AbstractThe sudden collapse of the lining refractory in a waste incineration plant during a regular maintenance has been investigated. Preliminarily visual inspection showed corrosion of the support hangers that have been installed to enhance the stability and the strength of the lining refractory. One of the ends of the 310 SS support hanger was exposed to the incinerator hot gas steam with a temperature of 1100 oC. Visual, optical and SEM inspection have been conducted on the failed support hangers to figure out the cause of the failure. Upon examination, the support hangers showed the formation of thick oxide scale and a phase transformation in some parts. The second phase particles are believed to be sigma-phase particles with some traces of -ferrite that gave the steel its magnetic properties. Sigma phase particles are very brittle, but cause no problem if the temperature is kept at the range in which sigma-phase forms. However, impact loading can lead to catastrophic failure for the sigmatized stainless steels.

Introduction:During routine shut down of a water incinerator plant to repair the worn areas of the refractory lining, approximately 12m2 collapsed without warning from one of the vertical walls of the plant. An initial examination of the failure revealed that the refractory support hangers made from 310 SS had corroded leading to the collapse of the refractory lining on the incinerator wall. When in operation, the incinerator had been used to burn a wide variety of hazardous chemicals at temperature above 1100 oC. The refractory lining is made up of three layers; an inner insulating board, a second layer of light insulating refractory and a third outer layer of heavier and more erosion resistant hot face refractory. Running through the refractory lining are stainless steel support hangers, which are screwed into the incinerator wall, figure1. The hangers give the refractory lining the strength it requires to support itself [1].

Before the failure, the lining and the supports were in service for five years and some of the lining has worn out already so the tips of the hangers are exposed to the hot gas steam temperature of about 1100 oC. The failure occurred while the operators were jack-hammering the lining to remove the heat-affected refractory.

Analysis:The Hot Steam:The exhaust gas generated from the incineration of solid wastes contains corrosive substances, such as dust, acid gases (SOx, HCl, NOx, CO2, and other gases), and air which is readily available for the combustion process. Such oxidizing environments are highly corrosive to metals and especially stainless steels with the most corrosive substances are HCl and SO2.

310 Stainless Steel:The support hangers are made of 310SS which is an austenitic stainless steel with high creep strengths and high resistance to high-temperature corrosion [2]. The composition of the 310SS is shown in table1.

Table 1: Composition ranges for 310 grade stainless steel [3]

Grade C Mn Si P S Cr Mo Ni N310 min.

max.-

0.25-

2.00-

1.50-

0.045-

0.03024.026.0

- 19.022.0

-

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This type of stainless steel is generally used at temperatures starting from about 800 or 900 °C [3] and designed to resist oxidation in continuous service at temperatures up to 1150°C [4] in both oxidizing and carburizing atmospheres. The typical heat treatment for such stainless steel is annealing for maximum corrosion resistance. Typical applications for 310SS include: furnace parts, heat exchangers, and oil burner parts.

Visual Inspection of the Support Hangers:The support hangers, and as mentioned above, are made of 310SS with a nominal cross section of 8mm and shaped in a V-shape as shown in figure 1. Different specimens have been taken from the support hanger to be tested under the optical and scanning electron microscopes. The locations of the different specimens are shown in figure 2. After the failure of the support hangers, the thickness measurements were noted down and tabulated in table 2; table 2 shows the measurements for only one of the support hangers. The measurements of thickness were taken each 30mm from the end of the support hanger near the incinerator wall. Most notably, most of the support hangers have failed on the same location where the hot face refractory contacts the thermal insulating refractory as shown in figures 3 and 4. The failed side is the lower side in figure 2. At the location of failure, we notice thick oxide film formed and this can be noted from table 2 at a distance of 150mm from the exposed end of the support hanger. Unfortunately, the fracture surface have oxidized already and no further visual inspection can be carried out. When the support hangers were tested with a magnet, they showed a high magnetism with the magnetic properties increase from the pivoted end toward the failed end near the hot gas steam. This indicates that there was a phase change in the stainless steel since stainless steels are non-magnetic. Building on that and on the thickness of the oxide scale formed, we can conclude that the corrosion product is not a protective Cr2O3 layer.

Table 2: Measurement of thickness of support hanger 2 after failure (distances taken from the exposed end)Support Hanger 2

Left- side (Upper side) Right-side (Lower-side)Intervals (mm) T1 (mm) T2 (mm) T (mm) Intervals (mm) T1 (mm) T2 (mm) T (mm)

30 3060 6 5.9 5.95 6090 6.9 7 6.95 90

120 8 8.1 8.05 120150 8.2 8.1 8.15 150 11.6 12.6 12.1180 8.1 8.1 8.1 180 8.2 8.2 8.2210 8.1 8.1 8.1 210 8.2 8.1 8.15240 8.4 8.2 8.3 240 8.2 8.3 8.25

Average 7.67 7.64 7.66 Average 9.05 9.30 9.18

Optical Microscope:For testing under the microscopes, different specimens have been cut out from one of the failed hangers; the location of each specimen is shown in figure 2. The specimens were painted to distinguish them; white, yellow, blue, and red starting from the exposed end toward the pivoted one as shown in figure 2. The observations for each specimen are as follows:

White: this specimen is from the nearest location to the exposed end. Visual inspection of the specimen shows that it is with the smallest diameter among all specimens. This indicates high corrosion product has formed in this area; most of the oxide scale has fallen off so we can only see very thin intermittent oxide layer under the microscope. Some

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internal oxidation has been noted; this internal precipitates will most probably be Cr2O3. We can notice the formation of second phase particles all the way to the centre of the specimen with the shapes vary between needle, big needle, and some globular shapes (figures 5 and 6).

Yellow: if we move further toward the pivoted end of the support hanger, we will notice similar observations with very thin intermittent oxide layer with small amount of internal precipitates of Cr2O3. However, a distinct layer of second phase particles have formed beneath the oxide layer with fewer particles toward the centre.

Blue: if we take another step toward the pivoted end of the support hanger; we can notice almost similar observations to those found in the yellow specimens. The layer of the formed second phase particles is almost the same width of the oxide layer.

Red: this is the last specimen that has been taken from the support hanger near the pivoted end. This specimen is with the largest diameter where only small amount of oxide layer has formed on the outer surface of the specimen. Similarly, only small dispersed amount of internal precipitates has formed. Most notably, there is no second phase particle formation in this specimen. This microstructure is typical to that of ordinary stainless steels.

We tested one more specimen under the optical microscope which is the cross section of the support hanger from the nearest location to the incinerator wall. From this specimen we noticed that the crack is brittle; no elongation lips, and had propagated along the grain boundaries rather than across them. (Unfortunately, no figures provided since I could not find the specimens to take more micrographs!)

From the optical micrographs we have concluded that the metal of the hanger was not showing typical stainless steel behavior at the time of the failure due to the very thick oxide layer formed on the metal. In addition, we noticed the formation of the second phase particles in the bulk of the metal, and the presence of the second phase particles increases with the increase of temperature along the refractory from no formation at the pivoted end to 'the highest density with equal distribution' at the exposed end. Not all of the hangers failed at the same time; some of them failed prior the main failure since there were already oxidized. The other ones failed because they were not able to withstand the excessive load applied on them. No clue has been found on the effect of the refractory type on the corrosion morphology; although the corrosion increases as the temperature increases. The crack propagation was intergranular rather than transgranular.

Scanning Electron Microscope:We examined the white and yellow specimens under the SEM to get better micrographs and to conduct an elemental analysis of the different phases we obtained using the optical microscope. Figure 7 shows the cross section of the white specimen when tested under the microscope. Different spectra have been used to figure out the composition of the different parts and phases present. From figure7-spectrum1 we notice that the dense oxide layer formed is mainly composed of Cr, Fe, and O. this indicates that this layer is a spinel layer with some chromia. This spinel is what caused the fast growth of the thick oxide layer with a thickness of about 3.4mm (figure 12). Internal precipitates present in the alloy are chromia as shown in figure7-spectrum3. When the second phase particles were examined, they showed high concentration of Cr and the area adjacent to these particles are Cr-depleted zones which indicates that there was Cr migration in the alloy as shown in figure7-spectra4 and 5. To find

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what exactly the compositions of the different second phase particles are, we conducted an elemental analysis for these particles. Although they have different shapes; globular, big needle, and small needle shapes, they have exactly the same composition as shown figure8-spectra1, 2, and 4. Again, the adjacent zones are Cr-depleted zones (spectrum 3).

Also, we have conducted similar analysis for the yellow specimen. The observations for this specimen are similar to those found for the white specimen. The thick oxide layer is a spinel with some traces of chromia (figure9-spectrum1). Internal precipitates are chromia (figure9-spectra 2, 3, and 4). The dark internal precipitates at location 5 are not a new phase; they are Si-rich particles in which Si was introduced during the polishing process of the specimen as shown in figure9-specturm 5. Like the case for the white specimen, the different-in-shape second phase particles are with the same compositions which are Cr-rich particles (figure10-sepctra 1, and 3) and the adjacent zones are Cr-depleted zones (figure10-spectrum 2).

Finally, we took a part from the support hanger that was embedded in the refractory left with a thick oxide layer not fallen off yet and we tested that under the SEM. Figure11 shows the location from which the sample has been taken. From figure 12, we notice that the thickness of the oxide scale is 3.4mm which is very thick and unusual for stainless steel as discussed earlier. When we tested several locations across the oxide scale we got exactly the same composition which is spinel with high amounts of Cr, Fe, and O; therefore, only one spectrum is shown in figure 12. In a closer look (figure13), we notice that the oxide scale starts to spall; this explains the small amount of oxide scale observed for the white and yellow specimens under the optical microscope as discussed above. Spectrum 1 shows high C present in this area because of the epoxy resin entered the interface between the oxide and the metal when the oxide scale started to detach. Spectra 2, 3, and 4 show the spinel composition of the oxide layer, Cr-depleted zone, and Cr-rich particles respectively.

Second Phase Particles:As discussed above, the stainless steel has a single phase, austenite. However, the specimens tested showed the formation of second phase particles and their presence depends on the temperature distribution; as the temperature of the support hanger increases, the presence of the second phase particles increases. Phase changes in the stainless steel can influence the mechanical as well as the chemical properties of the stainless steel [5]. The possible phase change at high temperature for stainless steel is sigma-phase.

The sigma-phase is a brittle, Cr-Mo [4] or Cr-Fe rich [6] intermetallic phase. At temperatures between approximately 600 and 900 °C and for prolonged exposure time, sigma phase precipitates in high Cr, Mo, or Si-containing stainless steels; Grades 310, 314, 316, and 317 [2] are among the most susceptible austenitic stainless steels. Hau et al state that: "[…] it would appear that there should be little consequence as long as the affected components continuously operate at the elevated temperature. However, cracking could occur if the components were impact loaded or excessively stressed during maintenance work" [7]. Sigma phase depletes the chromium from the matrix and the main characteristics of these sigma-phase particles are: non-magnetic, very low impact strength, and very low intergranular, pitting and crevice corrosion resistance [4], [8]. Table 3 shows the second phase particles that are most likely to form in stainless steels. From this table and for sigma-phase, we notice that the formation of sigma phase is faster when formed from -ferrite. This is very important since -ferrite is magnetic and the steel showed high magnetization as mentioned earlier. However, the mechanism of nucleation is still a matter of controversy, particularly on the role of -ferrite and M23C6 in the nucleation process. Studies have reported its formation

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associated with or without the dissolution of carbides [6]. This is strongly supports our conclusion about the type of the second phase particles since we did not observe any carbides in the steel. Therefore, we can conclude that the second phase particles are mainly sigma-particles with some traces of -ferrite phase in the alloy that gave the steel its magnetic property.

Table 3: Most common second phase particles in stainless steels [4]

Recommendations:The main causes of the failure are the formation of second phase particles and the impact loading caused by jack-hammering of the lining refractory during maintenance. Therefore:

Normal austenitic stainless steels should not be used anymore. A good alternative of 310SS is 310LN with nitrogen and 347L with the Nb that will prevent the formation of carbides and second phase particles. Cold working of the stainless steels was found to initiate the formation of second phase particles whereas annealing is found to suppress their formation.

Maintenance interval should be reduced for better inspection and anticipation of proper failures in the refractory.

The use of hammering should be stopped as the excessive impact loading caused the sudden failure of the lining refractory.

The failure occurred after five years of service at which the ends of the support hangers were exposed to the hot gas steam with very high temperature almost near their limits (1150 oC); therefore, the replacement schedule should account for the minimum thickness of the lining refractory that will not cause the exposure of the support hangers to the hot gas steam.

ConclusionFrom the analysis of the failed support hangers of the waste incinerator plant, we found that the cause of the failure is the corroded support hangers which are made of 310SS. However, the oxide layer thickness is unusual for austenitic stainless steels. Closer investigation revealed that there are second phase particles in the metal. No consensus about the type of these phase particles has been approached, but depending on many literatures and published

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papers, they are believed to be sigma-phase particles with some -ferrite that gave the steel the magnetism. The use of the 310SS or any austenitic stainless steel should be stopped in the incinerator plants. A good alternative is the 310LN with the nitrogen addition or the 347L with Nb alloying that both will suppress the formation of second phase particles and carbides in the austenitic stainless steels. Also, the maintenance schedule and practice should be improved to minimize the intervals between maintenances and the hammering technique for removal of the worn lining refractory should be prohibited.

References:1. Paul Jordan, "Case Study: High Temperature Components"; University of

Manchester, 20072. "Stainless Steel - High Temperature Resistance"; [http://www.azom.com/details.asp?

ArticleID=1175]3. "Stainless Steel - Grade 310"; [http://www.azom.com/details.asp?

ArticleID=966#_Composition]4. ASM Handbook, Volume 13: Corrosion5. "Phase Changes in High-Temperature Service ";

[http://httd.njuct.edu.cn/MatWeb/gas/ka_ht/ht_phcha.htm]6. T.Sourmail; "Precipitates in creep resistant austenitic stainless steels";

Cambridge University; [http://www.msm.cam.ac.uk/phase-trans/2003/sourmail.review/index.html]

7. Jorge Hau et al; "Sigma Phase Embrittlement of Stainless Steel in FCC Service"; [www.cap-eng.com/news.asp?show=art&artid=235]

8. "Examinations to the sigma-phase" [http://www.metallograf.de/start-eng.htm?untersuchungen-eng/sigmaphase/sigmaphase.htm]

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AppendixFigures addressed in the report:

Figure 1: Schematic of a refractory support hanger and its relationship relative to the layers of refractory.

Figure 2: Schematic diagram showing the locations of the samples cut from the failed refractory support hangers.

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Figure 3: The failure location for support hanger 2 Figure 4: The failure location for support hanger 3

Figure 5: White specimen cross section under optical microscope (10x)

Figure 6: White specimen cross section under optical microscope (20x)

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Figure 7: SEM micrograph of the white specimen with EDX analysis

9

1

2

3

4

5

Figure 8: SEM micrograph of the second phase particles in the white specimen with EDX analysis

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Figure 9: SEM micrograph of he yellow specimen with EDX analysis

11

1

2

3

4

5

Figure10: SEM micrograph of the second phase particles in the yellow specimen with EDX analysis

Figure 11: Refractory from which the specimen with thick oxide layer has been taken

12

1

2

3

Figure 12: The specimen took from the refractory (figure11)

Figure 13: Oxide/Metal interface forspecimen in figure 12

13

12 3 4

1

2 3

4