CA1184403A

CA1184403A - Low interstitial 29 percent chromium - 4 percent molybdenum weldable ferritic stainless steel containing columbium or titanium

Info

Publication number: CA1184403A
Application number: CA000391706A
Authority: CA
Inventors: Thomas H. Mccunn; Harry E. Deverell
Original assignee: Allegheny Ludlum Corp
Current assignee: Allegheny Ludlum Corp
Priority date: 1981-01-16
Filing date: 1981-12-08
Publication date: 1985-03-26
Also published as: KR880001356B1; EP0057316B1; MX156238A; BR8200150A; ATE12527T1; KR830007871A; AU7829481A; DE3169748D1; ES508364A0; EP0057316A1; JPS57137455A; ES8305049A1

Abstract

LOW INTERSTITIAL 29% CHROMIUM - 4%
MOLYBDENUM WELDABLE
FERRITIC STAINLESS STEEL CONTAINING COLUMBIUM OR TITANIUM

ABSTRACT OF THE DISCLOSURE

A low interstitial, corrosion resistant, weldable ferritic stainless steel has an addition of between 0.05%
and 0.50% columbium or titanium to give the steel improved toughness when the maximum achievable cooling rate is limited. The steel consists essentially of the following composition by weight percent: between 25.0% and 35.0%
chromium, between 3.6% and 5.6% molybdenum, between 0.05%
and 0.50% of at least one element from the group consisting of columbium and titanium, less than 3.0% nickel, less than 2.0% manganese, less than 2.0% silicon, less than 0.5%
aluminum, less than 0.5% copper, less than 0.050% phosphorus, less than 0.050% sulfur, less than 0.01% carbon, less than 0.02% nitrogen, the carbon plus nitrogen content being less than 0.025% and the balance iron.

Description

1 2. _escription of the Prior Art A ferritic stainless steel must have superior pitting and crevice corrosion resistance in order to be used in certain chemical environments as for example in po~er plants exposed to sea water, and pulp and paper process equipment. Stainless steels containing 29% chromium and 4%
molybdenum are highly resistant to crevice corrosion. These steels require 3 low level of interstitials, for example a total carbon plus nitrogen content of less than 0~025~ by weight, to have good post-welding ductility and intergranular corrosion resistance.

The applications mentioned above often require heavy gage supporting products such as plate, as well as light gage welded tubing such as condenser tubing. This equipment is often assembled through a welding process. The shape and si~e of the assembled equipment usually prevents the use of a final heat treatment or, if capable of a ~inal heat treatment, the shape and size often severely limit the ability of the assembled equipment to cool rapidly from the heat treating temperature. Moreover, the toughness of the alloy decreases as the thickness increases and ac the cooling rate decreases. This is illustrated in Figure 12 in a paper by H. E. Deverell entitled "Toughness Properties of Vacuum Induction Melted High-Chromium Ferritic Stainless Steels", published in ASTM STP 706, Touqhness of Ferritic Stainless Steels, R.A. Lula, Ed., American Society For Testing and Materials, 1980. The decrease in toughness decreases weldability such that the plate, which in some instances may ~1~

; :

1 ~e incapable of being water-quenched because of its size, rnight exhibit cracking during welding. Or, if the plate is water-~uenched, the cooling rate because of the thickness of the plate may not be rapid enough to achieve suitable toughness such that the plate may exhibit cracking during welding. Therefore, better ~oughness must be achieved by some other means where watee-quenching is impractical or where water-quenching does not achieve suitable toughness to improve the weldability of the various components co~prising the final assembled structure.

Even if the final product is to be of light gage7 the conventional production methods re~uire the cooling of thicker slabs and bands during the processing. The cooling rates of these heavier section sizes is slow. Water-quenching would speed up the cooling process however, water-quenching is often impossible or impractical due to shape and size.

As the thickness of the product section increases, the toughness as measured by Charpy impact transition temperature decreases. Toughness is the ability of a metal to absorb energy by deforming plastically before fracturing. The transition temperature is the temperature at which the fracture which occurs from the impact is 50 percent shear (ductile~ and 50 percent cleavage (brittle~.

23 The toughness of the 29~ chromium - 4~ molybdenum alloy is lo~l compared to substantially lower chromium alloys of an equivalent carhon and nitrogen content because f LJ3~ 3 1 of the high alloy content. The toughness of the 29%
chromium - ~ molybdenum alloy is improved by water-~uenching to speed up the cooling process instead of the slo~er air-cooling, However, in many cases the water-quenching process is not possible or ractical to use, so a method is needed to improve the toughness of the steel in those situations where the maximum cooling rate is limited.

~ 29% chromium - 4~ molybdenum ferritic stainless steel with a maximum carbon plus nitro~en content of 0.025%
is disclosed in Patent No. 3,929,473. Patent No. 3,932,174 is a modification to which small amounts of other elements are added to achieve the same range of corrosion properties as 3,929,473~ However, these patents do not teach the use of columbium or titanium.

lS Patent No. 3,807,991 teaches the addition of between 13 and 29 times the amount of nitrogen or between 0.065~ and 0.363% columbium to a steel of 1% molybdenum for improved toughness and intergranular corrosion resistance in the air-cooled condition. Patent 8,957,544 discusses the addition of titanium and columbium according to the equation /6 + %Cb/8 = (~C + ~N). The molybdenum content of the steels in these patents is lower than that of the present invention.

The presence of titanium and columbium in the ~S steel reduces the susceptability of a steel to intergranular attack, but the weldability of the steel is poor unless the level of interstitials is low. Molybdenum improves pittlng and crevice corrosion resistance, but according to VOS~

1 Patent 4,119/765 if molybdenum is present in an amo~nt of over 3.5~ and is combined with chromium, titanium, silicon or columbium, the notch toughness is reduced, especially in the as-welded condition. U.S. Patent 4,119,765 adds from 2% -4.75% nickel to improve the weldability of the steel. The amount of nickel must be regulated carefully so as to improve notch toughness and acid corrosion resistance without interferlng with other properties~

A final reference is a paper entitled 'IFerritic Stainless Steel Corrosion Resistance and Economy" by Remus A. Lula. The paper appeared on pages 24-29 of the July '76 issue of Metal Progress. This reference does not disclose the ferritic stainless steel of the present invention~

For the reasons noted hereinabove, the present invention is distinguishable from the cited references.

3. Summary of the Invention Accordingly, an object of the present invention is to provide a low interstitial, corrosion resistant, weldable ferritic stainless steel with a high molybdenum content which exhibits improved toughness in heavier section thickness when the maximum achievable cooling rate is limited.

A further object of the present invention is to provide a low interstitial, corrosion resistant, weldable ferritic stainless steel with a high molybdenum content which exhibits improved toughness due to a small addition of either columbium or titanium.

1 In particular, this invention provides a 10~7 interstitial ferritic stainless steel which is corrosion resistant and weldable at room temperature. The improvement of the present invention being an addition of a small critical amount of columbium or titanium to improve the toug'nness of the steel in situations where water quenching is impossible or impractical.

The steel consists essentially of by weight percent: between 25.0% and 35.0~ chromium, between 3.6% and 5.6% molybdenum, between 0.05~ and 0.50~ of at least one element from the group consisting of columbium and titanium, less than 3.0~ nickel, less than 2.0% manganese, less than

2.0~ silicon, less than 0.5~ aluminum, less than 0.5~
copper, less than 0.050% phosphorus, less than 0.050~ sulfur J
lS - less than 0.01% carbon, less than 0.02~ nitrogen, the carbon plus nitrogen content being less than 0.025% and a balance of iron.

Chromium and molybdenum are preferably present in respective amounts of 28.5% to 30O5% and 3.75% to 4~75~O At least one element from the group consisting of columbium and titanium is present preferably in the amount of 0.05% to 0.20~. Manganese and silicon are each usually present in amounts of less than 1.0%. Aluminum, copper, phosphorus, and sulfur are present usuall~ in amounts of less than 0.1~.
Carbon and nitrogen are present preferably in amounts of less than 0.008~ and 0.016% respectively.

The advantages of the steel of this invention will be a parent from the following descri~tion ~hich is illustrative of several aspects of the invention.

--S--1 Descr_ption of the Preferred Em _diments This invention relates to a low interstitial ferritic stainless steel having a chromium content of between 25.0% and 35.0~ and a molybdenum content of bet~7een

3.6% and 5~6%. ~ small amount of columbium or titanium of between 0.05~ and 0.20% is added to the steel composition to improve its toughness when the maximum achievable cooling rate is limited.

Ingots from four heats were vacuum-induction melted to the co~npositions given in Table I.

~ ~8~3 Ln ~ o ~
.~ o o o o o o o o I
~ ,. ~ ~
U~ o o o o o o o o o o o o P~ o o o o o o o ~, C~ o o o .,~ ,, ~ ,, Z
o o o o ~ In ~D ~
Z o o o o o o o o t`; cn O -D a~
~--dP ~ ~ ~r ~n 3 ~ o . o i~ ~ o o ,~ o O
,1 C~
. , . . I
~1 E~ C) ~ ~ R Q Q
O o o ~ o C ) C~ o o o o ,_, o o o o C) .,, ~ ~ o ~ o a~ ~ ~ o~ o s::C~
OD a7 ~ a~

~ o~ ,1 ~o Q O O r-~
V
O O O O

1_ ~ 1-- Ul `~ O O O O
'O O O O
O O O O
~`I o --I N ~ ~
r-l O O O O
O O O O
~'1 a~
~ ~ m c~ c~

1 The ingots ~ere conditioned, heated to a temperature of 2050F and hot rolled to a strip about 0.140 in. thick.
The hot rolled band was annealed at a temperature of 1850~F, and water-auenched and cold rolled to a ~hic~ness of 0.062 in. The strip was then annealed at a ~emperature of 1850F, water-quenched and TIG welded.

Four sets of transverse Charpy V-notch impact subsize specimens were taken, two from the hot rolled band and the others from the 0.062 in. thick stripsA After annealing the specimens at a temperature of 1850F, the specimens were either water-quenched or air-cooled.
The cooled specimens were then tested for toughness characteristics. The results of the tests are shown in Table II.

Table II
Impact Transition Temperature (F) 0.140 in. Thick 0.062 in.Thick Heat Air-Cooled Water-Quenched Air-Cooled Water-Quenched B 165 45 -45 -go The transition temperature decreases with increasing columbium content in the air-cooled condition thus indicating that columbium acts against the detrimental effects on toughness of slow cooling~ Although the impact transition temperatures of the air-cooled specimens are higher that those of the water-quenched specimens it can be seen from Table II that the difference in impact transition 1 temperat~res of an air-cooled and a water-quenched 0.062 in~
thick steel strip is less than 100F. ~hereas the difference in impact transition temperature for prior art compositions is 240F. ~owever, in situations where water-quenching is impractical or in situations where the cooling rates achieved by water-q~enching of heavy thickness sections approach of are slower than those of air-cooling lighter thickness sections, the addition of columbium improves the toughness.
The impact transition temperature of the 0.062 inch steel strip of a composition according to our invention is below 0F, whereas that of prior art composition A is 130F. It is essential that the impact transition temperature of such a steel strip be below room temperature so that the steel strip will not crack upon welding. Steels 1~ of prior art compositions had to be water quenched beEore cold rolling in order to achieve the necessary toughness characteristics. The composition of this invention enables ~ ~
us to achieve good toughness characteristics by air-cooling the steel before cold~rolling instead.

Corrosion tests were also performed on the 0.0~2 in. thick strip in the as welded condition and on the base metal specimens which were heat treated at 2250F and air-cooled to simulate the heat affected zone upon welding a heavier thickness. The specimens, 1 in. x 2 in. were exposed to a boiling solution of ferric sul~ate-50% sulfuric acid for 120 hours according to ASTM A262 Practice B for intergranular corrosion testing. The corrosion rates are given in Table III~
-10~

1 Table III

Corrosion Rates in Ferric Sulfate - 50~ S~furic Acid Interqranular Corrosion Test (ASTM A262, Practice B) .
0.062 in. TIG Welded 0.062 in. Base ~etal (Heat Treated at 2250F -Air-Cooled) Heatinches/month inches/month .000496 0.000580 B0.000478 0.000330 C0.000390 0.000395 D0.000483 0.000556 The results of additional tests of the mechanical properties of the steel in the welded condition are shown in Table IV.
Table IV
Welded Mechanical Properties_of 0.062 in Thick Strip Heat 0.2~ Yield Tensile % Elongation in - ----Strength, ksi Strength, ksi 0.5 in. 2.0 in.
A 81.0 92.2 24.0 6.0 B 85.4 98.6 24.0 9.0 C ~.5 100.0 9.0' 15.5 D 88.8 101.~ 2300 6.0 'fracture in base metal, not in weld From the above paragraphs 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 should not be limited to the specific examples described herein.
We claim:

Claims

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

1. A low interstitial, corrosion resistant, weldable ferritic stainless steel with improved toughness consisting essentially of by weight percent: 25.0% - 35.0% chromium, 3.6% - 5.6% molybdenum, less than 3.0% nickel, less than 2.0%
manganese, less than 2.0% silicon, less than 0.5% aluminum, less than 0.50% copper, less than 0.050% phosphorus, less than 0.05% sulfur, less than 0.01% carbon, less than 0.02%
nitrogen, the sum of the carbon and nitrogen being less than 0.025%, 0.05 - 0.50% of at least one element from the group consisting of columbium and titanium and the balance of iron.

2. A low interstitial, corrosion resistant, weldable ferritic stainless steel with improved toughness according to claim 1 with a chromium content of between 28.5% and 30.5%.

3. A low interstitial, corrosion resistant, weldable ferritic stainless steel with improved toughness according to claim 1 with a molybdenum content of between 3.75% and 4.75%.

4. A low interstitial,corrosion resistant, weldable ferritic stainless steel with improved toughness according to claim 1 with 0.05% - 0.20% of at least one element from the group containing columbium and titanium.

5. A low interstitial, corrosion resistant, weldable ferritic stainless steel with improved toughness according to claim 1 with 0.05% - 0.20% columbium.

6. A low interstitial, corrosion resistant, weldable ferritic stainless steel with improved toughness according to claim 1 with 28.5% - 30.5% chromium, 3.75% - 4.75%
molybdenum, and 0.05% - 0.20% of at least one element from the group containing columbium and titanium.

7. In a process for the manufacture of ferritic stainless steel of improved toughness wherein said steel is hot rolled, annealed and cold-rolled to strip thickness, said ferritic stainless steel consisting essentially of by weight percent: 25.0% - 35.0% chromium, 3.6% - 5.6%
molybdenum, less than 3.0% nickel, less than 2.0% manganese, less than 2.0% silicon, less than 0.5% aluminum, less than 0.5% copper, less than 0.5% phosphorus, less than 0.05%
sulfur, less than 0.01% carbon, less than 0.02% nitrogen, the sum of the carbon and nitrogen being less than 0.025%, at least one element from the group consisting of columbium and titanium and the balance iron, the improvement com-prises providing additions of between 0.05 to 0.20% of at least one element from the group consisting of columbium and titanium and air cooling the steel from annealing temperature to cold rolling temperature; said steel characterized by a Charpy impact transition temperature of below 0°F as cold-rolled to a thickness of 0.062 inch.

8. A process for making a low interstitial, corrosion resistant, weldable ferritic stainless steel with improved toughness according to claim 7 with the additional step of welding.

9. A low interstitial, corrosion resistant weldable, ferritic stainless steel with improved toughness made according to the process of claim 7.

10. A low interstitial corrosion resistant, weldable ferritic stainless steel with improved toughness made according to the process of claim 8.

11. A process as set forth in claim 7 including welding the steel and air-cooling the steel from the weld heat temperature of the welded steel.