Oxidation failure of radiant heater tubes

\ PERGAMON

Engineering Failure Analysis 5 "0887# 090Ð001

Oxidation failure of radiant heater tubes K[B[ Yoona\ D[G[ Jeongb a

Department of Mechanical En`ineerin`\ Chun`!An` University\ 110 Huksuk\ Don`jak\ Seoul 045!645\ Korea b Samsun` Heavy Industries Co[\ 39!0 Woon`nam\ Chan`won\ Kyun`nam 530!189\ Korea Received 29 July 0887^ accepted 00 September 0887

Abstract A radiant heater tube with a burner installed inside designed to transfer the heat generated by the burner from the outside of the tube by radiation[ Accordingly\ the tube metal must endure a high temperature of approximately 899Ð0999>C[ The radiant tube was manufactured by centrifugal casting with high NiÐCr alloy steel[ In this study\ a failure analysis of a radiant heater tube was performed by careful visual inspection of the tube cracks\ metallographic observation of the near crack region and chemical analysis of tube metal and oxide scales[ It is argued that the principal cause of the cracking is progressive oxidation of the tube metal beneath cracked thick oxide scales attached to the inside of the tube[ The oxide scales are generated by abnormally high operating temperatures which can be veri_ed by the aged microstructure and internal void formation[ Þ 0888 Elsevier Science Ltd[ All rights reserved[ Keywords] Cracks^ Overheating^ Oxidation^ Process!plant failures[

0[ Introduction For heat treatment of moving steel plates in a hot rolling process of an iron foundry\ the plates are usually allowed to pass between two radiant heater tube rows that are located above and below the pathway[ The plates are heated by radiation from the radiant heater tubes with inside burners[ Since the burner is located inside the tube and hot ~ue gas pass through the tube\ the radiant tubes are continuously operated at high temperature which causes materials degradation due to thermal aging and corrosion:erosion problems due to the combustion gas[ As a result\ thinning of the tube and crack initiation occur frequently ð0Ł and leaking of the gas through the crack may cause serious surface _nish problems to the rolled plate product as well as operating problems[ Since the operating temperature of the heater tubes is expected to be well above 899>C\ they are generally fabricated by centrifugal casting with HK or HP steels having high contents of Ni and Cr[ In this study\ a failure analysis of locally fabricated radiant heater tubes was performed[ A  Corresponding author[ S0249Ð5296:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved PII] S 0 2 4 9 Ð 5 2 9 6 " 8 7 # 9 9 9 2 2 Ð 7

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Fig[ 0[ Schematic illustration of the U!type radiant heater tube structure[

metallographic observation near the crack region of a failed tube was made[ Also\ a chemical composition analysis of the oxide scales and the base tube metal in the vicinity of the crack was carried out[ Using these results\ the cracking mechanism was found[ Methods of preventing the tube cracking problem are also discussed[ 1[ Tests and results 1[0[ Material and specimens The arrangement of the U!type radiant heater tube is schematically shown in Fig[ 0[ At one end of the tube\ a burner is installed inside the tube[ Flue gas passes through the tube and is exhausted

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Fig[ 1[ Samples of a cracked radiant tube for failure analysis[

through the other end of the tube[ The tubes are hung from the ceiling of the furnace and laid at the bottom of the furnace[ Steel plates moving between the tube rows are heat!treated by radiated heat from the tubes[ As shown in the _gure\ the U!type tube is supported by three hangers[ Cracking problems have been frequently reported near the locations where supporting hanger A\ which is close to the burner\ is in contact with the tube[ Cracks initiated at the inside of the tube and propagated to the outside inducing internal combustion gas to leak and cause problems to the heat!treated rolled plates[ The cracking direction was random and not related to the principal stress direction[ From this observation it can be argued that stresses generated in the tube due to internal pressure or due to thermal gradients are not the major cause of cracking[ Hence\ for the analysis of the cause of failure of this case\ a metallographic investigation of the failed component seems more appropriate than stress analysis[ Cracked specimen D was sampled from the cracked area around the supporting guide A as shown in Fig[ 1 for metallographic analysis[ Specimen D contained a big primary crack and several secondary cracks which were smaller than the primary crack[ Considerable wall thinning was observed at the inside of the cracked tube[ Since it is known that a failure analysis with a secondary crack "which usually shows the initial stage of cracking# is preferred to that with a primary crack "which shows almost the _nal stage of the failure#\ a failure analysis on the secondary crack which is 7Ð8 mm long was conducted[ The failed radiant tube was fabricated locally by centrifugal casting with 14 CrÐ19 Ni steel[ The chemical composition of the tested sample is shown in Table 0 with the standard composition[

Table 0 Chemical composition of the 14 CrÐ19 Ni radiant tube steel "in wt)# Elements

C

Si

Mn

Test sample Manufacturing standard

9[32 9[19Ð9[59

0[72 * ³1[99 ³1[99

P

S

Ni

Cr

Mo

Fe

9[914 9[997 11[17 11[54 9[957 bal[ ³9[93 ³9[93 07[99Ð11[99 13[99Ð17[99 ³9[49 bal[

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1[1[ Visual inspection To make a detailed observation of the cracked specimen\ specimen D was cut in a direction perpendicular to the primary crack as depicted in Fig[ 1\ generating two small sub!specimens\ D0 and D1[ With the specimen D0 the primary crack was opened to fracture with intention of observing the crack surface[ The crack tip region of the specimen D1 which was not opened was observed by a microscope[ Figure 2"a# and "b# show the inner and outer surfaces of the other specimen D1[ All other failed tube samples as well as specimen D showed many locally thinned areas at the inside of the tubes[ As observed from Fig[ 2"b#\ which shows the inner surface of the specimen D1\ thinning was fairly localized and occurred irregularly[ It should be noted that thinning did not occur gradually with distance from the burner[ From this observation\ we can rule out the possibility of erosion damage by solid particles included in the burner combustion gas as the main cause of tube thinning[ Hence\ it may be predicted that local oxidation or local corrosion is the main causes of thinning[ A thick oxide scale indicated by an arrow in Fig[ 2"b# was attached at the locally thinned area[ Similar oxide scales were also observed in a number of local oxidation pits[ Most of the oxide scales contain several cracks formed in random directions[ The oxide scale in Fig[ 2"b# is enlarged in Fig[ 3[ Cracking of the scale must be mainly due to the di}erence in thermal expansion coe.cient between the oxide scale and the tube metal on which the scale is attached[ Generally at the initial stage of oxidation\ an oxide _lm forms on the metal to prevent further oxidation[ Therefore\ when a stabilized oxide _lm is formed on the surface\ the resistance to oxidation under high temperature conditions is increased[ As for the radiant tube of this failure analysis\ the high content "14)# of Cr enables the formation of a Cr1O2 _lm that increases resistance to high temperature oxidation[ If this oxide _lm is removed\ the base metal of the tube will undergo repeated oxidation which results in continuous thickness reduction ð1Ł[ When cracking occurs in the oxide scale\ as illustrated in Fig[ 3\ the crack tip area loses the protective e}ect of the oxide _lm and the base metal beneath the crack tip will be repeatedly oxidised[ As a result\ a sharp oxide spike will be gradually formed in the base metal under the oxidation layer where oxide cracking occurred[ Observation of the opened fracture surface of specimen D0 showed an oxide layer extending to the crack tip[ 1[2[ Metallographic observation To con_rm the forementioned crack initiation mechanism\ a small metallographic sample was taken from the location of a local oxidation pit of specimen D1 where the oxide scale is attached\ as shown in Fig[ 3[ It was mounted and the scale was ground out until the tube metal right beneath the cracked oxide scale appeared[ An observation was made to see if any tube metal cracking occurred at the location beneath the oxide scale crack[ The specimen preparation procedure is shown in Fig[ 4[ From this observation\ a small crack of 3!mm length was found in the base metal right beneath the oxide scale and this crack was oriented in the same direction as the crack in the oxide scale[ Each crack tip area of the tube metal was observed by a scanning electron microscope and is shown in Fig[ 5"a# and "b#[ The crack tip was not sharp but looked like a blunt notch[ Also\ Fig[ 5"b# shows that the matching crack surfaces are separated from one another[ These two observations con_rm that the crack was not initiated by mechanical loading such as fatigue load

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Fig[ 2[ Appearance of internal and external surfaces of the cracked tube specimen D1[

but rather by repeated oxidation[ Hence\ the crack initiation mechanism assumed in the previous section is con_rmed[ The primary crack in the specimen D0 which was opened to fracture is illustrated in Fig[ 6[ The top line of the specimen represents a fracture surface of the primary crack where a deep oxide layer is present[ Beneath the primary crack\ the formation of a secondary crack with a shallow oxide layer is observed[ It is evident that the characteristics of the crack are similar to the crack shown

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Fig[ 3[ Enlarged view of a cracked oxide scale located in thinned area[

Fig[ 4[ Preparation of metallographic sample for studying crack initiation mechanism[

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Fig[ 5[ Crack tip morphology[

in Fig[ 5[ Thus\ it can be argued that the main cause for the formation of cracks in the tube is oxidation[ 1[3[ Chemical analysis To investigate the cause of the severe oxidation\ a chemical analysis was performed on tube metal and oxide scale[ Composition analysis was carried out both on the sound part of the tube

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Fig[ 6[ Oxide spike under the fractured surface of primary crack[

which was located far from the oxide layer and on the tube metal in the vicinity of a primary crack whose composition may be a}ected by oxide scale formation[ These two analysis results were compared\ to predict the cause of oxidation[ Also\ a piece of oxide scale was detached from the tube|s interior surface and analyzed[ Compositional analysis of the oxide layer at the crack tip was also conducted[ EDAX analysis in the electron microscope was used for composition analysis and the results are summarized in Table 1[ It is shown that the Cr content of the metal right beneath the primary crack decreased compared to the sound part of the tube[ This is due to the formation of Cr1O2 at the crack surface resulting in a decrease of the Cr content in the neighboring base metal[ Analysis results of three oxide scales "two pieces detached from di}erent locations and one attached at the crack tip# showed that the Ni content did not vary while the Cr content increased and the Fe content decreased considerably[ Thus\ it can be predicted that Cr1O2 is the main

Table 1 EDAX analysis results of the radiant tube at several locations "in wt)# Composition Locations

Ni

Cr

Fe

Si

Mo

S

V

Sound tube metal far from the cracked region Tube metal near to the cracked region Crack tip oxide scale Detached oxide scale 0 Detached oxide scale 1

07[56 08[45 06[08 19[11 07[94

29[05 16[23 22[41 20[13 30[13

38[63 40[46 33[75 33[73 24[37

0[32 0[42 1[33 0[30 2[95

* * 0[10 0[42 0[22

* * 9[36 9[43 9[50

* * 9[29 9[11 9[11

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Fig[ 7[ Ni distribution map of crack tip region "same location as shown in Fig[ 5"b##

oxidation product in the oxide scale[ Other minor elements appeared in the scales such as 9[3Ð 9[5) of S and 9[1Ð9[2) of V apparently produced by contact with combustion gas[ A high content of S may generate a corrosion problem^ vanadium usually forms V1O4\ which\ together with high velocity combustion gas\ may cause erosion problems[ In this tube failure\ however\ S and V seemed not to contribute to the failure[ Figure 7 is a composition map obtained by EDAX that shows the Ni distribution at the crack tip region of Fig[ 5"b#[ This picture shows that the oxidation layer is not correlated with Ni[ Figure 8 shows the Cr distribution at the same location[ Focusing on the Cr concentration of the oxide scale in Fig[ 8\ it is evident that the oxide layer at the crack tip is of Cr oxide product[

2[ Discussion Abnormal oxidation problems of high temperature steels are generally caused by improper operating temperature exceeding the recommended temperature range[ The manufacturing process of the radiant tube of this study is similar to a well!known centrifugally!cast HK steel\ but the chemical composition of the tube is di}erent from that of the HK steel since Si was added to the tube metal[ If we consider the composition only\ which is Cr  12Ð15)\ Ni  08Ð11)\ C  9[14)\ Si  0[4Ð2[9)\ and Fe  balance\ it is a typical composition of 203 stainless steel "UNS20399 steel#[ This material is known to have excellent high temperature oxidation resistance by forming Cr1O2 protective oxide _lm[ However\ if the service temperature exceeds 0999>C\ the stabilized Cr1O2 _lm becomes unstable and transforms into volatile CrO2 losing its protective e}ect[ There! fore\ the current radiant tube material should be used in a temperature range which does not exceed 0999>C\ to prevent abnormal oxidation[ Also\ it was reported that for high CrÐNi

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Fig[ 8[ Cr distribution map of crack tip region "same location as shown in Fig[ 5"b##

steel\ excessive oxidation can occur in a short period if the operation temperature is higher than 0989>C ð3Ł[ In order to know whether the failed radiant tubes had been in service below or above 0999>C\ the degradation level of the microstructure due to thermal aging was assessed[ In most of the cases\ the service temperature of the tube can be predicted by comparing the microstructure with that under known service conditions[ Figure 09 shows a typical microstructure of the failed tube[

Fig[ 09[ Microstructure of uncracked region of the tested radiant tube[

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000

Fig[ 00[ Microstructure of cracked region of the tested radiant tube showing void formation

Referring to the known reference microstructures of HK tubes degraded at high temperature ð2Ł\ the microstructure shown in Fig[ 09 corresponds to the microstructure similar to one aged at 849Ð 0999>C for 59\999 h[ Since the service period of the failed tube was only 04\999 h which is much less than the 59\999 h of the corresponding microstructure\ it can be predicted that the service temperature of the tube was above 0999>C[ Figure 00 shows a microstructure near the cracked area which shows internal void formation[ Voids of this kind were reported to be formed when the service temperature reaches 0989Ð0129>C in the case of NiÐCr steel ð3Ł[ Hence\ it can be argued that the local metal temperature during the service must go up to this high temperature[ This overheating can be induced by touching of the ~ame to the tubes near the supporting guide A in Fig[ 0[ Therefore\ to prevent radiant tube failures methods should be sought to lower the tube metal temperature below 0999>C\ particularly in the vicinity of supporting guide A[ Modi_cation of burner tips or improving combustion systems can be considered[ 3[ Conclusions By conducting a failure analysis on the cracked radiant heater tubes used in a high temperature furnace\ the following conclusions are derived[ The radiant heater tube which was centrifugally!cast with the same chemical composition as a typical HK steel except additional Si could be used without problems if it is operated at a temperature less than 0999>C so a protective Cr1O2 oxidation _lm forms[ However\ as the operating temperature exceeded 0999Ð0099>C\ the stabilized Cr1O2 transformed into volatile CrO2 and abnormal oxidation or rapid oxidation occurred[ The failed tube of the current study must have been used at or above the recommended temperature range and as a result\ locally thinned areas were formed by excessive oxidation[ Some of the oxidation pits were _lled with oxide scales formed

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by rapid oxidation[ This thick oxide scale was usually cracked because the heat expansion coe.cient of the oxide was di}erent from that of the tube metal on which the scale is attached[ Through the opening of the oxide crack\ fresh tube metal which was located beneath the oxide crack tip su}ered repeated oxidation resulting in small cracks initiating in the tube metal[ Tube failure _nally occurred as a result of propagation of these small cracks to the outer surface of the tube[ The failure could be prevented by maintaining the temperature of the tube at the ~ame side of the burner\ that is\ in the vicinity of supporting guide A in Fig[ 0\ below 0999>C by improving the existing combustion system or by modifying the burner tips[ Acknowledgements The authors are grateful for the support provided by a grant from the KOSEF "Korea Science and Engineering Foundation# through Safety and Structural Integrity Research Center in Sung Kyun Kwan University[ The authors also would like to thank POSCO "Pohang Iron and Steel Co[# for providing samples[ References ð0Ł Williamson J\ Shipley M[ Life assessment and monitoring of furnace heaters\ improving reliability in petroleum re_neries and chemical and natural gas plants[ Houston\ TX\ USA\ November 8Ð01\ 0881[ ð1Ł Walter M\ Schutze M\ Rahmel A[ Oxidation of Metals\ 0882^39]26[ ð2Ł Life prediction of tubes for steam reformer and cracker[ Document for Information Document No[ 74\ KHK\ 0872[ ð3Ł Lai GY[ High temperature corrosion of engineering alloy\ ASM International\ 0889[