Strong ferritic alloys of the Fe-CR-Al type containing 0.4% to 2% tantalum were developed. These alloys have improved fabricability without sacrificing high temperature strength and oxidation resistance in the 800 C (1475 F) to 1040 C (1900 F) range

Blankenship, C. P.

Oldrieve, R. E.

English

NASA Technical Reports Server

United States Patent [191 
Oldrieve et al. 
11 11 4,055,416 
[MI Oct. 25, 1977 
TANTALUM MODIFIED FERRITIC IRON 
BASE ALLOYS 
Inventors: Robert E. Oldrieve, Bay Village; 
Charles P. Blankenship, Rocky 
River, both of Ohio 
The United State of h e r b  as 
represented by the AdministrPtor of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
Assignee: 
Appl. No.: 651,009 
Filed: Jan. 21,1976 
Int. Cl.2 ...................... C22C 38/06; C22C 38/26; 
C22C 38/48 
U.S. Cl. ................................... 75/124; 75/126 D; 
75/126 F; 75/128 G, 75/128 T 
Field of h c h  ................. 75/124, 126 C, 126 D, 
75/126 F 148/37; 115/128 G, 128 T 
[561 References Cited 
U.S. PATENT DOCUMENTS 
2,191,790 2/1940 &ranks ................................ 75/126 F 
3,890,143 6/1975 Skoglund et al. ................. 75/126 D 
3,499,802 3/1970 Lagneborg ............................. 148/37 
3,852,063 12/1974 & h i  et al. ........................ 75/126 D 
Primary Examiner-Arthur J. Steiner 
Attorney, Agent, or Firm-N. T. Musial; G. E. Shook; 
John R. Manning 
[571 ABSTRACT 
Strong ferritic alloys of the Fe-Cr-A1 type containing 
0.4% to 2% tantalum have improved fabricability with- 
out sacrificing high temperature strength and oxidation 
resistance in the 800" C (1475" F) to 1040" C (1900" F) 
range. 
5 Claims, No Drawings 
https://ntrs.nasa.gov/search.jsp?R=19780010239 2020-03-20T12:49:32+00:00Z
4,055,416 
1 2 
vated temperatures. Another object of the invention is 
to provide improved alloys by the addition of about 
0.4% to about 2% by weight tantalum to ferritic iron- 
base alloys containing about 1% silicon, 18% chro- 
5 mium, 2% aluminum, 0.4% titanium, and 0.04% carbon 
TANTALUM MODIFIED FERRITIC IRON BASE 
ALLOYS 
ORIGIN OF THE INVENTION 
The invention described herein was made by employ- 
ees of the United States Government and may be manu- 
factured or used by or for the Government without the 
payment of any royalties thereon or therefor. 
BACKGROUND OF THE INVENTION 
This invention relates to improved iron base alloys 
having ferritic body-centered cubic micro-structures. 
The invention is particularly directed to tantalum modi- 
fied ferritic iron base alloys having improved high tem- 
perature mechanical properties and oxidation resis- 
tance. These alloys are particularly useful in high tem- 
perature applications including furnace linings, flue 
45 
requirement. 
The principal disadvantage of these prior art ferritic 
iron base alloys is that they lost their ultimate strength 
and rupture strength at temperatures in excess of 650" 
C. When present day ferritic iron-chromium-aluminum 
alloys are utilized at higher temperatures, the alloys 
lack sufficient strength to support their own weight, 
and they have unsatisfactory corrosion resistance when 
combined with even moderate loads. Thus, such alloys 
have not been used in high temperature applications 50 
above 800" C. 
such that the alloy is strengthened by carbides as well as 
by solid solutioning and with freedom from formation 
of damaging phases upon long term exposure at ele- 
vated temperatures. 
A further object of the invention is to provide novel 
alloys having improved strength properties at elevated 
temperatures while retaining the excellent oxidation 
resistance of ferritic iron-chromium-aluminum alloys 
without loss of fabricability or weldability. 
These and other objects of the invention will be ap- 
parent from the specification which follows: 
DESCRIPTION OF A PREFERRED 
EMBODIMENT stacks, and the like. 
addition of aluminum, have been available as Series 400 
stainless steels, chromium-molybdenum-vanadium Chromium - 15.0% to 20.0% 
steels with less than 0.1 weight percent carbon and 
iron-nickel magnetic compositions. Aluminum added to 25 
these materials in the amount of a few percent generally 
assures that the alloy remains ferritic and free of damag- 
ing allotropic phase transformations. This aluminum 
addition further contributes to corrosion resistance as 
well as resistance to scaling, and the presence of alumi- 3o 
num is sometimes used to control grain size. 
Chromium added in addition to aluminum provides 
resistance to oxidation and corrosion at very high tem- 
exclusively as resistor heating elements and have com- 35 ing composition by weight: 
positions containing chromium in excess of 23% to- 
gether with 5% aluminum. Recently developed ferritic 
alloys with 18% chromium and 2% aluminum, as well 
as AIS1 405 stainlss steel have received attention for 
high temperature applications where strength is not a 40 
Ferritic iron base alloys, both with and without the 2o ~ l l ~ ~ ~  made in accordance with the present invention 
have compositions by weight in the following ranges: 
~ l ~ ~ i ~ ~ ~  _. 2.0% to 4.0% 
silicon - 0.4% to 1.0% 
Titanium - 0.4% to 1 .o% 
Nickel _. 0.4% to 1 .o% 
Carbon - 0.01% to 0.05% 
M~~~~~~~~ - 0.0% to 0.5% 
Phosphorous - o . ~ %  to 0.02% 
Sulphur - 0.0% to 0.01% 
Tantalum - o.4% to 2.0% 
Iron - Balance 
peratures to are used The preferred alloy in the above range has the follow- 
Chromium - 18% 
Aluminum __ 2% 
Silicon - 1% 
Titanium __ o.4% 
Carbon __ o.04% 
Tantalum - 1.3% 
Iron - Balance 
Another preferred alloy has the same as above with the 
exception that the tantalum is reduced to about 0.5%. 
The 15-20% chromium is required for the mainte- 
nance of oxidation resistance for long term exposure. In 
combination with at least 2% aluminum, the chromium 
provides a stable alpha phase alloy structure. 
Tantalum in the amount of at least 0.4% provides 
strengthening by solid solution of a large diameter 
atomic constituent. Tantalum further provides for the 
fine distribution of carbides which offer resistance to 
ily at temperatures in the 800"-1040" c range. The al- s5 tages heretofore not recognized, as Will be Seen by a 
loys have excellent oxidation resistance with greatly further description Of the 
improved high temperature strength. The iron base Carbon in amounts of 0.01-0.05% is required to pro- 
alloys of the present invention are ferritic body-cen- vide for precipitated carbides essential to strengthening. 
tered cubic micro-structure with the following compo- Titanium, nickel, and silicon in the amount of about 0.4 
sition weight percent: 15~20% chromium, 2-4% alumi- 60 - 1.0% each are representative of the type of additional 
num, 0.&2.0% tantalum, 0.01-0.05% carbon, manga- elements which'are included to provide resistance to 
nese about 0.5%, phosphorous about 0.02%, sulphur grain boundary oxidation in iron base alloys with less 
about 0.01%, and the balance iron. than about 3.0% aluminum. In addition to the aforemen- 
tioned constituents, the alloys contain up to about 0.5% 
OBJECTS O F  THE INVENTION 65 manganese, up to 0.5% nickel, and small amounts of 
It is, therefore, an object of the present invention to phosphorous and sulphur. 
provide new alloys having excellent oxidation resis- The actual compositions of alloys made in accordance 
tance and greatly improved strength properties at ele- with the invention are set forth in Table I. 
SUMMARY OF THE INVENTION 
The alloys of the present invention are for use primar- grain-boundary sliding. Tantalum offers other advan- 
4,055,416 
3 
TABLE I 
COMPOSITIONS OF FERRITIC IRON-BASE ALLOYS 
Alloy Composition, wt. % 
No. C Mn Si Cr Ni Ta Mo Nb AI Ti P S 
1 ,038 37 1.14 17.74 .I7 0.45 0 0 2.10 .44 ,008 ,012 
2 ,040 .37 1.28 17.76 .20 1.25 0 0 2.10 .45 .007 .014 
3 .041 .5 1.01 17.91 0 0 2.04 0.58 2.19 0 0 0 
4 ,010 ,001 .05 14.72 0 0.91 0 0 4.27 0 .002.003 
5 ,012 ,001 .05 15.23 0 1.97 0 0 5.35 0 .002.004 
6 ,041 .3  . I  14.37 0 0 2.02 0.53 4.72 0 0 0 
4 
Commercial alloy B had a nominal composition of 
A comparison of the as-fabricated stress to rupture 15% Cr, 4% A], 0.007% C and the balance Fe. 
properties of several of the alloys shown in Table I with A comparison of the data in Table I11 shows that new 
those of an existing commercial product are shown in alloy 2 provides about 25 times the life of commerical 
Table I1 wherein the samples were die punched from 15 alloy A at 800" C and 20 times its life at 1,ooo" C. Also 
as-rolled, sheet-sheet stock of 1.6 mn thickness. The new alloy 2 has only one half the cyclic oxidation 
commercial Fe-Cr-AI alloy (A) had a nominal composi- weight change of commercial alloy A at 1140" C. 
tion of 18% Cr, 1% Si, 2% AI, 0.4% Ti, 0.04% C, and Alloy 1 privdes a 10-fold and 7-fold longer life than 
the balance Fe. commercial alloy A at 800" and looo" C, respectively. 
TABLE I1 2o Alloy 1 also provides increased oxidation resistance 
over the existing commercial alloys. STRESS TO RUPTURE DATA FOR FyCr-AI ALLOYS 
R 1i-m Table I11 also shows a comparison of the increased 
IOOO" c 800' c life and remarkably increased oxidation resistance 
which the addition of 2% Mo and 0.5% Nb provides a 
25 ferritic Fe-Cr-AI when about 0.04% C is added in com- 
binaion with these elements. 
A 3 0  1 9  11 4 7.0 Alloy 2 has exhibited superior performance charac- 
teristics in automobile pollution control devices. Full 
A tantalum addition to the preferred alloys clearly 3o size exhaust manifold thermal reactors were fabricated 
provides a substantial increase in the high temperature Of as as A. The 
load carrying capacity Over that of a commercial alloy reactors were operated until the reactor core cracked or 
(A) which contains no tantalum. A comparison of the was penetrated by oxidation. The alloy 2 reactor core 
data in Table 11 for 100 hour rupture life shows an was removed from test, unfailed, after 760 hours of 
improvement of 82% at 1,000" c and of 100% at 800" c 35 exposure. The alloy 2 lost less than one third the weight 
for the preferred alloy (No. 2) containing 1.3% Ta over due to Oxidation Of the core than that lost by the com- 
the data obtained for the commercial alloy (A). The mercial 
rupture strength improvement is retained for 1 , ~  While a Preferred compositon range has been de- 
hours of life at which alloy N ~ .  2 can sustain an 85% scribed for the alloys, it will be appreciated that various 
greater stress at 1,OOO" C and 100% at 800" C. modifications may be made to these compositions with- 
The second preferred composition, alloy No. 1 in 40 out departing from the spirit of the invention or the 
Table I which contains only 0.5% Ta, exhibited a rup- scope Of the 
ture strength advantage of 45% to 51% for 100 hours at What is 
1m" and 800" C, respectively. Alloy 1 further showed 1. A ferritic steel alloy having improved high temper- 
a rupture strength advantage of 52% to 55% for 1,000 45 ature strength at temperatures to 190400 c, improved 
hours at the test temperatures of ~ , O O O O  c and 800" C, oxidation resistance to 1,150" C, and good cold form- 
respectively. ability consisting essentially of, in weight percents; 
weight loss of the alloys for a fixed exposure time using 
cyclic oxidation test conditions are shown in Table 111. 5o 
All the oxidation data are cyclic furnace test results for 
one hour at temperature for each cycle. 
TABLE I11 
RUPTURELIFE AND OXIDATION RESISTANCE 
IO00 hrs 100 hrs IO00 hrs 100 hrs 
Alloy life life life life 
10.7 1 4 4  2 8  17 2 
2 5.5 3 5  22.8 14.0 
A reactor. 
is: 
Comparisons of rupture life at temperatures and the 15.0% to 20.0% chromium, 2.0% to 4.0% aluminum, 
0.4% to 1.0% silicon, 0.470 to 1.0% titanium, 0.01% 
to 0.05% carbon, 0.4% to 1.5% tantalum and the 
balance iron. 
2. An alloy as claimed in claim 1 containing about 
18% chromium, 2% aluminum, 1% silicon, 0.4% tita- 
nium, 0.04% carbon, 0.5% to 1.3% tantalum and the 
balance Fe. ' 
3. An alloy as claimed in claim 2 containing about 
1.3% tantalum. 
Alloy Hrs Hrs mg/cm2 4. An alloy as claimed in claim 2 containing about 
1 2010 340 f 2  8 0.5% tantalum. 
2 5500 loo0 + 3  0 5. An alloy as claimed in claim 1 containing about 0.4 
6o to 1.0% nickel, 0.5% manganese, 0.02% phosphorous b 1x00 100 + 4  0 A 210 50 +8 0 
B 230 16 - 17 0 Failed at and about 0.01% sulfur. 
- ___ 
-- __ RupiKre Life Oxidation RssEE-  55 
800" c dt IOOO" c dt 600 hrs at 1140" C 
I O  Mn/m' 3 5 Mn/m2 wt change 
~ - - . _ ~ . _ _ _  
485 hours * * * * *  - _ _ _ _ _ _ _ ~ -  -_  _ _ _  
65 


Tantalum modified ferritic iron base alloys

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