Effects of Grain Size and Cooling Rate on Magnetoacoustic Stress Measurement in AISI 4140 Steel
Effects of microstructures on the uniaxial stress dependence of low-field magnetoacoustic response were studied. Test samples were cut from a single length of AISI 4140 steel bar and heat treated to obtain both coarse- and fine-grained specimens for study. Within these two classifications, the specimen cooling rates included both air- and furnace-cooling to contrast coarse and fine pearlitic development. The specific test procedure involved measurement of the fractional frequency shift of phase-locked ultrasonic waves propagated in a sample simultaneously being magnetized. Curves of frequency shift vs. magnetic induction were then obtained for each specimen at various applied test loads.
The results indicate that both the permeability of the samples and the quantitative behavior of the fractional frequency shift curves are dependent on heat treatment and microstructure. The most pronounced difference in behavior was found between the as-recieved sample and all of the heat treated samples.
Qualitatively, the test results from this study agree with previous investigations of various low- and medium- carbon steels. It is concluded that the stress dependence of acoustic fractional frequency shift exhibited in the magnetoacoustic test method is more dependent on carbon content than on microstructure.
KeywordsDomain Wall Motion Medium Carbon Steel Increase Cool Rate Heat Treated Sample Applied Tension
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