Characterization of Type 304L Stainless Steel: Comparison of ASTM A262 Practice A and Analytical Electron Microscopy Techniques

Abstract

The ASTM A262 Practice A procedure is frequently used to assess whether Type 304/304L austenitic stainless steels are “sensitized”. In this study, Type 304L steel containing 18 ppm boron examined in the as-received (mill solution-annealed) condition exhibited a “dual” structure after the Practice A test despite its low C content. Detailed characterization of this alloy was performed to assess the precipitation behavior in this steel. Intergranular, Cr-rich M₂B- borides, identified by electron diffraction, were observed in the as-received condition. Samples aged at 700°C (nominally “sensitized”) produced fully “ditched” grain boundaries having Cr levels in excess of 14 wt.% and concomitant Cr-depleted zones less than 50 nm in extent. Despite exhibiting fully “ditched” grain boundaries, Cr levels of ∼18–20 wt.% with no Cr-depleted zones were detected in specimens aged at 900°C (nominally “stabilized”). These results show the Practice A test can be mis-used/mis-interpreted for assessing sensitization.