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STRUCTURE INVESTIGATION AND PHASE
ANALYSIS OF Fe-Cr CARBIDES
E. Kuzmann, E. Bene, L. Domonkos, Z. Hegedüs, S. Nagy, A. Vertes
To cite this version:
STRUCTURE INVESTIGATION AND PHASE ANALYSIS OF Fe-Cr CARBIDES
E. KUZMANN, E. BENE, L. DOMONKOS and Z. HEGEDUS
Csepel Iron and Metal Works, Research Institute for Physical Metallurgy and Technology, Hungary S. NAGY, A. VERTES
Departement of Physical Chemistry and Radiology Lorand Eotvos University of Sciences, Budapest, Hungary
R6sum6, - On a 6tudik par spectroscopie Mossbauer les positions possibles de l'emplacement du chrome dans le carbure de type M3C, qui se forment au cours de la mktallurgie d'acier. L'analyse de phase a Bte exkcutee en connexion de quelques carbures d'autre type (M7C3, MsC, M23C6) aussi.
Abstract. - The possible positions of chromium incorporation into M3C-type chromium containing carbides formed in course of steel production were studied. The phase analysis of this and of some other types (M7C3, M6C, M23C6) of carbides was performed by means of Mossbauer spectroscopy.
1. Introduction.
-
The quantity, quality, composi- tion and structure of carbides [I, 21 formed in course of the production of differently alloyed steels, as well as the carbide transformations taking place during the various heat treatments are known to have a significant influence on the properties, but mainly on the mecha- nical properties of steels [3].The crystal structure of carbides of the type M,C has been studied (because of their metallurgical importance) by means of a number of methods, among others by electron diffraction [4], X-ray diffrac- tion [ 5 ] and Mossbauer spectroscopy [6]. In the orthorombic cementite structure of Fe3C the Fe atoms might occupy two positions. Iron atoms in general positions have 11 Fe and 3 C nearest neighbours at average distance of 2.60
A
and 2.04A
respectively. Iron atoms in special positions have 12 Fe and 2 C nearest neighbours at average distance of 2.62A
and 1.97A.
There are 8 general and 4 special positions per elementery cell. The question arises whether the substitutionally incorporated alloying elements have a preference for one or the other of these positions. Huffman et al. [6] found that when Mn is incorporated into cementite there is a greater possibility of the Mn-atoms occupying the general position. In the first part of our investigations we tried to establish whether when Cr-atoms are incorporated these too have a preference for one or the other of the two positions.M3C, M6C, M,C3 and M,,C6 occuring in steels with the help of standard spectra [7].
2. Experimental. - The carbide samples were pre- pared by chemical isolation of appropriately chosen steels of special compositions after preliminary heat treatment. The unisolated samples (situated in the matrix) were mechanically thinned down after heat treatment. The elementary composition of the samples was determined by chemical analysis. The types of the carbide phases in the standards and in the samples chosen for structural analysis were checked by X-ray diffraction. All samples chosen for phase analysis were examined both light and electron microscopically and subjected to supplementary mechanical tests.
The heat treatment of the steels was carried out as follows :
Steel of the quality W3' (C : 0.3
%,
Cr : 2.7%,
W : 5.1%,
V : 0.35%,
Mo : 0.12%)
was heat treated at 850 OC for one hour, austempered at 10600 to 1 080 OC for 30 min. and quenched in oil, then annealed at 550° to 600 OC for 2 to 5 hours.Steel of the quality K13 (C : 0.3
%,
Mn : 0.5%,
Cr : 5.5
%,
V : 0.9%,
Mo : 1.5%)
was annealed a 860 OC for 3 hours, austempered at 1 000 OC for 30 min. and quenched in oil.The steel K14 (C : 0.25
%,
Mn : 0.3%,
Cr : 2.8%,
- In the second part of our work we have subjected to
(1) The qualities are designated according to the Hungarian
phase the mixture of various
-
from the Standard, the approximately corresponding DIN Norms are : metallurgical aspect interesting-
isolated (and partly w3,
X ~ O W C ~ V ~ ~ ; K13,
X40CrMoVSl ; ~ 1 4 ~ 3 2 ~ - situated in the steel matrix) carbides of the types MoV33.C6-410 E. KUZMANN, E. BENE, L. DOMONKOS,
V : 0.5
%,
Mo : 2.7%)
was annealed at 860 OC for3 hours, austempered at 1 000 OC and 1 040 OC for
2 hours, then quenched in oil, that is, from a 520 OC salt bath, on air.
The carbides were isolated electrolytically with ammonium sulphate-citric acid, i. e. with a hydro- chloric acid electrolyte.
With the exception of those in the matrix, the samples for Mossbauer spectroscopy were prepared by account- ing for optimum surface densities [8]. The measure- ments were carried out in the 4.5 K to 550 K range.
In our paper all data on isomer shift refer to the 5 7 ~ o source diffused into platinum.
3. ResuIts and interpretation. - 3.1 INVESTIGA-
TION OF THE STRUCTURE OF CARBIDES OF THE TYPE
(Fe,-,Cr,)3C.
-
Of the spectra of (Fe,-,Cr,)C, carbides studied in the range 0<
x<
0.1 two taken above their Curie temperature are shown in figure 1. Figure la is the spectrum of Fe3C recorded at 520 K.VELOCITY irnrnlsl
FIG. 1.
-
Mossbauer spectra of a) Fe3C at 250 "C and b) (Feo.9aCro.o8)3C at room temperature.This spectrum was the basis of computerized evalua- tion. The spectrum was decomposed into two qua- drupole pairs of lines with the following restrictions :
3.1.1 The widths of all four lines are the same ;
3.1.2 The overall area of the first pair of lines representing the iron atoms in general position is twice the overall area of the second pair of lines correspond- ing to the special position ;
3.1.3 Lines pertaining to the same pair of lines have identical areas.
These restrictions seem justified partly because of the low surface density of the absorbent used, and are also supported by the results of other authors [6]. In order to obtain information about the uncertainty involved in
the determination of the ratio of the number of iron atoms in general position to the number of those in special position from the line areas of the spectrum, we have made runs in which restriction 2 was omitted. The obtained isomer shifts correspond to the results of Huffman et al. [6].
In the evaluation of the other M,C-spectra we again considered two positions (general and special) of the iron atoms, thus neglected the fact that the incorporat- ed Cr-atom will alter the environment of the neigh- bouring Fe-atoms, so that at least two more positions (thus at least two other pairs of lines) ought to be reckoned with. This, of course, is still a modest estima- tion, even if we exclude the possibility of one iron atom having two chromium neighbours.
The spectra were evaluated in two stages. The first stage involved all three restrictions with three varia- tions of restriction 2 :
-
In the first run the ratio of the areas was 2 : 1, in the second this ratio was greater (corresponding to the incorporation of the Cr-atom into special positions) and in the third run the ratio of the areas was less than 2 : 1 (when Cr was considered to be incorporated in general position).- In the first stage of the evaluation the parameters of the Fe,C spectrum served as input parameters. In the second stage of evaluation restriction 2 was omitted. The output parameters of stage one served as input parameters of stage two. It is noteworthy that the data obtained by the second stage were independent of the concrete values of restriction 2 in the first stage. On the basis of our current measurements we are not in a position to decide whether similarly to the Mn- atoms the Cr-atoms too prefer to some degree one of the positions in the cementite lattice. The probable reasons for this are the following :
1) The maximum Cr-content which can be introdu- ced into an M3C-type carbide is about 17
%,
while a67
%
Mn content can be achieved, consequently a substantially smaller effect has to be detected in the case of chromium.2) The shortcomings of the model might be res- ponsible for the finding that the relative X2 value
obtained when decomposing the spectra of M3C-type carbides containing chromium was generally above 2 to 3. Figure 2 which shows the spectra of a Fe3C and a Fe,.,Cr,.,C carbide recorded below their Curie temperatures supports the assumption of iron positions of more than one kind. It appears from the broadening and merging of the outer lines of Fe,.,Cr,.,C that iron atoms (( sense )) a great variety of magnetic field
strengths within a fairly wide range which might be attributed to the interference by the Cr-atoms.
L I
-5 -4 -3 -2 -1 o
ti
t i+i
Li +j I V E L O C I T Y (rnmls)FIG. 2. - Mossbauer spectra at 77 K
a) Fe3C ; 6) Fe2.gCro.1C.
3 . 2 PHASE ANALYSIS.
-
We have studied Fe-Cr carbides of various compositions and types by means of Mossbauer spectroscopy in order to determine the quality and quantity of the iron carbides formed in course of steel production, and to follow carbide transformation taking place during the various heat treatment procedures.FIG. 3.
-
Mossbauer spectra of a) 60 % of M6C f 40 % of M3C; b) 20 % of M6C+
80 % of FesC; c) 20 % of M6C+
80 % of Fez.gCro.lC ; d ) 60 % of M6C f 40 %of Fez.gCr0. IC at room temperature.
FIG. 4.
-
Mossbauer spectra of the standards of carbides and their mixtures. Composition of the samples from thetop to the bottom: FesCr4C3, F ~ ~ W ~ V O . ~ C ~ O . S M O O . I C , Fe3WzCro. ~VO.~MOO.IC, Fe6Cr17C6, 60 % of M6C 4- 40 %
of M23C6, 80 % of M7C3 Jr 20 % of M6C, 40 % of M7C3
+
60 % of M6C.The spectra of pure carbides serving as basis of phase analysis [7] and those of some of their mixtures of known compositions are shown in figures 2-6.
Our experiments have shown that M,C carbides containing less than 5
%
of alloying elements have even at room temperature magnetically split spectra, while at the temperature of liquid nitrogen all carbides with higher alloying element contents have split spectra. Allother carbides are non-ferromagnetic and their spectra are not magnetically split even at the temperature of liquid helium (Fig. 5). This fact offers a possibility of distinguishing between M3C and the other carbides. The sensitivity of the method for M3C is shown rela- tively to M6C in figure 3 and table I.
The Curie point of M3C depends upon the amount of alloying elements present which can be used for the classification of the M3C-carbides according to their alloying element content.
C6-412 E. KUZMANN, E. BENE, L. DOMONKOS,
FIG. 5. - Mossbauer spectra at liquid helium temperature :
a) MsC ; b) 40 % of M7C3
+
60 % of M6C ; c) 80 % of M7C3 4- 20 % of M6C.Some of the data used in the phase analysis of M,C and M6C
M3CIMsC
Weight Iron atom line area ratio
Figure ratio ratio
-
-
-
-
3a 0.67 2.2 3.3
+
0.53b 4.0 13.3 12.2 f 1
3c 4.0 12.9 11.2 f 1
3d 0.67 2.2 3.0 _f 0.5
noted that within the same carbide type isomer shift depends slightly also upon the content of alloying substances, consequently for the evaluation of a series of measurements standards prepared in the same way as the samples should be used
-
a condition which was satisfactorily met in our case.It appears from figure 4 that the Mossbauer method alone does not solve the distinction between carbides of type M6C and M,,C,. However, if we have at our disposal information which narrows down the, varia- tion possibilities, such as information about the history, i. e. preliminary heat treatment of the sample, about the possible carbide transformations, as well as some other spectroscopic information, e. g. informative X-ray
diffraction results, it is possible to perform in certain cases the required phase analysis. In their current state of development the X-ray and the Mossbauer methods are satisfactory supplementations of one another.
At the bottom of figure 6 the spectrum of a carbide
sample of unknown composition, isolated from W3
VELOCITY (mm/sI
FIG. 6. - From the top to the bottom : Mossbauer spectra of M7C3, MsC, FesC, M7C3
+
M6C+
Fe&, Fe2.gCro.lC andM6C
+
M3C isolated from W3 steel.steel in course of metallurgical research can be seen. The sample was examined by electron and X-ray diffraction methods which identified the main phase reliably as M6C. The Mossbauer tests, however, showed beside the M6C phase the presence of M,C. A correla- tion was further found between the mechanical para- meters (useful life at high stress) of the steel samples and the composition of the carbides in them. We found that the highest M3C : M6C ratio will ensure the most favourable mechanical properties.
Our next investigation was directed at the phase analysis of carbides in K13 and K14 steels. Data on the heat treatment of the steel samples, on the isolation of carbides and on the composition of the carbide phases as determined by us are given in table 11.
TABLE 11.
-
Production parameters and Miissbauer results of phase analytical samples No. of sample-
51 55 55 55a 56 57 57 70 70 61 62 63 63 64 Heat treatment Annealing Temp. Time ' OC m n parameters Quenching Austem- pering Temp. Timt OC min TemperingyFP,
z~
Electrolyte IsolationMassbauer Spectres- COPY Temp. oC Type of steel Secondary phases indicated by the spectra I I I I 1 Manner of Quenching Fig. showing spectrum 6 7e 7d 7a 7c 7b 8c 8d 8a 9a 8b 8e 9b oil 600 240 (NH4)2S04+ citric a c ~ d HCl HCI in the matrix HCI (NHp)2S?4 + citrlc a c ~ d (NH4)2S?4 + citric acid HCl HCl HCI HCI HCI lHCl
1
H C ~ I 1000 30 oil 1000 30 oil 1801 040 120 from salt bath 520 OC in air
1 000 120 from salt bath
520 O C in air
1 000 120 from salt bath 520 OC in air 1 040 120 oil -4 -3 -2 -1 0 +I +2 +3 +4 VELOCITY Imm/e) ': ... ... ..." ::
..,...'
.2:+..
:;:. ..:::;. . . /
:. ..: .:::..
... i:. , ....
..
.:..
.:
. .
,.
.. . . ;. . :
:.
. . . ::.
;
; .:.!:../
. .
. .
.
...:
." .,::. . . .. . .
. .
......
:.
.
. . . ..., '3. .. ....
. . .
I I I I I I I I -5 -4 -3 -2 -1 0 +i +2 +3 +4 +5 VELOCITY lmmlslFIG. 7.
-
Mossbauer spectra of carbides isolated from steels annealed at 860 "C, 3 hr. See table 11..
.
.
.,-..
... ".
.
z>ck%.:;;jz.:;$ :...-,.
.
:...
.:
c ,.:... , ...
:.
...; ..:
a: .:.t.::.\.
.:
" 2. d.
.;:;:.::;:
.;'a:.. :.
'.. . .
.
.
..
....>.:
.
': 0..
..>.
.,.:..
. :.
" .
FIG. 8. - Mossbauer spectra of carbides isolated from : a) K14 steel annealed at 860 "C, 3 hr ; b) and e ) K14 steel hardened at 1 000 OC, 2 hr ; c) and d ) K13 steel hardened at
Citric acid electrolyte was used to check whether the lack of M,C (found in all K13 samples) might be
attributed to the solution of cementite in the hydro- chloric acid electrolyte [9].
Comparison of the spectra of the annealed (Fig. 7b)
and the hardened (Fig. 8c) states reveals that in the hardened state M6C predominates, while in the anneal- ed state the M6C phase is only a part of the who1 carbide quantity. Since the samples had been prepared with identical surface densities, from the far less marked effect in case of the hardened sample we might conclude that during the transition from this annealed into the quenched state a carbide transformation
. 8
-3 -2 -1 0 +1 +2 +3
VELOCITY ImmJs)
takes place, where M' would be any eventual increase in the concentration of the alloying metals at the expense of iron.
Our results are in agreement with those of earlier metallurgical studies.
FIG. 9. - Room temperature Mossbauer spectra of carbides
isolated from K14 steels hardened at 1 040 "C, 2 hr ; a ) cooled Acknowledgment.
-
We wish to express our thanks from salt bath in atmosphere ; b) quenched in oil.to Dr. F. Parak and Dr. L. Bogner of the Department of Physics of the Munich Technical University for sent M,C carbide with more or less than 5
%
of having kindly put at our disposal their spectra recordedalloying element content. at the temperature of liquid helium (Fig. 5).
References
[I] Kuo, K., J. Iron Steel Inst. 173 (1953) 363 ; 182 (1953) 223. [7] KUZMANN, E., NAGY, S., SZT~REK, A., NAGY, F., DOMON- [2] BUNGARDT, K., M ~ ~ D E R S , O., LENNARTZ, G., Arch. Eisenh. KOS, L., HEGEDUS, Z., V~~RTES, A., Proc. Conf. on the
32 (1961) 823. Application of the Mossbauer Effect, Cracow, Poland 1 [3] WOODHEAD, J. H., QUARREL, A. G., J. Iron Steel Inst. 203 (1975) 123.
(1965) 605. 181 NAGY, S., LEVAY B., VBRTES, A., Acfa Chim. Acad. Sci. Hung.
[4] DUGGIN, M. J., COX, D., ZWELL, Trans. AIME236 (1966) 342.
(51 FASISKA, J., JEFFREY, G. A., Acta CrystalIogr. 19 (1965) 463. 85 (1975) 273.
