US3043680A

US3043680A - Method of preventing carburization in molten metals

Info

Publication number: US3043680A
Application number: US854601A
Authority: US
Inventors: Walter C Hayes; Orson C Shepard
Original assignee: North American Aviation Corp
Current assignee: North American Aviation Corp
Priority date: 1959-11-23
Filing date: 1959-11-23
Publication date: 1962-07-10
Anticipated expiration: 1979-07-10

Description

United States Patent tanford, Calif., assignors to North American Aviation,

No Drawing. Filed Nov. 23, 1959, Ser. No. 854,601 Claims. of. 75-66) Our invention relates to a method of protecting metals from carburization in liquid metal coolants.

Liquid metals have excellent heat transfer characteristics, and are employed as coolants in industrial processes and in nuclear reactors. The liquid metals will, in service, take up carbon from' various sources, such as graphite, hydrocarbons and certain carbon-containing steels. Metal compositions containing carbide-forming elements become carburized and damaged when exposed to carbon-contaminated liquid metals. Heretofore, in order to use stainless steels in sodium systems, elaborate precautions have been necessary to limit the carbon concentration of the sodium. For example, the Liquid Metals Handbook, Sodium-NaK Supplement ('1955), page 177, states:

The carburization of stainless steel is a cumulative process, and the total amount of hydrocarbons that can be added to a system during its lifetime must be restricted on the basis of exposed area and allowable depth of carburization. In one application, a total of 0.0002 lb./ft. of exposed steel area has been set as a limit for carbon addition of any kind to the system. Prior removal of trace hydrocarbons from the cover gas is important, since, unlike oxygen, removal during operation is not feasible.

An object of our present invention, therefore, is to provide a method of protecting structural metals from carburization.

Another object is to provide a composition for removing carbon from a liquid metal coolant system.

Another object is to provide a method of protecting alloy steels from carburization in liquid metal systems.

Still another object is to provide a low cost gettering composition for protecting stainless steel from carburization in sodium.

The above and other objects and advantages of our invention will become apparent from the following detailed description taken together with the accompanying claims.

In accordance with our present invention, metals may be protected from carburization in a liquid metal system containing carbon by placing in said system a metal having a low carbon activity. 7

The materials which may be used in our invention as getters for protecting all-0y steels from carburization are those metals which have a lower carbon activity than iron. Metals with low carbon activity have a strong affinity (high driving force) for carbon and form stable carbides relatively rapidly. Iron itself is substantially neutral with respect to carbon in carbon-saturated sodium.

It has been found that the carbon activity level, rather than the carbon concentration, is the controlling factor in carbon diffusion. Thus, between steels initially satuice iron, and which may be satisfactorily used in our invention for carbon gettering, and for protecting alloy steels from carburization, are manganese, chromium, molybdenum, tantalum, vanadium, niobium, and titanium. These elements will preferentially form more stable carbides than iron.

Our gettering compositions operate effectively in all liquid metals and are not limited to use in any single liquid metal or eutectic system. Examples of the liquid metals are sodium, lithium, tin, bismuth, and bismuthtin.

The foregoing low carbon activity metals may be used singly or in combination for service in gettering carbon and inhprotecting alloy steels from carburization. We found, however, that surface layers of carbide or oxide form on the pure metal or metal combinations which act as diffusion barriers, and tend to progressively decrease the effectiveness of the gettering action. We have discovered that such effects can be avoided by distributing the gettering element in a diluent material. This allows further diffusion of carbon into the composition, formation of surface carbide diffusion barriers is avoided, and the gettering action is not impaired with the passage of time. As an example of such behavior, we found that chromium metal, which has a very low carbon activity, is not as effective in protecting alloy steel from carburization as is chromium alloy.

The criteria for a satisfactory base or alloying material include a fairly high rate of-c'arbon diffusion therein, solubility for carbon and getter, and a neutral or slightly lower activity relative to carbon. The solubility requirement is important because if a continuous carbide layer is formed on the surface of the getter, we have found that it forms a diffusion barrier to further absorption of carbon, and hence the effectiveness of the getter would rapidly diminish after a short time. Among the practical, commercially-available diluents are iron and nickel; alloys of these metals with the low carbon activity metals comprise useful hot trap carbon getters.

However, even in a diluent material, the effective range of the alloy constituents varies with concentration. Below about 5 weight percent total alloy additive concentration, carbon gettering is low; between about 5-30 weight percent total alloy in either nickel or iron base getters, efficient carbon gettering is achieved; but beyond about 30 weight percent the effectiveness again drops. The drop off beyond 30 Weight percent may be due to the formation of nndiffusible surface layers of metal carbides, or to the formation of intermetallic compounds rated with respect to carbon, carbon diffuses down an which act as abarrier to carbon diffusion to the interior. For example, the effectiveness of chromium alloys as hot trap carbon getters increases with increasing chromium content and then decreases. The optimum chromium range within the 5-30 weight percent range is about 912 weight percent, and a very effective alloy is a 9 Cr-l Mo ferri-tic steel.

The effective ranges of the low carbon activity metals in iron and nickel base alloys are indicated as follows. The effectiveness and component ranges of our gettering compositions are not significantly influenced by the particular liquid metal from which carbon is being removed, or by the diluent base.

The alloy constituents, Within the above concentration Remainder essentially Fe Quarternary Alloys Remainder essentially Ni Quaternary Alloys Nb 0.5-10 V 0.5 V 0.5- Ta 0.5- 5 MO 0.5-10 Ta 0.5- 5 CI 0.5 MO 0.5- 5. Ti 0.5-10 Ti 0.5- 3 v 01s- 5 Mo 0.5- 3 Ta 0.5-10 Mo 0.5- 3 V 0.5- 5

Remainder essentially Fe Our gettering material may very satisfactorily be used in a hot trap in an operating liquid metal system. The hot trap would be of conventional design and contain thin sheets or ribbons of the getter in a large surface-tovolume ratio. The material is fabricated in the form of a cartridge which can be inserted and removed from the hot trap. There could be full liquid metal flow through the hot trap, or, more likely, the hot trap would be situatedin a bypass loop of the system. The temperature in the hot trap is desirably maintained at a higher temperature than the liquid metal system, for instance about 150- 400 F..higher, in order to hasten the rate of reaction with the getter materiaL. In a sodium-cooled nuclear power reactor, the current designs of which operate at peak temperatures of about 1000 F., the side stream hot trap getter would, for example, satisfactorily operate at about 1200 F.

The following examples are ofiered to illustrate our carbon gettering invention in greater detail. Metal samples 0.060 inch thick were exposed for 200 hours in 1300 F. sodium saturated with carbon. The metals and test results are shown below.

spaaeeo e 3 4 range for each metal, may be used in any combination in binary, ternary, and quaternary iron or nickel base alloys, Carbonyl carbon in provided the total alloy content does not exceed about Mammal Percent) ggg g gg igggg weight percent. Examples of binary alloys would be any (wt. Percent) (wt. Percent) of the foregoing metals in the indicated concentrations 111 5 iron or nickel. Examples of ternary and quate nary (1L5 (lg-0.551%; .25 r O. 0- nickel and non alloys weight percent) are as follows. 225 CH Mom 0.12 1. a as ran-:2 5 r-0. 0- 1 Ternary Alloys 10 5 (Ir-0.5 Mo-Si 0 o 1-1. Cr 0.54 M 5 lgi i ifiifj 8:13 litfi M0 5 5 Ta 0,5 15 304 stainless stce 0.06 0. 72-0.

12 Or (410 stainless steel). 0.10 1. -1. Cr T1 5 1? 8r 430 stainless steel) o. 1.

r 0. 0. 7 Ta 0.5 25 V -5 3 or nil OOHM M0 -5-1 T3- 15 Z 11 O Q5-{) (]G Remainder essentially Ni ff 8-8; 5:3 Ternary A [lays LgRtjmaiu der essentially Fe) 0 03 1 08 I I t 21.6 (Dr-2.2 M0 0. 07 1.8 Mo 05-10 V 05-10 11.5 gr1.3 %o-l.l Ti 10.3 r-0.5 1 0 Ta 8'258 t 8? 3 a; a; C1- a 2. o- .5 a5. Cr Mo 5 5 (Remainder essentially Ni) V 0.5-10 Ta 0.5-10

It should be appreciated that the foregoing examples are illustrative rather than restrictive of our invention. Our invention should be understood, therefore, as being limited only as is indicated in the appended claims.

We claim:

1. A method of preventing carburization of an alloy steel in a hot molten metal system selected from the group consisting of sodium, lithium, tin, bismuth, and bismuth-tin, which comprises placing in said molten metal an alloy consisting essentially of approximately 5-30 weight percent of at least one metal selected from the group consisting of manganese, chromium, molybdenum, tantalum, vanadium, niobium, and titanium, and the remainder at least one metal selected from the group consisting of iron and nickel, thereby gettering carbon from said molten metal in said alloy.

2. The method of claim 1 wherein said first-named metal in said alloy consists of about 9-12 weight percent chromium.

3. The method of claim 1 wherein said alloy is a ferritic steel containing about 9 Weight percent chromium and about 1 weight percent molybdenum.

4. The method of claim 1 wherein said alloy, in a form having a high surface are'a-to-volume ratio, is placed in a hot trap, said trap being maintained at a temperature of about ISO-400 F. higher than the portion of said liquid metal system outside of said hot trap.

5. The method of preventing the carburization of an alloy steel structural material in a liquid sodium system containing carbon at a temperature of about 1000 R, which comprises placing a ferritic steel containing about 9 weight percent Cr-1 weight percent M0 in a high surface area-to-volume ratio in a side stream hot trap maintained at a temperature of about 1200 F., and passing said sodium through said hot trap, thereby gettering carbon on said ferritic steel.

References fiited in the file of this patent UNITED STATES PATENTS Sibert Dec. 21, 1954 Olsen Nov. 12, 1957 Batutis et a1 Mar. 24, 1959 OTHER REFERENCES Bain: Alloying Elements in Steel, A.S.M. 1939, page 75 relied on, American Society for Metals, Cleveland, Ohio.

Brick and Phillips: Structure and Properties of Alloys, 2d edition, McGraw-Hill, New York, 1949, page relied on;

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,043,680 July 10, 1962 Walter C. Hayes et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, lines 9 to 50, the table should appear as shown below instead of as in the patent:

TERNARY ALLOYS 5-25 Cr; 0.5- 5 Mo Remainder essentially Ni 5- 5 M; 0.5-15 Ta -25 Cr; 0.5- 5 Ti 5-25 Ta; 0.5- 3 V 5-10 M0; 0.5-15 Ta 5-1O M0; 0.5- V Remainder essentially Fe 5-20 Ta; 0.5- 5 Ti 5-20 Cr; 0.5-10 Ta 52O Cr; 0.5- 5 M0 5-10 V 0.5-10 Ta 0.5- Cr; 0.5- 5 Mo; 0.5-10 Ta Remainder essentially Ni 0.5- 5 M0; 0.5-10 Ta; 0.5- 5 Ti 0.5-15 Cr; 0.5- 5 M0; 0.5- 3 Ti 0.5-l5 Cr; 0.5- 5 M0; 0.5- 3 V 0.5-15 Mn, 0.5-10 Ta; 0.5- 3 Ti O.5*-10 T1 0.5-10 Ta; 0.5- 3 V 0.5-15 Cr 0.5- 3 Ti 0.5- 3 V QUATERNARY ALLOYS O.5-l0 Nb; 0.5- 5 V 0.5- 5 Ta Remainder essentially Fe 0.5-10 V 0.5-10 Ta; 0.5-- 5 M0 0.5-10 M0; 0.5- 5 V 0.5- 3 Ti 0.5- Cr; 0.5- 5 V 0.5- 3 M0 0.5-l0 Ti; 0.5- 5 Ta; 0.5- 3 M0 Signed and sealed this 29th day of January 1963. (SEAL) Attest ERNEST W. SWIDER DAVID L LADD

Claims

1. A METHOD OF PREVENTING CARBURIZATION OF AN ALLOY STEEL IN A HOT MOLTEN METAL SYSTEM SELECTED FROM THE GROUP CONSISTING OF SODIUM, LITHIUM, TIN, BISMUTH, AND BISMUTH-TIN, WHICH COMPRISES PLACING IN SAID MOLTEN METAL AN ALLOY CONSISTING ESSENTIALLY OF APPROXIMATELY 5-30 WEIGHT PERCENT OF AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OF MANGANESE, CHROMIUM, MOLYBDENUM, TANTALUM, VANADIUM, NIOBIUM, AND TITANIU, AND THE REMAINDER AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OF IRON AND NICKEL, THEREBY GETTERING CARBON FROM SAID MOLTEN METAL IN SAID ALLOY.