Duplex stainless steels (DSSs) are multicomponent Fe-Cr-Ni-based alloys with high strength, corrosion resistance and weldability. The name, duplex, refers to the two-phase microstructure of these materials, which comprise roughly equal proportions of ferritic and austenitic grains (different crystal forms of iron). DSSs are widely used as engineering materials, particularly for corrosive environments, such as gas pipelines and chemical reaction vessels. High-alloy DSSs, known as super duplex stainless steels (SDSSs), are more expensive but their higher chromium, nickel and molybdenum content offers even greater resistance to stress corrosion cracking.
Despite their useful properties, the application of SDSSs is limited to temperatures around 250 °C due to changes in mechanical properties caused by decomposition of the ferrite phase. This poses an even greater issue when welding SDSS, as welds are more susceptible to thermal embrittlement.
Tungsten inert gas (TIG) welding is commonly used to create SDSS weldments, but the products are extra sensitive to thermal embrittlement. To understand how TIG welding affects the microstructure of SDSS in the longer term, a team from Sweden used the LOQ instrument at ISIS Neutron and Muon Source to study phase separation in an SDSS base metal and two weldments by small-angle neutron scattering (SANS).
"The application of SANS in metallurgy is experiencing something of a revival right now," says Steve King, the LOQ instrument scientist. "For studying the bulk substructure of a piece of metal, as opposed to just the surface structure or metal foils, with high statistical confidence, there isn't anything to beat the penetrating power of the neutron. And sometimes we can also do the measurements at in operando temperatures. Here at ISIS, we have the most modern and most extensive suite of SANS instruments in the world."
The SANS data showed that aging the base metal (300 °C, 48,000 hours) and weldments (300 °C, 35,000 hours) increased their hardness due to Cr clustering in the ferrite phase. The effect was more pronounced in the weldments than in the base metal, showing that TIG welding accelerated phase separation, especially during prolonged aging.
The pronounced phase separation in the weldments was attributed to two factors: the accelerating effects of their higher Cr and Ni content, and residual strains caused by the welding process. It was found that internal residual strain was exacerbated when using a higher heat input in each welding pass.
Already, the information from neutron scattering is enabling the design of SDSSs that are less prone to thermal embrittlement. “The application of SANS to study phase separation in Fe-Cr based alloys has provided significant new fundamental knowledge and we are currently exploiting this know-how for innovations in the field," remarked principal investigator, Peter Hedström. “So far, we have submitted one patent application for a new duplex alloy and one patent for a new process route, both aiming at mitigating the thermal embrittlement."
The development of new alloys could help to expand the engineering applications of SDSSs, enabling their use under even more demanding conditions.