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Research describing the variation of magnetic properties in steels is presented. In particular the effects of grain size and dislocation density are examined by means of the Jiles-Atherton model.

A model for characterizing the influence of noncoaxial stress and field on hysteretic magnetic properties was presented. It was shown that at an angle of 0 degrees, coercivity tends to decrease in the transition from negative to positive stress. At 90 degrees, the opposite behavior was observed when coercivity increased as positive stress changed to negative.

Microstructural attributes such as grain size d and dislocation density [[zeta].sub.d] affect the hysteretic magnetic properties of steels because they affect domain wall movement and pinning. In an earlier paper, a model was proposed for computing hysteresis loops based on the effect of grain size and dislocation density. In that paper, hysteresis loops were compared that all had the same maximum field [H.sub.max]. The result was that coercivity departed from linear relationships with inverse grain size (viz. 1/d) and [[zeta].sup.1/2.sub.d] for large values of 1/d and [[zeta].sup.1/2.sub.d].The same was true of hysteresis loss [W.sub.H], except that hysteresis loss even showed a peak, first increasing and then decreasing with increasing 1/d and [[zeta].sup.1/2.sub.d]. This kind of behavior had not been seen by experimenters, particularly core-loss people. It was learned that the core-loss experimenters compared hysteresis loops of the same maximum flux density [B.sub.max], instead of the same [H.sub.max]. In this paper, we use the model previously formulated to produce hysteresis loops with the same [B.sub.max]. Indeed, the appropriate linear relationships are found. The paper also addresses effects of uniaxial anisotropy on these microstructural magnetic effects and why two hysteresis parameters are affected by microstructural variation. Index Terms--Hysteresis modeling, magnetic materials, microstructural effects, uniaxial anisotropy effects.

This paper describes experiments with 2Cr-1Mo-grade steel at various stages of creep damage. Properties of magnetoacoustic emission (MAE) were tested and compared to other magnetic properties such as hysteresis loop B(H), Barkhausen effect (HBN), and magnetostriction [lambda]. The creep-damage stage was evaluated from microstructural analysis and mechanical properties tests. It was revealed that incipient creep damage leads to decrease of MAE and magnetostriction as well to slight decrease of coereivity Hc and to nonmonotonic behavior of HBN intensity. These features are discussed qualitatively using models of magnetic hysteresis and of MAE. Index Terms--Barkhausen effect, creep damage, magnetoacoustic emission, nondestructive evaluation.

The design of magnetoacoustic emission (MAE) instrumentation for boiler tube inspection is discussed. The new design is useful for detecting extent and asymmetry of creep damage in power plant boiler tubes. The MAE declines monotonically with the increase in creep damage and is therefore useful for measuring creep damage in the outer walls of boiler tubes.

The use of magnetic test for determining the annealing of weld is examined. The significant decline in emf or permeability from base metal to heat-affected zone (HAZ) to fusion zone results in unannealed weld whereas on increase in the emf and permeability from base metal to HAZ to fusion zone indicates that the weld is annealed.

Research into the dependence of the hysteresis parameters of the Jiles-Atherton model as a function of grain size and dislocation density is presented. It is concluded that the harmonic amplitudes drop monotonically with inverse grain size and the square root of dislocation density.

Research into the evaluation of magnetoacoustic emission due to a nonuniform distribution of creep damage at a seam weld using finite element modeling is presented. There was found to be a difference in current density range between where magnetoacoustic emission is sensitively detected and where secondary emf changes are sensitively detected.

A magnetic hysteresis model is used to map the effects of magnetic hysteresis on GMR and on accompanying magnetic noise in a GMR magnetic field sensor by using a model for giant magnetoresistance (GMR) hysteresis and a model for Barkhausen noise. It is found that biasing fields halfway down the GMR curve does indeed produce a much quieter measurement than near zero bias fields.

A study was conducted to analyze a model based on the domain wall pining magnetomechanical hysteresis framework of M.J. Sablik and D.C. Jiles that was developed to determine changes in magnetic hysteresis under biaxial stress conditions. The model supported the Villari effect and other types of asymmetric stress effects. Experimental results indicated that the angular dependence of Barkhausen noise.

Creep damage in steel causes a reduction of magnetic properties. A mathematical model, previously formulated, accounts for this. Recently, this model was used in finite element modeling (FEM) of a magnetic C-core signal due to creep damage at a seam weld in Cr-Mo steam pipe. The FEM assumed unrealistically that in the absence of creep damage, the weld material and heat-affected zone (HAZ) and base metal all had the same magnetic properties. In this paper, new finite element simulations are presented for worst case relative permeabilities of 1271, 784 and 571 for base metal, HAZ, and weld material. Reduced permeability at the weld results in a considerably reduced emf at low probe magnetic fields. However, creep damage does produce an additional emf reduction that is large enough to be detected, even when the creep damage does not extend to the pipe wall surface. A method is suggested for calibrating the magnetic signal for weld, HAZ, and base metal effects. Index terms - creep damage, magnetic NDE, seam weld, finite element modeling

This paper presents a way to relate two standard models for magnetomechanical hysteresis - the Sablik-Jiles-Atherton model and the Schneider-Cannell-Watts model - while at the same time presenting physical models for the asymmetry in magnetic property behavior under tensile and compressive stress in steel, including, for oH processes, the presence of an extremum in the dependence of magnetic properties on tensile stress, an effect which is sometimes known as the Villari effect.

[Co.sub.90][Fe.sub.10](2.5 nm)/Cu(2 nm, 3 nm, or 4 nm) multilayers (MLs) were DC magnetron-sputtered in the presence of a trace [N.sub.2] sputtering gas constituent during CoFe layer growth. A reduction in saturation magnetization and easy axis coercivity coupled with an increase in resistivity was observed for the nitrided MLs compared to metallic MLs with identical Cu layer thickness.

Using appropriately modified magnetization curves for each element of creep-damaged material, a finite element calculation has been carried out to simulate magnetic detection of non.uniform creep damage around a seam weld in a 2.25 Cr 1 Mo steam pipe. The magnetization curves for the creep-damaged elements were obtained from an earlier model for the magnetic effect of a uniformly creep-damaged material as given by Chen, et al. In the finite element calculation, a magnetic C-core with primary and secondary coils was placed with its pole pieces flush against the specimen in the vicinity of the weld. The secondary emf was shown to be reduced when creep damage was present inside the pipe wall at the cusp of weld and In the vicinity of the cusp. The C-core detected the creep damage best if it completely spanned the weld seam width. Also, the current in the primary needed to be such that the C-core was not magnetically saturated.

A micromagnetic model involving deviatoric normal stress factors and Schneider-Cannell-Watts model evaluates the effect of biaxial stress on the hysteretic and magnetoelastic properties in polycrystalline-composed steel structures. Computer and experimental results are consistent with the kind of magnetic changes induced by the stress. Different biaxial stresses are determined nondestructively by the new model.

A modified Barkhausen noise spectrum model of irreversible permeability reveals a coincidence between the permeability and Barkhausen peaks. A hysteresis model indicates a higher peak for Barkhausen noise amplitudes and irreversible permeability in comparison with total permeability. A combination of the two models demonstrates the dependence of Barkhausen noise behavior on magnetic field and stress.

A modification to the Jiles-Atherton hysteresis model makes it possible to model magnetic effects of plastic deformation. With this model, it is shown how a narrow hysteresis with an almost steplike hysteresis curve for an undeformed specimen is sharply sheared by plastic deformation.

The effects of biaxial stress on the Barkhausen noise amplitude when the magnetic field acts in the stress plane but is noncoaxial with the biaxial stress axes were investigated. The Barkhausen noise signals were measured under conditions of biaxial stress in steel pipe. Experimental and modeling results showed that the Barkhausen noise amplitude increases as the stress is increased from negative to positive when the field direction is at small angles from the axis. At large angles, however, the reverse is true for the Barkhausen noise amplitude.