Microstructural assessment of nitrogen-strengthened austenitic stainless-steel welds using thermoelectric power

The electronic properties of an alloy correlate directly to its microstructure. The thermoelectric power coefficient has been determined in this investigation to be a rapid and accurate measurement that can be used for microstructural assessment and correlated to achieve the maximum material performance of structural alloys. In high-nitrogen-strengthened stainless-steel welds, nitrogen partitions into solidsolution nitrogen and nitrides during the welding thermal cycle. The formation of nitrides results in a degradation of the mechanical properties and the corrosion resistance. Thermoelectric power measurements offer a means of assessing the weld-interstitial nitrogen content, which allows for a better correlation between the nitrogen content and the weld-metal microstructure and properties. Using pressure-composition-temperature (PCT) (activity) diagrams, thermodynamic models are developed to describe the relationship between the thermoelectric power coefficient as a function of the interstitial nitrogen content and the formed nitride content. These correlations between the electronic property measurements and the interstitial nitrogen content will allow for a faster, nondestructive, and improved property prediction of nitrogen-strengthened stainless steels.