能扩展到夏比和断裂韧性测试）。

At present, the bolts are produced from SA540 steel, which is very similar in composition to the common AISI 4340 steel. Previous work on SA540 has suggested that its composition is not well optimised for use in very large bolts - results suggest that, when oil quenched, through-thickness transformation to 100% martensite is unlikely to be achieved on a consistent basis. If processing is performed perfectly then the grade is capable of an exceptional combination of strength and toughness; however, the grade is not robust to variability in processing. The component size is at the limit of the hardenabilty of the composition. Hence, properties are non-uniform through the section. Furthermore, it has been demonstrated that small differences in cooling rate around a critical value can lead to significant changes in microstructure evolution, which in turn result in significant scatter in hardness values. This project will seek to compare SA540 to another grade of steel, A723, which is likely to be more hardenable in thick sections. It will involve applying industrial heat treatment processes to both SA540 and A723 and monitoring the evolution of microstructure and properties. A number of techniques will be utilised, including dilatometry, electron microscopy (both scanning and transmission) and mechanical testing (which is likely to extend to Charpy and fracture toughness tests).