Acoustic emission during deformation of dual-phase steels
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I.
INTRODUCTION
A C O U S T I C emission (AE) is a transient elastic wave generated by a rapid release of strain energy within a material. During deformation, dislocation motion, crack nucleation and propagation, inclusion fracture, twin formation, etc. can produce AE. Therefore, the studying of AE during deformation can help us to understand the dynamic processes of deformation. Though some scientists t1'21 have studied AE during deformation of some steels, finding that there was an AE peak in the yielding region, the theoretical analysis of the peak was not satisfactory. In addition, no studies of AE during deformation of dual-phase steels have been performed as yet. In the quenched condition, yielding of the dual-phase steel is continuous. After tempering at certain temperatures, the yielding plateau may return. The yielding stress and the L~iders strain vary with tempering temperature. Therefore, it is expected that the AE in the yielding region will vary significantly as the heat treatment condition varies. The purpose of the present work is to study the variation of AE during deformation of dual-phase steels as the heat treatments vary and to give an explanation of the results. II.
EXPERIMENTS AND RESULTS
A. Materials and Tensile Tests The 20C and 13MnVN steels were selected for this study. Their chemical compositions are given in Table I. For tensile testing, a sheet specimen of 2.6 • 6 mm 2 crosssectional area and 30-mm gage length was used. The tensile direction was parallel to the rolling direction. The speed of the machine's crosshead was 0.5 mm/min. Tests were carried out at room temperature.
Q.Y. LONG, Associate Professor, and YIN HUAZI, Technician, are with the Institute of Metal Research, Academia Sinica, 2-6 Wenhua Road, Shenyang 110015, People's Republic of China. Manuscript submitted November 8, 1988. METALLURGICAL TRANSACTIONS A
B. AE Measurements An AE measurement instrument with a 'center frequency of 120 KHz and a bandwidth of 48 KHz was used. The total gain of the measurement system was 97 db, and the threshold voltage was set at 0.5 V. The total AE energy, EE, and the energy rate, A E / A t , were determined in these AE measurements. In this work, At is 1 or 0.3 second. The AE energy was recorded by counts; the larger the counts, the higher the AE energy. When total counts of EE reach 104 or 105, the counts automatically reset to zero. The details of the technique to determine the AE energy are described in Reference 3. C. Heat Treatment and AE during Deformation 1. FP treatment The FP treatments are those heat treatments in which the samples are heated in the temperature range of 740 ~ to 815 ~ for 30 minutes, followed by isothermal transformation for 60 to 90 minutes in a 550 ~ salt bath, then air-cooled (AC). After the FP treatments, a microstructure consisting of ferrite (F) and pearlite (P) was obtained. The details of the FP treatment are given in Table II. The typical variation of load and AE energy with time during deformation is shown in Figure 1. The characteristics of the AE
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