Monitoring of Steel Microstructures using Electromagnetic Sensors

The characterization of steel microstructures is an important tool for metallurgists as mechanical properties are controlled by microstructural parameters such as grain size, phase balance and precipitates. This paper descr-ibes multi frequency electromagnetic(EM)sensors that have been designed to detect changes in the relative permea-bility and resistivity of steel on-line during steel processing, which can be directly related to changes in microstructure. Examples presented in this paper include both laboratory hot tests and industrial field trials for monitoring ofphase transformation in steels, detection of decarburization on a steel rod surface, and imaging of molten steel in the submerged entry nozzle (SEN) during continuous casting

Non-destructive characterisation of N/Al level in P91 steels using electromagnetic sensors

P91 steels with different N/Al ratios short term tempered or long term aged at a series of temperatures have been characterised using a multifrequency electromagnetic sensor. It was found that the low frequency inductance value is sensitive to the intralath MX precipitates that determine the mean free path for domain wall motion and hence the initial relative permeability of the steels. The electromagnetic sensor is capable of separating P91 steels with low N/Al ratio in the service entry or early service state. In contrast, the hardness measurements were found to be insensitive to the N/Al ratio for the short term tempered samples

Quantification of the effect of changes in steel microstructural parameters on EM sensor signals

In an accompanying paper being presented in NDESAI, the utility of Multifrequency electromagnetic sensors has been shown to be able to detect steel microstructure changes, for example the austenite to ferrite transformation and the presence of decarburisation in high carbon steel rods. In the present study, steels with various carbon contents have been used to study the effect of phase balance changes (ferrite, pearlite, un-tempered martensite and tempered martensite) on the EM readings. With an increase in pearlite content in ferrite/ pearlite microstructures, the relative permeability and hence inductance value dec-reases. Changing the microstructural state from pearlite to martensite, in a high carbon steel, decreases the rel-ative permeability and hence inductance value, whilst tempering increases these values. In addition, steel wires, with a fully pearlitic microstructure have been used to determine whether the EM sensor can be used to quantify interlamellar spacing changes. The low frequency inductance value was found to increase approximately linearly with an increase in the interlamellar spacing for the range of values investigated

Non-destuctive Evaluation of Power-Generation Steel Microstructure Changes Using Electromagnetic Sensors

This paper presents results from a multi-frequency electromagnetic (EM) sensor used to characterise the microstructural changes in P9 and T22 power generation steels after tempering and elevated temperature service exposure. The EM sensor can detect the microstructural changes in both steels due to the corresponding relative permeability values increasing from 33 to 67 and then 133 for P9 during the tempering and the long service exposure respectively while for T22 the permeability changes from 61 to 75 and then 86, as well as the relatively small per-centages of resistivity change. The changes in the rel-ative permeability values have been qualitatively corre-lated to the changes in microstructural features, which affect the domain wall movements during the multi- freq-uency EM sensor tests. The real inductance value at low frequencies Lo is particularly affected by differences in the relative permeability of the steels studied, which is found to increase exponentially with the Lo values in the range studied

Non-destructive characterisation of N/Al level in P91 steels using electromagnetic sensors

P91 steels with different N/Al ratios short term tempered or long term aged at a series of temperatures have been characterised using a multifrequency electromagnetic sensor. It was found that the low frequency inductance value is sensitive to the intralath MX precipitates that determine the mean free path for domain wall motion and hence the initial relative permeability of the steels. The electromagnetic sensor is capable of separating P91 steels with low N/Al ratio in the service entry or early service state. In contrast, the hardness measurements were found to be insensitive to the N/Al ratio for the short term tempered samples

The effect of alloy content on the microstructure of as-deposited weldmetal in 9Cr-based ferrous weldments

The more widespread use of 9Cr-1Mo-based steels (modified and un-modified) is limited, in part, by a lack of understanding of the welding behaviour of this class of materials. A series of weldments, based on modified 9Cr-1Mo alloy steels, with varying Si and Mn contents was investigated using transmission electron microscopy (TEM). All as-deposited samples were found to consist of laths of heavily dislocated martensite separated by an interlath phase. Addition of Si was found to increase the amount of retained austenite (γ) present in the interlath region, whilst increasing Mn content promoted carbide formation. At high levels of both additions, a banded structure containing both phases was formed. These additions had little effect on the carbides (M23C6 type) present within the laths

Effect of Strain-induced Precipitation on the Recrystallisation Kinetics in a Model Alloy

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