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Concluding Remarks Gun tube erosion can set a limit to the development of more desirable weapons systems. Two things should be done if this limit is to be avoided. Research drawing on existing technology can provide materials better suited to withstand the severe mechanical, thermal, and chemical environment in a gun tube. Longer term, more basic studies can develop, at least empirically, pre- dictive models for the erosion process. The ability to predict offers the hope of being able to propose solutions. The Committee was impressed by the quality of the investigations con- ducted on the many facets of the gun tube erosion problem in the past. However, it appears obvious that the problem of defining the mechanism of erosion cuts across the missions of the various Army arsenals. It is therefore felt that future effort in this area would be more effective if a single leader and a single source of funding devoted exclusively to this technology area be designated. DESCRIPTION OF GUN SYSTEM The components of the gun system included in this report are primarily the gun barrel, the projectile and the propellant charge. Nomenclature is indicated in the first two figures. A gun barrel for fixed ammunition is shown in Figure 1. Rifling consists of helical grooves cut in the bore of the tube extending from the forcing cone to the muzzle end. The ridges between the grooves are known as lands. The forcing cone includes the origin of rifling where the grooves begin, and the commencement of rifling where the lands reach full height. Nomenclature for the various regions of projectiles and complete rounds of ammunition is given in Figure 2. In addition cannelures, partially visible under the grommet "O", are ringlike grooves cut in the rotating band in order to lessen the resistance offered to the rifling as the projectile begins to move along the

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Classes of Ammunition A-FUZE • -BOOSTER C-FUZE WELL LINER D-SHELL E-OGIVE F-BOURRELET G-BURSTING CHARGE H- ROTATING BAND I-CRIMP 1- BASE COVER K-CARTRIDGE CASE L-PROPELLING CHARGE M-PRIMER N-LIFTINGPLUG O-GROMMET P-COTTER PIN WITH PULL RING Q-IGNITER FIXED AMMUNITION SEMIFIXED AMMUNITION M SEPARATE.LOAMNG AMMUNITION RA PD 40671A Figure 2 Types of Complete Rounds of Artillery Ammunition

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10 gun tube. Also shown in Figure 2 are propellant charges and primers. Around is the short name for all components of ammunitions necessary to fire a gun once. Erosion is the enlargement and wearing away of the bore interface of gun barrels by the movement of high temperature gases and residues generated from the burning of propellant powder, by chemical action between constituents of propellant powder gases and gun material, and by friction between projectile and bore interface. Erosion is most often measured by the change in dimensions of the bore. The wear is frequently given as the change in diameter rather than change in radius. Erosion is seldom uniform around the circumference of the bore or along the length of the bore. Especially in large separate loading guns, more erosion tends to occur at the twelve o'clock position in the region of the origin of rifling than at the six o'clock position. Erosion tends to be worst at the vicinity of the origin of rifling and decreases as the distance increases from the origin of rifling until about mid-length. Erosion then begins to increase slightly again as the muz- zle end is approached. If the projectile cants too much, the bourrelet bears on the same group of lands along the entire length of the gun. This may cause eccentric wear of the lands in particular and this is usually very noticeable at the muzzle end. Erosion has a deleterious effect on the performance of the gun, finally limiting its life. The muzzle velocity, the range, and the accuracy of the weapon are reduced, the variation in muzzle velocity and the variation in range are increased; in extreme cases fuses are damaged and malfunction, also bands on pro- jectiles are stripped. The extent of erosion is sometimes expressed in terms of extent of the drop in muzzle velocity or loss in range. The erosion limited life of guns is the number of rounds of ammunition that can be fired until any one of these characteristics becomes unsuitable for service. It is claimed that in a well de- signed gun system all these characteristics should reach condemnable limits at about the same number of rounds.

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11 Fatigue considerations affecting life of cannons, and stripping of lands, are not within the context of this report. HISTORY OF EROSION AND EROSION CONTROL Erosion is attributed to many chemical, mechanical, physical and thermal occur ranees. Very hot gases melt the gun steel and the molten metal is blown away. The constituents of the propellant powder gases react with the steel and diffuse into the metal. Compounds and eutectics of iron are formed. These are brittle and some have low melting points. The transformation temperature of the gun steel is altered and the gamma phase is stabilized at low temperatures. The volume changes of the bore surface metal due to alternate heating and cooling with each round leads to thermal cracking of the interface, so-called heat-checking. The surface of the bore is roughened and friction increases between not only the flowing gases and the gun steel but also the projectile and the barrel. The rotating band wipes off brittle layers, heat softened layers and molten layers. In addition volatile compounds are formed with the steel and carried away by the gases. The bore interface of a slightly worn 5" gun and that of an extremely worn 5" gun in the region of the origin of rifling are shown in Figures 3 and 4. The heat check system on lands and grooves is shown in greater detail in Figure 5. The cartridge cases used in these guns act as a rear seal and protect the chamber from the propellant powder gases. In separate loading guns, the walls of the chamber become heat checked. The loss of metal in the vicinity of the forcing cone results in the forward movement of the place where the projectile seats, known as the advance of the forcing cone. There are two consequences, the en- largement of the volume of the chamber, and the increased difficulty to seal the forward end of the chamber against escape of gases because of the roughened and uneven surface. Both of these effects contribute to loss of velocity and range. The problem is to eliminate or decrease erosion in the vicinity of the origin of rifling without shifting the region of fast erosion to another location.

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14

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15 Past approaches to the control of erosion may be grouped under five headings as follows: 1. The Gun Barrel System 2. The Projectile System 3. The Charge Assembly 4. Wear Reducing Additives 5. Firing Conditions No one is independent of the others.

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16 1. The Gun Barrel System The material is the important factor in the gun barrel system. Design of rifling, forcing cone and centering cylinder are critical considerations. The change from cast iron to steel for gun barrels took place over a twenty- year period ending in 1833. Since then the quality, strength and fracture toughness of gun steel have been improved on an industry-wide scale. This improvement still continues today with the introduction of new steel making processes. But the melting point of the gun steel has not changed much. One important aspect that has not received enough attention is the formation of heat checks on the bore interface and roughening of the surface. Decoppering agents are claimed to increase the rate of growth of these thermal cracks and to increase the rate of wear. The life of a 3 inch trial gun was doubled by elimina- ting the decoppering additive from the ammunition. In 1944 during W. W. II, Stellite 21 was developed for machine guns under the auspices of the National Defense Research Council /N. D.R. C.) in order to overcome the lack of ductility of Stellite 6 which was found to resist erosion but 2 3 cracked in testing. ' Stellite 6 was tested along with 18-4-1 high-speed steel and two types of die steel, which three steels were found unsatisfactory. The Stellites were tested especially because of their hot hardness. All efforts to improve Stellite 21 by raising its melting point were fruitless. Failure of the experimental alloys was by cracking. Stellite 21 liners were successful in the caliber . 50 chromium plated and nitrided machine gun barrels. It was not successful in the caliber . 60 machine gun, nor in the 37mm gun. 1 3 Nickel base hot hard alloys failed because of poor erosion resistance. ' Low and high alloy steels have been evaluated in many different ways for erosion resistance. In general the higher the alloy content, the lower was the resistance, except for low carbon heat treatable intermediate chromium content stainless steels.

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17 It has long been thought that the simplest approach to eliminating erosion would be to use inert metals of high melting point. Four such metals were always selected for trial, namely tungsten, molybdenum, chromium and tantalum. Con- siderations of cost and availability indicated the use would be as coatings or liners, if at all. 2'3 Molybdenum base and chromium base hot hard alloys were developed during 2 W. W. II by the N. D. R. C. The resistance to erosion was evaluated as adequate. Following W. W. II the development of these alloys was continued by the Department 4 5 of Defense. * After a decade or more of intensive effort no alloy suitable for cannon was developed. The fracture toughness and ductility were inadequate. "... Alloys containing up to 50% chromium have potentially good ductility in sections up to at least 1.125 in. square when cast as 4-in. or smaller ingots. Above about 40% chromium, the alloys appear to become more sensitive to direc- tional factors, and the best combination of longitudinal and transverse ductility in the high-chromium alloys depended on the amount of reduction during hot working and the method. Generally, ductility increased with increasing amounts of hot re- duction, and forging (rather than rolling) gave optimum ductility. Ductility in sec- tions larger than 1.125 in. square appears to require a fine grain size in larger ingots or a method of hot working to refine the grain. The impact transition tem- perature of a 50% chromium, 50% iron alloy (based on an average of 15 ft-lb. in a standard V-notch Charpy test) was found to be approximately 180°C. (356°F). " As a result of these characteristics, in spite of a very large program supported by the U.S. Army Ordance Corp. and promising results on laboratory samples, it was not feasible to produce large sections with a fine grain size. Therefore, the ductile/brittle transition temperature was always above ambient. The alloys may have been inherently erosion resistant, but never survived in the gun barrel long enough for one to find out.

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25 by wear further down the barrel. Soft iron wire rotating bands welded in place have also been studied. Plastic rotating bands have been tried in both machine guns and cannon and 1 8 some trials have given encouraging results. ' Wear was reduced at the origin of rifling and muzzle wear was eliminated. The dimensional stability of some plastics has been found to be faulty. High temperature characteristics of plastics affecting their ability to spin reliably the heavy projectiles in hot large guns at high muzzle velocities have not been fully established by the Army. The lack of simple but controlled erosion tests procedures inhibits the establishment of necessary characteristics of plastic products which might be suitable. However, some success has been obtained with plastics in large cannon and it is evident more attention should be given to plastics as a substitute for metal in rotating bands, or for use in combination with metal rotating bands.

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28 3. The Charge Assembly (including additives) The charge assembly consists of the propellant, the primer, several ad- ditives and the container. Propellants are made of mixtures rather than single constituents. Ma- terials such as nitrocellulose, nitroglycerine, nitroguanidine, etc., are used. The flame temperature is an important characteristic and affects the severity of the erosion. Different propellants having the same flame temperature may cause variations in erosion by a factor of 4. Other aspects important to the mechanisms of erosion are the proportions of the products of combustion in the gases, such as CO, CO , HO, H , N , and a wide variety of dissociation products. & m lu m Looking first at the heat loss from the propellant gases to the gun tube, g Corner gives a semi-empirical formula; 9 = T0 - 300 I A2 0.86 1.7 +0.38 d2 (£-) \j where 9 = maximum rise in bore temperature above ambient at the origin of rifling, °K d = bore diameter, in. C = weight of propellant, Ibs. To = flame temperature of the propellant, ° K As would be intuitively obvious, higher propellant flame temperature and increased charge weights make for higher gun temperatures. Larger diameter gun tubes run cooler. Using the above, one may estimate the tube temperature for different systems as shown on Table I on the following page entitled "Calculated Forcing Cone Temperatures."

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27 oo r-. o in r- o ro S ON o • « (O 00 CM £ ' a o OT X) CJ O co o s CO o CO m vO O Sf ro O sj- co 2 st st CM oo 00 \O ON CO oo o H T3 CO O. X cd E o o oo O m 00 O 1-1 10 o 05 * o o r^. CN * o o m O o m m o c en o > iH O -D T-< CJ ° "3 1 •*J H^ CO § 2 » w •a a» c s O TO i; £ §

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The above is admittedly simplistic and ignores the effect of chamber pres- sure, tube geometry and chemistry but does illustrate the apparent importance of propellant flame temperature. The issue is also confounded by the fact that the 8" M110E2 and 175MM rounds use the wear reducing additive. Such rounds without additive presumably would have much lesser wear life: such data were not available. Since for the same muzzle velocity, roughly, the weight of the propellant charge (C) times the force constant of the propellant (FQ) must be the same. that is: CF = CnRT = constant o o where- F o = force constant ft Ibs/lb n = gas volume, moles of gas/gm of product T o = isochoric flame temperature R = gas constant C= weight of charge Since C is more or less limited by chamber geometry, at least in high performance systems where erosion is a problem, the only avenue open is the development of propellants with lower flame temperatures and lowered mean molecular weight combustion products i.e., the number of moles of gas/gm is increased. This has been achieved in the triple base cool propellants as shown below: Propellant M6 M5 M30 Type Single base Double base Triple base F , (ft Ibs/lb) 317000 355000 364000 T °K *o* 2570 3245 3040 n, moles gas/gm .04432 .03935 .04308 M, mean molecular 22.6 25.4 23.1 weight

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29 Thus M5 would probably be a more erosive propellant than M30 in a given system. While M6 would be less erosive, a larger charge weight would be necessary. Since the systems are generally volume limited M6 could probably not be used. The development of the "cool" triple base propellants was a significant advance. The requirements for ever higher and higher performance systems however, requires another such step. Currently, in development, the nitramine base systems offer the promise of high force constants, (400, 000 ft Ibs/lb) and low flame temperatures (approaching 2500°K). The mean molecular weight of the gases is less than 18 in some instances. There is some evidence from vented vessel firings, measuring wear in a nozzle, that these propellants are more erosive than the current standard formulations. One explanation that has been ad- vanced is that the particulate nitramine in the propellant is acting as an abrasive. However, in the less erosive triple base propellants, a significant fraction of the formulation is particulate nitroguanidine. This would seem to question this interpretation of the increased erosivity. Also the additives, like TiO , are not m considered abrasive. It would seem that the chemistry of the combustion gases of the nitramine formulations might be of some significance with regard to the increased erosivity. Evans et al, using a vented vessel, investigated the effect of the CO/CO ratio with added trace amounts of other gases. They were able to demonstrate an increase in erosivity due to reduced CO/CO ratios. The severity of the effect m increased with increasing temperature. It is also interesting to note that Lenchitz and Silvestro report that the various additive formulations increased the CO/CO ratio when added to a propellant fired in a bomb calorimeter. Par- ti ticularly interesting (Evans) is the observation that hydrogen in low concentrations, increases the rate of erosion. They postulate the formation of volatile iron carbonyl catalyzed by the addition of hydrogen. It is interesting to note that the lower mean molecular weights of the product gases of the nitramine propellant combustion is, in part, due to higher hydrogen concentrations. Thus, one could

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30 suggest that while the erosion mechanism at high flame temperatures is princi- pally thermal, at lower flame temperatures, the change in product gases may make the chemistry the first order cause. The CO/CO ratio and the iron carbonyl mechanism have also been ,-4.ii 12, 13 investigated by Frazer. Contributing to the chemical species in the propellant powder gases are the products from the primer which consists usually of black powder (carbon, sulfur, and potassium or sodium nitrate) and the percussion element which may be mixtures of mercury fulminate, potassium chlorate, antimony sulfide, and lead styphnate. Such compounds and their combustion products act as catalysts to chemical reactions between gun steel and hot propellant powder gases. Among the reaction products which have been identified are iron carbide, two iron oxides, two iron nitrides, iron sulfide, copper sulfides, zinc sulfides, chromium carbide, nickel carbide, stable austenite, iron carbonyl. Iron carbide and iron oxides form low melting point eutectics. There are many iron carbonyls some of which are volatile and decompose readily with slight heat to form carbides, oxides, carbon, etc. In one investigation enough carbonyl was recovered after rapid cooling of propellant powder gases to account directly for 0. 03% of the weight loss of the barrel, and it was estimated after taking into account the decomposition products that half of the weight loss of the gun could be attributed to this carbonyl reaction. Evidence of the loss of gun steel by some volatile process is found in the metallographic study of the bore interface of worn plated gun barrels. 2 The CO/CO ratio is higher in the combustion products of single base m cool propellants than it is in the double base (hotter^ propellants. A deep understanding of the role of the propellant in gun tube erosion is not a trivial problem. The hydrodynamics, heat transfer, and chemical kinetics individually are not readily amenable to analytical or experimental treatment. Coupling of these descriptions as a unified "erosion" model represents a formid- able challenge. Higher performance gun systems (higher muzzle velocities and

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31 chamber pressures) can be expected to be more subject to tube erosion. An under- standing of the mechanism of erosion should point to a relief from the problem. It would seem that this task, as a multidisciplinary effort, should be a matter of high priority, and should begin as soon as possible; in anticipation of future user re- quirements. In the short term the nitramine propellants with their lower flame tempera- tures seem to offer some hope. It is not clear that they are all more erosive. (It is known that the Air Force is involved in this research area. No data from this source was available at this writing). Perhaps trade-off's in formulation are possible which will be acceptable in terms of erosion but which do not yet yield the full potential of the nitramine compositions. Additives which are incorporated within the propellant powder, in order to retard deterioration in storage are diphenylamine, ethyl centralite, etc. The additive commonly used to suppress muzzle flash is potassium sulfate. Additives which are not incorporated within the powder, but may be included in the ammunition are the decoppering agent (usually lead foil) and wear additives. Reference has previously been made to the decoppering agent accentua- ting roughness of the bore and increased wear, and to potential catalytic actions of black powder and potassium sulfate. It was first noted that there was considerable difference in the wear rate, (at the origin of rifling}, between guns in which the propellant charge was contained in a cloth bag versus those in which the charge was in a metal cartridge case. The "cartridge case" gun wore faster. This was attributed to the cool gase^ from the pyrolysis of the cloth bag flowing in a laminar layer down the gun tube for a few calibers. This presumably served to insulate the origin of rifling from the high temperature gases generated by the propellant combustion.

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32 This was extended by the British to placing a silicone oil filled tampon on the base of the projectile. In moving down the tube oil was smeared on the tube surfaces to insulate it from the hot gases. Reduction in erosion was observed. The Canadians applied the laminar cooling effect to cartridge case rounds by a polyvinyl chloride or polyurethane foam jacket placed in the forward portion of the case. This served to reduce the rate of erosion by a factor of about 2-3. 16 The effort in the U. S. centered about the use of the "Swedish additive". As finally applied, it consisted of a mixture of TiO and paraffin wax coated on m cloth. A scroll of this material was placed immediately to the rear of the projec- tile in the cartridge case. Extension of the wear life of gun tubes was remarkable. For example, in the 105 mm gun with the M392 ADPS round the useful wear life of the tube was reported as extended from 100 to 10, 000 rounds. In other systems the effect was not so marked e. g. the increase in the 90 mm systems was a factor of three, (700 to 2100 rounds). One is unable, to date, to predict the degree of success of wear reducing additives in a particular gun system. 17 Picard extended the "Swedish additive" concept to include materials other than TiO . The preferred material is a very fine talc, again dispersed in a wax a matrix on cloth and applied in a similar fashion. In certain guns this appears to be more effective than TiO . That this is true in all guns is uncertain. Lt There is evidence from vented vessel firings (measuring orifice wear) that wax alone is an effective material. This has led some to comment that the principal function of the particulate material (TiO or talc) is to provide stress risers to a assist the wax in breaking up upon firing the charge. This does not seem to fit with the specification expressed in the above quoted patents to use the lower melting point waxes; presuming that the lower range melting points would represent more "plastic" waxes, less prone to "shattering." It should also be noted that the particulate matter alone, without the presence of the wax, can effect a reduction in wear in some situations. It is reported (unconfirmed) that in the 40 mm grenade launcher the TiO or talc/wax 26

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38 combination did not prevent erosion. Yet a small amount of talc (?) contained in a plastic packet was effective. The launcher is a M-low type system where the propelling charge is burned at high pressure in a "chamber" built into the cartridge case. The hot gases are then vented through a pair of orifices at low pressure into the space behind the projectile. The patents of Jacobson imply that the particulate matter and the matrix in which it is contained affect the efficiency of the additive. The inorganic particles reported were: Na0B O , Na0WO., CrfNO0)0, CrF., MoO,, A1F_,3H 0, WO 24324 33 3 6 & s Ta O and TiO . Lt 5 *_> The reduction in wear amounted to 25 percent for the Na B .O to greater £i TT O than 95 percent for WO , Ta O and TiO , in the order shown. The issue is 0 A O £l confounded by the method of application: various substrates ranging from sheets or propellant, paint, dispersed as a powder, or contained in a layer of paraffin wax near the mouth of the case. This latter method used, with the WO , Ta O <3 A O and TiO , was the most effective. The work at Picatinny Arsenal investigated m the use of Group IV oxides as the particulate matter in a wax matrix. Silica (SiO ) was found to be the most effective. This was attributed to its large surface 2 area and high heat capacity. Smaller particle sizes (200U) were preferred. The Picard patents specify particles sizes 3 to 60 a No mention is made of particle size in the Jacobson patent. 18 The Navy , as late as 1969. appeared to favor the use of polyurethane 19 cylinder as a wear reducing additive. However, recent data has shown talc to be superior for use in 5"/54 guns. Navy data also imply that a residual wear reducing effect persists when rounds with additive are followed by non-additive rounds. Experience, measuring the wear of an orifice in a vented vessel and firing propellant-additive mixes i.e., without wax, may be summarized as follows:

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34 The reduction in wear is inversely proportional to the particle density and directly proportional to the thermal diffusivity. The additive should be segregated from the bulk of the propellant and placed in the forward part of the charge. No effect of particle size could be demon- strated. Phenolic microballons (50 \i diam) were almost as effective as the smallest talc particles: the best additive. These results were derived from a series of tests designed to verify the thesis that the particulate matter acted to damp the turbulence. In so doing, they would cause the laminar layer, an effective barrier to hear transfer, to become thicker. The validity of this supposition was not demonstrated. It is evident that there is a large body of information concerning control of gun tube erosion using additives. These data apparently have never been collated and examined with a view to developing a rationale for understanding the phenomena. The main thrust of the research and development effort has been the development of better additives. Yet "better" additives have not been universally successful. It is submitted that an attempt should be made to elucidate, in depth, the mechanism by which additives reduce erosion. This appears to be necessary in anticipation of future requirements for high performance gun systems possibly using exotic propellants. There does not appear to be any short-term solution. "Quick fixes, " i.e., variations on the current theme, will probably continue to have some measure of success.

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35 4. The Firing Conditions The firing conditions have an effect upon erosion. Two temperature systems are significant, namely the temperature of the bore interface and the temperature of the wall of the barrel. Neither is independant of the other. Since W. W. II extensive studies have expanded the understanding of barrel heating. Much can now be predicted and when needed useful equipment is available. Reference has already been made to the interdependence of flame temperature of the propellant powder and the temperature of the bore interface. The rate of fire is also an important factor and it has several parameters, namely number of rounds per minute (rpm); number of rounds in the burst of fire; number of bursts; time between bursts; time between repetitions of cycle before complete cooling of weapon to ambient temperature. The number of parameters add to the complexities of standardizing erosion testing of any one model especially since large guns are being used in substained firing programs. A high rate of fire raises the temperature that the bore interface can reach and presumably can at times overcome the benefits of cool propellants. The converse is also true that cool propellants can permit high rates of fire to be tolerated. But it follows that hot hardness and the retention of hot hardness after cycles of heating and cooling are two more parameters to be included in consider- ations of new materials for erosion resistance.