Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to a ~erritic
stainless steel.
United States Patent Nos. 3,932,174 and 3,929,473
describe ferritic stainless æ~eels having superior crevice
and intergranular corrosion resistance. The steels described-
therein con~ain 29~ chromium and 4% molybdenum. ~hey alsohave a maximum carbon plus nitrogen content of 250 parts per
million. Carbon and ni~rogen are limited as the corrosion
- resistance of the steels deteriorates with incerasing levels
thereof.
- -
1 The low carbon and nitrogen requirement for the
alloys of Patent Nos. 3,932,174 and 3,929,473 is
disadvantageous in that it necessitates more expensive
melting procedures, such as vacuum induction melting.
Through the present invention, there is provided
an alloy having properties comparable to that of Patent Nos.
3,929,174 and 3,929,473, yet one which does not require the
expensive melting procedures referred to hereinabove. The
alloy of the present invention can, for example, be melted
and refined using argon-oxygen decarburization (AOD) procedures.
The alloy of the present invention has up to
2.00% of elements from the group consisting of titanium,
zirconium and columbium in accordance with the following
eguation:
%Ti/6 ~ ~Zr/7 ~ %Cb/8 > ~C ~ %N
and a carbon plus nitrogen content in excess of 275 parts
per million. It is characterized by superior crevice and
intergranular corrosion resistance, by good weldability and
by satisactory toughness both prior to and after welding.
For the reasons noted hereinabove, the alloy of
the present invention is clearly distinguishable from that
of Patent Nos~ 3,932,174 and 3,929,473. It is also
distinguishable from that of two other alloys, that of
Patent No. 3,957,544 and that of Patent No. 4J119,765. Both
of these alloys have maximum molybdenum contents below that
specified for the present invention.
1 Another reference of interest is a paper entitled,
"Ferritic Stainless Steel Corrosion ~esistance and
Economy". The paper was written by Remus A. Lula and
appeared in the July 1976 issue of Metal Progress, pages
24-29. It does not disclose the ferritic stainless steel of
the present invention.
It is accordingly an object of the present invention
to provide a ferritic stainless steel.
The ferritic stainless steel of the present
invention is characterized by superior crevice and
intergranular corrosion resistance, by good weldability and
by satisfactory toughness both prior to and after welding.
It consists essentially of, by weight, up to 0~08~ carbon,
up to 0.06% nitrogen, from 25.00 to 35.00% chromium, from
3~60 to 5.60 molybdenum, up to 2.00~ manganese, up to 2.00%
nickel, up to 2.00~ silicon, up to 0.5~ aluminum, up to
2.00% of elements from the group consisting of titanium,
zirconium and columbium~ balance essentially iron. The sum
o carbon plus nitrogen is in excess of 0.0275%. Titanium,
zirconium and columbium are in accordance with the following
equation:
%~i/6 ~ %Zr/7 + %Cb/8 > %C ~ ~N
Carbon and nitrogen ar~ usually present in respective
amounts of at least 0.005% and 0,010%, with the sum being in
excess of 0.0300~. Chromium and molybdenum are preferably
--3--
1 present in respective amounts of 28~50 to 30~50% and 3.75 to
4.75~. Manganese, nickel and silicon are each usually
present in amo~nts of less than l~00%. Aluminum which may
be present for its effect as a deoxidizer is usually present
in amounts of less than 0.1%.
Titanium, columbium and/or zirconium are added to
improve the cravice and intergranular corrosion resistance
of the alloy, which in a sense is a high carbon plus nitrogen
version of Patent Nos. 3,932,174 and 3,929,473. It has been
determined, that stabilizers can be added to high carbon
and/or nitrogen versions of Patent .~OSr 3,~32,174 and
3,929,473, without destroying the toughness and/or weldability
of the alloy. Although it is preferred to add at least
0.15% of titanium inso~ar as the sole presence of columbium
can adversely affect the weldability of the alloy, it is
within the scope of the present invention to add the required
amount of stabilizer as either titanium or columbium.
Columbium has a beneficial effect in comparison with titanium,
on the toughness of the alloy. A particular embodiment of
the invention calls for at least 0.15% columbium and at
least 0.15~ titanium. Titanium, columbium and zirconium are
preferably present in amounts up to 1.00% in accordance with
the following equation:
~Ti/6 ~ %Zr/7 + %Cb/8 = 1.0 to 4.0 (%C ~ ~N)
The ferritic stainless steel of the present
invention is particularly suited for use as a welded
article having a thickness no greater than 0.070 inch
(usually no greater than 0.049 inch), and in particular, as
7~
1 welded condenser tubing with a wall thickness which typically
ranyes from 0.026 to 0.037 inches.
The following examples are illustrative of several
aspects of the invention:
Ingots from fifteen heats (Eleats A through O~ were
heated to 2050F, hot rolled to 0,125 inch strip, annealed
at temperatures of 1950 or 2050F, cold rolled to strip of
from about 0.062 to 0.065 inch and annealed at temperatures
of 1950 or 2050F. Specimens were subsequently evaluated for
crevice corrosion resistance. Other specimens were TIG welded
and evaluated for crevice and intergranular corrosion resis-
tance. The chemistry of the heats appears hereinbelow in
Table I.
--5--
~3~
l TABLE I
COMPOSITION (wt.~)
... ... _ . . _ . . . . . . , . _
Heat C N Cr ~o Mn Ni Si Al Ti Cb Fe
A 0.042 0.022 29.09 4.00 0.24 0.31 0.34 0.039 0.31 - Bal.
B 0.064 0.022 28.98 4.01 0. 24 O~ 29 0.34 0O050 0.34 - Bal.
C 00020 0~021 29~08 4~00 0~24 0~29 0~33 0.023 0~26 ~ Bal.
D 0.037 0.019 29~05 4002 0~24 0~29 0~34 ~053 0.40 - Bal.
E 0.039 0.014 28.88 4~02 0~24 0~30 0.33 0.055 0.61 - Bal.
F 0.064 0.013 28~91 4~01 0~24 0~29 0.32 00055 0~66 ~ Bal.
G 0.015 0.015 29.10 4.02 0.35 0.41 0.38 0.010 - 0~38 Bal.
H 0.030 0~016 29~10 4~04 0~36 0~45 0~40 0~014 ~ ~ 0~53 Bal.
I 0.029 0O019 28.92 4.04 0.35 0~54 0~39 0~016 0~20 0~39 Bal.
J 0~030 0~025 28~96 4.2G 0.34 0.45 0.36 0. 029 0~ 50 - Bal.
K 0.030 0.026 29~05 4~18 0~34 0~46 0.37 0~029 0~20 0~32 Bal.
L 0.031 0.025 28.96 4.06 0~36 0~45 0~29 0~027 0~09 0~45 Bal.
M 0~034 0~027 28~95 4~20 0~43 0~46 0.37 0.040 0.19 0.41 Bal.
N 0.035 0.026 28.75 4020 0~40 0~47 0~45 0~025 0.20 0.42 Bal.
O 0~032 0~024 29~52 4010 0.37 0~51 0~28 0.030 0~31 0.44 Bal.
Additional data pertaining thereto appears hereinbelow in
Table II.
~3~7~
l TABLE II
Heat %C + %N ~Ti/6 ~ %Zr/7 ~ ~Cb~8
A 0.064 0.052
B 0.086 0.057
C 0.041 0.043
D 0.056 0.067
E 0.053 0.102
F 0.077 0.110
G 0.030 0.048
~ 0.046 0.066
I 0.048 0.082
J 0-055 0.083
K 0.056 0.073
L 0.056 0.071
M 0.061 0.083
N 0.061 ~086 :
O 0.056 0.107
Note that Heats A and B are outside the subject invention.
They are not in accordance with the following equation:
%Ti/6 ~ ~Zr/7 ~ ~Cb/8 > ~C + %N
Crevice corrosion resistance was evaluated by
immersing 1 inch by 2 inch surface ground specimens in a
10% ferric chloride solution for 72 hours. Testing was
performed at temperatures of 95 and 122F. Crevices
were created on the edges and surfaces by employing
. polytetrafluoroethylene blocks on the front and back, held in
position by pairs of rubber bands stretched at 90 to one
another in both longitudinal and transverse directions. ~he
l test is described in Designation: G48-76 of the American
Society For Testing And Materials.
The results of the evaluation appear below in Table
III.
TABLE III
10~ FERRIC CHLORIDE CREVICE CORROSION TEST
_ _ WEIGHT LOS5_(GRAMS)
Base Metal As Welded As Welded
Heat 122 F 95 F 122~
A 0.0 0.0 0O419
B 0.8519 0.0198 0.5783
C 0.0 0.0001 0.0004
D 0.0 - 0.0
E 0.0 D.0 0O0
F 0.0 0.0001 0.0
G - - 0.0
H - _ _
I - - 0.0
J - - 0.0003
K - - 0.0
L - - 0.0
M - - 0.0
N - - - 0.0
o - - 0.0013
- From Table III, it is noted that the crevice corrosion
resistance of Heats C through G and I through O is superior
to that for Heats A and B. Base metal from Heat B lost as
much as 0.8519 gram. ~elded metal from Heats A and B
respectively lost as much as 0.4195 and 0~S783 gram.
7:~
1 Significantly, Heats A and B are outside the subject invention~
On the other hand, Beats C thrvugh G and I through O are in
accordance therewith.
Intergranular corrosion resistance was evaluated by
immersing 1 inch by 2 inch surface ground specimens in a
boiling cupric sulfate - 50% sulfuric acid solution for 120
hours. The usual pass-fail criteria for ~his test are a
corrosion rate of 24.0 mils per year (0O0020 inches per
month) and a satisfactory microscopic examination. This
test is recommended for stabili~ed high chromium ferritic
stainless steels.
The results of the evaluation appear hereinbelow
in Table IV.
i34~
1 TABLE IV
CUPRIC SULFATE - 50% SULFURIC ACID CORR0SION TEST
,
CORRO5ION RATE - AS WELDED
~ ~ICROSCOPI~
EXAMINATION AS
Heatmils~year ~ WELDED (AT 30X)
A 8~21 OrO00684
B 141 0.011786
C 6.82 0.000568
D 9.94 0~000828
E 5.5~ 0.000466
F 11~0 0.000914
G 5.76 0.000480 NA*
H - _ _ -
I 6.29 0.000524 NA
J S.61 0.000551 NA
X 5.59 0.000466 NA
L 5.24 0.000437 NA
M 5.78 0.000482 NA
N 5.28 0.000440 NA
O 6.35 0.000529 NA
*NA: NO INTERGRANULAR ATTACK OR GRATN DROPPING
From Table IV, it is noted that only Heat B failed the
subject test. Heat B had a corrosion rate of 141 mils per
year. As stated hereinabove, it is one of the two heats
outside ~he present invention. The other heat, being Heat
A. It is, however, further outside the subject invention
than is Heat A in that:it has a lower titanium to carbon
plus nitrogen ratio.
Toughness was evaluated by determining the
transition temperature using Charpy Y-notch specimens for
.
--10 -
1 hot rolled and annealed material (0.125 x 0.394 inch
specimens) and for as welded material (0.062 to 0~065 x 0.394
inch specimens)l Transition temperature was based upon a 50%
ductile - 50~ brittle fracture appearance. The transi~ion
temperatures appear hereinbelow in Table V.
: TABLE V
TRAN
_, _
Hot Rolled
And
Heat .As Welded Annealed
A 25(1) 16st3)
B 60(l) 185(3)
C 80(1) 155(3)
D 115(1) 185(3)
E 245(1) 195(3) -~
F 220(1) 190(3)
G _35(2) 95~4)
H __ 120(4)
I 95(2) 160(4)
J 110(2) 13~4)
K 60~2) 120~4)
L 90(2) 113(4)
M 105(2) 135~4)
N 155(2) 140(4)
O 130(2) 210(4)
(1) Strip annealed prior to welding at 2050F - air cooled
(2) Strip annealed prior to welding at 1950F - water quenched
(3) Annealed at 2050F - water quenched; transverse test
(4) Annealed at 1950F - water quenched; trans~erse test
1 The transition temperatures indicate that the ~teel of
the present invention can be cold rolled, formed and welded,
although some preheating might at times be desirable.
The columbium-bearing specimens had lower transition
temperatures than the titanium-bearing specimens. The
specimens containing bo~h titanium and columbium had
transition temperatures between that of the columbium-
bearing and titanium-bearing specimens.
It will be apparent to those skilled in the art that
the novel principles of the invention disclosed herein in
connection with specific examples thereof will suggest
various other modifications and applications of the same.
It is accordingly desired that in construing the breadth
of the appended claims they shall not be limited to the
specific examples of the invention described herein.
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