TWI, one of the world’s foremost independent research and technology organisations, with expertise in materials joining and engineering processes as applied in industry, has successfully joined a type of steel alloy, which was earlier considered to be an impossibility using conventional fusion methods. TWI has welded a number of samples of oxide dispersion-strengthened (ODS) steels as part of its core research programme into the friction stir welding of steel. These specialised alloys have been developed to deliver excellent strength and creep performance at elevated temperatures, making them suitable for applications in nuclear reactors and power generation equipment, where they are exposed to extreme heat.
These steels derive their properties from a distribution of fine particulates, usually ceramic-based, which impede deformation and so give the steel its strength and resistance to creep. Unfortunately, conventional fusion welding techniques destroy these strengthening particulates, preventing the steel from being effectively fabricated. Being a solid-state process, friction stir welding does not melt the steel being joined and would therefore allow ODS steels to maintain their properties after fabrication. It is believed that the friction stir welding process, besides being able successfully to weld ODS steel, may also render it less prone to hydrogen cracking – a significant advantage for welds operating in a nuclear environment.