Ultrasonic and magnetic analyses of porosity in iron compacts
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I. INTRODUCTION
THIS paper describes work done as part of an evaluation of the potential of magnetic methods and ultrasonic measurements to characterize porosity in materials. Because the strength and ductility of porous materials can usually be related to the porosity, F~l measurement of the porosity by nondestructive methods is highly desirable. Therefore, a series of iron compacts with various porosities were analyzed by magnetic methods, and the effects of the pores on coercivity, hysteresis loss, and permeability were measured.t2J In addition, the effects of the pores on acoustic and magnetic Barkhausen signals were analyzed.t2] In similar compacts, the effects of pores on ultrasonic wave attenuation and velocity were determined, t3~ This paper extends these studies of the effects of porosity on the magnetic and ultrasonic properties of iron compacts to the characterization of their mechanical properties.
phasize the effect of initial porosity. Porosity, pore sizes, and pore shapes were characterized on sections parallel to the tensile axis at the mid-diameter of the gage length, t41 The grain structures were equiaxial. Because of the different processing conditions, the average grain size of the compacts varied, but only the 0.3 pct porosity compact had a significantly different grain size. Magnetization measurements were made using a state of the art magnetic hysteresisgraph that has been described previously, tsl Barkhausen measurements were made by using a search coil. Magneto-acoustic emission measurements were made using a broadband ultrasonic transducer which was coupled to the specimen. These methods have been described elsewhere. [61 The ultrasonic velocity and attenuation were measured in a pulse-echo configuration in an immersion system. The procedures used have been presented earlier, t3'71
III. EXPERIMENTAL RESULTS II. MATERIALS AND PROCEDURES The iron used in this study was prepared from - 3 2 5 mesh water atomized iron powder. Compacts 50 cm by 50 cm by 150 cm were prepared from the powder using varying compacting pressures to achieve different densities. The processing and heat treatment conditions for the iron compacts used for the magnetic and ultrasonic wave analyses are given in Table I. The chemical composition of the iron compacts in wt pct after the final treatment of 100 hours in dry hydrogen at 1120 ~ was 0.005C, 0.001N, 0 . 0 1 3 0 , 0.013P, and 0.015S. The resultant porosities in the different iron compacts were 0.3, 1.5, 3.7, 6.2, and 11.1 pct (Table I). Details on the microstructure and mechanical properties are given in Table II. The mechanical properties of the iron compacts have been described previously t41 but are presented here in slightly different form to emW . A . SPITZIG, Senior Metallurgist, R.B. T H O M P S O N , Director and Professor, and D.C. JILES, Associate Scientist, .Center for N.D.E., are with the Ames Laboratory, Iowa State University, A m e s , IA 50011. This paper is based on a presentation made in the s y m p o s i u m "Nondestructive Evaluation--Predicti
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