Engineering Failure Analysis 34 (2013) 166–173

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Engineering Failure Analysis

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Cracking of an austenitic stainless steel lance pipe in a limecalcining plant

1350-6307/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.engfailanal.2013.07.033

⇑ Corresponding author. Address: Metallurgical Laboratories and QA Group, R&D and Scientific Services, Tata Steel Ltd., Jamshedpur 831001, In+91 9204651924; fax: +91 6572345407.

E-mail addresses: goutam.mukhopadhyay@tatasteel.com, goutam2007@yahoo.co.in (G. Mukhopadhyay).

Goutam Mukhopadhyay ⇑, Sandip BhattacharyyaR&D and Scientific Services, Tata Steel, Jamshedpur 831001, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 16 February 2013Accepted 24 July 2013Available online 11 August 2013

Keywords:Lance pipeAustenitic stainless steelSulphidationCarbide networkSigma phase

Kiln of a lime plant, a part of an integrated steel plant, converts limestone (CaCO3) into lime(CaO) by heating at a temperature of around 900 �C using coke oven gas as a fuel. Cokeoven gas is a by-product of the coke plant, which contains mainly hydro-carbons. The cokeoven gas is lanced into the kiln through a number of pipes attached to the kiln. The lancepipes are cracking prematurely within 6 months of their service against an expected ser-vice life of 3 years. The analysis of cracking of the lance pipe has been presented. The inves-tigation includes visual observation, chemical analysis, characterization of microstructuresusing optical and scanning electron microscopes, EDS analysis, and measurement of hard-ness. The lances are found to be made of AISI 302 grade of austenitic stainless steel. Visualobservation shows longitudinal cracks on the pipe wall associated with irregular yellowishsurface having multiple pits. Analysis of the deposit on the pipe wall shows the presence ofsulphur. Microstructural examination shows intergranular corrosion with thick porouslayer of scale on the surface. Microstructural and EDS analysis indicate sulphidation andchromium carbide networks along the grain boundary as well as sigma phases withinthe grains. Sulphidation, sensitization due to grain boundary carbides and sigma phasesled to intergranular corrosion causing cracking of the pipe wall.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The cracking of lance pipes in a lime calcining plant of an integrated steel plant has been investigated. Lime calciningplant is associated with most of the integrated steel plants to meet the demand of calcined lime (CaO) for various processeslike sinter making, pellet making and steel making. The basic oxygen steel making of integrated steel plant puts high demandfor calcined lime with high percentage of CaO (�98%) for desulphurization process of steel. Lime plant converts raw lime-stone (CaCO3) into lime (CaO) in the kiln by heating at a temperature of around 900 �C using coke oven gas as a fuel. Thelime plant has 7 calcining kilns each with a production capacity of 300 T/day for Kiln#1-5 and 425 T/day for Kiln#6 and7. Lime kiln is a parallel flow regenerative unit with two shafts having alternating burning and non-burning operations. Atypical kiln, as shown in Fig. 1, has two shafts which are connected to each other at the bottom. Both shafts are alternativelycharged with limestone of size range +25 mm to �80 mm, and the calcined lime, the final product, is discharged continu-ously from the bottom of both the shafts. Coke oven gas (H2: 52–55%, CH4: 22–25%, CO: 8.5–9.5%, CO2: 3–4%, CmHn:3–4%, N2: bal.), the fuel used for firing/burning in the kiln, is fed at a pressure of 1 atm from a main ring through a numberof lance pipes which are evenly distributed throughout the cross-section of the kiln. The average requirement of coke ovengas for a production rate of 300 T/day is around 3000 Nm3/h. Air is introduced at the throat of the shaft above the charge for

dia. Tel.:

Service Hopper

Rotating Hopper

Shaft closing trap

Chimney

Chimney trapBag filter

Jet loader Fan

Gas Booster

Discharge table

Discharge trap

Shaft1

Fuel Valve

Skip

Shaft2

Lance

Conveyer to LD shop

Product BinsConveyorsLance cooling blowersDischarge feeder

Blowers for cooling airBlowers for combustion air

Cooling air relief trap Combustion air relief

trap

Hopper

Fig. 1. Schematic diagram of a kiln in lime calcining plant.

G. Mukhopadhyay, S. Bhattacharyya / Engineering Failure Analysis 34 (2013) 166–173 167

combustion and the system is throughout pressurized. Air is preheated in the regenerator (preheating zone) before it mixeswith the fuel coke oven gas. The flame passes through the burning zone from top to bottom. The temperature at the burningzone is around 900 �C.

Fig. 1 shows the operation of a typical lime kiln. Shaft 2 is connected to the chimney through a bag filter and an induceddraft (ID) fan, when shaft 1 is in firing/burning operation. The waste gas generated in shaft 1 goes to shaft 2 through theconnecting channel. This waste gas at a temperature of around 900 �C is used for preheating the limestone in shaft 2; mostof the calcination takes place in this preheating zone. The temperature of waste gas drops to 150 �C after preheating and goesto bag filter and then to atmosphere through chimney. The process of firing continues for 720 s and then reversal processshifts from shaft 1 to shaft 2.

The lance pipes through which the coke oven gas (fuel for firing) is fed is cracking prematurely within 6 months of theirservice while the expected life is around 3 years. There are a total of 18 lances in each shaft; 6 lances are arranged in an innerring and 12 are in an outer ring. The lances enter at the upper part of the preheating zone and hang freely in the midst of thedescending charge. The lance pipes are usually made of austenitic stainless steels. Premature failure of the lance pipes is amajor concern in most of the plants. Austenitic stainless steel pipes are generally subjected to sulphidation [1] because ofsulphur (S) bearing compounds present in the service environment. Apart from sulphidation, austenitic stainless steel suffersfrom grain boundary carbide and sigma phase (r) formation in the temperature range 600–1000 �C and 480–815 �C, respec-tively. Sulphidation, grain boundary carbide and sigma phase formation followed by intergranular corrosion, depending onthe environmental condition at the application, lead to cracking of the stainless pipes. Metallurgical analysis of the prema-ture failure of a lance pipe followed by recommendation has been presented in the present article.

2. Experimental procedure and results

2.1. Visual observation

The lance pipes are uniformly arranged in inner and outer rings and enter at the upper part of the preheating zone of thekiln and hang freely in the midst of the descending charge. The inner diameter of the lance pipe is around 45 mm with a wallthickness of 5 mm. Certain portion of lance pipe was found broken from its tip within 6 months of its service while the ex-pected life is around 3 years. Visual observation shows longitudinal cracks on the pipe wall (Fig. 2a). The pipe surface exhib-its irregular porous appearance or multiple corrosion pits associated with yellowish deposits at places (Fig. 2(b–d)). The

Fig. 2. Samples collected from the failed lance pipe: (a) failed pipe shows longitudinal crack, (b–d) closer view shows irregular/porous surface withyellowish deposit. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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deposit adherent to the pipe wall was collected for analysis. The failed pipe was collected for detailed analysis to find out theroot cause of cracking.

2.2. Compositional analysis

A small specimen was cut from the failed lance pipe and prepared for the determination of its chemical analysis. Thechemical analysis of the sample was carried out using X-ray fluorescence spectroscopy (XRF); carbon (C) and sulphur (S) con-tent of the sample were determined using combustion infrared technique (LECO, TC600) as per standard IS 228 Part 20 [2].The chemical analysis of the lance pipe is compiled in Table 1. The chemical analysis indicates the pipe material as AISI 302grade of austenitic stainless steel. The chemical composition of the deposit collected from the pipe wall was carried out bywet chemical method using inductively coupled plasma emission spectrometer (Model: ARCOS, Germany) and is shown inTable 2. The analysis shows higher levels of Cr2O3 followed by CaO, SiO2, and MnO in the deposit. The analysis also showssignificant amounts of sulphur (S) in the deposit. Cr2O3 is generated due to the oxidation of the alloying element Cr present

Table 1Chemical analysis (wt.%) of the lance pipe material.

Sample C Mn S P Si V Cr Mo Cu Ni Nb

Lance 0.15 1.54 0.034 0.027 0.42 0.045 17.8 0.37 0.49 8.7 0.04AISI 302 0.15 max 2 max 0.03 max 0.045 max 1 max 17/19 8/10

Table 2Analysis (wt.%) of the deposit collected from the surface of the failed pipe.

Sample S P Cr2O3 TiO2 Al2O3 MnO SiO2 CaO Fe (T)

Deposit 4.31 0.008 29.40 0.013 0.10 2.80 2.81 4.56 42.03

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in the stainless steel pipe at the service condition while S comes from H2S and CaSO4 present as impurities in coke oven gasand raw limestone, respectively. Seong et al. [3] and Bhattacharyya et al. [4] have also mentioned H2S gas present as impurityin the coke oven gas as a potential source of S attack. Stanmore and Gilot [5] has reported CaSO4 present in the raw limestoneas a source of S.

2.3. Microstructural examination

A sample was cut from the failed lance pipe in the transverse direction of its length for microstructural examinations atthe cross-section. The sample was then mounted with resin, ground, and polished using standard metallographic techniques.The cross-section was examined under optical microscope (Leica, model: DMRX, Germany). The unetched microstructureshows severe intergranular corrosion near the surface associated with a thick layer of scale (Fig. 3). The scales on the surfacewere observed to be porous (Fig. 3a) which is typical of a reducing scale observed due to sulphur attack [3,4,6,7]. Variousresearchers [3,4,6,7] have reported the presence of porous scale on stainless steel pipe surface with outermost porousiron-rich oxide layer as a result of sulphur attack. Significant amount of intergranular corrosion was observed on the grainboundaries below the layer of scale as shown in Fig. 3(b and c). The intergranular corrosion was primarily due to oxidationand sulphidation which were confirmed by EDS analysis in the next section. Sulphidation is basically a corrosive attack byvarious sulphur bearing compounds at temperatures between 260 and 540 �C [1] The polished sample was then etched withVillela’s reagent (1 g picric acid, 5 mL hydrochloric acid, and 100 mL ethanol). Etched microstructure shows extensive grainboundary network of carbides (Fig. 4a) below the region of intergranular corrosion. The sample was colour etched withGroesbeck solution [8,9] to identify sigma phases [10,11] within the microstructure. Sigma (r) phase is basically aniron–chromium intermetallic compound that develops in stainless steels under prolonged exposure to high temperature(600–1000 �C) [10–12]. Groesbeck solution (4 g NaOH + 4 g KMnO4 + 100 g H2O) was prepared, and the ground and polishedsample was dipped into the solution and heated at 90 �C for 150 s. Microstructures reveal brown to black coloured sigmaphases (Fig. 4b) along the grain boundaries as well as within the grains in the form of plate or needle-like precipitates.

Fig. 3. Unetched microstructures at the cross-section of lance pipe: (a) microstructure (�100) shows thick porous layer of scale at the outer surface, (b)microstructure (�100) shows intergranular corrosion near the outer surface, and (c) microstructure at higher magnification (�200) shows formation ofscale at the grain boundary near surface.

Fig. 4. Etched microstructures at the cross-section of lance pipe: (a) microstructure etched with villela’s reagent reveals austenitic matrix along with grainboundary network of carbides, and (b) microstructure etched with Groesbeck solution reveals brown to black coloured sigma phases at the grain boundaryas well as plate or needle like precipitates within the grains. (For interpretation of the references to color in this figure legend, the reader is referred to theweb version of this article.)

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Similar microstructural features showing plate or needlelike precipitates of sigma phases have been observed by Babakret al. [10] and Łabanowski [13] in HK 40 (25%Cr, 19%Ni) and IN-519 (24%Cr, 24%Ni, Nb) cast high alloy steels, respectively.Formation of carbides (M23C6) and sigma phase are intimately related. Many researchers [14–17] reported that M23C6 acts asthe precursor to the sigma phase but this view has also been disputed by a lot of other researchers [18,19].

2.4. Scanning electron microscopy and EDS analysis

The polished sample was examined with the help of a scanning electron microscope (SEM) operated at an acceleratingvoltage of 15 kV for its microstructural as well as elemental characterizations. Energy Dispersive Spectroscopy (EDS) analysiswas carried out on the scale at the outer surface followed by at the adjacent region of intergranular attack located under-neath. The results of EDS analysis (point 10 as shown in Fig. 5a and Table 3) indicates the scale on the outer surface primarilyas oxidation corrosion product of chromium and iron (Cr2O3 and Fe2O3). The EDS analysis at the grain boundary regions be-neath the layer of scale indicates intergranular corrosion primarily due to sulphidation and oxidation as illustrated in Fig. 5(aand b) and Table 3. The EDS analysis indicates the presence of higher amounts of sulphur (S) which aggravates the corrosion.The main source of S is hydrogen sulphide (H2S) gas and calcium sulphate (CaSO4) which are present as impurities in the fuelcoke oven gas and raw material limestone, respectively as discussed earlier. Below the region of intergranular corrosion,there is grain boundary network of chromium carbides as indicated by the EDS analysis. EDS analysis was carried out fromthe grain boundary to the centre of a grain as shown in Fig. 6(a). The analysis shows higher amount of Cr at the grain bound-ary with a drop at the adjacent location as depicted in Fig. 6(b). The drop in Cr content at the location adjacent to grainboundary is a result of the formation of Cr carbide network at the grain boundary. The EDS analysis of needle-likeprecipitates within the grains as shown in Fig. 7 is compiled in Table 4. The results of EDS analysis confirm the needle likeprecipitates as sigma phase.

Fig. 5. Microstructure under SEM showing locations of EDS analysis: (a) locations of EDS analysis on the scale at the outer surface and at the grainboundary, and (b) locations of EDS analysis on the scale at the grain boundary underneath the layer of surface scale and severe intergranular corrosion.

Table 3Results of EDS analysis (wt.%) at locations as shown in Fig. 5(a and b).

Locations O Si S Ca Cr Mn Fe Ni

EDS analysis at locations as shown in Fig. 5(a)Point 1 41.91 3.93 1.470 6.68 17.78 28.21Point 3 31.16 4.16 1.977 1.660 16.26 33.61 12.80Point 6 27.20 3.87 1.741 1.738 9.19 34.12 22.12Point 9 2.46 24.68 1.797 51.83 19.22Point 10 37.64 4.27 36.28 17.79 3.05

EDS analysis at locations as shown in Fig. 5(b)Point 1 21.06 11.96 17.58 23.95 9.08 11.69 4.05Point 2 6.31 2.64 14.45 2.36 59.08 14.83Point 4 28.23 17.81 4.08 6.38 15.10 1.19 20.17 6.21

(a) (b)

Fig. 6. (a) Microstructure under SEM showing the locations of a typical EDS analysis carried out from the grain boundary to the centre of a grain,(b) diagram showing the variation of Cr content from the centre of a grain to its boundary.

Fig. 7. Microstructure showing the locations of EDS analysis on the needle like precipitates of sigma phases.

Table 4Results of EDS analysis (wt.%) at locations (sigma phases) as shown in Fig. 7.

Locations Si Cr Mn Fe Ni

Point 1 – 34.61 – 54.75 10.63Point 2 1.70 32.39 1.50 53.95 10.43Point 3 0.98 33.41 – 57.48 8.11Point 4 1.37 30.21 0.95 55.99 11.46

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2.5. Measurement of hardness

Hardness values were determined at the cross-section of the pipe wall. The hardness was measured by a Vickers hardnesstesting machine using a load of 10 kgf. Five indents were taken to get the average hardness value. The average hardness valueof the pipe wall is measured to be 200 ± 0.8 HV.

3. Discussion

Lime calcining plant is a part of most of the integrated steel plants to meet the requirement of calcined lime (CaO) for itsvarious processes like sinter making, pellet making and desulphurization during steel making. In a lime plant, limestone(CaCO3) is converted into lime (CaO) in a Kiln through heating at a temperature of around 900 �C using coke oven gas asa fuel which is a by-product of coke plant of the integrated steel plant. The coke oven gas is lanced into the kiln througha number of pipes attached to the kiln. The lance pipes are usually made of austenitic stainless steels. Austenitic stainlesssteel pipes are generally subjected to sulphidation, grain boundary carbide, and sigma phase formation followed by inter-granular corrosion, depending on various environmental conditions during application, leading to cracking of the pipes[3,6,12,20,21]. Premature failure of the lance pipes is a major problem in most of the plants. Metallurgical analysis of thepremature failure of a lance pipe has been carried out to find out the root cause of cracking.

Visual observation of the pipe broken from its tip end shows longitudinal cracks. The pipe exhibits an irregular surfacewith multiple pits having yellowish appearance at places. The pipe is found to be made of AISI 302 grade of austenitic stain-less steel. The microstructure of the pipe wall shows a thick porous layer of scale at the surface followed by a region of severeintergranular corrosion (Fig. 3). The EDS analysis of the scale (Fig. 5 and Table 3) primarily indicates oxidation corrosionproduct of chromium and iron (Cr2O3 and Fe2O3). The porous nature of scale is typical of sulphide attack [3,6,7]. The analysisof the deposit collected from the surface shows presence of sulphur in the level of 4.3 wt.%. EDS analysis also indicates a layerof sulphide (Fig. 5b and Table 3) between the porous scale (oxides of chromium and iron) and the substrate. Microstructuralanalysis indicates grain boundary corrosion. The EDS analysis at the grain boundary regions beneath the layer of scale showssignificant amount of oxygen (O) followed by sulphur (S) indicating intergranular corrosion primarily due to sulphidationand oxidation (Fig. 5 and Table 3). Yellowish appearance of pipe surface, presence of sulphur in the surface deposit, porousscale, and EDS analysis suggest that sulphur has a significant role in the cracking of the pipe. Kane [20] has discussed sul-phidation attack, similar to the present case, for a stainless tube.

Sulphidation is basically corrosion by various sulphur compounds at temperatures between 260 and 540 �C [1]. The mainsource of S responsible for sulphidation is hydrogen sulphide (H2S) gas and calcium sulphate (CaSO4) which are present asimpurities in the fuel coke oven gas and raw material limestone, respectively. The waste gas analysis of the kiln also shows aSO2 concentration of 94 mg/Nm3. The mechanism of sulphidation has been described in various literature [1,22]. Once sul-phur has entered the alloy pipe, it combines with chromium as sulphides, effectively redistributing the protective scale-forming elements near the alloy surface and thus interfering with the process of formation or re-formation of the protectivescale. Sulphur can also transport, under certain conditions, through the protective scales of Cr2O3 to the oxide–metal inter-face and forms precipitates of metal–sulphides (usually rich in Cr and Mn) beneath the scales where the oxygen potential issufficiently low. Once sulphides have formed in the alloy, there is a tendency for the sulphide phases to be preferentiallyoxidized by the encroaching reaction front; sulphur is displaced inward forming new sulphides deeper in the alloy, oftenin grain boundaries or at the sites of other chromium- or aluminum-rich phases, such as carbides. In this fashion, the reactionfront proceeds into the alloy-matrix with a front of sulphides followed by oxides. The relative corrosivity of sulphur com-pounds generally increases with temperature. The formation of chromium sulphides and oxides depletes the chromium con-tent directly under the oxide layer. Depending on the process particulars, corrosion occurs in the form of uniform thinning,localized attack, or erosion–corrosion.

Microstructure (Fig. 4a) below the region of intergranular corrosion shows extensive grain boundary network of chro-mium carbides as confirmed by EDS analysis (Fig. 6). The region adjacent to the grain boundary shows a drop in Cr contentwhile that at the grain boundary exhibits a sharp rise (Fig. 6b). Austenitic stainless steels suffers from grain boundary net-work of carbides leading to intergranular corrosion in the temperature range of 480–815 �C; this susceptibility has beentermed as sensitization and has been attributed to the precipitation of M23C6 carbides on the austenite grain boundaries[12,19,23]. The chromium content adjacent to the grain-boundary carbides dropped below some critical limit [24,25], whichrendered the alloy susceptible to severe localized attack by the corrosive environment. The susceptibility to sensitization in-creases with increasing carbon content of the base stainless steel [26,27]. Therefore, the carbon content of the steel must bekept lower (C < 0.03%) to avoid the formation of carbides, i.e., sensitization [26]. But the present material (AISI 302) of thepipe contains higher level of carbon (0.15 wt.%) which promotes an early sensitization.

Further, microstructure and EDS analysis confirm the presence of sigma phase along the grain boundaries as well as with-in the grains in the form of plate or needle-like precipitates (Fig. 4(b), Fig. 7 and Table 4). Sigma phase is an iron–chromiumintermetallic compound and it forms in stainless steel with long-time exposure in the temperature range of 600–1000 �C.This temperature range varies somewhat with composition and processing. Small amounts of Si, Mo, Ni, Nb and Mn increasethe rate of r formation [28]. In ferritic stainless steel, sigma phase is composed of iron and chromium only, but in austeniticstainless steel alloys, it is more complex and includes nickel, manganese, silicon, niobium, and so forth in addition to iron and

G. Mukhopadhyay, S. Bhattacharyya / Engineering Failure Analysis 34 (2013) 166–173 173

chromium [28]. Therefore, the compositions of sigma phase obtained in this investigation as shown in Table 3 are in goodagreement with those reported in the existing literature [10,11,29]. Sigma phase is a hard brittle compound and it depletesCr in the grain boundary regions; only a small amount of sigma phase precipitation is required to reduce the mechanical andpitting corrosion resistance of alloys. Formation of sigma phase along with carbides depletes primarily Cr from the matrixadjacent to grain boundary promoting intergranular corrosion leading to cracking. Apart from these metallurgical phenom-ena, the lance pipes are also subjected to abrasion with the descending burden of limestone. But sulphidation and sensiti-zation are the two most dominating mechanisms governing the cracking of the pipe.

4. Conclusion

Analyses of the above results led to the following major conclusions:

(i) The material (AISI 302) of the pipe is found to be improper for the application as it contains higher amount of carbon(0.15 wt.%) which led to sensitization enhancing intergranular corrosion.

(ii) Apart from sensi, presence of sulphur (S) in the application environment causes sulphidation of the stainless piperesulting intergranular corrosion leading to cracking of the component.

5. Recommendation

The carbon (C) content of the stainless steel alloy used in the pipe must be kept below 0.03 wt.% to avoid the formation ofalloy carbides. Resistance to sulphidation and oxidation may be achieved by adding Al and Si in the Fe–Cr–Ni alloy. Ferriticstainless steel 12Cr–5Ni–4Al alloy (C < 0.03 wt.%) with minor additions of Ti, Mn, and Si alloy may be used for theapplication.

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