25 mm 009 in diameter lath can produce a detectable

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2.25 mm (0.09 in.) diameter lath can produce a detectable horizontal displacement even when oriented so that the weakest signal propagates to the receiver. Dynamics of Transformations Acoustic emission techniques are very useful for following the kinetics and measuring the dynamics of martensitic transformations. For example, Fig. 28 shows the acoustic emission that occurred during the continuous cooling of a high carbon steel. The M s temperature for this steel was about 200 °C (400 °F); only a few acoustic emission counts were generated above this temperature and these might have been associated with a local stress assisted transformation. As the temperature continued to decrease below M s , increasing rates of emission were observed, each emission presumably associated with the rapid growth of a martensitic plate across an ( ) = Δσ t t 2 0 0 –0.176 0 0.085 0 0 0 0.550 ( ) = Δσ t t 2 –0.0287 –0.0159 –0.0244 0.0157 –0.0623 0.0890 0.0244 –0.0089 0.2032 ˙ v t = 10 2 v t = 3 2 3 . π 76 Acoustic Emission Testing F IGURE 28. Acoustic emission counts detected on cooling a high carbon steel: (a) total; (b) rate. Total acoustic emission (10 7 counts per cubic centimeter) 12 8 4 0 100 150 200 250 (212) (302) (392) (482) Temperature, °C (°F) Rate of acoustic emission (10 6 counts per cubic centimeter per second) 100 150 200 250 (212) (302) (392) (482) Temperature, °C (°F) 1.5 1.0 0.5 (a) (b)
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austenite grain. The maximum transformation rate appeared to be about 60 °C (108 °F) below M s and emission ceased entirely at a temperature difference of about 100 °C (180 °F) below M s . The observation that intense acoustic emission signals are emitted at the beginning of martensite transformations has been used to measure M s values as part of an alloy development program. 106 The M s values were reported for numerous steel alloys with different compositions and microstructures. The technique was reported to be a simple and accurate means for deducing martensite transformation temperatures. The acoustic emission accompanying the transformation was reported as extremely sensitive to the microscopic processes involved in the transformation. The carbon concentration of low alloy steels 102 has a strong effect on both the temperature dependence of the emission during continuous cooling and the number of detectable signals per unit volume (Fig. 29). The temperature dependence of the emission is consistent with the decrease in M s with increasing carbon concentration. The effect of carbon concentration on the number of detectable signals, however, is more likely to be a manifestation of changes in the dynamics and morphology of the martensite transformation. Effect of Cold Working Cold working also influences the acoustic emission during the martensitic transformation of steel. The nature of the cold work effect is very sensitive to the carbon and nickel concentrations that control martensite morphology and the transformation kinetics. The cold work effect, like the carbon concentration effect, resides in changes to the morphology and dynamics of individual martensitic transformations.
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  • Fall '19
  • Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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