Areas of increased hardness, or hard spots, are a great concern to the safe operation of high pressure gas pipelines, since they are more susceptible to hydrogen induced cracking (HIC). Utilizing the material effects on magnetic hysteresis, we will examine the ability to calculated hardness by comparing flux signals at high and low applied fields to B-H hysteresis curves that are a function of hardness. A finite element model will be used to generate flux signals from regions with known magnetic properties in order to evaluate the theoretical feasibility of the technique. The results of our modeling show that there is a dramatic jump in the MFL response after reaching saturation values and decreasing the field. We should make finer steps in the applied field in this region to see if it is an artifact of the modeling or not. The conclusions of our models are severely limited by the lack of measured BH curves for the martensitic pipeline steels. The BH curves of our models did not show a large divergence as shown in figure 3, but there is an assumption that the remanence will increase with hardness. Another detail is the cross over point near saturation where the harder materials become less permeable. If the jump in amplitude is a result of the coercive nature of the steel then its high correlation with hardness can be used to calculate the hardness. Geometry can be derived by the profile of the signal (i.e. length and width). The main influence on signal amplitude is then thickness and hardness. If there is a strong correlation between these two variables, that is harder spots are also thicker, then it should be possible to develop algorithms to correlate hardness to MFL signals. Future work will require more data on the BH curves of martensitic steels and would have to take into account the actual magnetizer being used.
Calculation of hardness using high and low magnetic fields
Berechnung der Härte mittels großen und kleinen Magnetfeldern
2006
9 Seiten, 9 Bilder, 1 Tabelle, 5 Quellen
Aufsatz (Konferenz)
Datenträger
Englisch
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