US2498052A - Method of retarding erosion of gun barrels - Google Patents

Description

Feb. 21, 195% N. H. SMITH 2,493,052

METHOD OF RETARDING EROSION 0F GUN BARRELS Filed Jan. 5, 1946 Rounds Fired 0 60 I20 I60 240 360 420 480 540 600 660 720 780 840 900 I020 Rounds INVENTOR M6 HAL? 5M! W BY ATTORNEY Patented F eb. 21, 1950 METHOD OF 'RETARDING EROSION OF GUN BARRELS Nicol H. Smith, Philadelphia, Pa., assignor to the United States of America as represented by the Secretary of the Navy Application January 5, 1946, Serial No. 639,461

1 Claim. (01. 89-14) This invention relates to ordnance of the type which includes a rifled gun adapted to fire ammunition having a, rotating band, or equivalent structure, coacting with the gun rifling to cause rotation of the issuing projectile for stabilizing it in flight. More particularly, the invention relates to a novel method of prolonging the useful life of such guns in their use with ammunition of the type described.

An object of my invention is the provision of a simple, direct and thoroughly practical method for retarding the erosion of guns.

Another object of my invention is the provision of a method for retarding gun erosion which may be successfully practiced in connection with guns of a wide variety of sizes for automatic or single shot fire at high or low velocities.

As conducive to a clearer understanding of certain features of my invention, it may be noted that gun barrel erosion is a complex phenomenon involving a combination of numerous effects, predominantly thermal efiects, chemical and metallurgical effects, and mechanical effects, all of which contribute as the result of gun use to shortening the useful life of the gun. The individual intensities of the various effects are determined by such factors as barrel material, type and quantity of powder and type of projectile employed, rate of fire, and velocity of projectile.

As the result of high temperatures reached by powder gases, a thin layer of metal near the bore surface of the gun is rapidly heated to a temperature which under certain circumstances may exceed melting range of the metal. Even if the bore surface is not melted, the temperatures reached are such as to reduce the strength of the steel and facilitate its removal. In cases where gas leakage occurs, the material may be simultaneously melted and carried away by scouring action of the escaping gas.

At the high temperatures attained by the powder gases in the gun, the products of the powder explosion are chemically active. Furthermore, the raising of the bore surface temperature increases the reactivity of the gun material. As a result, there is a very complex set of reactions between the gun material and the powder gases. .The importance of chemistry in the erosion process has not as yet been fully evaluated, but the existence of a considerable variety of reaction products has been established.

. In firing a gun, the temperature attained by the steel bore surface is well above the alphagamma transformation point. A transformed surface layer frequently develops to some thou- .resides in the provision of a method for improvsandths of an inch thickness which differs markedly in its metallurgical characteristics from the underlying steel. This layer has lower strength, probably a lower melting point due to chemical effects, and is less resistant to erosion than the underlying steel.

There are several mechanical effects which are known to contribute to the erosion process. In

the first place, simple frictional or abrasive wear is a factor the importance of which depends on both design and materials. When the surface of a steel projectile bears on a bore surface of steel, galling may occur in addition to abrasive wear, with the consequent removal of small masses of metal from the bore surface. Both the abrasive wear and the galling tendency are affected by the'surface temperature attained in the bore. This temperature, at the point of rubbing,

is influenced by the instantaneous heating effect caused by friction.

It is also apparent that plastic displacement of metal adjacent the gun bore surface, by hammer- .ing band is accompanied by an upward displacement of theland surface ahead of it. In effect,

a waveconsisting of a depression of the land material near the origin of rifling and'a raising of material in front of this region proceeds down thebore as firing progresses.

Accordingly, another object of. my invention ing the resistance of guns to plastic deformation .and erosion normally produced by thermal, chemical and metallurgical, and mechanical effects combined.

These and other objects of the invention may be better understood by reference to the following detailed description and the accompanying drawing, in which 1 Fig. 1 is an enlarged cross-sectional view of part of a rifled gun with a projectile therein, illustrating the relation of the rifling to the rotating band on the projectile, and

Figs. 2 and 3 are graphs illustrating the comparison of the gun life in the practice of the present invention with the gun life in the practice of prior methods, with respect to changes in projectile velocity and changes in gas pressure in the gun, respectively.

Referring now more particularly to the practice of my invention, I provide a gun of any desired size, to be employed for automatic or single shot fire at high low velocity, having a bore surface lining of refractory metal, which may be chromium or other erosion resistant material, such as molybdenum, tungsten or tantalum in normal or in hardened condition, which is capable of withstanding gun temperature effects and chemical attack. The protective lining presents a grooved or rifled surface as a result, for example, of depositing the protective metal onto the bore surface of a rifled barrel, or-ofrifling the liner before or after it is placed in the barrel. According to my invention, I provide against rapid wear of this protective metal, due to mechanical effects such as friction, abrasion and galling, by bringing into immediate contact with the protective lining in the process of firing, only projectiles having a pre-engraved rotating hand or equivalent structure, so that the engraving is not performed by the action of the rifiing. The protective lining preferably is employed in such thicknesses as to insulate the underlying gun steel from heat of explosion to a measure which will prevent the, formation of a transformed layer in the steel.

' As illustrative of the practice of my invention, I provide a gun barrel l of steel which is hellcally grooved or rifled to provide internal lands H and grooves l2, the gun being electroplated at the bore surface with a thin adherent deposit [3 of substantially pure chromium deposited as by a standard commercial or U. S. Navy plating process, or by the method disclosed in a patent to Olin et al., No. 1,886,218, granted Nov. 1, 1932, for Gun barrel and process of finishing the same. The plating I3 is preferably though not necessarily equal to or greater than established minimum value of approximately 0.003 inch. For longest life, the minimum thickness of the chromium I3 is important in preventing the formation of transformed or alteredlayers in the underlying gun steel [0 normally caused by sudden heating in combination with chemical attack and subsequent cooling, and prevents undercutting and stripping and lessens cracking of the chromium. The lining l3 itself is more. reliably adherent and will endure longer when the gun steel I0, beneath it does not transform. It will be understood that it may be necessary to vary somewhat the effective minimum thicknessof the plating l3, depending upon such factors as the size of barrel l0, character or rate of fire, type of powder, and muzzle velocity of projectile to be employed with the plated barrel.

Although an improvement in the life of guns is brought about by chromium platingsor platings of other erosion resistant material, it has been found that when a substantial number of rounds have been fired usin standard ammunition, the stresses due to the usual engraving of the projectile by the rifling cause such platings to be destroyed mechanically. For example, a gun having a bore which is chromium plated, as described, and from which ball bullets, are fired, shows removal of chromium from the forcing cone, obliteration of the origin of rifiing, and removal of metal from the driving edges of the lands. It thus can be. seen that before long the plating [3 will be removed mechanically. The

removal occurs predominantly at the breech end of the gun where erosive influences of heat and chemical attack are most severe, leaving the underlying steel l0 inadequately protected.

I have discovered that much can be added to the useful life of guns bore-plated with chromium or other erosion-resistant material, by employing pre-engraved projectiles in such guns. More particularly, the projectile I4 is pre-engraved, as shown at l5, so that the engraving is not performed in the usual manner by the action of the rifiing lll2 on the projectile as it passes through the gun bore. It will be understood that the projectile I4 may be pre-engraved directly onthe, core itself, as in the case of small caliber bullets which do not have a rotating band as such, or the pie-engraving may be performed on a rotating band IE on the projectile. The preengraving may be effected in any desired manner, as by machining the projectile. The pre-engraved projectiles M are preferably so dimensioned as to form a somewhat loose fit with the plating l3 or to provide a small clearance, as for example, 0.006 inch on the diameter across gun lands. The loose projectiles further prevent mechanical effects on the chromium or other plate.

A phenomenal increase in-the erosion lifeof a caliber 0.50 machine gun barrel, fired under hypervelocity conditions, was obtained by the use of pre-engraved bullets in loose fitwithabore surface plate of chromium having about 0.003 inch thickness to prevent thermal alteration of the underlying steel. The height of plated lands from the bottoms of theplated grooves was approximately 0.010 inch, which is somewhat greater than that employed with respect to conventional caliber 0.50 machine gun lands. On the other hand, the individual width of plated lands in the gun approximated 0.060 inch, while the width of groove in the pre-engraved projectiles fired was about 0.071 inch. A clearance maintained betwen tops and bottoms of interfitting lands and grooves of bul let and. gun amounted to about 0.006 inch on the diameter. The clearance between lands of the bullet and grooves of the gun was made slightly gerater than the clearance between grooves of the bullet and 'landsof the gun, which as a general principle gives improved accuracy of fire. With these conditions, the plate was just beginning to chip off the lands when testing had been discontinued after 440 rounds. There was only a slight decrease in velocity. A similar barrel, with an unprotected steel bore surface, eroded to such an extent when copper banded projectiles without pre-engraving were fired with the same powder charge as used above, that velocity decreased 200 feet per second after only rounds.

Referring now to Fig. 2 I have shown a graph illustrating a comparison of the projectile velocity changes occurring in the use of a gun in accordance with prior practices, with the projectile velocity changes occurring in the use of a gun in accordance with the present invention, As there shown, the ordinates represent projectile velocity changes in feet per second, and the abscissas represent the number of projectile rounds fired from the gun. In Fig. 3, I have shown a graph similar to that in Fig. 2, except that the comparison is based upon pressure changes occurring in the gun barrel, the ordinates representing gun pressure changes instead of projectile velocity changes, On Figs. 2 and 3, theline (a) represents the changes in a gun having an unplated bore and using standardammunition which is not preengraved; the line (1)) represents the changes in a gun having an unplated bore but using preengraved ammunition; the line (0) represents the changes in a gun having its bore chromium plated to a thickness of .005" and using standard ammunition which has not been pre-engraved; and the line (:1) represents the changes in the gun chr0- mium plated as described above and using preengraved ammunition, in accordance with the invention. The lines (a), (b), (c) and (d) were derived from plots of values obtained from the actual firing of a standard type gun using standard type ammunition, under the conditions specified.

From an examination of Figs. 2 and 3, it will be seen that the conditions represented by the lines (b) and (0) improve somewhat the useful life of the gun over that which is normally obtained under the standard conditions represented by line (a). However, in the use of the gun according to the present invention, as represented by line (d), the improvement in the useful gun life is not merely an approximation of the algebraic sums of the improvements represented by lines (b) and (c), as might be expected, but is many times as great as the improvements represented by lines (17) and (0). Moreover, in the practice of the new method, the projectile velocity actually increases at first and remains greater than the normal velocity for a substantial number of rounds (180 as shown), and the pressure in the gun actually increases at first and remains greater than the normal pressure for a much longed period of use (780 rounds as shown).

Numerous results of other tests show that erosion resistant refractory metal linings on the bore surface of guns contribute much to the useful life of a gun, particularly when provided in such thickness as to prevent heat alteration of the underlying steel and when protected against mechanical wear and distortion normally caused by engraving stresses. Chromium, in particular, I find, is resistant to thermal and metallurgical efiects in guns. It is protected from wearing excessively due to mechanical influences when pre-engraved projectiles are used. Thus, it will be seen that in the present invention there is provided a method of prolonging the useful life of guns, in which the objects hereinbefore noted together with many thoroughly practical advantages are successfully achieved. It will be observed that the method alleviates many dilficulties encountered in repair and replacement of eroded guns and the associated problems of production and transportation.

It will be understood that while the use of chromium or other protective metal or metal alloy linings, such as plates, coatings or claddings which cover the entire bore surface of the gun, are preferred, relatively short linings covering the breech zone which is exposed to most severe thermal, chemical and mechanical attack may also be used with good results. The linings preferably are deposited electrolytically or by other depositing methods as from the metal carbonyl, but it will be understood that certain alternatives such as claddings, or removable sleeves may be employed where practical. It will also be understood that the linings, such as the platings, claddings, sleeves and the like, may be either composite or of all one metal.

In way of illustration, my method may be practiced in connection with guns which have been modified as by electropolishing or machining from standard finished dimensions to receive the necessar lining as by electroplating back to finished dimensions, as well as with respect to guns which have been made to finished dimensions initially-with the lining included.

In the following claims, it is to be understood that the expression plating the rifled bore of the gun is intended to denote the application, by plating, electroplating, coating, or other means, of a relatively thin layer of erosion-resistant material to the rifled bore, whether the rifled bore is in a liner or is directly in the gun barrel proper.

As many possible embodiments of my invention may be made, and as many changes may be made in the embodiments hereinbefore set forth, it will be understood that the matter described herein is to be interpreted as illustrative and not as a limitation.

I claim:

A method of prolonging the useful life of a rifled gun in the firing of projectiles, which comprises plating the rifled bore of the gun with an erosion-resistant material, and ore-engraving said projectiles to provide grooves and lands which fit loosely with the lands and grooves, respectively, on the plated, rifled bore surface of the gun, and with the clearance between lands of the projectile and grooves of the gun slightly greater than the clearance between grooves of the projectile and lands of the gun.

NICOL H. SMITH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,659,625 C'owan Feb. 21, 1928 1,777,519 Flowers Oct. 7, 1930 1,886,218 Olin et a1 Nov. 1, 1932

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