Study of magnetic anisotropy in API 5L steels from crystallographic texture and barkhausen noise measurements

Magnetic anisotropy was studied through the characterization of magnetocrystalline energy (MCE) in API 5L steels from crystallographic texture and Barkhausen noise (BHN) measurements for different sample geometries and microstructural and texture characteristics. Experimental measurements were carried out to determine the angular dependence of the BHN signal in the band from remanence to saturation (SR) of the hysteresis loop of these steels. The samples texture was analyzed by two methods: X-ray diffraction and Electron Back-scatter Diffraction (EBSD). From the X-ray texture data, the orientation distribution function of the polycrystal was determined to estimate the average MCE in the investigated steels using the series expansion method. In the case of EBSD measurements, the average MCE was determined by averaging this energy over the individual grain orientations of the raw EBSD data. The EBSD-derived MCE was found to be in good agreement with the predictions made from X-ray texture for all the investigated steels. The statistically sufficient number of grains for the accurate EBSD-derived MCE estimation in the studied steels was determined to be in the order of several hundreds to a thousand. The ability of BHN measurements to determine the MCE was validated for different sample geometries, microstructures, and textures. This was achieved by comparing the BHNderived MCEs with the predictions from X-ray and EBSD measurements. In the studied materials, the average BHN-derived MCEs were found in close agreement with the predictions made from X-ray and EBSD measurements. A stochastic physical model and a simulation mathematical framework were developed in order to explain the observed correlation between the MCE and BHN signals in the SR band. The proposed stochastic model helps evaluate the angular dependence of the MCE for API 5L steel samples from their texture, grain size distribution, and carbon content. These parameters were used to model the magnetic free poles distribution at grain and interface boundaries in each steel and, consequently, the number and strength of reverse-domain nucleation and growth events as the material goes from SR. The statistical distributions of the reversed-domain nucleation and irreversible growth fields were obtained for a large number of grain boundaries and used to estimate the BHN signal at each angular position of interest. The modeled average MCE was in good agreement, both in shape and relative magnitude (from one material to the next) with the average BHN-derived MCE and also predicted from texture measurements. This points out to the fact that the strong correlation between the BHN signal in the SR band and the MCE is due to role that the MCE plays in the formation of magnetic free poles at grain boundaries. The results obtained in this thesis prove that the proposed model is capable of reproducing previous experimental results and explaining the relationship between the MCE and BHN signals, based on the microstructural and texture characteristics of the investigated steels. Study of magnetic anisotropy in API 5L steels Chapter I 1 CHAPTER I: INTRODUCTION Low carbon steels such as API 5L steels are widely used in pipelines for the transportation of oil and gas products in the petroleum industry, not only due to their excellent mechanical properties, but also because they have very good magnetic properties. It is well known that the magnetic properties of pipeline steels can be affected by several metallurgical parameters such as physicalchemical composition and phase, grain size, microstructure [1, 2], crystallographic textures [3], stress, temperature, and magnetic field [4]. Through the application of this principle, the vast majority of nondestructive methods have been developed based on the magnetic properties of functional materials. There are several magnetic measurement techniques for the in-line inspection (ILI) of operating pipelines such as magnetic flux leakage (MFL) ILI, Barkhausen noise (BHN), magnetoacoustic emission, and the method of metal magnetic memory [5]. These techniques, which are known as Non-destructive Testing (NDT) methods as ILI classifies, are used to detect macro and microdefects associated with metal loss and cracks in steel structures and residual stress (by means of BHN) through the application of an intense magnetic field to the component under inspection. Although MFL is an effective method for pipeline inspection [6-9], it still has some limitations with respect to MFL signals such as the strong variation of MFL signal amplitude and subsequent difficulty in sizing defects when the pipe suffers high stress levels (up to 70 % of the yield stress of the pipeline material) generated during operating conditions associated with oil and gas transportation. In order to improve the MFL detection efficiency and its ability to assess the stressdependent magnetic properties, the determination of the magnetic easy axis and magnetic anisotropy in pipeline steels was proposed through the study of the angular-dependent method of BHN measurements [6]. It was concluded in reference [7-9] that pipeline steels may exhibit bulk magnetic easy direction along the pipe axis and the effect of cold working during manufacturing process is responsible for this magnetic anisotropy rather than the influence of crystallographic texture. Many other papers also deal with the correlation of magnetic anisotropy and magnetic easy axis with BHN measurements in plastically deformed low-carbon steels [10] and materials under stress [11-16]. It can be observed that in the reviewed works, the analyzed anisotropies are associated with the magnetomechanical effects by considering the main peak of BHN signals around the Study of magnetic anisotropy in API 5L steels Chapter I 2 coercivity point in the hysteresis loop. This effect includes all the contribution of other anisotropic components, which cannot be separated in the analysis. According to reference [17], magnetic anisotropy includes several forms such as magnetocrystalline energy (MCE) and other induced anisotropies such as shape, surface, and stress anisotropies. The use of an anisotropy component during any analysis depends on the practical consideration and specific situation found in the material (for example: the material state can be stressed or unstressed; the magnetization within the material is not along an easy direction, etc.) in which the competence of energy forms is determined by minimizing the total energy of the magnetic system, which in fact is not necessarily to be a minimum energy condition [18]. It has been known that in an unstressed sample under the Curie temperature the magnetic anisotropy is mainly contributed by MCE, which is an intrinsic magnitude associated with the crystal lattice [19]. The domain structure of the material is determined by the MCE, which is often larger in magnitude (by several orders) than the contribution of any induced anisotropy [20]. By consequence, MCE has a strong influence on the way that ferromagnetic materials can be used and manufactured so that the study on the MCE has attracted a great deal of practical interest in the last decades and has been the focus of recent researches as well. It is important to note that, in the case of ILI of operating pipelines, the pipe wall remains magnetized after the inspection. This so-called remanent magnetization can play a significant role in the degradation mechanisms of the pipeline [21]. This fact was reported in reference [22] in which the residual magnetic fields were measured close to 0.3 0.4 T in API 5L steels inspected by a MFL tool, while along the axial direction of the pipeline can reach 0.85 T after a saturation magnetization of 2.07 T is applied. The strength of both the applied and remanent magnetic field along different directions of the pipe wall depends of the magnetic properties of the steel used to make the pipeline [23]. In addition, the combination of microscopic parameters such as microstructure, grain orientation, crystallographic texture, and other induced magnitude like stress may change the behavior and character of the anisotropy within the material. During a MFL inspection the pipe wall is magnetized near to saturation. This fact may cause changes in the position of the net magnetization vector of the pipe with respect to its original direction before the inspection. In such a case, it results very difficult the determination of the bulk magnetic easy axes in the pipeline, as magnetic properties also depend on the history of the material magnetization due to hysteresis. In this regard, the best way to accurately determine the magnetic easy axes in ferromagnetic materials is by means of the average MCE. Although several Study of magnetic anisotropy in API 5L steels Chapter I 3 methods such as torque curves, magnetization curves, and crystallographic textures [24] (commonly from X-ray global texture) exist for the MCE measurement and estimation, they are not easy to handle and impossible to carry out in field conditions either nondestructively or destructively. The latter requirements can be satisfied perfectly if one considers the ability of BHN measurements to investigate magnetic anisotropy in steels as has been reported by most of the reviewed works [3-16, 23]. However, a relationship between the MCE and BHN measurements, which may open up a promising branch for the non-destructive application of BHN signals, is actually a new research field with a very little number of publications until this moment. The influence of crystallographic texture on the determination of the magnetic easy axis in pipeline steels is still not well understood. Without doubts, an appropriate way to evaluate this relationship can be achieved through the determination of the average MCE. The following review of related works, which have been done about the MCE and BHN up to the present, can help clarify in depth this obse