NACE Standard TM0498-98Item No. 21235StandardTest Method

Standard Test Methods for Measuring theCarburization of Alloys Used for Ethylene

Cracking Furnace Tubes

This NACE International standard represents a consensus of those individual members who havereviewed this document, its scope, and provisions. Its acceptance does not in any respectpreclude anyone, whether he has adopted the standard or not, from manufacturing, marketing,purchasing, or using products, processes, or procedures not in conformance with this standard.Nothing contained in this NACE International standard is to be construed as granting any right, byimplication or otherwise, to manufacture, sell, or use in connection with any method, apparatus,or product covered by Letters Patent, or as indemnifying or protecting anyone against liability forinfringement of Letters Patent. This standard represents minimum requirements and should in noway be interpreted as a restriction on the use of better procedures or materials. Neither is thisstandard intended to apply in all cases relating to the subject. Unpredictable circumstances maynegate the usefulness of this standard in specific instances. NACE International assumes noresponsibility for the interpretation or use of this standard by other parties and acceptsresponsibility for only those official NACE International interpretations issued by NACEInternational in accordance with its governing procedures and policies which preclude theissuance of interpretations by individual volunteers.

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Approved 1998-06-26NACE InternationalP.O. Box 218340

Houston, TX 77218-8340+1 (281) 228-6200

ISBN 1-57590-058-0 1998, NACE International

TM0498-98

NACE Inte

_______________________________________________________________________Foreword

This standard test method establishes two recommended methods for measuring the relativecarburization resistance of alloys for tubes intended for service in ethylene cracking furnaces, orfor assessing the performance of these tubes after service. Application procedures for the twomethods are defined in detail. The combustion analysis method is preferred because it isquantitative. The chemical etching method is simpler and less expensive, but is only semi-quantitative. Other methods considered and the reasons they are not recommended arediscussed in Appendix A.

This standard is intended to assist designers, operators, producers, fabricators, users, and testinglaboratories in the measurement of the carburization of alloys used for ethylene cracking furnacetubes.

This standard was prepared by Task Group T-5B-11, a component of Unit Committee T-5B onHigh-Temperature Materials Performance. It is issued by NACE International under the auspicesof Group Committee T-5 on Corrosion Problems in the Process Industries.rnatio

___In NACE standards, the terms shall, must, should, and may are used in accordancewith the definitions of these terms in the NACE Publications Style Manual, 3rd ed.,Paragraph 8.4.1.8. Shall and must are used to state mandatory requirements.Should is used to state that which is considered good and is recommended but is notabsolutely mandatory. May is used to state that which is considered optional.nal i

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_______________________________________________________________________NACE InternationalStandard

Test Method

Standard Test Methods for Measuring theCarburization of Alloys Used for Ethylene

Cracking Furnace Tubes

Contents

1. General..................................................................................................................... 12. Definitions................................................................................................................. 13. Combustion Analysis Method.................................................................................... 14. Chemical Etching Method ......................................................................................... 4References..................................................................................................................... 5Bibliography ................................................................................................................... 6Appendix A: Other Carburization Measurement Methods................................................ 7FiguresFigure 1: An uneven magnetic response line .................................................................. 2Figure 2: A relatively uniform magnetic response line..................................................... 3Figure 3: Four sections of internally finned tubing........................................................... 4NACE International

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____________________________(1) Materials Technology Institute of the Chemical Process (2) Alloy Casting Institute (ACI) has become part of the Stee(3) American Society for Testing and Materials (ASTM), 10Section 1: General1.1 Currently there is no standardized test method toevaluate the relative carburization resistance of variousalloys for possible use in furnace tubes for ethylenecracking service. MTI(1) Publication No. 521 providesdefinitive guidelines for developing standard carburizationtesting methods for broader applications, and proceduresfor measuring the test results. This NACE standardaddresses the latter but not the former, and the latter onlyfor alloys and tubes for ethylene cracking furnaces. Thisstandard does not attempt to standardize carburizationtesting methods.

1.2 Carburization data for proprietary alloys and forstandard alloys such as UNS J942042 have generallybeen developed by tube suppliers. Because of variationsin testing and measuring methodologies, attempts tocompare carburization data from one supplier to anotherfor specific temperature ranges have not always providedconsistent results. The measurement methods estab-lished by this standard provide a means to evaluate any________

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Industries (MTl Founders So

0 Barr Harbor disparities involved in interpreting supplier carburizationdata. The user of this standard is cautioned to ensurethat the carburization testing methods used to rank alloysare equivalent.

1.3 The methods for measuring relative carburizationresistance established by this standard should not beassumed to give any information regarding the possibleconsequences of carburization in general or for anyparticular furnace design. Nor should they be assumedto provide any basis for projecting the life of partially orfully carburized tubes.

1.4 Users of carburization data determined by thesemethods are responsible for developing their own criteriafor applying the data to their needs.

1.5 The evaluation of mechanical properties and theextent to which they might be affected by various degreesof carburization are not within the scope of this standard.____________________________________Section 2: Definitions2.1 Carburization is defined as the absorption of carbonby an alloy from its surrounding test or serviceenvironment resulting in an effect on alloy properties andperformance.

2.2 Alloys used for ethylene cracking furnace tubes aregenerally considered to include wrought and cast heat-resistant materials with varying major quantities of Ni, Cr,and Fe, and small additions of other elements.Examples include but are not restricted to UNS N08811,UNS J94204, ACI(2) HP-45 (with various modifications),and variations of a 45Ni-35Cr alloy.____________________________________Section 3: Combustion Analysis Method3.1 General

3.1.1 This test method involves a standardprocedure for determining the carbon profile ofcarburized material by means of combustionanalyses of layers consecutively machined from atest specimen. The end result is a carbon profilecurve developed by plotting the average carboncontent of each layer versus the depth of themidpoint of that layer from the surface exposed tothe carburizing environment. This procedure isgenerally defined in ASTM(3) G 79,3 which should beI)c

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followed in carrying out this method. This method ismore involved and more expensive than alternativeprocedures, but is the most accurate andquantitative.

3.1.2 There are three main methods for combustionanalysis of carbon. These include the infraredabsorption, thermal conductivity, and gravimetricmethods. This standard specifies use of the infraredmethod which ASTM G 79 recognizes as moreprecise. It also requires less time and samplepreparation than other methods.1

, 1215 Fern Ridge Parkway, Suite 116, St. Louis, MO 63141.iety of America, 455 State St., Des Plaines, IL 60016.r., West Conshohocken, PA 19428-2959.

TM0498-983.2 Test Specimen Preparation

3.2.1 If the test specimen is to be removed from alaboratory-exposed tube, the primary exposedsurface shall be identified. Layer removal shall beginwith this surface.

3.2.2 If the test specimen is to be removed from aservice-exposed tube, a magnet or dual-frequencycarburization meter shall be used to locatecarburized areas. A representative section of tubeshall be removed using a cool cutting technique toavoid overheating. Cut surfaces shall be smoothed2

by grinding and/or sanding, and the service-exposedsurface shall be bead blasted. The section shall beplaced in a magnetic field and dry magnetic particles(or those suspended in acetone) shall be applied tothe bead-blasted surfaces. The magnetic responseline will define qualitatively the extent and pattern ofcarburization. Carburization generally extends be-yond the magnetic response line. Therefore, layerremoval shall not stop at that line. Representativemagnetic response lines are shown in Figures 1 and2.FIGURE 1An uneven magnetic res ponse line. The distribution of magnetic particles defines themagnetic carburized area as being around the smooth-bore inner surf ace of this typical tube section.

For combustion analysis, layers from this tube section shall be removed in l ongitudinal strips.NACE International

TM0498-98

NAFIGURE 2A relatively uniform magnetic response line is shown by this distribution of magneticparticles. Layers from this tube section shall be removed in circumferential sections.3.2.3 If the magnetic response line is not uniformaround the tube section (Figure 1), a longitudinalstrip of tube shall be removed from the area of mostinterest. Layers shall be removed from this strip. Ifthe response line is uniform around the tube section(Figure 2), circumferential layers shall be removedfor analysis.

3.2.4 Prior to layer removal, the test specimen shallbe cleaned to remove any surface contaminants.Glass bead blasting is preferred, but other similarlymild cleaning methods may be used. The testspecimen shall be degreased prior to machining.

3.3 Layer Removal

3.3.1 Layers shall be removed by milling or turning.No lubricating or cooling media shall be used duringmachining.

3.3.2 The thickness of the first three layers removedshall be 0.050 mm (0.020 in.) each. The thickness ofsubsequent layers may be increased to 1.3 mm(0.050 in.) each. At least two layers beyond themagnetic response line shall be removed. If moresensitive results are desired, the layer thickness mayCE Internationalbe reduced to between 0.1 and 0.2 mm (0.004 to0.008 in.).

3.3.3 Chips from each layer shall be collected asseparate samples and labeled according to testspecimen designation and depth from the exposedsurface. The minimum sample size for combustionanalysis is one gram per analysis. Sufficientmaterial should be available for at least two analysesper layer, or for one analysis and one reserve sampleper layer. The size of the test specimen should beselected accordingly.

3.4 Carbon Analysis and Data Presentation

3.4.1 The sample from each layer shall be analyzedby the infrared method. The thermal conductivitymethod may be considered if it is the only alternativemethod readily available. ASTM E 3534 and ASTM E10195 illustrate the analytical methods fordetermining total carbon content using thesecombustion methods.

3.4.2 The analytical results shall be plotted in graphform as percent carbon versus the midpoint distanceof each layer from the exposed surface wherecarburization was initiated.3

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_______________________________________________________________________Section 4: Chemical Etching Method4.1 General

4.1.1 This test method involves a standard chemicaletching test to semi-quantitatively measure the extentof carburization of alloys used for ethylene crackingfurnace tubes. This test method may be used to rankthe relative carburization of different alloys exposedto laboratory carburization tests or removed fromoperating furnaces.4.1.2 By semi-quantitatively defining the extent andprofile of carbon uptake, this test method providessufficient accuracy to compare the suitability ofdifferent alloys for new construction followinglaboratory carburization tests. It can also be used forevaluating the carbon profiles of service-exposedtube sections representing different temperaturesalong the length of the tubing, as shown in Figure 3.FIGURE 3Four sections of internally finned tubing etched to show the degree of carburization alongthe length of a single tube section as a function of location and tube-metal temperature.4.1.3 This method represents a relatively simple ap-proach that can be carried out by most laboratories.More precise methods, while more quantitative, maynot necessarily result in improved design oroperational control decisions.

4.2 Test Specimen Preparation

4.2.1 Removal of test specimens from service ex-posed tubes shall be done by cool cutting techniquesthat will not overheat the cross sections to beexamined.

4.2.2 A minimum of two test specimens from thesame heat and carburized under identical conditionsshall be tested using this etching method. Testspecimens shall be of uniform geometry, surfacefinish, and thickness. Unexposed materials from thesame heat shall be retained for reference.4.2.3 After carburization according to normal labor-atory techniques, test-specimen cross sections shallbe prepared by grinding with successive grit papermaterials through at least 400 to 600 grit.

4.2.4 Continued polishing using diamond pastes ornapless cloths for metallographic microstructureexamination may also be applied.

4.3 Etching Test

4.3.1 The etchant solution shall be a mixture of 20wt% nitric acid (HNO3) and 4 wt% hydrofluoric acid(HF). This solution shall be freshly prepared usingnormal laboratory techniques and procedures,including appropriate safety precautions, by mixing200 cc of concentrated 70 wt% HNO3 with 70 cc ofconcentrated 49 wt% HF and 670 cc of distilledwater. An alternative etchant, glyceregia, has beenNACE International

TM0498-98used in the past, but shall not be used with thisstandard.

4.3.2 The premixed etchant solution shall be trans-ferred to a suitable plastic (polypropylene orpolyethylene) tray.

4.3.3 The test specimens shall be immersed in theetchant solution. The polished surfaces of the testspecimens shall face upward. This orientationensures full exposure and avoids the creation ofcrevices on the polished surfaces.

4.3.4 The test specimens shall be immersed for twohours at room temperature (20C [68F]). Followingimmersion, the test specimens shall be removedfrom the solution, rinsed with water, and dried.

4.3.5 This procedure sufficiently attacks all carbidesso that a zone of carburized versus noncarburizedmaterial is visually evident.

4.4 Evaluation of Test Results

4.4.1 The polished surfaces of the test specimensshall be examined visually. This may be augmentedby magnification of up to 100X. The test specimensshall be ranked according to the depth of thecarburization zones revealed by the etchant.NACE International

_____________________________________4.4.2 Results of the test shall be recorded along withphotographs of the cross sections. An appropriatescale by which the thickness and relative depth ofcarburization can be judged shall be included.

4.4.3 The photographs should preferably be at least75 by 130 mm (3.0 by 5.0 in.) and the wall thicknessin the photographs should be no less than 80% ofactual thickness. For example, a 6.4-mm (0.25-in.)wall thickness should appear to be no less than 5.1mm (0.20 in.) in the photographs.

4.4.4 The depth of carburization reported for eachtest specimen shall be as measured from the surfacewhere carburization was initiated. The total test-specimen thickness shall also be reported. If testspecimens have been exposed on all sides, initialand final caliper or micrometer readings should betaken for reference.

4.4.5 For tubular test specimens, the depth ofcarburization shall be reported as a percentage ofwall thickness from the inside, exposed surface. Fortest specimens that have been exposed to thecarburizing environment on all surfaces, the degreeof carburization shall be reported as a percentage ofthe cross section tested.

4.4.6 Written descriptions of the etched testspecimens shall include any appearance of spottyor nonuniform carburization.__________________________________References1. H.J. Grabke, Carburization: A High TemperatureCorrosion Phenomenon. Part II: Best Practices forTesting Alloys, MTI Publication No. 52 (St. Louis, MO:MTI).

2. Metals and Alloys in the Unified Numbering System(latest revision), a joint publication of the AmericanSociety for Testing and Materials (ASTM) and the Societyof Automotive Engineers Inc. (SAE), 400 CommonwealthDr., Warrendale, PA 15096.3. ASTM G 79 (latest revision), Standard Practice forEvaluation of Metals Exposed to CarburizationEnvironments (West Conshohocken, PA: ASTM).

4. ASTM 353 (latest revision), Standard Test Methodsfor Chemical Analysis of Stainless, Heat-Resistant,Maraging, and Other Similar Chromium-Nickel-IronAlloys (West Conshohocken, PA: ASTM).

5. ASTM E 1019 (latest revision), Standard TestMethods for Determiniation of Carbon, Sulfur, Nitrogen,and Oxygen in Steel and in Iron, Nickel, and CobaltAlloys (West Conshohocken, PA: ASTM).5

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_______________________________________________________________________BibliographyAgarwal, D.C., and U. Brill. Combating HighTemperature Environmental Degradation By TestingExisting New Nickel and Iron Base Alloys.CORROSION/94, paper no. 403. Houston, TX:NACE, 1994.

Agarwal, D.C., U. Brill, and U. Heubner. Alloys and HighTemperature Environments - The Cost EffectiveApproach for Material Selection. CORROSION/92,paper no. 309. Houston, TX: NACE, 1992.

ASM Handbook, Vol. 11: Failure Analysis and Prevention.9th ed. Elevated-Temperature Failures. MaterialsPark, OH: ASM International, 1986: pp. 263-297.

ASM Handbook, Vol. 13: Corrosion. 9th ed. High-Temperature Corrosion. Materials Park, OH: ASMInternational, 1987: pp. 99-100.

ASM Metals Handbook, Desk ed. Heat-ResistantMaterials. Materials Park, OH: ASM International,1985: pp. 16:16-26.

ASTM A 340 (latest revision). Standard Terminology ofSymbols and Definitions Relating to MagneticTesting. West Conshohocken, PA: ASTM.

ASTM E 3 (latest revision). Standard Practice forPreparation of Metallographic Specimens. WestConshohocken, PA: ASTM.

ASTM E 340 (latest revision). Standard Test Method forMacroetching Metals and Alloys. WestConshohocken, PA: ASTM.

Baboian, R. Corrosion Tests and Standards. WestConshohocken, PA: ASTM, 1995.

Bagnoli, D.L., and J.J. Krupowicz. Experiences withEthylene Heater Tube Carburization. COR-ROSION/92, paper no. 307. Houston, TX: NACE,1992.

Brill, U. Mathematical Description of CarburizationBehavior of Various Commercial Heat-ResistantAlloys. Proceedings of the First InternationalConference on Heat-Resistant Materials, heldSeptember 23-26, 1991. Fontana, WI: ASMInternational, 1991.

Collins, P.A. Effect of Elevated Temperature Exposureon the Mechanical Properties of a Cast Niobium-Containing Alloy. CORROSION/80, paper no. 170.Houston, TX: NACE, 1980.Fontana, M.G., and N.D. Green. Oxidation and OtherHigh-Temperature Metal-Gas Reactions. CorrosionEngineering. New York, NY: McGraw-Hill, 1967.

Grabke, H.J., and A. Schnaas. Review on High-Temperature Gaseous Corrosion and MechanicalPerformance in Carburizing and OxidizingEnvironments. Proceedings of the PettenInternational Conference. Petten, Netherlands:North-Holland Publishing Company, 1978.

Hall, D.J., and J.J. Jones. The Carburization Behavior ofSteels for Petrochemical Plants. Proceedings of thePetten International Conference. London, England:Elsevier Applied Science, 1985.

Hall, D.J., J.J. Jones, and M.K. Hossain. FactorsAffecting Carburization Behavior of AusteniticSteels. MP 24, 1 (1985): p. 25.

Heubner, U.K. Drafahl, and J. Henrich. Creep Strengthand Creep Ductility of Welded Heat ResistantMaterials in a Carburizing Environment. Proceedings of the First International Conference onHeat-Resistant Materials, held September 23-26,1991. Fontana, WI: ASM International, 1991.

Jaske, C.E. The Effects of High Temperature Exposureon the Properties of Heat-Resistant Alloys. COR-ROSION/94, paper no. 397. Houston, TX: NACE,1994.

John, R.C. Alloy Carburization and Testing in SimulatedProcess Gases at 1800-1950F. CORROSION/95,paper no. 460. Houston, TX: NACE, 1995.

Kane, R.H. Carburization of Cast Heat-Resisting Alloysin Synthetic Petrochemical Environments. COR-ROSION/83, paper no. 266. Houston, TX: NACE,1983.

Kane, R.H. Effects of Silicon Content and OxidationPotential on the Carburization of Centrifugally CastHK-40. CORROSION/80, paper no. 168. Houston,TX: NACE, 1980.

Krikke, R.H., J. Hoving, and K. Smit. Monitoring theCarburization of Furnace Tubes in Ethylene Plants.CORROSION/76, paper no. 19. Houston, TX: NACE,1976.

Lai, G.Y. A New High Temperature Alloy for theChemical Process Industry. CORROSION/92, paperno. 310. Houston, TX: NACE, 1992.NACE International

TM0498-98Lancaster, J.F. Materials for the PetrochemicalIndustry. International Metals Reviews 23, 3 (1978):pp. 127-132.

Lin, J.Y., F.C. Chiang, H. Gang, and C.C. Su. The FieldMeasurement of Carburization and Oxidation ofEthylene Furnace Tubes by Multi-TechniqueMethod. CORROSION/94, paper no. 184. Houston,TX: NACE, 1994.

Mitchell, G.R.D., D.J. Young, and W. Kleeman.Carburization-Oxidation of Heat Resistant AusteniticSteels. CORROSION/92, paper no. 302. Houston,TX: NACE, 1992.

Mucek, M.W. Laboratory Detection of Degree ofCarburization in Ethylene Pyrolysis Furnace Tubing.CORROSION/83, paper no. 296. Houston, TX:NACE, 1983.

Saori, M., and S. Ohta. Carburization and Its Preventionin Ethylene Cracking Furnace Tubes. Proceedings ofJIMIS - 3: High Temperature Corrosion, Transactionsof the Japan Institute of Metals. Tokyo, Japan:Japan Institute of Metals, (1993): pp. 691-698.NACE International

____________________________________Smith, P.J., O. Van der Biest, and J. Corish. InitialStages of High-Temperature Corrosion ofCommercial Fe-Cr-Ni Alloys in a CarburizingAtmosphere of Low Oxygen Partial Pressure.Oxidation of Metals 24, 5/6 (1985): pp. 277-314.

Su, Chun-Chi, and Kuo-Zein Chen. 17 Years OperatingExperience with the FeCrNi Centrifugally CastFurnace Tubes in Process Plants. CORROSION/93,paper no. 228. Houston, TX: NACE, 1993.

Thomas, P., D.J. Yound, and D.L. Trimm. Carburization-Oxidation of Austenitic Steels. Metallic Corrosion 1,4 (1984): pp. 58-65.

Van den Bruck, U., and C.M. Schillmoller. Selecting HighPerformance Castings for Petrochemical Furnaces.CORROSION/85, paper no. 23. Houston, TX:NACE, 1985.

Wolf, I., H.J. Grabke, and P. Schmodt. Carbon TransportThrough Oxide Scales on Fe-Cr Alloys. Oxidation ofMetals 29, 3/4 (1988): pp. 289-305.___________________________________Appendix A: Other Carburization Measurement Met hodsA1 Several carburization measurement methods wereevaluated during the process of selecting the twomethods recommended in this standard. The others werenot recommended for various reasons as discussedbelow.

A2 Alternative methods considered but not recom-mended:

A2.1 Mass gain. This method measures total car-bon increase but does not evaluate depth ofpenetration or gradient. Because these issues areimportant in the operation, maintenance, and repairof ethylene cracking furnace tubes, this method is notconsidered a viable alternative for the purposes ofthis standard.

A2.2 Changes in mechanical properties. Carbur-ization causes changes in tensile, yield, elongation,hardness, etc. The degree of change depends on theextent of carburization. However, it is difficult tomeasure and correlate these changes with carboningress and its distribution. Thus, this alternativemeans of measurement is not considered suitable forthis standard.A2.3 Microhardness measurement. Available datashow that there is inconsistent correlation ofcarburization with hardness. Therefore, this methodis not recommended.

A2.4 Magnetic response. When alloys are carbur-ized and attain a certain amount of carbon, theybecome magnetic. This characteristic has been usedto determine the presence of carburization in an alloyand to estimate its extent by measuring the level ofmagnetic permeability. However, this change inpermeability can vary with the composition of thealloy, its wall thickness, and its geometry. Moreexperience with this nondestructive technology mayultimately make it a viable measuring method.However, for the current purposes of this standard, itis not recommended.7

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