Note: Descriptions are shown in the official language in which they were submitted.
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1 BACKGROUND OF THE INVENTION
2 This invention relates to a method of thermal
3 mechanically treating cast austenitic heat resistant
4 alloy structures to produce structures having superior
strength and superior ductility at elevated temperatures
6 and which also exhibit improved creep properties ~hen
7 exposed to carburizing or oxidizing environments at high
8 temperatures.
g Various industrial processes, especially chemi-
cal processes, create an insatiable demand ~or alloys and
11 alloy products which can withstand higher and higher
12 temperatures and environments deleterious to the alloys.
13 Such deleterious environments include both carburizing and
14 oxidizing environments, both of which are known to signif-
icantly affect plant performance and efficiency in many
16 industrial processes. These effects are evidenced in su~h
17 heat treatment equipment as, ethylene pyrolysis tubing,
18 carbon dioxide and helium cooled nuclear reactors, coal
19 processing plants, hydrocarbon reformers, and steam
generators.
21 A variety of alloys and ~lloy products have
22 been designed ~or application in such environments.
23 More particularly, austenitic alloy steels exhibiting
24 heat resistance and carburization resistance have been
developed for use in pyrolysis furnaces for the thermal
26 decomposition or organic compounds~ such as the steam
27 cracking of hydrocarbonsO Generally, the pyrolysis
28 ~urnace contains a series of heat-resistant alloy steel
29 tubes in which the reaction occurs. The term "tube" as
used herein also includes fittings, pipes and other parts
31 used to contain carburizing and oxidizing materials at
32 elevated temperatures.
33 When casting austenitic alloy steel into struc-
34 tures such as tubes, a microstructure develops which
3S consists primarily of columnar grains oriented radially
3~ through the thickness of the tube wall. ~uring high
37 temperature service, this type of grain structure encour-
38 ages the nucleation and propagation o~ cracks, which
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- 2 --
l once initiated, have a tendency to run throughout the
2 thic~ness of the structure. Because of this serious
3 detriment/ it is highly desirable to develop a me~hod of
4 treating such structures so as to inhibit the initiation
5 and propagation of such cracks. Furthermore, i~ would
6 be even more desirable to inhibit the initiation and
7 propagation of such cracks while improving other high
8 temperature properties such as creep and ductility.
g SU~IMARY OF THE INVENTION
1~ In accordance with the present invention, there
ll is provided a thermal mechanical treatment for improving
12 the high temperature properties of cast austenitic heat-
13 resistant chromium-containing alloy steel structures,
14 which method comprises (a) heating the structures to at
15 least the temperature at which chromium carbides go into
16 solution, but below the temperature where incipient
17 melting occurs; (b) maintaining the structures at such a
18 temperature long enough so that at least 50~ of the
lg chromium carbides qo into solution; (c) applying from
about 15% to 60~ plastic deformation by hot forming
21 operations; and (d) cooling the structures to room tem-
22 perature at such a ~ate to allow complete recrystal-
23 lization of the grains to occur.
24 DETAILED DESCRIPTION OF THE INVENTION
Austenitic alloy structures which can be treated
26 in accordance with the invention are those structures
27 which are fabricated by casting methods and which have
28 been developed for high temperature application~ Gener-
29 ally these structures are nickel-based or contain up
to about 30 wt.% iron. The structure employed herein will
31 contain from about 20 to about 30 wt.% chromium and about
32 0.25 to about 0.55 wt.% carbon, preferably about 0.3 to
33 0.5 wt.4 carbon. The structure may also contain minor
34 amounts of such elements as silicon, tungstenl rnolybdenum,
35 manganese, niobium, hafnium, aluminum, yttrium, etc. as
36 well as both tramp elements and minor amounts of impuri-
37 ties typically found in such alloys.
38 By treating the structures in accordance with
1 the present invention, the as-cast microstructure is
2 modified such that a relatively coarse equiaxed grain
3 structure is developed thereby minimizing the number of
4 grain boundaries which are oriented transversely to the
principal stress. By relatively coarse equiaxed grains,
6 we mean equiaxed grains having a grain size of about ASTM
7 6 to 2 that is about 45~m to about 180~m. Very fine
8 grains are undesirable because they maximize grain boun-
g dary sliding during creep, thereby lowering the strength
of the alloy and contribu~:ing to the nucleation and
11 propagation of cracks.
12 After thermal mechanical treatment according to
13 the invention, structures are obtained having:
14 (a) superior high temperature strength;
(b) superior high temperature ductility;
16 (c) improved creep properties at high temper-
17 atures;
18 (d) increased grain boundary area, thereby
19 decreasing the volume fraction of contin-
uous carbides and minimizinq the sites of
21 crack nucleation;
22 (e) grain boundaries of the required orienta-
23 tion which will minimize crack propagation,
24 and
(f) blocky carbides at the grain boundaries
26 which provide grain stability and prevent
27 grain growth during re-exposure to high
28 temperaturesO
2~ In treating the structures in accordance with
the present invention/ the structures are heated to a
31 temperature at which the chromium carbides go into solu-
32 tion, but below the temperature where incipient melting
33 occurs. The term incipient melting as used herein means
34 those temperatures at which the lower melting phases of
the alloys employed begin to melt. Generally, the struc~
3~ tures are heated to a temperature of about 1050C to
37 1300C, preferably about 1100 to 1200C. The structures
38 are maintained at that temperature for an effective amount
s~
1 of time~ By effective amount of time we mean that amount
2 of time required to allow at least 50% of the chromium
3 carbides to go into solution. While still maintaining the
4 structures at such high temperatures, controlled plastic
deformation is applied to the structures by hot forming
6 operations so ~hat about 15% to 60% deformation occurs,
7 preferably the deformation is applied in stages of about
8 10 to 15~ pe~ stage~ Non-limiting examples o hot forming
9 operations suitable for use in the instant invention in-
clude rolling, extrusion, dra~winq and forging. In general,
11 any hot forming operation is suitable which will cause
12 defor~ation at the temperatures where chromium carbides go
13 into solution. 8elow those temperatures the compatability
14 stress in the vicinity of the carbide particles are not
relaxed by creep in the matrix, instead cracks are gen-
16 erated as an alternative relaxation mechanism.
17 On completion of deformation, the structures are
18 transferred to a furnace and cooled at a rate not to
19 exceed about 100C/hr to allow recrystallization of the
grains to occur.
21 A further understanding of the invention can be
22 obtained by reference to the following examples which are
23 presented for purposes of illustrating the present inven-
24 tion and are not intended to be limiting unless otherwise
specified.
26 Comparative Examples A-E
27 Five coupons having the dimensions 1.25 cm x
28 5 cm x 20 cm were taken from the wall of a cast austenitic
29 steel tube comprised of about: 0.44 wt% C, 1.35 wt~ Si,
0.6 wt% ~n, 25.1 wt~ Cr, 21.2 wt% Ni, 0.03 wt% Mo, and
31 the balance being Fe. The original as-cast microstructure
32 of each coupon consisted of a mixture of equiaxed and
33 columnar grains of about 1.5 mm average diameter, which
3~ grains are heavily cored with a continuous network of
chromium carbides.
36 Each of the coupons was deformed by about 60~
37 by cold rolling and subsequently annealed in a tubular
38 furrlace at a temperature of about 1000C ~ 5C, except
s~
1 coupon E which was subjected ~o an additional annealing
2 s~ep at 800C~ All annealiny was performed in a high
3 purity argon atmosphere~ Table I below sets forth the
4 temperatures and times for which each coupon was annealed.
TABLE I
6 ls~ Anneal 2nd Anneal
7 Comp. Ex~ Temp. C Time hr Temp. C Time hr
8 ~ 1000
g B 1000 ~
C 1000 24
11 D 1000 120
12 E 1000 24 800
13 All coupons evidenced substantial recrystalliza-
14 tion after the annealing treatment and the microstructure
of each was found to contain a discontinuous carbide
16 network havinq recrystallized equiaxed grains of about
17 lOj~m in size. Although these coupons were comprised of
18 equiaxed grains having a size of about lO~m and contained
19 a discontihuous network of grain boundary carbides, they
were undesirable because the continuous carbides present
21 during the cold rolling operation were cracked and is~
22 sured and are inheri~ed by the refined recrystallized
23 microstructure. The presents of preformed cracks in the
24 modified structure render the material unsuitable for high
temperature service because of its lack of ductility and
26 Strength.
27 Comparative Examples F - N and Examples 1 - 7
28 Coupons measuring 1.25 cm x 5 cm x 20 cm were
2g taken from the wall of a cast austenitic tube having
the same compositîon as that of the tube in the previous
31 Comparative Examples. All the coupons were first heated
32 for one hour at 1200C and subjected to hot working at
33 various temperatures by passing them through a single
3~ stand mill at least twice. Each pass caused about 10%
reduction of the coupon. After deformation, the coupons
6 --
were tested for creep rupture. Table II below sets
2 forth the experimental conditions for each coupon and
3 Table III below sets forth the conditions and creep data
4 for each coupon.
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TABLE III
CREEP RUPTURE DATA
Temp. StressRupture P1. Strain% Reduction ~eas.
Ex. (C) (psi)Time hr. on loading (%1Area Elong.
~-* 1000 787~ 2.1 - 31.2 (1)
G* 1000 500025. 3 o. oo 9.1 6.1
~* lOûO 3000177. 2 o. oo o . 2 ~1)
I* 1050 50Q0 3.9 Ø12 51.7 10.4
J* 1050 3000 54.3 OOOQ (1) ~2
1000 5000 3.3 0.26 45.7 32.0
L 1000 3000 23~4 0.00 51.1 12~8
M 1000 5000 1.5 0.47 50.0 (1)
N lQ00 3000 5.0 0.00 33.4 40.0
1 1000 5000 52.4 0.00 (3) (3)
2 1000 4000163.9 o.oo 6.5 (1) ,
3 1000 30~0716.5 o.oo 2.2 1~6 ~1
4 1050 5000 ~.g 0.00 37.2 9. 6
1050 30002Q3. 3 0.00 8.~ 6.5
6 1000 5000 23.4 0.~0 27.5 lO.g
7 looo 30~0505.0 0.00 ~2) ~2)
(1) fragmented edges
(2! specimen destroyed
(3) equipment malfunction
* as received material
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l The data of the above tables illustrates that at
2 relatively large grain sizes the coupons are subject to
3 creep rupture as opposed to the coupons having a grain
4 size as claimed herein~
Comparative Examples 0 - Q
6 Three coupons having the same measurements and
7 composition as those of the above examples were heated for
8 one hour at 1200C then hot worked at 900C by passing
g twice through a single stand mill. The coupons were
annealed for various times and temperatures. Table IV
ll below sets forth the conditions under which the coupons
12 were treated.
13 TABLE IV
14 Rolling
No. Total % Anneal Anneal Grain
16 Ex. Passes Reduction Temp C Time hr. Size (f~m)
_._
17 Comp. 0 2 19 llO0 1 59
18 p 2 l9 1100 120 60
l9 Q 2 lg lO00 l 51
Hot rolling of the coupons represented in this
21 Table IV resulted in massive cracking and fissuring.
22 Therefore, hot working must be accomplished at tempera-
23 tures greater than 900C.
Example 8
A cast austenitic steel having the composition
2~ as the coupons set forth below is heated for 1 hour a~
27 1200C and subjected to deformation by extruding to cause
28 a 30% reduction~ The ~ube is annealed for l hour at
29 1100C and cooled to room temperature at a rate less than
100C/hr. The tube will be found to have superior high
31 temperature strength and ductility as well as improved
32 creep properties.
