Note: Descriptions are shown in the official language in which they were submitted.
WO 90/05792 PCT/SE89/00630
2039584
IRON-, NICKEL-, CHROMIUM BASE ALLOY
TECHNICAL FIELD
The present invention relates to an iron-, nickel-, chromium base
alloy having an austenitic structure and good high temperature fea-
tures, including a very high resistance against oxidization in oxi-
dizing atmosphere and against carburizing in carburizing atmosphere at
high temperatures, as well as a high creep fracture resistance.
BACKGROUND OF THE INVENTION
High alloyed, stainless, austenitic steels or nickel base alloys con-
taining up to 60% nickel conventionally have been used for objects
which during a long period of time are subjected to high temperatures
in combination with mechanical loading in oxidizing environments.
These alloys usually have a high oxidization resistance and often also
a very high creep fracture resistance, but because of the increasingly
high demands which are raised upon materials for the present field of
use there has arosen a need of materials having still better oxidiza-
tion resistance in oxidizing environment in combination with very good
creep fracture resistance, a combination of features which has not
satisfactorily been achieved with presently known alloys.
Another problem with known alloys of the above mentioned kind is that
they have a comparatively great tendency to take up carbon and nitro-
gen when exposed in carburizing atmosphere or in environments which
involve a risk for the taking up of nitrogen at high temperatures.
This particularity concerns austenitic steels but to an essential
degree also nickel base alloys. Also attacks from gaseous halides and
metal oxides in certain environments ma;; involve problems.
The above mentioned problems ~,vill be particularity accentuated in
those cases when the material is subjected alternatingly to carburi-
zing and i;o oxidizing media at high temperatures, or, which sometimes
even may occur, in environments which at the same time may act oxidi-
zing as well as carburizing. Those situations when the material in hot
CA 02039584 2000-10-24
-~ 26927-71
2
condition is exposed to ambient air after having been subjected
to caburizing in a furnace at a high temperature are examples
of alternatingly carburizing and oxidizing exposures. Similar
conditions may occur in furnaces where for some reason it is
difficult to maintain a balanced atmosphere. Further may be
mentioned furnace linings which are subjected to coke
depositions. It is conventional to remove such depositions by
burning them off, wherein air is supplied for the combustion,
which is a further example of exposure to alternatingly
carburizing and oxidizing media. Finally, treatment of poorly
degreased goods in oxidizing atmosphere at high temperatures is
an example of a situation where carburizing and oxidizing may
occur at the same time.
SUMMARY OF THE INVENTION
According to the present invention there is provided
iron-, nickel, chromium base alloy having an austenitic
structure and good high temperature features, including a very
high resistance against oxidization in oxidizing atmosphere and
against carburizing in carburizing atmosphere at high
temperatures, as well as a high creep fracture resistance,
characterized in that the alloy has the following composition
in weight-%:
0.01 - 0.08 C
1.2 - 2.0 Si
from traces up to 2 Mn
22 - 29 Cr
32 - 38 Ni
0.01 - 0.15 rare earth metals
0.08 - 0.25 N
CA 02039584 2000-10-24
26927-71
2a
balance essentially only iron and unavoidable impurities, said
rare earth metals in combination with the said content of
silicon improving the growth of a protecting Si02-layer on the
metal surface, when the metal surface is subjected to high
temperatures in oxidizing atmosphere, which counteracts the
transportation of metal ions, in the first place chromium, out
of the alloy, so that scaling is minimized.
BRIEF DISCLOSURE OF THE INVENTION
The invention aims at providing an alloy having a
composition which brings about an improved resistance at high
temperatures against carburizing as well as against oxidizing,
and which also gives a good creep fracture resistance. The
material according to the invention also has a good resistance
against the taking up of nitrogen and also against attacks from
gaseous halides and metal oxides. It can advantageously be
used in the form of sheets, plates, bars, rods, wires and tubes
in various kinds of furnaces, as for example carburizing
furnaces, sintering-, annealing-, and tempering stoves, where
also non degreased goods is heat-treated, and it can also be
used for accessories for furnaces and stoves, for example
charging-basket, -grates and -buckets. Further it can be used
in burners, combustion chambers, radiant-tubes, reaction rooms
in petrochemcial industry and in fluidized beds, exhaust gas
filters for motor cars, etc.
The following table shows the broad range for the
elements which are included in the alloy according to the
invention, and also the preferred, and the suitably chosen
ranges. The contents are expressed in weight-~. The balance
is iron, unavoidable impurities in normal amounts and normally
existing accessory elements. For example there is a negligible
amount of aluminum and calcium in the steel as a rest due from
the finishing metallurgical operation prior to casting. The
con-
WO 90/05792 PCT/SE89/00630
2039~8~
3
tents of phosphorous and sulphur are very small, max 0.040%, and max
0.008%, respectively.
Table 1
Broad Preferably Preferred
ranges chosen ranges composition
C 0.01 0.08 0.02 - 0.08 0.035 - 0.065
-
Si 1.2 - 2.0 .1.3 - 1.8 1.3 - 1.8
Mn from tracesto max 1.3 - 1.8
2
Cr 22 - 29 23 - 27 24 - 26
Ni 32 - 38 33 - 37 34 - 36
Rare earth
metals 0.01 0.15 0.02 - 0.12 0.03 - 0.10
-
N 0.08 0.25 0.1 - 0.2 0.12 - 0.18
-
The carbon content has importance for the features of the steel, as
far as the strengtt: is concerned, and shall therefore exist in an
amount of at least 0.01%, preferably at least'in an amount of 0.02%,
and suitanly not lass than 0.035%. If the alloy shall be used for the
2~ production of plates, sheets, rods, wires, and/or tubes, the carbon
content, hovrever, should not exceed 0.08%, suitably not exceed
0.065°/.
Silicon. i~ required in an amount of at leas 1.2% in order that a com-
bination effect between silicon and the rare earth metals shall be
~ achieved with reference to the oxidization resistance. This will be
explained more in detail in connection with the description of the
cerium content. Silicon also is favourable for the carburizing
resistance. From these reasons, the silicon content should be at least
1.3%. The upper silicon limit, 2.0°/, preferably max 1.8%, is due to
3C circumstances which has to do with technical circumstances relating to
the manufactoring and also to the fact that higher silicon contents
may cause difficultes in connection with welding.
Manganese generally improves the strength but impaires the oxidization
resistance. The content of manganese therefore should not exceed 2°,a
and should suitably be 1.3-1.8°~.
WO 90/05792 PCT/SE89/00630
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Phosphorous and sulphur in amounts exceeding the above mentioned maxi-
_ mum limits have an unfavourable influence upon the hot workability.
- The chromium content is high and lies within the range 22-29%, pre-
y ferably 23-27%. Herethrough there is achieved, in combination with a
high nickel content, a high silicon content, and a significant content
of rare earth metals, a good resistance against high temperature
damages, in the first place against carburizing and oxidization at
high temperatures.
Nickel is favourable for the oxidization resistance and also for the
carburization resistance and shall exist in an amount between 32 and
38%, preferably in an amount between 33 and 37%. A preferred composi-
tion is 34-36%.
Rare earth metal in the form of the lanthanum group of metals in an
amount, expressed in the amount of cerium which normally stands for
about 50% of the mischmetal, of 0.01-0.15%, preferably at least 0.02%,
and suitably at least 0.03% cerium, improves the formation of a thin,
elastic ai:d adhering oxide film, when the alloy according to the
invention is exposed to an oxidizing environment at high temperatures.
However, there is not obtained any further improvement of the oxidiza-
tion resistance in proportion to the addition of rare earth metals, if
the content of rare earth metals, in the first place cerium, exceeds
0.12%. The preferred range for the amount of rare earth metal therefor
lies between 0.03 and 0.10°/. Possibly the rare earth metals completely
or partly may be replaced by earth alkali metals.
Cerium and other lanthanides (rare earth metals) are suitably supplied
as mischmetal to the finished molten alloy together with silicon-
calcium or possibly lime as a final operation. Through the addition of
silicon calcium and/or by covering the melt with a layer of lime it is
possible to prevent major losses of cerium and other rare earth
metals, so that the rare earth metals, as expressed in amount of
cerium, will exist in a sufficient amount in the finished product in
order to bring about the desired effect. Through the influence of
WO 90/05792 PCT/SE89/00630
203984
cerium and other rare earth metals in the mentioned range of composi-
tion there will in combination with silicon in the above mentioned
range of composition be achieved a favourable impact upon the growth
of a Si02-layer on the metal surface, when the metal surface is sub-
s jected to high temperatures in an oxidizing environment. This
Si02-layer will form a barrier against the transportation of metal
ions, in the first place chromium, out of the alloy, so that scaling
is minimized.
Nitrogen has a favourable influence upon the creep fracture strength
of the alloy and shall therefore exist in an amount of at least 0.08%,
preferably at least 0.1°/, and suitably at least 0.12%. Nitrogen,
however, at the same time impaires the hot workability of the alloy
and shad therefore not exist more than in a maximum amount of 0.25%,
preferably max 0.2%, and suitably max 0.18%. Moreover, there may exist
traces of other elements, however, not more than as unavoidable
amounts of impurities or as accessory elements from the melt metallur-
gical treatment of the alloy. Thus the steel may contain a certain
amount of calcium and aluminum as a residu~il product from the finish-
ing of the steel. Boron is an example of an element that shall be
avoided, since that element even in very small amounts may impaire the
oxidization resistance of the alloy by locating itself in the grain
boundaries, 4Jnere the existence of boron ray prevent oxygen from pene-
trating and be deposited in the grain boundaries in a form of oxides.
BRIEF DESCRIPTION OF DRAWINGS
In the following description of the results, reference will be made to
the attached drawings, in which
Fig. 1 is a graph in which the results after intermittent oxidiza-
tion annealing of a number of commercial alloys are compared
with the results from a first example of an alloy according
to the invention, and
WO 90/05792 PCT/SE89/00630
6 2039~8~4
Fig. 2 is a graph which illustrates the oxidization resistance of an
alloy according to a second example of the invention by show-
ing the increase of weight in a thermo-balance as a function
of the annealing temperature up to 1300°C.
OXIDIZATION EXPERIMENTS
In Table 2, alloys 1-7 are examples of the invention. Alloys A, B and
C are commercial reference alloys. Alloy 1 was manufactured as a 500
kg test charge. Alloys 2-6 were manufactured as 13 kg laboratory
charges. Alloy 7 was manufactured as a 10 ton full scale charge. As
far as alloys 1-6 are concerned, the molten alloy was analysed prior
to casting as well as the composition of the finished product. The
impurity contents in all the examples were low. The balance therefore
consisted essentially only of iron. The compositions of alloys A, B
and C were obtained from the specifications for these materials.
Table 2
Alloy Charge/
No product C Si Mn Cr Ni Ce N Remarks
1 052875 0.058 1.27 1.58 25.1 34.7 0.05 0.033
piat~ 0.054 1.19 1.59 " " " 0.032
2 8322 0.045 1.75 1.68 24.7 34.7 0.065 0.126
bar " " 1.67 25.0 34.9 0.03 0.121
3 332; 0.049 1.56 1.55 25.0 34.8 0.086 0.55
bar " 1.54 1.53 " " 0.034 0.56
4 B323 0.047 1.55 1.43 24.7 34.8 0.053 0.146
bar " 1.52 1.42 " 34.9 0.018 0.147
B321 0.047 1.78 1.67 24.7 34.7 0.059 0.077
ba:' 0.046 1.75 1.66 25.0 31.9 0.023 0.078
6 332 0.040 1.87 1.80 24.9 35.3 0.114 not analysed
bar " 1.83 1.78 " " 0..034 0.022
2281-'.'_
piat~ 0.048 1.52 1.74 25.7 34.6 0.045 0.130
A max max max
0.08 1.5 2.0 24-26 19-22
B 0.04 0.35 0.75 21 31 0.3 Cu
C max 1.5-
0.10 2.3 0.5 21 11 O.G5 0.15
~'O 90/05792 PCT/SE89/00630
203958
The oxidization resistance of alloy. No 1 was examined through oxidiza-
tion annealing. Test coupons 25x15x2 mm were taken out from the plate.
The coupons were planed and ground. The test coupons were oxidization
annealed during a total annealing time = 45 h and with five alterna-
S tions down to room temperatures. The test coupons were annealed at
varying temperatures between 1050 and 1200°C. The coupons were weighed
by means of a standard balance prior and after the annealing experi-
ments. The results are shown in Fig. 1 which also includes the results
from corresponding testing of the commercial alloys A, B and C. From
these results it can be stated that the scaling temperature may be
1200°C.
Thereafter also the full scale produced alloy No. 7 was oxidization
tested in a thermo-balance. The increase of meight was measured as a
function of the annealing temperature as in the proceeding experiment
but all the way up to 1300°C. The coupons were weighed with a standard
balance prior and after the annealing experiments as a complement to
the thermo-balance measurements.
2~ The thermo-balance value and the differences between the coupon prior
and after the ex.peri~;:en., for each individual sample is shov:~n in
Table 3.
The increase of weight in the thermo-balance as a function of the
annealing temperature is shown in the graph in Fig. 2. The limits 1.0
and 2.0 gr/m2 h has been indicated by a dashed line in Fig. 2 from the
reason that the scaling temperature is defined by the size of the
increase of weight in the following way: "The scaling must not exceed
ig/mZ h with the additional condition that 50°C higher temperature
must not hive more than at the most 2g/mZ h".
T.he result frc,a t~e testing of alloy No. 7 shok~s that the alloy of the
invention. resists also a scaling temperature a'~ove 1200°C.
WO 90/05792 PCT/SE89/00630
'"~ 8 2039584
Table 3
Table over each individual 17.7 mm plate,
sample of
alloy No.
7,
- charge 2282 -71. Intermittent five alternations
annealing; during
45 h.
Test Experiment T-balance Loss of Total take
temperature No. values weight up of 02
o C g/ mz g/ mz
g/mZ
1100 B451 7.43 6.64 14.08
1150 B452 7.80 21.24 29.04
1200 8453 11.87 23.08 34.95
1200 B454 18.65 19.56 38.21
1250 B455 54.19 32.09 86.28
1250 B458 61.94 27.15 Bg,Og
1300 8456 35.95 47.90 83.85
1300 B457 56.57 42.22 98.78
CREEP FRACTURE STRENGTH EXPERIMENTS
2~- In these experiments the same alloys were used as in the oxidization
experiments, Table 2.
i:-re creep frGcture st~ength of a 20 m;~ plate made o: a'~loy No. _ i~o~
a S00 kg test charge was examined at the tempera;.ures 600, 750 and
25 900°C. Table 4 shows obtained Rkm-values and (within brackets) refe-
rence date including min/max-data from three full scale charges of the
commercial. steel grade C, Table 2. The examir:ed test material with the
iov~ vitro~;en content as expected has lower values than alloy C, ~:rhich
is kno~:.~n .o have an extremely high creep fractur a str ength.
WO 90/05792 PC?/SE89/00630
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Table 4
Temp Creep fracture limit, Rkm, N/mmz
- °C 102h 103h 104h 105h~"
600 250 175 105 62
(3G0-315) (235-240) (145-155) ! - 88- = 100)
750 78 45 24 13
(105-125) (67-73) (38-42) ( - 21- = 24)
900 28 16 ~ 10 5
(36-40) (23) (14-16) ( = 8- = 12)
*The values for 105h have been derived through manual (graphical)
extrapolation about one 10-pov:~er of time.
The five 13 kg laboratory charges, alloys 2-6, were manufactured in
,5
order to examine the impact of the nitrogen content upon the creep
fracture strength of the alloy according to the invention. The ingots
from these small laboratory charges were forged to size p 20 mm. The
nitrogen conte-its varied from min. 0.022% to max. 0.147%. The measured
creep fracture limit values at 900°C are shown in Table 5.
2v
T_aale 5
Charge N Ce Creep fracture limit,Rkm, N/mm2
25
Rkm/100 h Rkm/1000 h Rkm/10 000
h*
p 322 G.12'_ 0.030 33 20 (12)
B 325 G.G56 0.034 31 19 (lij
B 323 0.147 0.018 34 l.g (lG;
321 6.078 0.023 33 1" r c1
30 B 320 0.022 0.034 28 16 ~ cj
G
*The values for 1 G h have been derived ta.rough manual (grap;:vc~i i ext~a-
polation abo~it one 10-power of time.
T_n the continued experiments concerning the influence of the content
35 of nitroge:~, the best result was achieved wits alloy No. 2 containing
0.12% id. 'the improvement as far as the value of the creep fracture
WO 90/05792 PCT/SE89/00630
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l0 20~9~8~
limit at 900°C is concerned was about 20%. The experiments also show
- that also the content of cerium appears to have an impact upon the
creep fracture strength. The comparatively low values for alloy No. 4
' - in spite of a nitrogen content of about 0.15% - therefore may depend
on the fact that according to the control analyse the content of ceri-
um was only 0.018°/. This also indicates the importance of protecting
the lanthanides during the manufacturing so that these elementes are
not lost in connection with the finishing of the melt and the sub-
sequent casting. Also the rod material of alloy No. 5, which contained
about 0.08% nitrogen and 0.023°~ cerium, seems to get a larger reduc-
tion of the creep fracture values when the testing period is pro-
longed, probably depending on the moderate content of cerium, which
indicates that the content of cerium should be at least 0.03% in order
to bring about an effect not only upon the oxidizatiion resistance but
also upon the creep fracture strength. The investigation moreover
shows that the creep fracture strength is significantly increased with
increased nitr oge:~ content.
CARBURIZATION EX2ERIT~NTS1
G ~'
Ti:ese e>:p~r~ments concern studies if six different alloys in a redo-
cing, car::,ur:zing atmosphere. The dep~~:s cf carburization were
measured :,.nd from these measurements the carburization rates were
evaluated. ':he cue~ical compositio.~.s in ~::eight-are shor:r. in Ta'o:e
The compositions of alloys D-H relate to analysed compositions, while
the composition of alloy I is the nominal composition. Alloys D, E, G
and H are commercial, austenitic steels. Alloy F has a composition
according to the invention, and a'_lo;; ~ is a co;n;aercial, well-kno.;n
nickel 'oase alloy.
WO 90/05792 PCT/SE89/00630
11 203958
Table 6 Chemical composition, weight-%
Alloy Fe Ni Cr C Si N Mo Mn Other Ni/Fe-
elements ratio
D 69.6 9.6 18.4 .06 1.3 .15 .26 .53 .04Ce .14
.. 65.5 10.9 20.8 .09 1.7 .16 .24 .59 .04Ce .17
F 36.1 34.6 25.8 .05 1.5 .13 .05 1.74 .05Ce .96
G 53.8 19.1 24.7 .05 .5 .07 .25 1.50 - .36
H 62.7 12.6 22.2 .06 .39 .10 .37 1.51 - .20
I 15.5 60 23 _.5A1 3.87
The materials in all these cases had the shape of plates, and from
these plates coupons were taken, size 1Ox10x1-2 mm. The coupons were
1~ ground and carefully cleaned, vrhereafter they were subjected to a
reducing, carburizing atmosphere at the temperatures 850°C,
950°C,
1050°C and 1150°C during a period of exposure which lasted from
20 min
~.0 25 h. The reaction gases consisted of 89% H2 and ll~o C3H6, which
was flushed through the furnace at a flow rate of 160 m/min.
Lv
Tne carburization of the studied samples was analysed me~allograp:~i-
cally, and the carburization kinetics was found to be parabolic and
could be cescr ibed by the equa Lion xz -2r: ;,, where x=the depths of
P
penetration, kp=a rate constant and t=time of exposure. The obtained
25 data was plotted according to this equation, and the graphical rela-
tions then could be used to estimate the kD-values, which are listed
i n Tabl a 7 and 8.
T ~ vvas : o~:nd through metal iurgical s tudi es that the carburiza Lion
3~' region could be devided into t~.:o zones. First: is the so-called massive
carburizat:ion zone which is a zone just beneath the alle: surface. At
greater cep~::s there is a second zone o. cari.de precipi~ates along the
grain cou:idaries. The carburization rate constants, kp, are shovrn in
Table 7 fcr :oral, i.e. massive plus intergranular carbide formation,
3~ and in Tahie ° for massive carburization in the surface zone onl;.
i
~~v yuiV~iyG
PCT/SE89/00630
12 2039:~~34
Table ?
Values of carburization (10'um2/h) for t
rate constants, k t
l
p o
a
carbu-
rization depths.
Temp Alloy
C D E F G H I
850 5.9 1.4 - 3.0 4.0 _
i0 950 12.0 2.8 .1 3.8 8.4 .6
1050 43.1 48.3 10.8 27.5 38.8
1150 - 195.7 54.1 196.8 -
'"samples completely carburized
Table 2
Values cf carburization constants, (10'um2/h) f
rate k
or massive car-
p
burizatioi~.
2J
Temp Alloy
C D E F G H I
850 1.4 .05 - .8 2,p _
95C 4.3 - .3 4.4 7.0 1.7
1050 - 14.7 8.4 9.0 15.8 9.4
1150 - 38.4 11.0 19.5 - 31.2
Table 7 ar,d 8 shcw that ~ of the
alio~~ invention
had ~h
i
e s
gr.i:icanti;
lowest k -value as far as cerns massive
con carburization
P ~
r
as
'ctal carburization. :
ell as
J :7
