US3552938A - Security steel members with carbide inserts - Google Patents

Abstract

THE INVENTION RELATES TO A NOVEL STRUCTURE FOR SECURITY STEEL MEMBERS OF VASTLY IMPROVED TOOL CUTTING RESISTANCE, THE STRUCTURE CONSISTING OF A STEEL BAR HAVING A PLURALITY OF TUNGSTEN OR OTHER HIGH HARDNESS CARBIDES FILLED LONGITUDINALLY EXTENDING GROOVES, THE WHOLE BEING CLAD IN A STEEL SLEEVE AND BONDED.

Description

C. DRACA Jan. 5', 1911 SECURITY STEEL MEMBERS WITH CARBIDE INSERTS Filed Jan. 21, 1969 ATTORNEY United States Patent U.S. Cl. 29191.6 5 Claims ABSTRACT OF THE DISCLOSURE The invention relates to a novel structure for security steel members of vastly improved tool cutting resistance, the structure consisting of a steel bar having a plurality of tungsten or other high hardness carbides filled longitudinally extending grooves, the whole being clad in a steel sleeve and bonded.

The present invention relates to an improvement in the structure of so-called security steel members; security steel members comprising steel bars and flats of the type used in the construction of safety vaults, prison cells and the like. The invention also relates to the method for the production of security bars and flats presenting the abovementioned novel structural characteristics.

There are a few essential properties which steel members must exhibit in order to be considered suitable for security applications; from time to time, however, it is necessary to make certain adjustments in the minimum degree to which some of these properties must be exhibited in order to respond to changing conditions and particularly to the development of new or improved means and materials whereby to attack the security materials.

A case in point may be related to the development and widespread distribution of metal machining and cutting tools of vastly superior quality, wherein high hard-' ness metal carbides play an important role; whereas the hitherto used security steels had sufficient strength and hardness to resist prolonged and repeated attacks by means of the then existing tools, such as hack saws, files and the like; it was soon discovered that these cannot and will not resist to attacks by means of the new breed of carbide tools; the remarkable increase in the incidence of prison escapes and vault break-ins of the last few years is evidence of the obsolescence of existing security steel installations and proof of an acute need for a parallel improvement in the quality ofthe steel used in security applications.

Whereas it is generally recognized that security steels must be improved, it must also be recognized that such improvements must be directed to that particular property of the security steel members which has become insuflicient; this must also be achieved without loss of any of the other properties essential to security steel members. This means that the solution to the problem must result in a net improvement of the steel members ability to resist tool attacks and a retention of all other properties such as stiffness, ductility and others; this is precisely what the present invention promises to achieve. It must be remarked that in so doing, the present invention also results in producing some less essential improvement in most other properties such as stiffness as a result of which it may eventually be accepted by designers of security installations that where bars, in accordance with the present invention, are used, the diameter of each bar needs not be as large as in similar installations of the prior art.

The vastly improved saw resistance of the security bars of the present invention is achieved through an associaice tion of high hardness carbide and steel in the structural composition of the bars. It would be both impractical and uneconomical to make bars entirely from tungsten carbide; this material lacking the requisite bending and impact resistances. As a basis for comparison and for the purpose of illustrating the acute need for a bar material such as that of the present invention, it is useful to know that, on the average, it might take something like three thousand steel hack saw blades of standard quality to cut through a massive steel bar of Rockwell C hardness 62 to 65 and one inch diameter, whereas a single tungsten carbide saw blade is all that would be required to cut through the same bar in approximately one hour.

The carbide material of which use is made as inserts in accordance with the invention and to which reference is made throughout this disclosure is a product of powder metallurgy consisting of a sintered mixture of one or more refractory metal carbides with one or more of the iron group metals. The commercially available product usually consists of tungsten carbide and a nickel-cobalt binder, minor amounts of carbides of other refractory metals are often present particularly those of titanium and tantalium, but less frequently those of the other refractory metals such as columbium, molybdenum, vanadium, chromium, zirconium and hafnium. All such materials are characterized by their great hardness which is, in part, attributable to the hardness of the particular carbide as well as to the degree of compression prevailing at the time of the sintering. The practice of the invention is facilitated by the fact that the required carbide materials are generally available commercially; nevertheless, it must be understood that the invention also contemplates the possibility of utilizing mixtures of the carbides and binders and practicing the sintering of same in situ, i.e. in the cavities within which rods of sintered carbide would otherwise be inserted and secured by brazing.

Having so defined the nature and composition of the single most important product used in the practice of the invention and without which satisfactory results could not be obtained, namely the carbide; the invention proper will be disclosed in greater detail hereinafter with particular reference to the accompanying drawing of preferred embodiments, wherein:

FIG. 1 is a cross-sectional view through a round steel bar embodying the invention; and

FIG. 2 is a cross-sectional view through a flat security bar further embodying the invention.

In FIG. 1 it is seen that the improved bar consists of a core 1 of massive steel of the type commonly used in security applications, said core presenting a plurality of longitudinally extending peripheral grooves 2, each groove containing one, or more, continuous rod or insert 3 of carbide such as tungsten carbide, boron carbide, lithium carbide and the like as defined herinbefore. Over the bar 1 there is a close fitting tubular steel sleeve 4. The annular clearance space between the outside of the bar 1 and the inside of the tube 4 as well as the space remaining in each groove 2 after insertion therein of carbide insert 3, are entirely filled by means of a brazing metal composition.

Whereas the particular geometry and size of each carbide insert receiving groove 3 are factors of no importance, it is important that the number thereof exceed 3 and that these be uniformly distributed at equal angular intervals in the surface of the core 1.

The reason why the number of the grooves should exceed two can best be understood if one considers that any bar containing less than that number of grooves, i.e. two for instance, could be saw cut with relative ease and rapidity and without impinging upon the carbide inserts, to an extent such that the only metal remaining uncut would be the inserts and a narrow band of steel between them; such narrow band could not be sawed easily but would not need to be as it could much more easily be severed by applying a strong blow, as with a hammer. Where three inserts or more are used, the remaining steel after sawing through all the metal that can be sawed without touching the carbide inserts, would be a triangular portion having one insert at each apex; the area of such a triangle would obviously be too large to permit severing the bar by hammering.

The sleeve 4 provides the required confinement for the brazing powder used to bind the whole structure; sleeve 4 is also useful in providing protection against exposure to corrosive agents as well as in rendering the exact position of each carbide filled groove underneath, visually undetectable.

The universal principle according to which a chain canonly be as strong as the weakest of its links is worthy of consideration in the present instance; it could perhaps be stated as follows: In a security enclosure consisting entirely of security bars and security flats, the strength of the enclosure does not exceed that of the weakest of these. This therefore brings one to consider that regardless of the invulnerability of the bars of the present invention, most of its advantage would be lost if it were used in an assembly wherein the steel flats used as transverse members to secure the bars in parallel spaced apart relationship are not equally invulnerable.

In FIG. 2, it is observed therefore that the same principle applied to the production of a security fiat bar consisting of a fiat steel member 11 in the surfaces of which there are grooves 12 and carbide rods 13 brazed therein; a tight fitting steel sleeve 14 being adapted over the whole and secured in position by brazing.

The brazing methods and compositions are well known in the art and it is not intended that the present invention should be restricted to the use of a particular method or composition.

The net improvement in the security factor of jail bars and the like which result from the present invention can best be demonstrated and visualized from the following results of a number of tests on bars of various kinds:

The tests involved using a hack saw with a tungsten carbide rod like blade under 300 lbs. tension. In each case, the saw was used initially in a mechanical hack saw with a blade pressure of approximately 16 lbs., a stroke of 5 inches and frequency of 70 strokes per minute. The mechanical saw provided no lift of the blade during return strokes and no cutting fluid was used. Thereafter each test was completed by attempting the same manually.

TEST NO. 1

Tests conducted on a homogeneous steel bar of 62-64 Rockwell C hardness and 1 inch diameter.

Test results Machine cutting-blade penetration of 0.487 was obtained in one hour.

Manual cuttingbar was severed in 38. minutes.

N.B. Such bars are comparable to the best of the security bars commonly used in penal institutions and security vault construction.

TEST NO. 2

Test conducted on a bar similar to that used in test No. 1 but further presenting a flame sprayed tungsten carbide coating of 0.035 thickness.

Test results Machine cutting and manual cuttingpenetration of 0.417" was measured after 30 minutes of machine cutting followed by 10 minutes of manual cutting.

TEST NO. 3

Test conducted on a bar in accordance with the present invention similar to the bar used in test No. 1 but presenting a grade A (abrasive grade) tungsten carbide insert and a steel sleeve 0.060" thick.

Test results Machine cutting-complete penetration of sleeve was obtained in a few minutes but it took the remainder of 12 hours thereafter to penetrate a further 0.007" through the insert for a total penetration of 0.067".

On the basis of these figures, it is estimated that it Would take 72 hours to machine cut through a bar having four inserts.

Manual cuttingwas discouraged in this case but only after one hour and 7 blade breakages; penetration was of 0.012" through the first insert.

TEST NO. 4

Rod similar to that in test No. 3 but in which the insert was of tool grade tungsten carbide (grade B).

Machine cuttingno detectable eifect after 4 hours of mechanical cutting.

Manual cutting-no visible effect after one hour.

Tests No. 3 and 4 indicate that within the scope of the invention it is possible to arrive at making security bars of near absolute hack saw resistance or bars of lesser resistance, but invariably bars which could only be severed in considerable time even under the best conditions which are hardly those one would expect to find in jail cells and the like. In practice, the controlling factor in selecting between a bar of vastly improved resistance of test No. 3 and the almost perfectly safe bar of test No. 4, is one of cost; the abrasive grade (A) tungsten carbides being much less expensive than the costly tool grade (B) tungsten carbides.

The present invention also contemplates the possibility of realizing jail bars of the same general description by various other means such as, for instance, by using as core, steel bars in which the longitudinal grooves would have been previously formed by either machining, rolling extrusion and/or casting.

Although the use of preformed carbide rods is a convenient expedient, it would also be possible to make security bars of the invention by forming the carbide rods or inserts in situ, such as, for instance, by using pure carbides or sintered carbides in comminuted form with admixed brazing or sintering compounds or compositions, filling the grooves therewith and then binding the whole by practicing brazing or sintering.

Bars in accordance with the present invention should preferably be made individually in the exact lengths specified for each application, rather than be made in greater stock lengths from which bars of the required lengths could be cut; the costs and difficulties of cutting bars from prefabricated stock lengths thereof would be avoided if bars were made from precut materials. Cutting the steel core and steel sleeve presents no problem in such cases and trimming the carbide inserts also presents no difiiculties on account of the brittleness of the product which makes it relatively easy to simply break the slender rods.

For better results it is preferable to select the metal or alloy used for bonding the core, sleeve and carbide together. Among the metals or alloys which are known to have lower melting points than those of both the steel and the carbide and which have afiinity for each of these materials, in the sense that the molten phase thereof will wet both the steel and the carbide; copper and several of its alloys are such suitable brazing materials and these are to be preferred for their excellent thermal conductivities.

I claim:

1. Security steel members comprising a massive steel core, a tubular steel sleeve around said steel core, carbide filled longitudinally disposed grooves in the surface of said steel core; said steel core, tubular steel sleeve and carbide inserts being intimately bonded to one another.

2. Security steel members as claimed in claim 1, where- 5 in the said security steel members are security bars of circular cross-section and the number of said longitudinally disposed grooves is greater than two.

3. Security steel members as claimed in claim 2, Wherein said carbide is a sintered mixture of at least one refractory metal carbide and a metal binder selected from the iron group.

4. Security bar as claimed in claim 3, wherein said at least one carbide of a refractory metal is predominantly tungsten carbide.

5. Security steel members as claimed in claim 1, wherein said core, sleeve and carbide inserts are intimately bonded by brazing.

References Cited 5 UNITED STATES PATENTS 309,439 12/1884 Chillingworth et a1. 29191.6 504,431 9/1893 Green 29l91.6 3,421,866 1/1969 Palmer et a]. 29191.6

10 ALLEN B. CURTIS, Primary Examiner

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