TWI, Cambridge, UK

TWI friction stir welds ODS steel

TWI has successfully joined a type of steel alloy considered unweldable using conventional fusion methods. As part of its core research programme into the friction stir welding of steel, UK-headquartered research and technology organisation and engineering specialist TWI has welded a number of samples of oxide dispersion-strengthened (ODS) steels. 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 very high temperatures.

ODS steels derive their properties from a distribution of fine particulates, usually ceramic-based, which impede deformation and imbue the steel with its strength and resistance to creep. Unfortunately, conventional fusion welding techniques destroy these strengthening particulates, preventing the steel from being effectively fabricated.

TWI was asked to friction stir weld samples of these steels as part of a PhD project being undertaken by Huw Dawson of Manchester University. 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.

A number of samples of MA956, embedded with thermocouples to monitor process temperatures, were successfully welded at TWI’s Yorkshire Technology Centre. These samples are now undergoing extensive testing at Manchester University, to assess their performance in hostile environments, including their susceptibility to irradiation damage. 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.

TWI, Cambridge, UK