CA1163471A

CA1163471A - Ferritic stainless steel

Info

Publication number: CA1163471A
Application number: CA000348952A
Authority: CA
Inventors: Thomas H. Mccunn; Thomas J. Nichol
Original assignee: Allegheny Ludlum Corp
Current assignee: Allegheny Ludlum Corp
Priority date: 1980-01-03
Filing date: 1980-04-01
Publication date: 1984-03-13
Also published as: BR8001876A; NO800713L; DE3012957A1; PL124421B1; NO154585C; NO154585B; FR2473069A1; KR850000995B1; NL8001739A; JPH0321625B2; ATA255980A; SE436577B; CS216220B2; GB2066848A; AU5641880A; BE882792A; AT376706B; IT1188919B; IT8048299A0; SU1258328A3

Abstract

FERRITIC STAINLESS STEEL
ABSTRACT OF THE DISCLOSURE
A ferritic stainless steel characterized by superior crevice and intergranular corrosion resistance.
The steel 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 of carbon plus nitrogen is in excess of 0.0275%. Titanium, zirconium and columbium, are in accordance with the following equation:
%Ti/6 + %Zr/7 + %Cb/8 ? %C + %N

Description

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 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.

~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~

CUPRIC SULFATE - 50% SULFURIC ACID CORR0SION TEST
, ~ ~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 .

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.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A corrosion-resistant, tough, weldable ferritic stainless steel consisting essentially of, by weight, from 0.0050 to 0.08% carbon, from 0.0100 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 for deoxidizing the steel, up to 2.00% of elements from the group consisting of titanium, zirconium and columbium, balance essentially iron; said titanium, zir-conium and columbium being in accordance with the following equation:
%Ti/6 + %Zr/7 + %Cb/8 ? %C + %N
the sum of said carbon plus said nitrogen being in excess of 0.0275%; said steel being characterized by its superior as-welded crevice corrosion resistance at 50°C (122°F).

2. A ferritic stainless steel according to claim 1, having at least 0.005% carbon and at least 0.010% nitrogen, the sum of said carbon plus said nitrogen being in excess of 0.0300%.

.

3. A ferritic stainless steel according to claim 1, having from 28.50 to 30.50% chromium.

4. A ferritic stainless steel according to claim 1, having from 3.75 to 4.75% molybdenum.

5. A ferritic stainless steel according to claim 1, having up to 1.00% of elements from the group consisting of titanium, zirconium and columbium in accordance with the following equation:
%Ti/6 + %Zr/7 + %Cb/8 = 1.0 to 4.0 (%C + %N).

6. A ferritic stainless steel according to claim 1, having at least 0.15% titanium.

7. A ferritic stainless steel according to claim 6, having at least 0.15% columbium.

8. A ferritic stainless steel according to claim 1, having at least 0.005% carbon, at least 0.010% nitrogen, from 28.50 to 30.50% chromium, from 3.75 to 4.75% molybdenum, and up to 1.00% of elements from the group consisting of titanium, zirconium and columbium in accordance with the following equation:
%Ti/6 + %Zr/7 + %Cb/8 = 1.0 to 4.0 (%C + %N) the sum of carbon plus said nitrogen being in excess of 0.0300%.

9. A welded article having a thickness no greater than about 0.070 inches whenever made from a ferritic stainless steel consisting 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, said titanium, zirconium and columbium.
being in accordance with the following equation:
%Ti/6 + %Zr/7 + %Cb/8 ? %C + %N
the sum of said carbon plus said nitrogen being in excess of 0.0275%.

10. A corrosion-resistant, tough, weldable welded article having a thickness no greater than about 0.049 inches whenever made from a ferritic stainless steel consisting essentially of, by weight, from 0.0050 to 0.08% carbon, from 0.0100 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,

Claim 10 continued...

up to 2.00% silicon, up to 0.5% aluminum for deoxidizing the steel, up to 2.00% of elements from the group consisting of titanium, zirconium and columbium, balance essentially iron;
said titanium, zirconium and columbium being in accordance with the following equation:
%Ti/6 + %Zr/7 + %Cb/8 ? %C + %N
the sum of said carbon plus said nitrogen being in excess of 0.0275%, said steel being characterized by its superior as-welded crevice corrosion resistance at 50°C (122°F).

11. Corrosion-resistant, tough, weldable welded condenser tubing with a wall thickness between from about 0.026 to 0.037 inches whenever made from a ferritic stainless steel consisting essentially of, by weight, from 0.0050 to 0.08% carbon, from 0.0100 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 for deoxidizing the steel, up to 2.00% of elements from the group consisting of titanium, zirconium and columbium, balance essentially iron; said titanium, zirconium and columbium being in accordance with the following equation:
%Ti/6 + %Zr/7 + %Cb/8 ? %C + %N
the sum of said carbon plus said nitrogen being in excess of 0.0275%; said steel being characterized by its superior as-welded crevice corrosion resistance at 50°C (122°F).