CA1181267A

CA1181267A - Stabilized ferritic stainless steel with improved brazeability

Info

Publication number: CA1181267A
Application number: CA000383481A
Authority: CA
Inventors: George Aggen; Paul R. Borneman
Original assignee: Allegheny Ludlum Corp
Current assignee: Allegheny Ludlum Corp
Priority date: 1980-08-08
Filing date: 1981-08-07
Publication date: 1985-01-22
Also published as: ZA814922B; JPH034617B2; ATA345281A; EP0049033B1; AU7317081A; DE3172977D1; ES504584A0; BR8105025A; EP0049033A1; JPS5760056A; ES8302116A1

Abstract

Stablilized Ferritic Stainless Steel With Improved Brazeability ABSTRACT
A stabilized ferritic stainless steel is wettable by brazing materials used at temperatures of from 2000°F to 2100°F. The steel consists essentially of, by weight, 10.5% to 13.5% chromium, up to 0.1% carbon, up to 0.05% nitrogen, up to about 0.12% titanium and at least one other stabilizing element from the group consisting of niobium and tantalum in an amount in accordance with the relationship:

Description

6~
1 The invention relates to stabilized ferritic stain-less steels and is particularly use~ul for ferritic stainless steel articles which are joined by brazing.
Ferritic stainless steels possess excellent mechanical properties and oxidation and general corrosion resistance at elevated temperatures. These steels are ideal for use as the structural members of heat exchangers, exhaust systems, chemical process vessels and the like which are exposed to high tempera-tures ancl stresses and corrosive environments. Fabrication of these articles frequently requires the joining of the Eerritic stainless steel with either itself or with another dissimilar metal at sufficiently high temperatures for the joining method to be effective. Also, generally speaking, the steel must be joined in a temperature range exceeding the anticipated service temperature~ Brazing is a widely practiced me-thod of joining metals involving temperatures of from ~00 F to the 2000 F -2100F range which are above the melting point of the brazing filler material bu-t below the melting point of the base metal being joined. When the temperature of the brazing filler material is about the melting point, it becomes molten and wets the surface of the steel, and then flows by capillary action to fill a joint. Bonding results from the intimate contact produced by the dissolu-tion of a small amount of -the base metal in the molten filler metal.
Ferritic stainless steels to be joined at high temperatures contain low levels of carbon and small amounts o~
stabilizing elements for combining with carbon and nitrogen to maintain the ferritic phase and to maintain the oxidation and corrosion resis-tance of the steel. Stabilizing elements such as titanium, niobium or tantalum react with the carbon and /~t ,. '';

I nitrogen to prevent the formation and precipitation of chromium carbides and nitrides at grain ~oundaries and the simultaneous depletion of chromium in the surrounding areas. Stabilizing elements must be added in amounts e~ceeding the theoretical requirement to assure complete stabilization o~ carbon and nitrogen. Titanium is the preferred stabilizing element because of its very strong affinity for carbon and nitrogen, its low atomic weight and its availability. Other stabilizing agents including niobium and tantalum are not favored because they are 1~ more expensive and less effective on a weight basis than titanium and also because they are accompanied by a tendency toward weld cracking problems.
Titanium stabilized ferritic steels known in the prior art (see, e.g., Lula et al. U.S. Patent NoO 3,250,611) cannot be re~dily brazed with filler materials such as oxygen-Eree copper and nickel base alloys. These steels form a non-wettable surface film which prevents proper bonding between the férritic stainless steel base metal and the brazing filler material even when furnace brazing under vacuum or in an inert atmosphere. The oxygen-free copper as a high temperature brazing filler metal does not pene-tra-te this surface film~ Nickel alloy high tempera-ture brazing filler me-tals usually contain boron and silicon additions to penetrate the surface film. ~lthough the steel wettabilit~ is improved, these nickel base materials will also penetrate the grain boundaries thereby causing intergranular attack of the base metal. In addition, brazing operations are not aided by increased temperatures or by increasecd brazing times because the high temperature range is beginning to afect the grain size of the steel and prolonged time tends to increase 3~ film resistance. For these reasons, brazing with copper is . . ~, 1 impossible and bxazing with nickel base metals is not consistent enough to be of practical value from a quality assurance view-point. Thus copper clad ferritic stainless steels are used in brazing applications when the brazing temperature is to reach 2000F - 2100F. In this process, the copper cladding is brazed rather than the steel.
The present invention relates to a stabilized ferritic stainless steel composition which is wettable by conventional brazing materials used at temperatures of from 2000 F - 2100 F
in furnace brazing practices. In accordance with the invention, a ferritic stainless steel consists essentially of, by weight, 10.5% to 13.5% chromium, up to 0.1% carbon, up to 0.05% nitrogen, up to about 0.12% titanium and at least one other stabilizing element from the group consisting of niobium and -tantalum in accordance with the relationship:

Wt% Nb + Wt% Ta + Wt% Ti . ~ , . . . . . -.-- --- -- > 1 .
Wt% C + Wt% N

The presence of niobium, tantalum and titanium in accoxdance with this stabilization relationship are sufficient to effectively stabilize the inters-titial elements in the steel without forming a non-we-t-table surface film. The niobium and tantalum are presen-t as additions to the melt. Titanium may be present in the scrap feed or added to the melt. The titanium is responsible for the nature of the film which becomes non-wettable when titanium is present in amounts ~reater than about 0.12%.
Greater amounts of titanium could be tolerated and the effect of titanium on wettability could he neutralized if titanium compounds -1 stable at brazing temperatures such as TiO2, TiS and TiN are permitted to ~orm. However, oxygen, sulfur and nitrogen have an undesirable effect on other steel qualities and generally they will be kept as low as possihle. From a brazing viewpoin-t it is preferable to have a composition ~ree o~ titanium. For this reason the titanium is preferably present in an amount up 0.01%
by weight and, most pre~erably, up to 0.005%. The steel may also contain up to 0.1~ aluminum, up to 1.25% molybdenum, up to 1%
manganese and up to 1% silicon to enhance its mechanical and corrosion properties. Articles of this composition are wettable by fillers such as copper, nickel and their alloys and can be successfully furnace brazed according to conventional practices.
In some cases, however, it may be desirable to both weld and braze the same article. Therefore, titanium is tole-rated in controlled amounts up to 0.12% to prevent weld cracking while maintaining reasonable wettability during brazing operations.
Larger amounts o~ titanium render the steel unbrazeable for practical purposes.
To illustrate the beneficial results of the invention specimens from sixteen laboratory heats and two commercial heats were tested for wettability. The composition oE the laboratory heats and the commercial heats are identified in Table I as Nos. 1-16 and Nos. A and B respectively.

~ -- O ~ O ~ L'~ ~-- C ~
~ ~ V . .
Cl~
O 0~ ~ O O
~1 ~ ~ '~:c~ zz Z Z Z 2 Z Z Z Z Z Z 0 ~1 ~I O 0 r o o o f~ ~ O C O ~ ,~
o o ~ 1` o o o o o o o o ....
Z ~ ~ ~ O O O C O C O O
~ V

u~ N N C0 O O O
O O O o 0 ~1 ~ ~ ~ O o o O~ o _~ o o o o o ~I r~
E~ .. . . .. . . O G O -O ~ ~ N O ~ O ~ ) 0~ ~ r ~ ~ ~ ~
~`J t`l N N tN ~1 N ,~
N O O O O O O O O O O O O O O O O O O
OOOO ooO OOO OoO OoO Oo ~ r.~l r,~l ~ r~ r~) ~ r~ r~ r~ ~ r,~
3 oooo OOO O OO O O O OOO OO
~) ,o............ ... ... ... ... ..
0000 ooo 000 000 000 00 o o o o ~ 1--r- ~ ~ ~ r~) r~ r o ~ ~ ~ ~r r~ ~ N

~ . , . . . . , , . . . . ~ , . . . .

~1 o 0 o 1-- ,t r~ O N
OO_O OOO Oo~1 ~,~--( ~1~o r,~lr,~
OOOO OOO OOO OOO OCO OO
1~:;

r,~ 9 ~ ~ ~ r~7 ~ ~ O a~
Ll') ~ r~) ~ r~) r~) ~ ~1 ~ ~ ~ a~ r~ 10 ~ 11`~ r.~ o _ O O O O O O O O O O r,~ r~ l O O O~ r,~
Z .... ... ... ... ... ..

O ~ ~ ~ ~ r~ C 1~ ~ C~ o r; ~ ~ ~ ;
r,~ ro r- ~ r~ ~ r~ ~ ~ ~ ~D O ~ Cr~ o ~
L~ .... ... ... ... ... ..
O O O O O O ~ ~ ~ ~ Ltl U~ O O O_~ ~

N o ) C~ CD ~ O o~ a:) ~ N C ~ C~ OO ~9 .~ ~ c r~l r~ r ~ ~ r~) r~
U~ .... ... ... ... ... ..

~D ~ ~D ~D CO ~ ~ D ~ ~ ~ ~ C r~ r~~1 r,~l , 0000 000 00 0 0 00 000 00 ~n oooo ooo ooo ooo ooo oo o oo o o o o oo o o oo o o o oo O o o o N ~ N ~ N ~ r~ r~) ~ c ~ r~
: o o o o o o o o o o o o o o o o o o o o o o o o o c~ ~I c ~ ~ ~ u) ~ c ~ r~ to L'~
C ~:r L~ Lr~ r c r~- r~ r~- ~ r~
. ~ c o o o o o o o o o o o o o o o o o o o o a~ ~ ~ Lr~ Ln ~ r~
O O O Cl ~ ~ ~ ~ '' O ~ C` ~ O O O ~ O ~
o o o o o o o o o o o o o o c o o G C) C~

¦ ~ N 1~1 C Lt"l ~) r-- C~ C. o _ ~ ~ Lr ~ C~
Z ~ ~ ~ ~ ~ ~ ~ Z

1 Samples from the laboratory heats were hot rolled to about 0.100 inch and cold rolled to 0.020 inch. The commercial samples were also cold rolled to 0 020 inch. ~he cold rolled samples were then annealed and pickled in accordance with stan-dard practices. Circular specimens of 1 l/2 inch diameter were stamped from the cold rolled strips and tested for brazing wettabilit~ in a resistance heated cold wall vacuum furnace.
The test generally consisted of placing a brazing filler material on each specimen and heating the specimens and filler materials to the melting point of the filler mat0rial.
The wettability o~ the specimens were evaluated according to the parameter "d2/h", where "d" is the average diameter of the drop in inches which formed on the surface of the specimen and "h" is the height of the drop in inches, wettability being proportional to the area covered by the drop and inversely proportional to the height of the drop.
Specimens of the heats were tested at 2050F in conventional furnace atmospheres with oxygen-free copper as a brazing filler matexial. No flux was applied because this would be an uncommon practice in furnace brazing operations. Short l/8 inch lengths of 0.010 inch diameter wire with square ends were placedon end at the center of each specimen heated. In vacuum -testsr the furnace was evacuated cold, heated to 1050 F, held at a vacuum of one micron of mercury or less while heating to the brazing temperature. In inert gas tests, the furnace was evacuated cold, heated to 1050F, held at a vacuum of one micxon ox less while heating to 1200 F, pressurized with nitrogen to 1500 microns and heated to the brazing temperature.
In the reducing atmosphere tests r the furnace was evacuated cold, heated to 1050F, held at a vacuum of one micron ~. .

1 or less while heating to 1200 F, pressurized with dry hydrogen (having a dew poin-t of less than -80 F) to a pressure of 3no,000 microns and hea-ted to the brazing temperature. The wettability ratings (d /h) of the specimens are shown in Table II. The letter "C" indicates that the specimen was completely wetted.
TABLE II

Dry N2 Dry H2 No. ATMOSPHERE ATMOSPHERE Vacuum 1 C ~.292 C

2 C 4.836 C

3 C 5.55g C
lO 4 C ~.930 C
2.0~0 0.721 22.003 6 1.836 0.182 9.840 7 0.286 0.174 0.296 8 0.304 0.199 0.325 9 0.253 0.187 0.256 0.228 0.171 0.178 ~ 0.573 12 - - 0.579 13 - - 0.568 14 - - 30.502 - - 24.807 A 0.896 0.207 0.586 B 0.361 0.219 0.,25~

The wettability o~ the laboratory melted compositions can be compared with each other and with the prior art compositions of He~ts ~ and B to determine the adverse e~fects of titanium.
The prior art compositions are clearly non-wettable. The stabili~ed compositions of Heats 1-4 and 14-16 contain up to 0.005 of titanium ancl exhibit superior wettability under all atmospheres. The efEec-t of increasiny amounts of titanium i5 most clearly shown by the compositions of Heats 5-7. The composition of Heat 5 contains 0.008 wt% titanium and has superior wettability characteristics under all atmospheres.
The composition o~ Heat 6 contains 0.11 w-t~ titanium and has improved wettability characteristics under inert gas and vacuum atmospheres, however the adverse effect of titanium is evident in a reducing atmosphere. Heats 7-13 contain lar~e amounts of titanium and have no better wettability characteristics than do the prior art compositions.
The difference in wettability between the specimens brazed with oxyyen~free copper in a dry nitrogen atmosphere is seen from Figures 1 and 2. Figures 1 and 2 are the perspective and top views, respectively, of a brazing table supporting the specimens identified in Tables I and II. Specimens A and B are the cornmercial steels and illustrate the problem where the filler material does not wet the surface beyond the periphery of the molten drop. Similarly, specimens 7, 8, 9 an~ 10 are also not wetted by the filler material~ Specimens 1, 2, 3 and 4 are completely wetted by the oxygen-free copper. Specimens 5 and 6, although containing increasing titanium concentrations of 0.008%
and 0.11% respectively, are clearly wetted by the copper beyond the periphery of the molten drop.
Specimens of the heats were tested at ~000F under vacuum conditions with a nickel alloy as a brazing filler material. In these tests nickel alloy powder (A~S BNi-2) was mixed with a plastic cement which vaporized completely be~ore reaching 1000F. The mixture was formed into pellets of approximately 3/16 inch diameter by 3/16 inch height and the pellets were placed on the specimens. The furnace was evacuated cold and heated to the brazing ternperature~ No flux was applied because this is uncommon practice in furnace brazing at high temperatures. The wettability ratings of the laboratory melted specimens are shown in Table III~ The letter "C" indicates that the specimen was completely wetted.

i7 1 Table III

Heat d2/h C
6 35.917 7 5.329 9 2.~88 0 2.188 12 ~.929 1~ C
C

A

The prior art compositions were not tested but.they would have a rating approximating those of Heats 7 and 9 respectively in view of their titanium con-tents. The compositions o~ Heats 3 5 and 14-16 all contain less than .01 wt% titanium and have superior wet-tability characteristics. The composition of Heat 6 contains 0.11 w-t~ titanium and has superior wettability characteristics in comparison to the other compositions contain-ing 0.18 wt~ (Heat 12) or more tikanium (Heats 7 and 9).
It will be apparent to those skilled in the art that the novel principles o~ the invention disclosed herein in connection with specific examples thereof will suggest various other modifications and applications of the sc~me. It is accordingly desired that in construing the breadth of the appended claims they shall not be limited to the speci.fic examples of the invention described herein.

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Claims

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

1. A brazeable ferritic stainless steel consisting essent-ially of, by weight, 10.5% to 13.5% chromium, 0 to 0.03% carbon, 0 to 0.05% nitrogen, 0 to 0.10% aluminum, 0 to about 0.12% titanium, up to about 0.12% the sum of titanium and aluminum, 0 to 1.25% molybdenum, 0 to 1% manganese, 0 to 1% silicon, at least one other stabilizing element selected from the group consisting of niobium and tantalum in an amount in accordance with the relationship with the balance substantially iron, the steel being wettable by brazing filler materials.

2. The brazeable ferritic stainless steel of claim 1, comprising substantially no molybdenum, manganese and silicon,

3. The brazeable ferritic stainless steel of claim 1 wherein, nitrogen is present in amounts up to a, 0.03% and aluminum is present in amounts up to 0.020%, and comprising substantially no molybdenum.

4. The brazeable ferritic stainless steel of claim 1, 2 or 3 wherein the titanium is present in amounts up to about 0.01%.

5. The brazeable ferritic stainless steel of claim 1, 2 or 3 wherein the titanium is present in amounts up to about 0.005%.

6. The ferritic stainless steel of claim 1, 2 or 3 wherein the titanium is present in amounts of at least 0.001%.

7. The brazeable ferritic steel of claim 1 r 2 or 3 stabil-ized by niobium.

8. The brazeable ferritic steel of claims 1, 2 or 3 stabil-ized by tantalum.

9. The ferritic stainless steel of claims 1, 2 or 3 wherein the steel is wettable by molten copper.

10. The ferritic stainless steel of claims 1, 2 or 3 wherein the titanium is present in amounts up to about 0.01% and the steel is wettable by molten copper.

11. The ferritic stainless steel of claims 1, 2 or 3 wherein the titanium is present in amounts up to about 0.005% and the steel is wettable by molten copper.

12. A brazed ferritic stainless steel article having the composition of the steel of claims 1, 2 or 3.

13. A ferritic stainless steel article having the composition of the steel of claims 1, 2 or 3 wherein the article is brazed with copper.

14. The brazeable ferritic steel of claims 1, 2 or 3 stabil-ized with up to 1.0% niobium.

15. The brazeable ferritic steel of claims 1, 2 or 3 stabil-ized with up to 1.8% tantalum.

16. A method of brazing the ferritic stainless steel of claim 1, 2 or 3 comprising brazing the steel with a filler material.

17. A method of brazing the ferritic stainless steel of claim 1, 2 or 3 comprising brazing the steel with a copper filler material.