Effect of carbon content on eddy current response to sensitization and intergranular corrosion in simulated heat-affected zone of austenitic stainless steel

Abstract

AISI Type 316 stainless steel (C = 0.055 wt. %) and type 316L stainless steel (C = 0.016 wt. %) were subjected to various heat treatments between 873 and 1073 K for various durations of time. The aged specimens were then subjected to ASTM A262 Practice E test in a boiling 16 % H₂SO₄ solution containing 100 g/l of CuSO₄.6H₂O for 24 h and followed by a bend test. Based on the appearance at the bend portion, the specimens were classified into four categories viz. unaffected, fissured, cracked and broken. Electrochemical potentiokinetic reactivation (EPR) technique was used to determine the degree of sensitization (DOS) for the various categories of specimens. Eddy current (EC) tests were carried out to determine the eddy current amplitudes for the various categories of specimens in both the as-aged and Strauss-test exposed condition. The results of the EPR tests and the EC measurements were then classified into these four categories of specimens, in the as-aged condition and Strauss-tested condition (prior to bending). The results indicated that it was possible to predict DOS in the as-aged conditions using EC technique, though the overall change in the EC amplitude was small. The EC response from the Strauss test-exposed specimens was much higher than that observed in the as-aged specimens. The results indicated that EC testing was sensitive enough to detect both sensitization and IGC. The differences in the response of the two sensitized steels to eddy currents indicated that their carbon content played a major role in altering the chemical composition and the geometrical features of the depleted regions.