Magnetic characterization of 5cr-0.5Mo steel used in-process heater tubes

The magnetic properties of a process heater tube were determined to evaluate the structural integrity of its use in the oil industry. Carbon diffusion inside the tube during processing of petroleum is a cause of concern in the oil industry because it leads to brittle fracture. Both magnetic hysteresis and magneto-acoustic emission (MAE) parameters were determined for three specimens made from virgin, laboratory-aged and service-exposed materials. It was expected that the coercivity would be a good damage parameter by which to assess the servie-exposed material because it increased with carbon concentration. However, the observed insignificant difference in coercivity between service-exposed material and the virgin one ruled out the possibility of using coercivity as a suitable damage parameter. The present study indicated that saturation induction could be used to find the extent of carbon deposition inside the material. The RMS voltage (V-rms) of the MAE signal signal at low field was found to be higher in the laboratory-aged specimen due to greater interparticle distances which allowed domain walls to move longer distances before encountering a pinning centre. At high field, V-rms in the virgin specimen was found to be higher than that in laboratory-aged and service-exposed materials. That was attributed to the presence of higher anisotropy in the virgin sample, originating from the metallurgical processing.