The paper is concerned with the effect of applied macrostress on the magnetic characteristics of pipeline steels.Magnetic hysteresis parameters are examined as a function of tensile stress, tangential stress, and internal pressure.The study is performed on the hot-rolled 1010 steel alloyed with Mn and Si, widely used in Russia, on the X42SS steel designed specially for operation in hydrogen-containing media and on the X70 steel produced by controlled rolling and intended for heavy-duty trunk pipelines.The results obtained are discussed in terms of the development of nondestructive physical techniques for the diagnosis of the current state of pipe material in course of operation.

The reliable operation of trunk pipelines requires efficient monitoring of their technical state.At present, the issues involving evaluation of stress and strain in the most dangerous black spots of pipelines are urgent for the oil-and-gas industry.The problem is especially pressing for operations in corrosion-active media containing hydrogen sulfide, since hydrogenation leads to the formation of pores and cracks in steel.This paper is concerned with the magnetic, mechanical and structural characteristics of two advanced steel grades which are widely used for the construction of high-duty gas pipelines.The study shows that the numerical values of magnetic parameters depend on the magnetization direction about the control-rolling axis and correlate with the strength of X70 steel grade.It is found that for estimating the operating tensile stress, one should measure the magnetic characteristics in different directions about the pipe axis.Exposure to the hydrogen-containing medium leads to a significant decrease in the metal plasticity, an increase in the ultimate stress and yield stress, and a slight increase in the coercivity.The duration of the hydrogen charging dramatically affects the deformation behaviour of coercivity, thus providing a tool for magnetic monitoring of mechanical stress in steel pipes for transporting aggressive types of natural gas.

The evolution of the magnetic characteristics of case-hardened Steel 20 under the influence of elasto-plastic tensile, compressive, and torsional strains is analyzed. The field dependence of the differential magnetic permeability is shown to reflect a three-layer nature of the obtained micro-structure: a hardened surface layer, a transition zone, and a ferrite-pearlite core. It is proposed to evaluate the level of the applied stresses on the basis of the peak field and the height of the differential magnetic permeability peaks of corresponding layers of the case-hardened steel.

The paper studies the magnetic, mechanical and structural characteristics of X70 steel after controlled rolling. The coercive force H(c), residual induction B(r), maximum permeability mu(max) and maximum permeability field H(mu max) measured in situ in the loaded or unloading steel magnetized in different directions about the pipe axis arc analyzed for applicability as parameters from which the working tensile stress in a pipeline and its preceding overload with attendant transition to the plastic range can be determined. An approach is proposed for estimating the accumulated damage level in the steel under tension or torsion from the coercive force H(c) and/or saturation magnetization J(max)

Main pipelines are characterized by a highly indeterminate stress-strain state of pipes and complex conditions of thermomechanical loading, the principal loads being internal pressures and stresses in metal caused by differences in pipeline construction and use conditions resulting from soil mobility, non-design loads during floods etc. [1]. Therefore it seems urgent to develop reliable methods of estimating stress loading and damage of metal, which would offer advice on optimising pipeline service conditions and conclusions on the strength and safety of a system in the attempt to increase its durability [2]. The applicability of magnetic methods to the estimation of working stresses in pipelines is discussed in a number of studies made on hot-rolled pipe steels like St2, St4, 17G1S [3, 4]. However, much of pipeline steel is currently produced by controlled rolling, when higher strength and cold-resistance are attained due to lower temperature and higher extent of reduction in the final stages of rolling. Recrystallization and the growth of austenite grain therewith slow down, particularly, in the presence of dispersed precipitations of carbonitrides [1]. The fall of the rolling end temperature to the (γ+α)-region leads to the formation of ferrite grains with high dislocation density and a pronounced strain texture [5]; this affects magnetic properties considerably and one must taken it into account when developing magnetic techniques for estimating the state of pipelines. This paper seeks approaches to estimating internal stresses, as well as working stresses and damage, of control-rolled pipe steel by magnetic characteristics measured under uniaxial stress and torsion.

Paramagnetic VKNA superalloy on the base of Ni3Al intermetallic phase (L12 superlattice) under cold rolling deformation demonstrates superparamagnetic behavior associated with the formation of nanosized ferromagnetic clusters within the paramagnetic matrix. It may be assumed that these clusters correspond to the long-period phase with DO22 superlattice.

Introduction The development of the steels with increased nitrogen content (“high nitrogen steels – HNS”) is a promising direction of creation of economically alloyed high strength wearand corrosion-resistant materials [1-4]. The casting under nitrogen pressure (counter-pressure casting technique) allows to avoid using of expensive alloying elements (first of all, nickel) and, due to addition of strong austenite former nitrogen, form in Fe-Cr-N system steels fully austenitic structure with no deterioration of corrosion behavior and providing with low magnetic conductivity of austenitic steels. There is a great deal of data stored in scientific literature about strength, corrosive and tribological properties of HNS [1-7]. However, insufficient attention is devoted to physical properties of HNS and the possibilities of nondestructive testing of their chemical and phase composition, hardness and wear resistance. In the present work electromagnetic properties, phase composition, hardness and abrasive wear resistance of two high-nitrogen steels produced with counter-pressure casting technique are investigated. The steels are Kh19A1.0 cast steel (in wt.%: 1.00 N, 0.07 С, 19.02 Cr, 0.17 Mn) and Kh22GA1.24 hot deformed steel (in wt.%: 1.24 N, 0.08 С, 22.20 Cr, 1.38 Mn). For the purpose of comparison the industrially produced carbon-containing 95Kh18 steel (in wt.%: 1.00 С, 17.72 Cr, 0.48 Mn) is studied. Specimens with dimensions of 5.4×5.4×61 mm were oil quenched from temperatures of 950-1200°C. Hardness of the specimens was measured using Rockwell technique. The basic magnetic characteristics were determined by means of Remagraph C-500 setup, the electrical resistivity – using technique described in [8], the parameters of signal of double resonance electromagnetic-acoustic transduction (EMAT) – using technique described in [9], eddy-current characteristics – using technique described in [8]. Phase composition was determined using magnetic analysis method. Relative abrasive wear resistance (with respect to wear resistance of armco-iron) was investigated under wear over fixed abrasive (electrocorundum with 160 μm grain).

The wide use of products manufactured from structural steels made by powder metallurgy is first of all dictated by lower power and material consumption. The safe work of powder steel machine parts and structural members under operating loads requires reliable methods for nondestructive testing of the stress-strain state. To inspect the quality of products made of carbon powder steels, magnetic structuroscopy is extensively used [1], as this technique is noted for express information gaining and convenient measurement taking. There are practically no works on determining the stress-strain state of powder steels, although this subject-matter has long been discussed for conventional steels [2]. Porosity affects both the mechanical and magnetic properties of sintered steels, therefore it requires special attention. This paper presents investigations into the effect of elastic-plastic strains on the magnetic characteristics of powder structural steels with different residual porosity, including those in different structural states. Structural states were modelled by rolling deformation.

Special features of phase transformations due to tensile and torsional deformation of steel 12Kh18N10T are studied with the use of methods of magnetic structure and phase analysis including the Barkhausen method of magnetic noises (BMN).

The paper investigates the effect of tensile strains on the field dependence of differential magnetic permeability of double-layer composite steel 45 specimens simulative of surface hardening and steel 45 specimens subjected to laser beat hardening. It is shown that the field dependence of differential magnetic permeability is representative of the multilayer microstructure and of the changes occurring in each layer under tension. It is found that significant shifts of the magnetic permeability peaks toward stronger magnetic fields and rapid decreases in peak heights result from the involvement of different layers of the specimens in plastic deformation and proceed in a strictly defined sequence as the stress corresponding to the yield limit of each layer is reached. It is demonstrated that the lost bimodal character of the field dependence of differential magnetic permeability after unloading suggests that the material with a hardened surface layer nears fracture.

The effect of shear strain and accumulated damage on magnetic properties was studied on cylindrical steel rods subjected to torsion. The correlations obtained make it possible to evaluate the shear strain and current damage of metal, as well as to estimate the residual lifetime of an article under torsion, provided its magnetic parameters are available. Introduction: It is known that during plastic deformation, particularly in course of torsion, defects of metal continuity are generated (e.g. micropores, microcracks, vacancies, etc). The defects are observed as early as at early stages of plastic deformation. The formation of discontinuities is accompanied by partial relaxation of elastic energy [1], which leads to changes in the magnetoelastic energy of ferromagnetic material in the regions adjacent to the defects. This phenomenon is likely to affect the magnetic parameters of materials. Results: The influence of shear strain rate and torsion-induced damage on the magnetic parameters of metals was investigated. Analytical relationships were obtained for magnetic coercivity (from both major and minor hysteresis loops) as a function of strain and damage accumulated by a material. Commercial steel with 0.45% carbon content was studied. Measurements were made on hot-rolled rods 5mm in diameter. The test unit utilized made it possible to take magnetic parameters in the run of loading. Magnetic properties were determined from both major and minor cycles of magnetic hysteresis. As distinct from [2], all the magnetic properties were measured as a function of internal magnetic field. Measurements were made of the following magnetic parameters: maximum magnetic permeability μmax, coercivity Hc (hc) and residual induction Br (br) from major (Нmax= 60 kA/m) and minor hysteresis loops during magnetization in medium (bmax=0.4 T) and low (bmax=0.1 T) magnetic fields. The degree of shear strain Λ under torsion was estimated under the assumption that the sample cross-section radius is not subjected to distortion in course of loading [3], thus the shear strain degree on the surface of the sample is Λ = tgφ, (1) where φ is the angle between the printed mark on the sample surface and its generating line. The average degree of shear strain along the cross-section of the sample was calculated as [3] Λ = ⋅ = Λ ∫ ∫ 3 2 1 ~ 2 0 0 2 π φ φ π R dr d r tg R r R , (2) where R is the radius of the sample, r is its current radius ranging from 0 to R. The average integral degree of accumulated shear strain Λint was calculated by taking the summation over all the previous values of the strain degree. The parameter of material damage ω is used in mechanics to indicate the development of deformational microdefects. According to the phenomenological theory of fracture [4], ω = 0 before deformation, whereas ω = 1 when a fracture crack emerges. Since the state of strain was not subjected to changes in course of deformation, the parameter of material damage ω was calculated in accordance with the linear model by Kolmogorov for damage accumulation: [3] as