Method Of Making Stainless Steel Containing Borides

Abstract

A method for making stainless steel containing small, uniformly distributed boride particles and the product of such process having an improved absorption cross-section for thermal neutrons, in which a stainless steel containing about 0.1 to 4 percent by weight boron after being prepared in a convenient intermediate form is heated at or just above a critical temperature of from 2,275.degree. to 2,325.degree. F. at least long enough for it to be heated throughout to temperature and then it is rapidly cooled. The intermediate form is then worked to effect substantially uniform distribution of the boride particles. When necessary to prevent sagging and tearing of the intermediate form while it is being heat treated, it is supported while at heat. After cooling the support is removed before working.

Description

BACKGROUND OF THE INVENTION

This invention relates to a method of making a stainless steel alloy, and, more particularly, to a method for making a stainless steel alloy containing substantial amounts of boron having an improved absorption cross-section for thermal neutrons, and the product of such process. It is to be understood that here and throughout this specification and claims, the term "boron" when it appears alone is used in its generic sense to include naturally occurring boron (which usually contains about 18 percent boron-10), natural boron enriched with boron-10, or boron-10. The present invention is applicable to stainless steels containing any one or more of those forms of boron.

It has hitherto been known that naturally occurring boron is a desirable addition to stainless steel for use in the fabrication of nuclear reactor control rods because it has a favorable absorption cross-section for thermal neutrons. Its isotopic form boron-10 has a substantially higher absorption cross-section. Control rods have been made of austenitic stainless steel containing about 0.1 to 2.0 percent boron, but they left much to be desired whatever the form of the boron because of the difficulty hitherto experienced in ensuring the formation of small enough boride particles.

In practice, when control rods containing boron are in use in a nuclear reactor, thermal neutrons effect a transmutation of boron-10 to helium. To the extent that the boron-10 present in the alloy is converted to helium, the control rod is weakened, and the useful life of such control rods depends upon the distribution of boron-10 in the alloy. The more uniform the distribution of a given amount of boron-10, the less adverse is the effect of such helium formation.

Usually, boron, when present in stainless steel, is in the form of borides having a more-or-less complex structure, depending upon the composition of the steel. In the case of a chromium-nickel stainless steel, particles comprising M.sub.2 B are formed in the steel matrix with M equal to varying amounts of the elements Fe, Ni and Cr. While the size of such particles alone does not determine the degree to which the distribution of the boron departs from the desired uniformity, it is evident that the larger the size of such particles, the more boron each contains, and the less uniform is the distribution of the boron in the steel alloy. That is to say, when the borides are large, the boron is segregated even though the particles themselves may be uniformly distributed in the steel alloy. And, as a result of such segregation, the formation of helium during exposure of the steel to thermal neutrons has a greater weakening effect because the voids formed by the transformation are larger than those formed from the smaller particles.

In recognition of the desirability of more uniform boron distribution in such materials, the users thereof have required that the boron-containing particles be small, less than a calculated length of about 5 microns as determined by the intercept method of counting borides in stainless steel. However, in practice, I have found that the size of the boride particles could not be controlled by means of conventional manufacturing practices to provide the desired small boron-containing particle size.

SUMMARY OF THE INVENTION

It is therefore a principal object of my invention to provide a method for making stainless steel containing boron in which the boron is more uniformly distributed than hitherto.

Another object is to provide a process which ensures the attainment of boride particle sizes consistently less than about 5 microns long or even less than about 2 microns long as calculated from intercept counting of borides in stainless steel, with the particles substantially uniformly distributed throughout the matrix of the steel.

I have discovered that when the stainless steel containing relatively large boride particles is heated to a critical temperature, the borides undergo a high temperature reaction such that when the steel is rapidly cooled from that temperature, very fine boride particles are produced. Then the steel is hot and/or cold worked to the size required for fabrication into the desired-end products, and the working serves uniformly to distribute the fine boride particles.

The process of the present invention is preferably carried out by starting with the stainless steel alloy in an intermediate form such as a billet of convenient size. The temperature to which the intermediate form must be heated throughout for the required reaction to take place is sharply critical. The minimum temperature required can vary in practice from about 2,275.degree. F. to 2,325.degree. F. depending upon such variables as the composition of the steel and the accuracy of the temperature measuring equipment used. But in practice, the required temperature is readily determined as will be more fully pointed out hereinbelow.

Care must be exercised when the steel is subjected to the high temperature treatment that the intermediate form does not sag or tear to such an extent as to be unsuitable for hot or cold working to a finished shape. When necessary to prevent such damage, the form can be supported in a suitable tray or by means of a canister which is removed before the intermediate form is worked.

BRIEF DESCRIPTION OF THE DRAWING

Further objects and advantages of the present invention will be apparent from the following detailed description thereof and the accompanying drawing in which

FIG. 1 is a view of a photomicrograph showing the microstructure of a stainless steel alloy containing 1 percent boron under a magnification of 500 times treated in accordance with the present invention;

FIG. 2 is a similar view at the same magnification of the same type of alloy but with 0.37 percent boron at an intermediate stage of the method of the present invention;

FIG. 3 is a similar view at the same magnification showing the microstructure of the alloy shown in FIG. 2 after completion of the method of the present invention; and

FIG. 4 is a similar view at the same magnification showing the microstructure of the alloy having the analysis shown in FIG. 2 but without the benefit of the treatment in accordance with the method of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In carrying out the present invention, a stainless steel alloy is prepared comprising in weight percent within the tolerances of good commercial melting practices:

Percent Carbon up to 0.25 Manganese up to 10 Silicon up to 2 Chromium 12-26 Nickel up to 22 Molybdenum up to 4 Copper up to 4 Aluminum up to 4 Titanium up to 1.25 Columbium up to 1.25 Nitrogen up to 0.7 Boron 0.1-4

the balance iron except for incidental impurities and such other elements as may be desired which do not detract from desired properties or objectionably interefere with the formation and substantially uniform distribution of fine borides. For example, if better free-machining properties are desired up to about 1 percent of the customarily used free-machining additives can be included. In this category, I include up to about 1 percent of one or more of the elements phosphorus, sulfur, selenium and tellurium.

In carrying out the present invention, a steel of the desired composition within the foregoing broad range is prepared utilizing conventional melting and casting techniques. In general, the composition of the steel and the shape and properties required in the products to be formed therefrom will dictate whether the alloy should be prepared using air melting techniques or whether a controlled atmosphere should be used. It can also be readily determined by those skilled in the art whether the alloy is to be melted and cast as ingots which are then worked into intermediate forms or whether such intermediate forms are to be directly cast from the melt. For the production of control rods containing boron for use in nuclear reactors, I prefer to melt the steel in a vacuum-induction furnace and then cast the melt into ingots which are in turn hot worked to a suitably sized billet as the intermediate form. Up to this point my process does not differ in any way from conventional practices.

Then, in accordance with my invention the intermediate form of the steel, which can be a billet as was seen, is heated to 2,275.degree. F. to 2,325.degree. F. or just above long enough for the billet or other intermediate form to be heated throughout. Then the billet is rapidly cooled.

This results in the formation of extremely small boride particles, calculated as being less than 5 microns long. In practice, particle sizes calculated to be about 2 microns or less can be consistently provided. Now the thus treated intermediate form is processed by hot and/or cold working to the finished form using conventional working practices. The boride particles retain the small size, but may become spheroidized and are uniformly distributed.

The mechanism by which such small boride particles characteristic of the present invention are formed is not fully understood. However, though I do not wish to be bound thereby, I believe at this time that a eutectic reaction takes place and thereafter when the steel is rapidly cooled the extremely small boride particles are formed.

It is to be noted that unless the temperature is carefully controlled and/or, depending upon the size and shape of the intermediate form, the form may not be fully self-supporting at the treating temperature. In that event, the intermediate form is supported mechanically while it is at the treating temperature. For this purpose, a tray or other suitable support means can be used. A preferred form of support is provided when the intermediate form is enclosed in a canister formed of a material such as iron or another steel which remains solid and self-supporting at temperatures somewhat above the treating temperature. The shape of the canister should conform closely enough to that of the intermediate form so as to be able to support the latter at the elevated temperature used. The canister and the form within it are then heated to at least about 2,275.degree. F. to 2,325.degree. F., and the whole is maintained at that temperature until the intermediate form is heated throughout to that temperature. Then the assembly is rapidly cooled, and the canister is removed by chemical or mechanical means. As before, the intermediate form is then hot and/or cold worked to the desired finished form.

EXAMPLE NO. 1

As an example of my invention, a billet was prepared by hot working from an ingot, formed in an air induction furnace, containing 0.039 percent carbon, 1.69 percent manganese, 0.83 percent silicon, 0.016 percent phosphorus, 0.006 percent sulfur, 18.77 percent chromium, 15.11 percent nickel, 0.20 percent molybdenum, 0.17 percent copper, 1.00 percent boron, and the balance iron except for incidental impurities.

The thus formed billet after surface preparation, e.g. machining, was entirely enclosed in a canister formed of A.I.S.I. Type 304 stainless steel, and the assembly was heated at a temperature of about 2,310.degree. F. for 1 hour which was long enough for all the billet, about a 1.7 inch round in transverse cross-section, to be brought to that temperature throughout. Whereupon the assembly was rapidly cooled by quenching in water, and then the canister was removed from the billet by machining. In this condition, the boride particles have the desired small calculated size of less than 5 microns but are not usually uniformly distributed as is also required.

The billet was then forged to 11/8 inch square from a furnace temperature of 2,100.degree. F. followed by hot rolling to a 7/16 inch round cornered square rod, also from a starting temperature of 2,100.degree. F. Then, by cold rolling, the 7/16 inch round cornered square rod was then reduced to 3/16 inch strip. In FIG. 1 there is shown a photomicrograph of the structure of a portion of the resulting strip at a magnification of 500 times which can be compared to the structure of similar material treated in the conventional way and illustrated in FIG. 4 yet to be described in detail hereinbelow. Using a standard metallographic inspection procedure for intercept counting of borides, the boride size of Example 1 as shown in FIG. 1 was calculated to be less than 1.79 microns in radius.

EXAMPLE NO. 2

As a further example of my invention, a billet was prepared and treated as was described in connection with Example 1 except as follows. The billet had an analysis containing 0.035 percent carbon, 1.73 percent manganese, 0.53 percent silicon, 0.013 percent phosphorus, 0.008 percent sulfur, 18.57 percent chromium, 14.11 percent nickel, 0.11 percent molybdenum, 0.05 percent copper, 0.37 percent boron, and the balance iron except for incidental impurities. The microstructure of the billet at this stage showing the undesirably larger borides is shown in FIG. 4 at a magnification of 500 times. The billet was 1 in. .times. 2 in. .times. 6 in., was not enclosed in a canister and no special support was provided, and was heat treated at about 2,300.degree. F. for 1 hour followed by cooling rapidly by quenching in water. The resulting microstructure as shown in FIG. 2 at a magnification of 500 times is seen to contain the desirably small boride particles, but without the effects of working which bring about the desired more uniform distribution and also tend to spheroidize the particles.

Following the step of quenching in water, the billet was hot worked to one-fourth inch thick from a furnace temperature of 2,100.degree. F. and then cold rolled to 0.038 inch thick strip. A photomicrograph was prepared showing the resulting structure, also at a magnification of 500 times, and is shown in FIG. 3.

A comparision of FIGS. 2, 3 and 4 clearly shows the significant reduction in the size and improved distribution of the boride particles provided by the present invention. Though no special precautions need to be taken in carrying out my process, it is to be noted that the temperature at which the heat treatment is carried out is critical and relatively narrow. For any given composition the optimum temperature is readily determined. Test specimens of the desired composition are heated long enough for the high temperature reaction to take place at selected temperatures until substantially the minimum temperature for the reaction is found. Then an upper limit is determined by examining the effects of high temperatures on different specimens. For example, in the case of the composition of Examples 1 and 2, it was found that the heat treatment had to be carried out between about 2,300.degree. F. and about 2,340.degree. F. because at lower temperatures below about 2,275.degree.-2,300.degree. F. the desired reaction did not occur and above about 2,340.degree.-2,365.degree. F. the formation of shrinkage voids became objectionable and resulted in unsound material. When 1-inch cube specimens having the same composition as the billet of Example 2 were heat treated at about 2,200.degree. F. for 1, 4 and 8 hours, there was no apparent effect upon the size of the boride particles.

While a wide variety of compositions falling within the broad range hereinabove stated can be used in carrying out the process of my invention, the compositions of Example 1 and 2 are illustrative of my preferred range which consists essentially of, in weight percent, up to about 0.03 to 0.08 percent carbon, up to about 2 percent manganese, up to 0.045 percent phosphorus, up to 0.03 percent sulfur, up to about 1 percent silicon, 17 to 20 percent chromium, 7 to 15 percent nickel, 0.1 to 2 percent boron and the remainder iron except for incidental impurities. Within that range, nickel is included in an amount of at least about 12 percent when its effect on corrosion resistance and other properties is desired.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

Claims

I claim:

1. In the method for making stainless steel articles containing substantially uniformly distributed fine boride particles, the steps of making an intermediate form comprising in weight percent about

2. The method set forth in claim 1 in which the longest dimension of said boride particles as calculated by intercept counting is less than about 5 microns.

3. The method set forth in claim 1 in which the intermediate form is supported while being heated to prevent sagging and tearing.

4. The method set forth in claim 1 in which said intermediate form prior to heating is enclosed in a canister formed of a material which can support said intermediate form therein during said heating.

5. The method set forth in claim 4 in which said intermediate form is removed from said canister after cooling and before said working.

6. The method set forth in claim 5 in which the stainless steel article has an essentially austenitic microstructure and comprises in weight percent about

and the balance substantially iron.

7. The method set forth in claim 6 in which the longest dimension of said boride particles as calculated by intercept counting is less than about 5 microns.

8. The method set forth in claim 6 which comprises at least about 12 percent nickel and no more than about 2 percent boron.

9. The method set forth in claim 8 in which said assembly is heated to a temperature no higher than about 2,340.degree. F. to 2,365.degree. F.