Evaluation and correlation of residual stress measurement in steel

The stress patterns relevant for the assessment of the post production service behaviour of plain carbon hot strip mill plates were evaluated on the surface and at various depths. The hole drilling strain gage (HDSG) is a standard, well proven, semi-destructive technique and is employed for the purpose. However, this technique requires skill and is slow in operation. The magnetic Barkhausen technique (MBE) is non destructive and is still in an experimental stage, but has tremendous potential and has been tried here in steel plant environment. The objective is to establish whether MBE technique could be used as an alternative to HDSG technique. The calibration for MBE results was done under controlled compressive and tensile loading. The stress profiles were determined at six locations along rolling and transverse directions using these two techniques. The rolling stresses using HDSG method were found to be tensile near the surface, increase with depth and saturate at 0.5 to 1 mm depths. The transverse stresses were low tensile or compressive near the surface and saturate to low tensile values at similar depths. The stresses (averaged over a certain depth) by MBE are tensile in rolling direction and compressive in transverse direction. Correlation plots between residual stresses by HDSG method up to different depths and average stress by MBE show that that a good correlation exists at 0.35 mm depth. Beyond this depth, correlation is not good. It is true that for the material of the sheet and the frequency employed, the MBE results are not relevant beyond this depth. Also, the equivalent uniform stress as measured by HDSG method would deviate more and more from the actual stress as we go deeper.