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 coercivlty 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 (Kins) 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, Vms 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. © 1995 IOP Publishing Ltd.