Magnetic Hysteresis Loop Technique Research Articles

Magnetic Evaluation of Heat-Resistant Martensitic Steel Subjected to Microstructure Degradation.

The present paper investigates the use of the magnetic hysteresis loop technique to nondestructively evaluate microstructural degradation in heat-resistant martensitic (HRM) steels. The degradation impairs the safe operation of thermal power plants and it is thus essential to periodically assess it using nondestructive evaluation (NDE) techniques. In this contribution, HRM steels are thermally aged up to 16,000 h at 675 °C to simulate the microstructural degradation, then the changes in the magnetic coercivity, hardness, and microstructure are systematically characterized and the relations between them are determined. Both coercivity and hardness decrease with thermal aging duration, which can be interpreted in terms of the microstructure parameters’ evolution based on the pinning of crystal defects on domain walls and dislocations. Coercivity and hardness share the same softening trend with aging time, and good linear relations between coercivity, hardness, and microstructure parameters are found. These results provide a key to understanding the magnetic parameter evolution in HRM steels and suggest the possibility of using magnetic technologies for the NDE of microstructure degradation in thermal power plants.

Characterization of Creep Degradation of Modified 9Cr-1Mo Steel Using Magnetic Measurement

Abstract Microstructure evolution during the creep of a modified 9Cr-1Mo heat-resistant steel was characterized by a nondestructive method of magnetic hysteresis loop (MHL) technique at 893 K (620°C). Two parameters of coercivity and remanence were introduced to evaluate the magnetic properties of interrupted crept specimens. Experimental results suggested that both the parameters were significantly sensitive to the microstructures, such as dislocations, precipitates, and microcracks. During the primary and secondary creep stage, the coercivity and remanence increased with creep time, which was mainly attributed to the increase in pinned dislocations and number density of precipitates. However, because of the coarsening of M23C6 carbides and presence of a demagnetizing field induced by the nonmagnetic precipitates and microcracks, the coercivity and remanence decreased during the tertiary creep stage. It suggested that the MHL technique has great potential to evaluate the microstructure evolution during creep degradation of heat-resistant steels.

Quantitative Evaluation of Deformation Induced Martensite in Austenitic Stainless Steel Using Magnetic NDE Techniques

The goal of the present study is to examine the applicability of non-destructive magnetic hysteresis loop technique and Hall effect sensor to characterize the strain induced α′-martensite in an austenitic stainless steel. For this purpose, eight different levels of tensile deformation, from 0.05 to 0.44 true stain values, were applied and corresponding induced martensite fractions were determined, using X-ray diffraction method. Finally, magnetic hysteresis loop and Hall effect methods were applied. Results show that major outputs of hysteresis loop (maximum flux density, retentivity and coercivity) and Hall effect (real part, imaginary part, modulus and phase angle for the 3rd harmonic) are significantly affected by variations in the induced martensite fraction. Besides, a new parameter, amplitude and shape of raw received signal, was used in this study. The established relationships with high correlation coefficients between electromagnetic outputs and martensite fraction indicate the potential of the proposed methods to be used for detecting the microstructural changes in AISI 304 stainless steel subjected to different strains.

Application of magnetic hysteresis loop method to determine prior austenite grain size in plain carbon steels

Abstract The present investigation has been performed to explore the ability of magnetic hysteresis loop technique to detect prior austenite grain size changes due to various austenitizing conditions applied in plain carbon steels. To produce various prior austenite grain sizes, AISI 1045 steel specimens were austenitized at 950, 1000, and 1050 °C for 0.5, 1, and 1.5 h. Optical and scanning electron microscopic observations and X-ray diffraction method were conducted to characterize the microstructural features. It was observed that magnetic parameters extracted from hysteresis loop including maximum flux density, maximum differential permeability, and coercivity are affected by variation in prior austenite grain size. Therefore, these magnetic parameters could be used to estimate the prior austenite grain size. Three different functions introduced to do so, each gets a magnetic parameter and gives the estimated value of prior austenite grain size. To enhance the accuracy, another function which takes all three magnetic properties at the same time, were also introduced. It is suggested that the proposed electromagnetic nondestructive method is a useful and accurate enough tool for characterizing the microstructure and quantitative determination of prior austenite grain size of plain carbon steels.

Magnetic Hysteresis Loop Technique as a NDE Tool for the Evaluation of Microstructure and Mechanical Properties of 2.25Cr–1Mo Steel

This study intends to understand the effect of short-term overheating on microstructure modifications, and variation in mechanical and magnetic properties of boiler tubes. It is based on the hypothesis that short-term overheating on boiler tubes leads to microstructural changes degrading their mechanical properties, thus resulting in their failure. As part of this study, fresh 2.25Cr–1Mo boiler tube samples were heat treated in the range of 700–950 $$^{\circ }\hbox {C}$$ . Magnetic hysteresis loop (MHL) measurements were carried for the non-destructive evaluation (NDE) of the mechanical properties that get altered due to microstructural modifications. For comparison, MHL was also carried out on a service-exposed boiler tube, which had failed due to short-term overheating. The magnetic parameters were correlated with the change in microstructure and micro-hardness of the samples. A decrease in coercivity, remanence, maximum induction and micro hardness were found at the lower soaking temperatures, due to: easy magnetic domain wall and dislocation motions for the stress relaxation; annihilation and recovery of dislocations; increase in inter carbide distance; and the decrease in the number density of carbides for the coarsening of carbides. A subsequent increase in coercivity, remanence and maximum inductions along with hardness are due to the nucleation and growth of fresh bainitic phase, obstructing the magnetic domain wall and dislocation motion. A drastic decrease in coercivity for the service-exposed tube is due to the transformation of MX type carbides to $$\hbox {M}_{23}\hbox {C}_{6}$$ type and accumulation of such carbides at the grain boundaries along with the decrease in number density of the carbides. The presence of scale has less effect on the coercivity, but its demagnetizing effect largely decreases the remanence and maximum induction. The results of the study show that the MHL can be a suitable NDE technique for the evaluation of change in microstructure and degradation of mechanical properties in power plant steels.

Cure kinetics of epoxy/ β -cyclodextrin-functionalized Fe 3 O 4 nanocomposites: Experimental analysis, mathematical modeling, and molecular dynamics simulation

In this work: (i) magnetic nanoparticles of Fe3O4 are synthesized via precipitation method, functionalized with β-cyclodextrin hydroxyl-rich precursor, and characterized by FTIR, TEM and magnetic hysteresis loop techniques; (ii) epoxy nanocomposites containing bare and β-cyclodextrin-modified Fe3O4 nanoparticles are prepared, and their cure behavior/kinetics has been studied by isoconversional models applied on dynamic DSC data; (iii) adhesion energy between cure moieties and surface of β-cyclodextrin is quantified using molecular dynamics simulation. Activation energy of epoxy resin increased from 56 to 62 kJ/mol upon introduction of β-cyclodextrin-modified Fe3O4 to the epoxy, which suggests retarded crosslinking compared to the blank system, while a highly cured network was identified for the resin containing β-cyclodextrin-modified Fe3O4 particles due to additional chemical reaction of hydroxyl functional groups attached to Fe3O4 surface with cure moieties. Molecular dynamics simulation confirmed either quantitatively or qualitatively the strong adhesion between cure moieties and β-cyclodextrin surface.

Magnetic hysteresis loop technique as a tool for the evaluation of σ phase embrittlement in Fe–Cr alloys

Fe-48wt% Cr alloy was isothermally aged at 700°C up to 250h for the formation and growth of σ phase. Micro Vicker’s hardness and magnetic hysteresis loop (MHL) measurements were carried out at various lengths of time by interrupting the test to observe the change in mechanical and magnetic properties respectively. A small volume fraction of σ phase did not produce any change in the hardness whereas a drastic decrease in remanence was found for its demagnetizing effect. The existence of σ phase was confirmed by transmission electron microscopy. The maximum induction of the alloy decreased with thermal ageing as the volume of ferrites decreased for the formation of non-magnetic σ phase. The volume fraction of σ phase was estimated from the maximum induction. The results showed that MHL technique can even detect 1% of σ phase in the alloy considering remanence as a measuring parameter. Hence MHL would be a powerful non-destructive evaluation technique for the evaluation of σ phase embrittlement in Fe–Cr alloys.

Effect of Cr-Rich Phase Precipitation on Magnetic and Mechanical Properties of Fe-20% Cr Alloy

Magnetic Hysteresis Loop (MHL) and micro-Vickers hardness measurements were carried out in isothermal aged Fe-20 wt. % Cr alloy. The results were compared with the existing atom probe data at similar heat treated conditions. Coercivity and hardness of the alloy was increased and remanence of the alloy was decreased with the increase in ageing time. Such increase in the magnetic and mechanical hardness was due to the nucleation and growth of Cr rich α' phase. The size of the Cr rich α' phase precipitates were increased and the number density of the precipitates were decreased with the increase in ageing time observed by atom probe analysis. In this work a good correlation between hardness (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</sub> ) and coercivity (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) was found with the combined effect of precipitate size (r) and number density (n). A linear relationship was found with the change in coercivity and hardness of the alloy indicating that the MHL technique would be a good Non-Destructive Evaluation (NDE) tool for the evaluation of thermal embrittlement in structural components made of Fe-Cr alloys.

Evaluation of Embrittlement in Isochronal Aged Fe-Cr Alloys by Magnetic Hysteresis Loop Technique

Fe-Cr alloys with different Cr contents were prepared by an arc melting technique. The alloys were isochronally aged in the range from <TEX>$400^{\circ}C$</TEX> to <TEX>$900^{\circ}C$</TEX> with <TEX>$50^{\circ}C$</TEX> steps with a holding time of 100 hours. The ageing produced embrittlement in the alloys due to either the formation of a Cr-rich <TEX>${\alpha}'$</TEX> phase or a <TEX>$\sigma$</TEX> phase at high temperatures. Magnetic Hysteresis Loop (MHL) and Micro-Vickers hardness were measured at each step to correlate the magnetic and mechanical properties. Coercivity and hardness of the alloys were increased and remanence decreased up to 500-<TEX>$550^{\circ}C$</TEX> due to formation of a Cr-rich <TEX>${\alpha}'$</TEX> phase. Beyond 500-<TEX>$550^{\circ}C$</TEX> range, the coercivity and hardness decreased and remanence increased due to the coarsening or dissolution of the Cr-rich <TEX>${\alpha}'$</TEX> phase. In the Fe-48% Cr alloy, formation of the <TEX>$\sigma$</TEX> phase at <TEX>$700^{\circ}C$</TEX> reduced the maximum induction of the alloy significantly.

Study of thermal ageing behaviour of Fe-Cr model alloys by magnetic hysteresis loop technique

Fe-Cr alloys with different Cr content (5–20 wt. %) were prepared by arc melting technique. The alloys were solution annealed at 1050 oC/ 2h and air quenched and then tempered at 750 oC/ 1h followed by air cooling. Thermal ageing was carried at 475 oC for various lengths of time up to 1000 hours (hrs) to produce thermal embrittlement. The hardness and coercivity of Fe−20% Cr alloys were increased and the remanence was decreased, being due to the precipitation of Cr-rich a' phase and their subsequent growth. In Fe−15% Cr and Fe−10% Cr alloys, it was found that the coercivity decreased at the initial ageing period and then increased subsequently due to the competing effect of recovery of dislocations and precipitation of Cr rich phase. The change in magnetic properties in Fe−5% Cr alloy was different than other alloys since it was within the solubility limit of Cr in Fe. A linear relationship was found between the coercivity and hardness in Fe−20% Cr alloy, indicating that Magnetic Hysteresis Loop (MHL) technique would be a good tool for the Non-Destructive Evaluatiion (NDE) of embrittlement in Fe-Cr alloys.

Magnetic evaluation of creep in modified 9Cr–1Mo steel

Magnetic Barkhausen emissions (MBE) and magnetic hysteresis loop techniques have been used to correlate the magnetic properties with creep behaviour in modified 9Cr–1Mo steel. Formation of massive carbides like M23C6, Laves phase (Fe2Mo) and Z-phase are the cause of creep failure in the present material. The root mean square voltage of MBE signal increased from the secondary stage of creep. This was attributed to the growth of carbides. Microcracks generated by massive and brittle precipitates induced demagnetizing fields that restricted domain wall movement, resulting in continuous reduction of remanence during creep.

Ageing behavior study of 5Cr–0.5Mo steel by magnetic Barkhausen emissions and magnetic hysteresis loop techniques

Magnetic hysteresis loop and Barkhausen emissions were recorded and analysed for 5Cr–0.5Mo steel after ageing at 600 °C for various lengths of time. At the initial stage of ageing the interstitial carbon diffuses towards the grain boundary making the matrix magnetically softer. During this stage, extending up to 200 h of ageing, magnetic softness was found to be increasing. This was associated with a decrease in coercivity and increase in Barkhausen voltage. Beyond 200 h of ageing the precipitation of alloy carbides attain subsequent growth, making the material magnetically harder. The evolution of carbides was studied using SEM-EDAX analysis. It was observed that most of the carbides transformed into M 23C 6 type after ageing for 400 h.