514 zyxwvutsrqponmlk IEEE TRANSACTIONS ON MAGNETICS, zyxw VOL. MAG-22, NO. zyx 5, SEPTEMBER 1986 z EFFECT OF STRESS ON MAGNETIZATION AND MAGNETOSTRICTION IN PIPELINE STEEL* D,L, Atherton and J.A. Szpunar Department of Physics, Queen's Universfty Kingston, Ontario K7L 3N6, Canada Abstract Measurements of the effects of stress on the initial magnetizatiun curve of zyxwvutsrqp a sample of 1% Mn pipe line zyxwvutsrq steel and on its reversible component are presented. Stress-induced changes in the reversible and irreversible magnetization components are shown. Examples of magnetization changes induced by tensile and compressive stress cycles of various amplitudes and at different fields are given. Magnetostriction measurements made on the Same sample under various tensile and compressive stresses measured along anhysteretic and initial magnetization curves are included. A simple interpretation zyxwvutsrq is suggested based on the assumption that stress changes the alignments of magnetic domain easy axes with respect to the strain. Introduction Our previous investigations of the effects of stress on magnetization of pipe lines' have encouraged us to study specimens of pipe line steel in order to determine the effects of stress on magnetization and magnetostriction. Low field changes in magnetoelastic properties have already been discussed by Birss et a14. They demonstrated that for a range of carbon steels that have been subjected zyxwvutsrq to various metallurgical treatments the magnetization-stress curves are asymmetrical with respect to tension and compression, An understanding of the magnetization changes and magnetostatic and magnetoelastic interactions between defects is however difficult especially in construction steel where in- clusions, pearlite and cementite structures and also texture contribute to the complexity of such inte- ractions, Microstructure is responsible for the variety of the domain structures observed in steel. Szpunar and Szpunar5 have classified domain Structures in construction steel using four types of domain pat- terns. The observed domain configurations are complex and the proportion of different types of domains changes with the grain size. Stress will obviously modify such domain structure, To predict such changes we use the well known relation for energy assuming that a uniform tension of magnitude o is applied in the crystal direction, E = K zyxwvutsrqponml (a a + a2a2 + CL'C~~)-- - 2 2 zyxwvutsrqponmlkjihg h Q(cr2Y2 + 01 Y + a Y 1 2 2 2 2 1 1 2 2 3 31. 2 x00 I 1 2 2 3 3 ~X~~OD(~~CX~YIY~ + a2a3Y2Y3 + ~ s ~ ~ Y s Y I ) . where ax, az, a3 are the directional cosines of the magnetization directions and the YL, YZ9 Y3 are the directional cosines of the stress. Thus to the crystal energy is added a term crdl/l. known as a magnetoelastic energy. Since this energy is a function of the product zyxwvuts hcr we may expect that a crystal which has positive magnetostriction when deformed by tension will behave like a crystal having negative magnetostriction but deformed in compression I In line pipe steel the magnetocrystalline energy is a dominant part of the total energy. K, is high and the easy magnetic direction is <I OO>. For steel Ao is positive for tension and the corresponding energy term will be a minimum when the orientation of the domain magnetizations is parallel to the direction of the stress If the A0 product is negative, as in compression tests, the energy minimum corresponds to a domain magnetization orientation which is normal to the direction of the compressive stress, An understanding of the effect of stress on magnetization of pipe steel is important not only because of the need for funda- mental knowledge of magnetization processes but also because it may help to facilitate the magnetic detection of stress, - Experimental Results 1% Manganese line pipe steel has been subjected to tensile and compressive stresses. Magnetization changes and magnetostriction have been measured. The experimental apparatus has been described previously6. Reversible components of magnetization have been determined by assuming that small reversible changes in magnetization can be summed and that each magnetization state of the specimen is achieved as a result of the reversible and irreversible magnetization changes, An example of the method used for an unstrained sample is presented in Fig. 1 Fig, 2 shows the magnetization changes caused by tension and compression applied at a low field of lkA/rn. The magnetization is separated into reversible and irreversible components. Tensile stress increases the magnetization more than compressive stress. The changes in the irreversible components are much higher than in the reversible components, An increase in magnetization with compression or tensile stress is observed at low fields; however, at higher fields the situation becomes more complex. This is indicated by Fig. 3 which gives initial magnetization curves measured for zero stress and stresses of -500 kPa and 500 kPa, There are three transition points which indicate that at various field strengths the effect of stress on magnetization can be very different. This problem is further illustrated in Fig. 4 where stress induced changes in magnetization for three different values of the field strength are presented. The specimen was initially demagnetized then magnetized to the indicated field then tensile stresses of 200, 400 and 600 MPa were applied in three stages. Each stress was relaxed to zero before the next higher stress value was applied, The specimen was then demagnetized and similar compressive stresses applied. The increase in magnetization with tensile stress at low fields contrasts with the decrease at high fields, The Fig. 1: FIELD STRENGTH LkAIrn] Initial magnetization curve showing the small field reversals used to separate the reversible component (R). *Research supported by Canadian National Research Council (IMRI) and Department of Energy, Mines and Resources (CANMET),