Examination of defects and long term crack growth in rolled bars during long term storage

Abstract: This work has had the purpose to examine defects and the cause of crack growth during long term storage in rolled bars. The bars in question are rolled and stored at Ovako in Hofors. The problem is approached by theoretically examining the possibilities of hydrogen damage, room temperature creep and naturally occurring porosity. Seven bars of three steel type groups and with different rolling dates were selected and examined for defects with ultrasonic testing. Four of the bars show defects. Specimens containing the defects are cut out and grinded so that the defects can be examined in the scanning electron microscope and analyzed with energy dispersive x-ray spectroscopy. One of the specimens shows some form of cavity containing iron carbide. One of the others shows a surface with inclusions. The remaining two both shows inclusions and cracks that seem to originate from inclusions. The theoretical studies show that there are two types of hydrogen damage that could be in effect in the studied case and also that room temperature creep could facilitate but not cause crack growth and that porosity could serve as crack initiation points. Steps should be taken to minimize the problems mentioned above when they are suspected causes. Further examination of inclusions in different steel types should be made if considered a problem.