US3499802A

US3499802A - Ferritic,martensitic and ferriteaustenitic chromium steels with reduced tendency to 475 c.-embrittlement

Info

Publication number: US3499802A
Application number: US636107A
Authority: US
Inventors: Rune Gunnar Lagneborg
Original assignee: Sandvik AB
Current assignee: Santrade Ltd
Priority date: 1966-05-04
Filing date: 1967-05-04
Publication date: 1970-03-10
Anticipated expiration: 1987-03-10

Description

STEELS WITH REDUCED TENDE Filed May 4, 1967 R G. LAGNEBORG 3,499,802 OMIUM ITTLEMENT 3 Sheets-Sheet 1 FERRITE-AUSTENITIC can NCY TO 475 C.-EMBR I I I h l I v G.) g 57; cv l Go I ,29 I l I U) .E I I a:

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l I l l I o a R a a s 8 a Q March 10, 1970 R. G. LAGNEBORG 3,

FERRITIC, MARTENSITIC AND FERRITE-AUSTENITIG CHROMIUM -EMBRITTLEMENT lSgEEELS WITH REDUCED TENDENCY '10 475C 3 Sheets-Sheet. 2

Filed May 4,

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March 10, 1970 R. G. LAGNEBORG 3,499,802

FERRITIC, MARTENSITIC AND FERRITE-AUSTENITIC CHROMIUM STEELS WITH REDUCED TENDENCY T0 475C.-EMBRITTLEMENT Filed May 4, 1967 3 Sheets-Sheet 3 Fig.3

United States Patent M US. Cl. 14837 3 Claims ABSTRACT OF THE DISCLOSURE 475 embrittlement in ferritic chromium steels containing upwardly from 13% chromium is prevented by including in the steel composition an optimumrather smalladdition of aluminum or cobalt or both.

Ferritic chromium steels with chromium contents exceeding 13% show a tendency to so-called 475 C.- embrittlement. This embrittlement appears when these steels are being held at, or are slowly cooling down through, the temperature range of 400-550 C. This type of embrittlementwhich is clearly distinguished from the embrittlement caused by precipitation of sigma phase at higher temperatures up to 700 C.has been described e.g. in the following literature references:

1) Identification of the Precipitate Accompanying 885 F. Embrittlement in Chromium Steels, R. M. Fisher, E. I. Dulis, K. G. Carroll, J. Metals 197 (1953), p. 690.

(2) Further Studies of the Iron-Chromium System, R. 0. Williams, Trans AIME (1958) 497, vol. 212.

(3) Ueber das Verhalten von Stahlen mit 11 bis 20% Cr nach Auslagerung im Bereich der 475-Versprodung, E. Baerlecken, H. Fabritius, Stahl u. Eisen 78 (1958), p. 1389.

(4) Effect of 500 C. Ageing on the Deformation Behavior of an Iron-Chromium Steel Alloy, M. I. Marcinkowski, R. M. Fisher, A. Szirmae, Trans AIME 230 (1964), p. 676.

(5) Influence of Alloying Elements of the Impact Transition Behaviour of 12% Chromium Steels Aged at 900 F., E. J. Whittenberger, E. R. Rosenow, Trans ASM 48 (1956) p. 391.

(6) Metallography of an Iron-21% Chromium Alloy Subjected to 475 C. Embrittlement, M. J. Blackburn, J. Nutting, 1. Iron Steel Inst, 202 (1964), p. 610.

In these literature references some different theories are discussed regarding the cause of the phenomena which are usually designated 475 C. embrittlement. However, it cannot yet be considered that the cause of these phenomena has been made clear, nor is it considered possible to compensate foror to prevent-those phenomena by adding alloying components or in any other way. However, some of the investigations which are described or related in the above literature references indicate that the 475 C. embrittlement is connected with a precipitation of a ferrite, rich in chromium-probably, with about 80% chromiumin a miscibility gap in the system of ironchromium. The same phenomena of embrittlement appear in the strongly ferritic chromium steels; also, in chromium steels containing ferrite, e.g. ferrite-austenitic steels and such steels which contain a phase similar to ferrite, e.g. martensite or tempered martensite.

According to the present invention it has now appeared that the deterioration in properties, which follows a socalled 475 C. embrittlement of ferritic chromium steels 3,499,802 Patented Mar. 10, 1970 containing more than 13% and up to 18% chromium, preferably 15-17.5% chromium and steel of the corresponding composition containing a ferritic phase or a phase similar to ferrite, e.g. martensitic steels and ferriteaustenitic steels can be compensated for or prevented effectively by alloying the steel with aluminum and/or cobalt in an amount of at least 0.5% by wt., preferably at least 1% and especially at least 1.5% and at most 4.0%, preferably 3% and especially at most 2 to 2.5 wt. percent. The suitable upper limit of the proportion of aluminum and/or cobalt can be varied slightly depending on the remaining alloying constituents in the steel, and should in the cases of some martensitic steels be limited with regard to the capability as austenite formers.

The addition of aluminum and/or cobalt to ferritic chromium steels to compensate for or to prevent the deterioration of the properties of the steel which is connected with 475 C. embrittlement can be generally applied to steels which besides iron and chromium also contain other alloying components intentionally added or occurring as impurities, in so far as they do not abolish the advantageous effect of the addition of aluminum and/ or cobalt. As examples of such components, the following can be mentioned.

Carbon usually is a part of ferritic chromium steels to an amount of at least 0.0l -0.02% and in martensitic steels at least 0.15% by weight. The maximum carbon content varies with the chromium content and as for austeniteferritic steels it can be about 0.1%; as for ferritic steels, high in chromium, 0.20-0.25%, preferably 0.15%, and as for martensitic steels 0.5%, preferably 0.4% by wt., the upper limits usually being fixed by the risk of austenite formation at annealing temperature.

Silicon usually does not substantially impart brittleness, but it may influence the position of the phase boundary or the miscibility gap. The maximum silicon content can usually be chosen 0.6% and preferably 0.3%, the minimum content usually being 0.05%.

With regard to the risk of austenite formation the manganese content should be restricted to a maximum of 1.5%, preferably maximum 1% by wt. The content of nitrogen generally is at most 0.05%, but as for certain nitrogen-alloyed steels the content can amount to 0.20%. Average nitrogen content lies between 0.005 and 0.03% by weight.

Molybdenum improves the strength at elevated temperatures. The addition of molybdenum should be restricted to 3-4% as a maximum. As for ferritic steels and as for martensitic steels to a maximum 1.5% and minimum 0.5 respectively, 0.25% for ferritic respectively, martensitic steels.

Nickel may be included in varying amounts depending on the type of the steel. The nickel content in ferritic steels is usually defined to maximum 2.5%, preferably 1%, sometimes to 3.5%, the corresponding amount for martensitic and austenite-ferritic steels being maximum 2.5- 1%. However, nickel could also be eliminated from the composition.

Titanium, niobium and/or tantalum may be used in order to bind carbon and nitrogen as carbides and nitrides, preferably, up to a maximum content of 1% Ti and 1% Nb and/or tantalum.

Vanadium can be added to regulate the grain size preferably up to a maximum content of 0.5%. Tungsten can be added up to 1%.

According to the present invention an addition of aluminum and/or cobalt to steels inclined to 475 C. embrittlement compensates or prevents the deterioration of the properties of the steel which follows this phenomenon. Among the most important changes occasioned by 475' C. embrittlement should be mentioned an increased hardness,

some raising of the 0.2-limit, lowering the true tensile strength and fatigue strength, and especially decreasing contraction at fracture obtained in tensile test and a tendency to brittle fracture without necking. From the point of view of this application, this embrittlement is usually the most important change following the phenomena called 475 C. embrittlement and which appear in a temperature range of 400500 and especially 440-500 C., this range to a certain extent being dependent on the composition.

The contraction of steels inclined to 475 C. embrittlement usually is less than 15%. Steel with an addition of aluminum and/r cobalt according to the present invention usually shows a rather small decrease in contraction as measured on a static tensile test specimen and compared to non-aged condition, i.e. material which is not heattreated at 475 C.

In the following the invention will be described in more detail with embodiments referring to the appended drawing, in which FIGS. 1 and 2 are graphic charts illustrating the properties of the alloys of the present invention; and

Cir

alloy A. However, at times exceeding 200 hours, the yield strength is greater for the alloy B and after tempering for 1000 hours 0 is about 20 kg./mm. greater for B than for A. This shows that the 475 C. embrittlement cannot alone stand for the increase in yield strength.

Finally, in FIG. 3 the appearances of the breaks of the static tensile test specimens are shown, which specimens were used in the experiments according to the embodiment. The static tensile test specimens a-f were produced on alloy B with an addition of aluminum to prevent embrittlement according to the invention, while the test specimens g-m were produced on alloy B, which consisted of the same material but without this aluminum addition. The ageing times in hours at 475 C. were as follows: a, g=0, b, 11:10, 0, i=50, d, k=200, e, l=500, f, m=1000.

It is apparent from the figures that the test specimens of steel with addition of aluminum according to the invention showed a ductile break with a considerable contraction even at the longest ageing time.

As additional examples of the composition of similar steel alloys which are resistant to 475 C. embrittlement, the following can be mentioned:

0 Si Mn P max. S max. Cr A1 C0 N max Fe 0.05 0.3 0.3 0, 03 0.03 17.0 2. 5 0.03 Bal. 0.05 0.3 0.3 0.03 0.03 15. 0 2.0 a 0.03 Bal. 0. 05 0.3 0.3 0. 03 0. 03 17.0 a. 2.0 0. 03 Bal.

FIG. 3 is a photograph of specimens after testing.

Alloys with the following compositions were aged for FIGS. 1, 2 and 3 of WhlCh the results are related from 2000 hours and strll had the desired properties:

C Si Mn P max. S max. Cr Al N max. Fe

0. 036 o. 3 0. 3 0. 03 0. 03 16. o 2. 2 0. 055 Bal. 0. 033 0. 3 0. 3 0. 03 0. 03 16. 2 1. 0 0. 046 Bal' 0. 01s 0. 3 0. 3 u. 03 0. 03 14. 9 2v 9 0. 025 B211. 0. 02s 0. 3 0.3 0. 03 0. 03 15. 0 1. 1 0. 025 Bal.

experiments with two steel alloys A and B, both comprise 17.4% chromium and the latter also 1.9% aluminum. Both the alloys were produced in an induction furnace with an atmosphere of argon gas. The starting materials were two high-vacuum melted materials, to wit, pure iron and an iron-chromium alloy comprising 30% chromium. The chemical composition in percentage by weight of the steel alloys A and B is shown in Table I.

Regarding these four compositions, I have heat-treated the specimens at 1050 C. to achieve a grain size of 150p. which is most risky at 475 C. embrittlement. The contraction then was 4050%. The specimen (1) (C=0.036)=40% contraction (2) (C=0.033)=47% contraction (3) (C=0.018)=47% contraction (4) (C=0.025)=% contraction TABLE I Al dissolved in C M11 P S Or the steel N Fe A O. 007 0. 04 O. 003 0. 017 17. 1 0. 01 Bal. B 0. 006 0. 04 0. 009 0. 019 17. 1 1. 88 0. 03 B211.

Besides the contents stated, the alloys A and B contained only iron save for insignificant amounts of impurities.

The alloys were hot-worked to 15 mm. 5 and then coldworked to 11 mm. The grain size of the materials was 70a. Static tensile test specimens were produced and aged at 475 C. for periods of 10, 50, 200, 500 and 1000 hours. At the following tension test c true tensile strength, and contraction at fracture were determined.

The results for the alloys A and B are presented in FIGS. 1 and 2 of which the first shows contraction at fracture 4 in percent after ageing at 475 C., and the latter shows true tensile strength and a also after ageing at 475 C. As for the alloy A the contraction at fracture is rapidly decreased, and already after an ageing period of about 50 hours the fracture changes from ductile fracture to brittle fracture without necking. The contraction decreases insignificantly for alloy B during the first 200 hours and then it seems to assume a constant value of about 65%. FIG. 2 shows that the true tensile strength for the alloy A decreases rapidly with the tempering time which reflects the increasing tendency to brittle fracturewhile the alloy B does not show such a decrease. With short tempering times 0 is somewhat greater for the As examples of conventional steel alloys the resistance to 475 C. embrittlement of whichaccording to the present invention-can be increased by addition of aluminum and/or cobalt there can be mentioned steels with compositions corresponding to the standard specifications: A181 430, 434, 446, 442, 420, 431, 440A, 440B.

Examples of improved steels of the ferrite-austenitic type are the following:

C=max 0.15% (0.2%) Cr=18 a 2026% Si=01% Al and/ or Co =0.5-4%

l3-18% and preferably 15-17.5% chromium are objects meant to be exposed to the temperatures which cause embrittlement in a long time, as more than, 100 hours and particularly more than 1000 hours, especially to 400-550 C. and in particular 440-500 C. When used, the objects are also exposed to mechanical strain, eventually combined With oxidizing conditions.

The present invention is particularly applicable for objects of which it is demanded--when used-that the material has no tendency to brittle fracture.

Examples of applications are for purposes where there is demanded a contraction at fracture of at least 20% and preferablyv at least 30% based on experience. Among such purposes of applications are parts of gas or steam turbines of different kinds; jet engines; compressors; and similar (especially movable or rotating) parts of them, i.e. buckets and the like, especially if they are exposed during operation to a calculated tensile stress of at least 5 kg./mm.

Other examples are reaction vessels, conduits and similar articles which are used in chemical processes or the like and are exposed, or run the risk of being exposed, to temperatures within the range given above, especially if the material at the same time is exposed to tensile stresses of at least 5 kg./mm. i.e. tangential stresses in the tubeor container walls, caused by an inner gauge pressure.

Further objects are exhaust pipes of stationary and mobile establishments; heat exchangers; and similar articles, especially if they are exposed to stable or varying tensile stresses. These stresses usually are risky if they normally amount to at least 5 kg./mm. caused i.e. by an inner gauge pressure, by vibrations, etc.

According to the present invention, the steels to which aluminum and/or cobalt is added are usually suited for purposes of application as those mentioned above, of which a contraction at fracture (by tensile testing) of at least 20% and preferably at least 30% is demanded; also, when the steels are exposed to a temperature of 425- 550" C. during at least 200 hours and preferably at least 1000 hours. (Method for testing: By tensile testing of ferritic 17% Cr steel with a ferrite grain size 70;; aged at 475 C. during at least 200 hours and preferably at least 1000 hours a contraction of at least 30% must be measured out).

I claim:

1. Article of ferritic, ferrite-austenitic or martensitic chromium steel havinga structural condition produced by ageing at temperatures between 425 and 550 C. during more than 1000 hours and thereby shows a contraction of at least 30% at fracture as measured on a static tensile test specimen thereof and comprising, in percentages by weight, the following:

the total content of titanium, niobium, tantalum and vanadium being at most 2%, balance Fe and normal impurities.

2. Article according to claim 1, containing 14% of at least one member of the group consisting of Al and Co.

3. Article according to claim 2, containing 1.5-3.0% of said member.

References Cited UNITEDjftSTATES PATENTS 1,745,360 2/1930 Antoinette.

2,590,835 4/1952 lKirkby.

2,820,708 1/1958 Waxweiler -128 x 3,068,095 12/1962 i Anthony.

3,108,870 10/1963 Brady ...1 75 124 3,152,934 10/1964 Lula.

3,250,612 5/1966 Roy.

HYLAND BIZOR, Primary Examiner US. Cl. X.R. 75-126, 128