CORRELATION BETWEEN ULTRASONIC VELOCITY AND MAGNETIC ADAPTIVE TESTING IN FLAKE GRAPHITE CAST IRON

A recently developed nondestructive method, called Magnetic Adaptive Testin g was applied for investigation of flake graphite cast iron samples having various metallic matrices and graphite structures. MAT is typical by its low required magnetization of samples, because it is based on measurement of families of minor magnetic hyst eresis loops. The flat samples were magnetized by an attached yoke and sensitive descriptors of their magnetic/structural state were obtained from evaluation of the measured d ata. Ultrasonic velocity measurements were performed and results of the non -destructive magnetic tests were compared with these data. A very good correlation was found between the magnetic descriptors and u ltrasonic velocity. Cast iron is one of the most frequently used industrial construction materials. Low cost of production, good ma- chinability, and excellent possibilities of shaping the details by casting attract an intense interest of industry. The cast irons are generally many-component alloys of iron with large content of carbon. The cast iron structure is classified by its metallic matrix composition (ferrite, pearlite, carbides, etc.) and by morphology of its graphite inclusion. The mechanical properties are fundamentally dependent both on the matrix composition and on the graphite shape (flaky, spheroidal, vermicular, etc.), size and density (1). One of the types of cast iron - the flake graphite cast iron - is frequently used for mechanical components in bearings, brake shoes, etc. be- cause of its high wear resistance and damping capacity. The flake graphite cast iron is an ideal material for automobile brake disks since it has excellent damping properties and thermal conductivity just because of the flaky graphite. A nondestructive inspection of construction materials made of cast iron is highly desirable. Various non-destructive evaluation techniques have been examined so far, as eg alter- nating current potential drop (2), laser acoustic wave (3), ultrasonic back-scattering (4), or eddy currents (5). Graphite and other structures composing the cast iron matrices may be evaluated using electromagnetic properties such as conduc- tivity and permeability (6). Magnetic measurements are also frequently used for characterization of changes in structure of ferromagnetic materials, because magnetization processes are closely related to microstructure of the materials. This fact also makes magnetic measurements an obvious candidate for non-destructive testing, for detection and characterization of any defects in materials and manufactured products made of such materials (7). One of the most frequently used magnetic measurements is the detection of B-H curve. Structural non- magnetic properties of ferromagnetic materials have been non-destructively tested using traditional magnetic hysteresis measurement methods for a long time with fair success. A number of techniques have been suggested, developed and currently used in industry, see eg (8). They are mostly based on detection of structural variations via the classical macro- scopic parameters of hysteresis loops. An alternative, more sensitive and more experimental- ly friendly approach to this topic was considered recently, based on magnetic minor loops measurement. The survey of this technique can be found in (9). The method, called Magnetic Adaptive Testing (MAT) was presented, which introduced general magnetic descriptors to diverse varia- tions in non-magnetic properties of ferromagnetic materi- als, optimally adapted to the just investigated property and material. MAT was successfully applied for characteriza- tion of material degradation in different specimens and it seems to be an effective tool eg for replacement of the destructive hardness and/or ductile-brittle transition tem- perature measurements. In this work the direct correlation between MAT pa- rameters and ultrasonic velocity in flake graphite cast iron is studied using samples with different graphite structures and matrices. Although ultrasonic velocity is not consid- ered as the most important parameter of cast irons, it is a frequently measured quantity. The micro-structure of flake graphite cast iron is the most important parameter from point of view of cast iron properties. The correlation between graphite morphology and magnetic parameters has already been studied recently (10) based on the meas- urements performed on three as cast samples with differ- ent chemical compositions. It was also shown that ultra- sonic velocity depends on the area fraction and length of graphite (6) on the same samples. The purpose of the present work is to study the direct correlation between ultrasonic velocity and MAT parame- ters on three series of flake graphite cast iron samples, where apart from as cast samples two kinds of heat treat- ments (annealing to obtain a ferrite based matrix and nor-