General

This standard was developed to address a need for a standardized method to test the relative carburization of various furnace tube alloys used for ethylene manufacture. MTI⁽¹⁾ Publication No. 521¹ provides definitive guidelines for developing standard carburization testing methods for broader applications, and procedures for measuring the test result, but does not specify a standard method. This NACE standard defines a standard pack bed carburization procedure and standard methods for measurement of the relative carburization of alloy tubes intended for use in ethylene manufacture. The pack bed carburization procedure is specified because of its ease of application. Gas carburization procedures require a more complex apparatus. This procedure is designed so that alloys may be carburized without the use of expensive or complicated laboratory equipment. The user of this procedure is cautioned that the pack bed carburization procedure does not duplicate the carburizing conditions in the furnace tubes used for ethylene manufacture. However, it has been shown to be a reliable procedure that produces a standard carburizing environment that can be used to evaluate the relative carburization of alloys.

Tubular segments are specified as the test specimens to ensure that the test specimens have similar macrostructures, microstructures, and geometry to those typically used in furnaces for ethylene manufacture. A standard reference test specimen shall be used for each set of carburization exposures. The reference test specimen shall be alloy UNS⁽²⁾ N08810 (alloy 800H).

Carburization data for proprietary alloys and for standard alloys such as UNS J94204 (ACI⁽³⁾ HK-40) have generally been developed by tube suppliers. Because of variation in testing and measuring methodologies, attempts to compare carburization data from one supplier to another for specific temperature ranges have not always provided consistent results. Standardization of the carburization procedure and measurement methods allows data from different suppliers to be compared. Furthermore, by using a standard reference alloy with each set of exposures, difference in test severity between suppliers are apparent. This ensures a standard method is used to rank the carburization of alloys used for ethylene manufacture and provides a means to evaluate any disparities involved in interpreting supplier carburization data.

The carburization procedure and measurement methods established by this standard should not be inferred to provide any information regarding the possible consequences of carburization in general or for any particular furnace design. Nor should it be inferred to provide a basis for projecting the life of partially or fully carburized tubes. The test results should not be used for direct extrapolation to actual service in ethylene manufacturing plants. It is the responsibility of the user of this standard to determine the applicability of the data.

Users of carburization data determined by these methods are responsible for developing their own criteria for applying the data to their needs. In addition to the carburization test results, all other relevant material properties should be considered in evaluating the suitability or relative performance of a given tube alloy for service in ethylene manufacture.

The evaluation of mechanical properties and the extent to which they might be affected by various degrees of carburization are not within the scope of this standard.

This standard does not purport to address all the safety concerns associated with this carburization procedure. It is the responsibility of the user of this standard to establish appropriate safety and health practices.

⁽¹⁾ Materials Technology Institute of the Chemical Process Industries (MTI), 1215 Fem Ridge Parkway, Suite 116, St. Louis, MO 63141.

⁽²⁾ Metals and Alloys in the United Numbering System (latest revision), a joint publication of ASTM International and the Society of Automotive Engineers inc. (SAE), 400 Commonwealth Dr., Warrendale, PA 15096.

⁽³⁾ Alloy Casting Institute (ACI) is part of the Steel Founders' Society of America. 780 McArdle Dr., Unit G, Crystal Lake. IL 60014-8155.

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