Austenitic tubes are essentially nonmagnetic. Carburized areas of the tubes become magnetic, and if these areas are large, they can be detected with a magnet. A magnet on a string dropped down a tube will indicate areas that are magnetic but will not indicate the depth of carburization. There are several commercially available devices that are used for measuring the ferrite content of austenitic welds which may be suitable for identifying loacilized areas of magnetism in heater tubes. Some instruments and field services can relate the degree of magnetism to the depth of carburization. Most of the instruments are proprietary, and the field services are limited.
A rule of thumb states that up to 50% carburization can be tolerated on stream before loss of strength materially affects tube life. Although this rule of thumb indicates that a tube with 50% carburization should be replaced, it does not mean that less than 50% carburization will allow the tube to remain in service until the next shutdown. Factors including the rate of carburization, the expected service time until the next shutdown, the amount of excess metal, and changes in pressure and temperature must be taken into account.