An available analytical expression for irreversible variations of the remanent magnetization in an open-shaped ferromagnetic body resulting from mechanical loading is examined experimentally. The analysis was carried out using results of studies performed for chromium steels and Fe-Co alloys subjected to various heat treatments, as well as for R-Fe alloys.It is well known [1-5] that a ferri or ferromagnetic body characterized by remanent magnetization 1/ J and nonzero magnetostriction ( λ s ≠ 0) is demagnetized irreversibly during a deformation. In this case, the demagnetization value depends on applied external stresses σ and the number of loading-off-loading cycles (H-P).This phenomenon is the basis for the operation of magnetoelastic transducers for peak mechanical loads [6][7][8]. An open-shaped ferromagnetic body brought into the remanent magnetized state serves as a sensing element of such a transducer. Mechanical loading of the sensing element leads to an irreversible decrease in its magnetization. On the basis of this irreversible change in the magnetization, the load applied to the sensing element is determined.To improve the metrological properties of such transducers, the functional relationship between the irreversible decrease in the magnetization and the applied mechanical load need to be known.The aim of the presented work is to check experimentally, in a series of materials, an available analytical dependence of irreversible variations of the remanent magnetization of an open-shaped ferromagnetic body on applied mechanical stresses.The analysis was carried out using results of studies performed for chromium steels and Fe-Co alloys (Vicalloy) subjected to various heat treatments, as well as for R-Fe alloys [3,5,8].The investigation was performed using the following cylindrical samples: steel samples 5.5 mm in diameter and 15 mm in length, Vicalloy samples 6.0 mm in diameter and 14.7 mm in length, and samples of rare earth metal-containing alloys 10 mm in diameter and 31 mm in length. Measurements were carried out using the following scheme.The samples were magnetized to saturation along their length by an electromagnet and subsequently subjected to loading-off-loading using a test bed, which was designed on the basis of a tensile-testing machine. Then the magnetization of samples in the off-loaded state was determined.The magnetization was determined by a pulled-off coil method. The coercive force was measured by a vibrating-sample magnetometer. The magnetostriction was determined using bonded strain gages connected to a dc bridge circuit.The following expression, which relates the remanent magnetization of an open-shaped ferromagnetic body to the applied mechanical stress, was suggested in [7]:(1)where J r is the remanent magnetization of a sample before the loading, J is the magnetization of the sample subjected to loading and subsequent off-loading, σ is the absolute value of the mechanical stress, and β 1 is a coefficient. In this formula, β 1 is constant for a given material.To check the valid...
The main reason for pipeline weld failure is the formation of cold cracks in the heat-affected zone on account of the increased tendency to brittle failure. The cooling time after welding affects the tendency to cold cracking, and the critical cooling time is related to a parameter characterizing the cracking resistance for various steels. Basic principles are presented for welding low-alloy tube steels at low temperatures.The exploitation of new oil and gas deposits makes it necessary to construct industrial pipelines in northern regions of West Siberia, where building work can be done preferentially in the winter at low air temperatures (down to -50°C). A basic step is installation by welding, which largely determines the reliability, and in actual pipelines, cracking is most often due to welded joints.A survey of the reasons for welded joint failure in pipelines indicates that the main one is the formation of cold cracks in the heat-affected zone (HAZ) because of the elevated tendency to brittle failure.The cracking probability increases on welding at low air temperatures, which is due to the presence of quenched structures and extensive hydrogenation of the welded joint.Working reliability is largely determined by the proper choice of materials in combination with the best conditions for installation welding.There are no scientifically based criteria for choosing welding modes for negative temperatures, which hinders the definition of the best technology and often leads to unjustified complication and considerable expense on account of additional measures (heating before and after welding, thermal insulation of the installed components, and so on).Published data [1-5] and our studies show that one should use the parameter σ pmin characterizing the cracking resistance of steel in order to evaluate modes of welding and to define sound conditions for preventing coal cracks. For industrial pipelines of diameter 114-512 mm with wall thickness δ up to 16 mm, the critical value of σ pmin at low temperatures (down to -60°C) is 360-400 MPa [5,6].We have found that one should choose the conditions for heating installation joints in pipelines to eliminate cracking in the weld in the winter on the basis of the cooling time for the weld metal from 300 to 100°C, namely t 100 300 (Fig. 1), since as the cooling time for the metal in that temperature range increases, so does the release of hydrogen into the sur-
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