Revista Latinoamericana de Metalurgia y Materiales, Vol. 8 Nos. 1 & 2 (1988) Carburization of Fe-Ní-Cr Alloys in Ch.-H 2 Atmospheres between 900 and 1100°C. G. Matamala* and P. Cañete** The corrosión behaviour of Incoloy 800 in Ar-C~-H2 gas mixtures having a carbon activity of 0.95 was studied in the temperature range between 900 and 1l00'C. The kinetics, which can be described by the equation (b. m/A'f = 2kt + e, were determined gravimetrically. The kvalues obtained were 0.45,1.24 and 5.99 (mg2cm- 4 h+) at 900,1000 and 1l00°C respectively and the activation energy was determined to be 172(kJ/mol). Both M 2s O; and M7Ca carbides were observed, the former transforming to the latter upon an íncrease oí carbon activity in the sorrounding matrix. INTRODUCTION Carburization and metal dusting are eorrosion pro- blems experieneed by high temperature alloys in indus- trial proeesses sueh as ethylene production, natural gas reformation, eoal gasifieation, ete., whieh oeeur at tem- peratures higher than 700°C [1-3]. Our initial work in this area was earried out on iron [4-5], being extended now to the study of earburization of Ineoloy 800 in Ar-Ch 4 -H 2 gas mixtures in the tempera- ture range, 900-1100°C. The gas mixtures were relatively free of oxygen so that oxide formation [6-8] would not interfere with the earburization proeess. Several authors [9-13] have reported on the kineties and meehanisms of earburization of Fe-Cr, Cr-Ni and Fe-Cr-Ni alloys. The objeet of this work is to widen the studv on the Incoloy 800 which is representative of Fe- Ni-Cr refractory alloys. EXPERIMENTAL The equipment used of the carburiz ation experi- ments. described in detail in a previous papel' 141.consis- ted of a Lindherg electric furnace with a digital tem- perature eontroller, a vitreosil reaetion tube and two gas circuits. The weight gain of the sarnples was measured eontinously by the extension of a fused silica spring using a cathetometer; the sensitivity ofthe measurement was 0.05 mierograms. The Ineoloy 800 speeimens were of 10 x 10 x 3mm dimensions, and their surfaces were finished to 600 grit by polishing. The composition of the alloy was Fe-32Ni- 21Cr %wt. The experiments were carried out by carburizing the samples in a Ar-CH 4 -H 2 atmosphere with a carbon activity of 0.95. The reaction kinetics were determined at 900, 1000 and 1100°C for 200 hrs by measuring the weight gain of the specimens as a function of time. The surface morphology of the samples was investi- gated by scanning electro n microscopy. The progress of .carburization was observed by metallographic techni- ques using Murakami's reagent as etchant. •• Universidad de Concepción. Facultad de Ingeniería Concepción-Chile. Patromin Ltda., Santiago-Chile . KINETICS RESULTS Figure 1 shows the weight gain of the samples per unit area as a function of time. The weight gain begins after an incubation time, whieh was determined from a plot of the square of the weight gain per unit are a versus time. This plot was a straight line at short times; the interseetion of this straight line with the time axis gave incubation times of llO, 15 and 2 h at 900, 1000 and 1100°C respeetively. This behaviour is consistent with the presence of a thin protective oxide layer on the alloy surface, whieh must be converted to a carbide layer by heating in the carburizing gas mixtures. 30 '" E u - (]1 E o iIJ 20 '- o e :J t a. QJ 111 - O V 10 ... v s - .!: C1 iIJ 3; Fig. 1. Gravimetric measurements oí the carburization of Incoloy 800 in Ar·Cli4-H2 atrnospheres at different temperatures (a., = 0.95).
Transcript
Revista Latinoamericana de Metalurgia y Materiales, Vol. 8 Nos. 1 & 2 (1988)
Carburization of Fe-Ní-Cr Alloys in Ch.-H2 Atmospheres between 900 and 1100°C.
G. Matamala* and P. Cañete**
The corrosión behaviour of Incoloy 800 in Ar-C~-H2 gas mixtures having a carbon activity of 0.95 was studied in the temperaturerange between 900 and 1l00'C.The kinetics, which can be described by the equation (b. m/A'f = 2kt + e,were determined gravimetrically. The kvalues obtained were0.45,1.24 and 5.99 (mg2cm-4 h+) at 900,1000 and 1l00°C respectively and the activation energy was determined to be 172(kJ/mol). BothM2sO; and M7Ca carbides were observed, the former transforming to the latter upon an íncrease oí carbon activity in thesorrounding matrix.
INTRODUCTION
Carburization and metal dusting are eorrosion pro-blems experieneed by high temperature alloys in indus-trial proeesses sueh as ethylene production, natural gasreformation, eoal gasifieation, ete., whieh oeeur at tem-peratures higher than 700°C [1-3].
Our initial work in this area was earried out on iron[4-5], being extended now to the study of earburization ofIneoloy 800 in Ar-Ch4-H2 gas mixtures in the tempera-ture range, 900-1100°C. The gas mixtures were relativelyfree of oxygen so that oxide formation [6-8] would notinterfere with the earburization proeess.
Several authors [9-13] have reported on the kinetiesand meehanisms of earburization of Fe-Cr, Cr-Ni andFe-Cr-Ni alloys. The objeet of this work is to widen thestudv on the Incoloy 800 which is representative of Fe-Ni-Cr refractory alloys.
EXPERIMENTAL
The equipment used of the carburiz ation experi-ments. described in detail in a previous papel' 141.consis-ted of a Lindherg electric furnace with a digital tem-perature eontroller, a vitreosil reaetion tube and two gascircuits. The weight gain of the sarnples was measuredeontinously by the extension of a fused silica springusing a cathetometer; the sensitivity ofthe measurementwas 0.05 mierograms.
The Ineoloy 800 speeimens were of 10 x 10 x 3mmdimensions, and their surfaces were finished to 600 gritby polishing. The composition of the alloy was Fe-32Ni-21Cr %wt.
The experiments were carried out by carburizingthe samples in a Ar-CH4-H2 atmosphere with a carbonactivity of 0.95. The reaction kinetics were determined at900, 1000 and 1100°C for 200 hrs by measuring the weightgain of the specimens as a function of time.
The surface morphology of the samples was investi-gated by scanning electro n microscopy. The progress of.carburization was observed by metallographic techni-ques using Murakami's reagent as etchant.
••Universidad de Concepción. Facultad de IngenieríaConcepción-Chile.Patromin Ltda., Santiago-Chile .
KINETICS RESULTSFigure 1 shows the weight gain of the samples per
unit area as a function of time. The weight gain beginsafter an incubation time, whieh was determined from aplot of the square of the weight gain per unit are a versustime. This plot was a straight line at short times; theinterseetion of this straight line with the time axis gaveincubation times of llO, 15 and 2 h at 900, 1000 and1100°C respeetively. This behaviour is consistent withthe presence of a thin protective oxide layer on the alloysurface, whieh must be converted to a carbide layer byheating in the carburizing gas mixtures.
30
'"Eu-(]1E
oiIJ 20'-o
e:J
ta.QJ111
- OV 10...vs-.!:C1iIJ3;
Fig. 1. Gravimetric measurements oí the carburization ofIncoloy 800 in Ar·Cli4-H2 atrnospheres at different
Fig. 2 presenta a plot ofthe square ofthe weight gainper unit are a as a function of time from which the incuba-tion period has been discounted. The resulting datafollowed initially an equation of the type.
(6.m/Af = 2kt + e
For longer times the data deviate from this equa-tion, more so at higher temperatures, because the diffu-sion volume is progressively reduced as the carboningress takes place. This effect has also been observedby other workers [10, 11] and it is possible that suchdeparture occurs when the two carb urization fron ts fromopposing faces of the sample meet. Beyond this stage, aparabolícrate law would certainly not be expected sincethe boundary conditions have been altered.
N
N
Eu
O'E
o"<;
'"o.'"'"el~Ue
Carburization time (h)
Fig. 2. Parabolic plot of the carburization experiments inAr-C~-H2 atmospheres,
Rate constants k obtained by linear regressioanalysis of the points in the straight line región of Fig. :are presented in Table I.
TABLE 1
RATE CONSTANT FOR THE CARBURIZATION OF INCOLOY800 IN A AR-CIL-H2 GAS MIXTURES
rcc)
90010001100
0.451.245.99
(1)
An Arrhenius plot of these k values is presented inFig. 3, which includes the results obtained by Schnaasand Grabke who plotted 2k vs liT in their work [10]. Theactivation energy of 172 (kj/mol) obtained in this workagrees well with that of Schnaas and Grabke of 167(kj/mol).
'J: 1100 1000 900°C',' F 1O r---...,....-~--'T""'"-r----....,.,~_
u
LU..L..-,í.'-~Ju
o
•• Schnoos and GrobkelO
• Present work
e::JUiLou
~o~ 01 L- ~ ~ ~~
70 75 8.0
104 I T ( K )
85
Fig. 3. Arrhenius plot ofthe rate constantfor internal carbideformation in Incoloy 800 alloy, in Ar-C~-II¡¡ gas
mixtures.
DEVELOPMENT OF THECARBURIZATION FRONT
Fig. 4 presents a metallographic cross sectionalview of the carburized samples exposed at 1000°C to anatmosphere of carbon activity of 0.95, at 6, 10, 50, 100and 200 hours of carburization. Murakami's reagentwas used as etchant of the samples, after mechanicalpolishing.
b h 50 h
Fig. 4. Structural morphology oí Incoloy ~OO samples carburized at1000'C and carbon activity of 0.95, treated with Murakami's
reagent. Left side is the edge specírnen.
Revista Latinoamericana de Metalurgia y Materiales, Vol. 8 Nos. 1 .& 2 (1988) 69
The different structures observed, particularIy insamples with 100and 200 hours of carburization, are dueto the precipitation of carbides of varying compositions[10]. The central area of the sample decreases in widthwhen carburization time is increased, containing lessM28C6 carbides.
According to Schnaas and Grabke [10]the MoN8car-bide contains increasing amounts of chromium as tnecarburization front progresses inwards; in regions ofhigher chromium the attack by the etchant is less as evi-dent from Fi~. 4.
The metallographic attack by Murakami's reagentpermits the observation of an internal carburizationfront with M28C6formation in the samples carburized for6, 10and 50 hours and a transformation front of carbidesfrom M2SC6to M7Cg in samples carburized for 100and 200hours. The transformation reaction may be represented as,
From Fig. 4 it is possible to obtain kinetics data rela-ted to transformation front of M28C6 to M7C6 carbides.
TABLE II
ADVANCE OF THE TRANSFORMATION FRONTOF CARBIDES AT 1000·C
Time(s) 3.6 x l[J11.8x 111 3.6 x lOS 7.2 x lOS
Position (CM) 6.76 x 10"4 1.089 X 10.3 7.225 X 10"31.58 X 10.2
A linear regression of the data shown in Table Ilindícates a parabolic kinetic reaction with regressioncoefficient of 0.984 and a law which can be written as
¿=m+c ~¿ = 2.355 X 10'8t -1.433 X la3 (4)
where D representa the carbon diffussion coefficient incrrr/s, e is the advance of the transformation front of car-bides in (cm) at time t(s). The value ofD agrees well withthat reported by Harrison and col. [11], for the 25 Cr-20Ni-55 Fe alloy at 1000°C.
Fig. 5 shows the effect of temperature on the deve-lopment and composition of the carburization front, insamples carburized for 200h. At 900°C the transforma-tion front of carbides is at 0.34 mm from the surface. At1000°C the carbide transformation interface is at 1.30mm from the surface; the interface disappears at 1100°C,only coarser carbides of the M7C8type being present atthis higher temperature.
CARBIDE SURFACE MORPHOLOGY
Using scanning electron microscopy (SEM) it waspossible to observe carbide nucleation on the alloy sur-face after carburization for different times. Fig. 6 pre-sents photomicrographs taken by SEM on samples car-
burized in Ar-CIL-H2 gas mixtures of 0.95 carbon actí-vity at 1000·C and for different times.
Fig. 5. Structural morphology of Incoloy 800 sarnples carbu-rized at 900, 1000 and 1l00'C and carbon aetivity of0.95 for 200 hrs. Murakami's reagent was used as
the etchant.
Thin particles of carbides of 1 p.m size observedafter 0.5h of carburization cover the major part of thesample surface. After 2h of carburization, the formationof a surface carbide layer is observed, which after 10h istotally continous. After 50h, the continous layer becomesporous. After 100h oí carburization, the carbide layerdevelops into a discontinous network.
The morphology of the surface carbide changes asthe carburization temperature is changed. Figure 7 pre-sents scanning electron micrographs of Incoloy 800 sarn-pIes surfaces carburized for 200 hours at 900, 1000 and1100°C.At 900°C the carbide surface is porous, with adistribution of needles. As the diffusion of carbon isfacilitated by an increase of temperature, the carbidesurface becomes progressively less porous. At 1100°Cthe carbides formed after 200 hours of carburization stillhave a compact structure which, due to carbon satura-tion at the edge of the sample is broken beginning the"metal dusting" process,
SUMMARY
The carburization of Incoloy 800 in Ar-C~-H2atmospheres follows a parabolic law at the beginning,deviating at longer times from parabolie behavior, pre-sumably because of the hindrance to the diffusive flux ofcarbon provided by the precipitated carbides. Carburi-zation is observed only after an incubation period whichis shorter at higher temperatures.
The existance of this incubation period is probablyrelated to the presence of a surface oxide film whichblocks carburization in the initial stages.
70 LatinAmerican Journal oi Metallurgy and Materials, Vol. 8 Nos. 1 & 2 (1988)
U.5h
lOh
lOOh
2h
SOh
FIGURE 6.SEM Micrograhs of the surfaces of Incoloy 800 samples carburizedat 1000°C and 0.95 carbon activity.
Revista Latinoamericana de Metalurgia y Materiales, Vol. 8 Nos. 1 & 2 (1988) 71
a
e
M234 carbides are formed first which later aretransformed to M7Cs as the carbon activity increases.The transformation front progresses inwards as carbu-rization proceeds, with a parabolic kinetic reactionlaw.
The carbide morphology at the alloy surface chan-ges with temperatura, becoming less porous as the tem-perature is increased.
b
d
ACKNOWLEDGEMENTSThe authors are grateful for the support of the
Dirección de Investigación of the Universidad de Con-cepción through grants N° 2.10.92 and 20.95.12.
REFERENCES
1. R. Schueler, Hydrogen Processing (1972), 73.
72 LatinAmerican Journai of Metallurgy and Materials, Vol. 8 Nos. i & 2 (1988)
2. R.E. Gackenbach and J.F.Shay, Proc. 26'h NACE Conf., Phila-delphia (1970).
3. M.W. Clark, in NACE (ed) Process Industries Corrosion, Texas(1975) 329.
4. G. Matamala and P. Cañete: "Carburization and DecarburizationKinetics of Iron in C&-H2 mixtures between 1000-1100'C,"Materials Chemistry and Phvsics 12. 313-319 (19851.
5. G. Matamala: "Carburización de Fe, Ni y aleaciones Fe-Ní-Cr aaltas temperaturas en atmósferas CH4-H2", Tesis de M. Se., Uni-versidad de Concepción, chqk (19821.
6. Von K. Ledjeff, A. Rahmel, Monike Schorr, Werkstoffe undKorrosion 31. 83-97 (1980).
