Microstructural Analysis of Multi-Phase Coated Hot Stamped Steel Using Magnetic Barkhausen Noise

The microstructure of multi-phase coated hot stamped steel samples is analyzed non-destructively using the magnetic Barkhausen noise (MBN) testing method. The analysis includes examining the microstructural phases in the bulk of hot stamped samples and estimating the thickness of the Al-Si diffusion layer (DL), which exhibits a ferritic microstructure. The MBN of three sets of hot stamped ultra-high strength steel sheets are analyzed. The first set of samples with the same DL is used to study the effect of the microstructural phases of the bulk material and to qualitatively assess the amount of the phases present, while the second and third sets, both with an almost fully martensitic bulk, are used to analyze the capacity of MBN to characterize DL thickness. The results demonstrate that the first peak of the MBN envelope at a very low magnetic field level is due to the superficial ferritic DL, while the peaks at higher levels reflect the microstructural phases of the hot stamped steel. In cases where the ferrite is predominant in the bulk, a peak emerges at medium magnetic field levels, whereas if martensite is the predominant phase, a peak is reached at a higher magnetic field. When both phases are present in significant amounts, both peaks can be observed. Moreover, the amplitude of the first peak derived from the MBN envelope could be used to determine DL thickness. Therefore, an analysis of the MBN envelope signal could provide useful information about the quality of the hot stamping process.