Acoustic emission during tensile deformation of pre-strained nuclear grade AISI type 304 stainless steel in the unnotched and notched conditions

Acoustic emission (AE) generated during tensile deformation of notched specimens with varying notch lengths has been compared with those from unnotched specimens of a nuclear grade AISI type 304 stainless steel in the 5 and 30% pre-strained conditions. The results indicate that (a) nature of AE generation is different for different stages of deformation and (b) amount of cold work or pre-strain influences the magnitude of such AE generation. The observed results have been explained using the phenomena of varied localized deformation at the notch tip and deformation-induced α'-martensite formation in cold worked AISI type 304 stainless steel. An examination of the correlation between total AE counts (N) and stress intensity factor (K) has shown that the value of the exponent (m) in the relationship N = AKm decreases with increasing pre-strain. The formation of α'-martensite in unnotched specimens has been confirmed by equivalent δ-ferrite (%) content measurements. The examination of fracture surfaces by scanning electron microscopy (SEM) has indicated that the localized plastic deformation at the notch tip depends on the level of pre-strain

Study of the tensile behavior of AISI type 316 stainless steel using acoustic emission and infrared thermography techniques

Acoustic emission (AE) and infrared thermography technique (IRT) have been used to study the tensile behavior of AISI type 316 stainless steel. Strain rates of tensile testing were varied from 1.4 × 10−3 s−1 to 1.4 × 10−2 s−1. AE root mean square voltage increases with increase in strain rate due to the increase in source activation. Dominant frequency of the AE signals generated during different regions of tensile deformation has also been used to compare the results for different strain rates. The dominant frequency increases from elastic region to around 590 kHz during work hardening and 710 kHz around ultimate tensile strength (UTS) for all the strain rates. Temperature changes during different regions of deformation are monitored using infrared thermography. The temperature rise in the work hardening region is found to approximately increase linearly with time and from the slopes of the linear regression analyses the rate of temperature rise in the work-hardening region is obtained which is found to be very sensitive to strain rates. From the experimental results an empirical equation that relates the rate of temperature increase with strain rate and thermal hardening coefficient is obtained. The correlation between the variation of AE dominant frequency and temperature rise during different deformation regions provided better insight into the tensile behavior of AISI type 316 SS for different strain rates

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

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