STRUCTURAL EVOLUTION IN CAST STEEL REFORMER …konference.tanger.cz/data/metal2001/sbornik/papers/107.pdf · STRUCTURAL EVOLUTION IN CAST STEEL REFORMER TUBES DURING LONG-TERM SERVICE

METAL 2001 15. - 17. 5. 2001, Ostrava, Czech Republic

- 1 -

STRUCTURAL EVOLUTION IN CAST STEEL REFORMER TUBESDURING LONG-TERM SERVICE AT ELEVATED TEMPERATURES

Jerzy Łabanowski

POLITECHNIKA GDAŃSKA, Narutowicza 11/12, 80-952 Gdańsk, POLAND

Abstract

Catalytic tubes are the most important elements in reformer furnaces at ammonia chemicalplants. A steam reforming process converts hydrocarbons into mixtures of hydrogen, carbonmonoxide and carbon dioxide. This reaction proceeds at a temperature range 800-900 °C andunder pressure 3 - 4 MPa. This severe working condition causes a structural damage in thetubes. It is necessary to develop reliable methods for inspection of tube’s degradation and forrealistic prediction of its residual life. The paper presents effects of long-term service atelevated temperatures on microstructural changes of the alloy IN-519 (24%Cr, 24%Ni, Nb).The relationship between mechanical properties obtained in static tensile tests and the degreeof microstructure degradation is discussed.

1. INTRODUCTION

Prediction of residual lifetime of creep-resistant tubes and pipelines has been based oninvestigations performed during exploitation, including creep tests, tube deformation ordeformation velocity measurements, metallographic examinations, mechanical properties tests(static, impact and fatigue) and physical properties examinations (radiological, ultrasonic,magnetic and electric properties measurement).The basic criterion for the evaluation of the suitability of creep-resistant material for futureoperation is creep strength, which depends on the material’s structure and always decreasesduring the tube’s lifetime. Creep tests are expensive and long lasting, so there are attempts touse more cost-effective tests instead of creep tests; these also give useful and significantinformation [1,2].So far there is no reliable criterion describing the amount of degradation in creep resistantreformer tubes. Conventional Non-Destructive Testing (NDT) techniques such as eddycurrent and ultrasonic currently applied to reformer tubes are geared to finding creep damagein the form of internal cracking. These methods are still in progress but due to the complexityof factors influencing the life of the reformer tubes, present life prediction methods are limitedand are not able to detect and quantify reliably the damage present in the tubes [3,4].The structural criterion is very convenient but it can be accepted only when the structurechanges continuously with the working time [5]. This method, for example, is widely used forassessing amount of degradation in steels operated in elevated temperatures in power plants[6]. Similar correlation can be observed for other metallic alloys working under creepconditions.In this current study, the centrifugal cast tubes made of alloy IN-519 after various workingtimes are considered. The effects of long-term service at elevated temperatures onmicrostructural changes have been studied. The relationship between operation time andmechanical properties obtained in static tensile tests is discussed.


- 2 -

2. EXPERIMENTAL PROCEDURE

Investigations were performed on five tubes made of IN 519 cast steel taken from ammoniareformer furnace from POLICE chemical plant. The tubes worked at a temperature of 880°Cand under 3,2 MPa pressure. Samples were taken below 4 m from the inlet end of the tubeswhere temperature during operation is stable. Tested tubes worked for various times from24000 to 95000 hours. The tubes of a total length 12 meters (Fig.1) were assembled togetherby welding from the 3-meter segments, so the chemical composition of the alloy couldslightly differ along the tube.

Fig.1. Catalytic reformer tube used in POLICE chemical plant.

Table 1 shows he chemical composition of the samples. Since the new material was notavailable, the reference structure was detected on the sample “PM” taken from inlet end of thetube, where operational temperature did not exceed 540° C.The microstructure of the material has been examined by light optical microscopy and byscanning electron microscopy (SEM), and the chemical compositions of various phases havebeen examined by energy dispersive X-ray analysis (EDX). Metallographical samples weremechanically polished and etched by Murakami reagent 30g K3Fe(CN)6, 30g KOH, 60 mlwater. In addition, the mechanical properties at room temperature have been measured bytensile testing. Metallographical examinations were performed on the cross sections at thearea near 1/3 tube wall thickness from the inner surface. The surface condition of the tubes(carburisation, oxidation) is not discussed in this paper.

Table 1.Chemical composition of tested tubes

sampledesignation

distancefrom thetube inlet

Workingtime

Chemical composition wg.%

m h C Si Mn Cr Ni Nb S PPM 0,2 0,297 0,598 0,557 23,3 26,0 1,38 0,005 0,02924-1 6,2 24000 0,351 0,62 0,53 23,42 25,59 1,46 0,003 0,02124-3 6,5 24000 0,352 0,72 0,52 23,65 25,36 1,57 0,002 0,02030-1 6,2 30000 0,309 0,79 0,68 23,79 25,17 1,47 0,002 0,02430-3 6,5 30000 0,435 0,71 0,61 23,40 24,66 1,8 0,001 0,00844-2 4,0 44000 0,286 0,731 0,518 23,4 25,7 1,47 0,003 0,02372-2 4,0 72000 0,354 0,833 0,467 24,7 24,9 1,50 0,009 0,02572-1 8,0 72000 0,318 0,86 0,365 23,9 24,9 1,46 0,012 0,02395-1 5,0 95000 0,331 0,659 0,464 23,87 26,17 1,63 0,006 0,013


- 3 -

The microstructures of the 10 specimens of IN-519 cast steel after various working times ispresented in Figs.2-9.

Fig.2. Non-degradated structure of IN-519 cast steel, specimen “PM”. Magn. a) 200x, b) 500x

Fig.3. Microstructure of 24-1 (a) and 24-3 (b) specimen. Magn. 500x


a) b)

a) b)

a) b)


- 4 -

Fig.5. Microstructure of 44-2 specimen.Magn. 500x

Fig.6. Microstructure of 72-2 specimen.Electrolytic etch. SEM image.


Fig.8. Sample 95-1. Voids aligned alongdendrite boundaries. First microcracks started

at inner wall of the tube. Magn. 25x

Fig.9. Microstructure of 95-1 specimen.Magn. 500x

X-ray phase analysis revealed that apart of the austenite, NbC, Cr23C6 ,Cr15,58Fe7,42C6 carbidesand σ phase were present at the structures. The σ phase was detected in all specimens except

a) b)


- 5 -

the reference “PM” one. The highest diffraction lines occurred for 30-3 (30000 h) and 72-2(72000 h) specimens.Results of hardness tests and tensile tests performed at room temperature on flat testspecimens are given in Fig10. The specimens were taken parallel to longitudinal axis of thetubes and contained the whole tube wall thickness. The time axis at the figures is plotted inlogharytmic scale in order to obtain the straight line and estimate correlation betweenmechanical properties and tube working time.

2 3 4 5 6 7 8 910000 100000Working time, [h]

300

500

400

600

TS,

MPa

Y = -113.295 * log(X) + 1653.13Coef of determination, R = 0.9116

2 3 4 5 6 7 8 910000 100000

Czas pracy, godz.

3

8

13

0

5

10

15

A5,

%

Y = -3.60895 * log(X) + 42.3603Wspolczynnik krelacji, R = 0.9676

2 3 4 5 6 7 8 910000 100000Working time, [h]

180

220

260

160

200

240

280

HB2

,5/1

87,5

/15

Y = -10.8672 * log(X) + 328.185Coef of determination, R = 0.2118

Fig.10. Mechanical properties of IN-519 alloy after long-time service at 880°C in reformerfurnace. a) tensile strength, b) elongation, c) Brinell hardness

a)

b)

c)


- 6 -

3. DISCUSSION

The non-degraded “PM” specimen showed the dendritic columnar austenite grains locatedperpendicularly to tube walls. The structure consists of austenitic matrix with a proportion ofprimary inter-dendritic eutectic niobium carbide and chromium carbide. These massivelamellar carbides closely resemble pearlite. The thin semicontinous network of eutecticcarbides can also be seen in the iterdendric areas (Fig.2). An EDX analysis showed that someof eutectic carbides are Nb-carbides and others are Cr-carbides. The two types of carbidescannot be distinguished from each other in the optical microscope.

The original morphology of eutectic carbides has been modified during the long time ofoperation at elevated temperatures during which certain coalescence has taken place. Thestructure of 24-1 and 24-3 samples contain still lamellar eutectic of primary carbides, but thecoalescence process has formed not completely continuos network of primary carbides.Secondary carbides (mainly Nb carbides) are observed within the grains, but to a rathervarying degree, both in size and amount. In addition to the small and rounded carbides insidethe austenite grains, also coarser and plate or needle-like precipitates are observed. An EDXanalysis revealed that these precipitates have a chemical composition close to the sigmaphase. Presence of this phase in the alloy structure was unexpected because IN-519 cast steelhas low susceptibility to formation of this embrittling phase in the temperature range 600-950°C. To ensure low susceptibility to sigma phase formation, the chemical composition ofthe alloy was modified to conform to the upper limit of 1,0 per cent silicon and to maintainthe nickel/chromium ratio at unity or above. The amount of sigma phase and degree ofcarbide coalescence was different in samples 24-1 and 24-3 (Fig.3). These two samples weretaken from the different segments of the one reformer tube but very close each other (0,2 m)separated by welding joint. Obviously, the working conditions in this area of the furnace werethe same, the manufacturing technology was also similar, so the structural difference couldonly be explained through slightly different chemical compositions of tube segments.The degree of degradation in the 30-1 and 30-3 specimen (30000 h) was greater than in “24”samples (Fig.4). The carbide coalescence process is more advanced and quantity of lamellareutectic decreased but it still existed in both samples. The grain interior was filled with needlelike sigma phase precipitates and dispersed NbC carbides. These two samples were takenfrom two segments of one reformer tube very close each other and also in these structures theconsiderable differences in the amount of secondary phases was observed.The structure of 44-2 sample (44000 h) consists of continuos network of coalesced carbidesaround austenite grains with only the trace of primary eutectic structure. Very small amountsof sigma phase were detected inside the austenite grains.The similar structure was found in 72-1 sample (72000 h). In this case, the semicontinuouscoarse network along austenite grains consists of Cr and Nb carbides. Sigma phase is alsopresent in the structure in the form of blocky precipitates. In 72-2 (Fig.6,7) sample, thecorresponding coarse network of coalesced carbides existed but grain interiors were filledwith great amounts of needle-like sigma phase. A small number of creep voids are found instructure in “72” samples. The voids are formed at the interface between matrix and primarycarbides or between matrix and the blocky sigma. The voids were rather small (up to 8 µm)and limited in number. They are rather scattered with no tendency to lining.The structure of 95-1 was similar to 72-2 sample, but unetched structure revealed a greatnumber of creep voids. The voids were located mainly on the inside surface of the tube and atthe midwall. The voids are becoming aligned along dendrite boundaries. Aligned voidscoalesced into fissures (up to 800 µm) were detected close to the inside surface of the tube(Fig.8).


- 7 -

Generally, metallographic observations revealed that the following transformations took placein the structure during tube exploitation:

• coalescence of primary inter-dendritic austenite-carbide eutectic,• formation of continuos network of primary and secondary carbides around austenite

grains,• sigma phase precipitation in the needle-like form,• coalescence of sigma phase to the blocky shape.The amount of primary and secondary phases (NbC, M23C6, σ ) existing in the structures ispresented in Fig. 11.

0 20000 40000 60000 80000 100000Working time, [h]

5

15

25

0

10

20

30

Am

ount

of p

rimar

y an

d se

cond

ary

phas

es, %

��

��

��

��

��

��

��

��

��

��

Fig.11. Total amount of primary and secondary precipitates existed in the IN-519 alloystructure after long term service at elevated temperature

This simple quantitative description of the degree of structure degradation shows thatstructural changes during the operation time are not uniform and change in a non-monotonicmanner with operating time. Moreover, the structures of the samples taken from the samereformer tube, exploited in the same conditions also can differ in quantity and form ofprecipitated phases.

Results of mechanical properties tests showed in Fig.10 indicate that the most significant isreduction of plastic properties measured by unit elongation (E) at room temperature. The Evalues show the best correlation with working time logarithm. Taking this into account, thedegree of the degradation of E values could be useful for determining the tube’s degradationlevel and for estimating its residual lifetime. The method employing mechanical propertychanges can be applied after its modification through considering other working parameters,e.g. stress and temperature.The hardness test results show a large scatter and it seems that this method is not suitable forestimating reformer tube degradation.


- 8 -

4. CONCLUSIONS

1. The microstructural examinations of IN-519 cast steel reformer tubes shows that structuralchanges occur in non-monotonic way with operating time at elevated temperatures, thusthe evaluation of tube degradation can not be based only on the structure appearance.

2. Degradation of elongation values obtained in tensile test show the best correlation withthe working time of reformer tubes.

LITERATURE

1. BARCIK J.: Stopy na rury pirolityczne. Uniwersytet Śląski, Katowice 1995.2. CZYRSKA-FILEMONOWICZ A. et al. Analiza zmian strukturalnych staliwa HK 40

wywołanych eksploatacją rur w instalacji do parowego reformingu metanu. Hutnik 1981,No 11-12.

3. DA SILVA T.L., LE MAY I. Damage accumulation mechanisms in reformer furnace.Proceedings of Conference Materials Ageing and Component Life Extension. CIM.Montreal 1995.

4. JONES J.J.: Developments in heat-resisting alloys for petrochemical plants in Researchand Development of High Temperature Materials for Industry. Editor E. Bullock. ElsevierApplied Science. London-New York 1989.

5. TILLACK D.J.,GUTHRIE J.E. Wrought and cast heat resistant stainless steels and nickelalloys for the refining and petrochemical industries. NiDI Technical Series, No 10071.Toronto , 1992.

6. Atlasy degradacyjnych zmian struktury podstawowych stali energetycznych w wynikudługotrwałej eksploatacji. Instytut Energetyki, Warszawa, 1996.

Documents

STRUCTURAL EVOLUTION IN CAST STEEL REFORMER …konference.tanger.cz/data/metal2001/sbornik/papers/107.pdf · STRUCTURAL EVOLUTION IN CAST STEEL REFORMER TUBES DURING LONG-TERM SERVICE