Abstract
Grain size and precipitates have a significant effect on the mechanical properties of steels and it is desirable to be able to characterise these in a non-destructive manner. Grain and precipitate sizes and their spatial distributions in both an extra-low-carbon steel and a laboratory model steel have been individually varied and compared with a variety of characteristic magnetic parameters measured from major and minor magnetisation loops. These magnetic parameters are shown to be very sensitive to grain size distribution when there are no precipitates within the grains. However, the magnetic parameters exhibit complex behaviours with precipitate size distribution, which is linked to a critical precipitate size for effective pinning and another critical precipitate size for strongest pinning to domain walls. The interaction between grain size and precipitate distribution effects on the minor loop properties in the studied steels are discussed.
Highlights
Non-destructive evaluation (NDE) of steel microstructure using electromagnetic (EM) sensors can be used for process control during fabrication or damage monitoring in-service since microstructural features, such as grain size, phase balance, precipitation etc., affect the magnetic response
The average length of diameters measured at 2° intervals passing through the object’s centroid were taken as the equivalent circular diameter (ECD) of grains or particles
We have shown that grain size has strong influences on the μ profile when there are no precipitates or any other significant microstructural features within the grains as in the extra low carbon (ELC) samples
Summary
Non-destructive evaluation (NDE) of steel microstructure using electromagnetic (EM) sensors can be used for process control during fabrication or damage monitoring in-service since microstructural features, such as grain size, phase balance, precipitation etc., affect the magnetic response. Grain size effect may appear insignificant on some major loop properties i.e. coercivity or remanence when there are other microstructural features within grains (such as precipitates or dislocations) or there is a significant amount of second phase, as seen in some carbon or high-alloy steels [11,12]. Even in these cases, grain boundaries are still expected to interact with the processes of domain wall (DW) movement or domain rotation. This fundamental study will help separate their individual effects, rather than inferred effects from complex microstructures, so as to facilitate the application of this technique to the NDE of selected microstructural features of interest
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