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Metallic Armor Materials
Chapter 7
METALLIC ARMOR MATERIALS
Dino 3. Papetti
I. INTRODUCTION The history of armor materials and the role of metals in that history is vast, and a complete chronological account of all signifi cant events is impossible. However, it is well documented that through thousands of years of warfare, armored men have defeated unarmored men. As metals became more common and men more skilled in working them, it became possible for small armies to be fully equipped with armor so effective that few projectiles could penetrate it. In those early times, however, it was impossible to equip large armies with complete kits of armor. It was only during the early years of the Industrial Revolution that improved methods of providing more armor were developed [1]. This was due mainly to improved production techniques and the need to overcome continu ously escalating threats, many of which, ironically, were also a product of the Industrial Revolution. For thousands of years, warfare was conducted on foot and horseback as suggested in Fig. 1. During the last sixty years, however, drastic changes in warfare have taken place. Mobility changed from feet to wheels and wings. Despite these changes, the keys to victory in warfare including mobility, firepower, and protection remain unchanged [2]. Mobility and protection have always imposed particular demands on armor. Mobility requires the lightest possible armor. However, protection generally increases with increased armor weight, thereby impeding mobility. This enigma has perplexed armor developers for centuries. On the other hand, it has encouraged innovation and ingenuity in armor materials research and armor systems design. Metallic materials have always received special consideration in the develop ment of armor protective systems for many reasons including availability, fabricability, flexibility, and cost.
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II. CHARACTERISTICS OF METAL ARMOR Armor is intended to provide maximum possible resistance to penetration by all types of projectiles as well as by fragmentation, resulting from the detonation of high explosive (HE) shells, gren ades, mines, high velocity jets, kinetic energy penetrators, etc. In addition, the armor must resist cracking, spalling, and fracture upon multiple impacts while maintaining structural integrity if the armor is being used as a structural member. Metal armor falls into three general categories. These are steel, aluminum, and titanium, all of which through alloying, heat treat ing, or other processing techniques, result in armor materials of different characteristics to meet different threats [3]. The major metallic armor materials and systems which have been developed and incorporated to varied extent on material requiring ballistic protection are illustrated in Fig. 2.. Hardness and toughness, along with material soundness, greatly influence the ballistic performance of the armor. Armor resistance to penetration is also affected by
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METALLIC ARMOR MATERIALS
I l _
DUAL PROPERTY
TITANIUM
Fig. 2.
FACE-HARDENED PLATE
Classification of metallic armor.
the angle at which a plate is mounted (obliquity) and the angle of impact of the projectile. The greater the armor obliquity, the greater will be the distance the projectile must travel through it in order to perforate it. Figure 3 illustrates the penetration required for various thicknesses of metal armor at obliquities from 0° to 60°. Selection of the optimum obliquity to provide maximum ballistic protection at minimum weight has been widely investigated along with the kind of armor and amount which can be used. The limits within which those factors can be varied are set by the kinds of armor available, the weight restrictions, the shape, and the ease of manufacture. Metallic armor provides greater flexibility and a wider range of limits to the designer who wants to build maximum protection into a combat vehicle or other armored system.
20" Obliquity
45 Obliquity
T.54cm) 0 Obliquity
30 Obliquity
60 Obliquity
Fig. 3. Variation in the distance of p e n e t r a t i o n necessary for perforating a Z.54 cm (1 in.) plate at various obliquities.
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III. ARMOR TYPES Metallic armor is either ferrous or nonferrous and is produced by rolling, forging, or by casting. Rolling and forging are employed to produce plates and casting is used to produce special shapes such as tank turrets, e t c . In many instances wrought armor, after having been rolled or forged is further treated to achieve additional ballistic characteristics, such as face-hardened steel armor. Homo geneous armor plate is used alone as monolithic armor, or several plates can be used in spaced or layered configurations. Homogene ous metal armors are also used to produce metallurgically bonded dual property armor. Homogeneous plate can be fitted with spikes attached to the impact surface to defeat particular types of threats. Because of the wide variety of threats to be considered by armor developers and users, no one metal armor can be expected to cope with all of them efficiently. A. Steel Armor When discussing metal armor, the appropriate introductory discus sion would have to begin with homogeneous wrought steel armor plate. Such a material was introduced over sixty years ago as armor plate on the first tanks developed by the United States Army. Various modifications to the original homogeneous steel armor were developed and evaluated during the next twenty years, but with little or no improvement in ballistic protection. During World War II, rolled homogeneous steel armor underwent an important change due mainly to alloy conservation priorities brought about by the conflict. The resulting armor was predicated on low alloy material with low carbon content and adequate toughness characterized by military specification MIL-S-12560. The current rolled armor specification (MIL-S-12560B) is written for two types of armor, i.e. Class I and Class II. Each Class is heat treated to provide maximum resistance to penetration (Class I) and maximum resistance to shock (Class II). Rolled Homogeneous Armor (MIL-S-12560B) has become the standard steel armor material which is used for comparison to determine performance improvements (merit ratings) of new candi date armors. Prior to World War II only limited experience was available in the production of cast armor. In 1935, limited production of cast armor was started for the Watertown Arsenal. By the l a t e 1930s, both the United States Army and Navy began to show greater interest in armor castings, and work on heavier sections was started [4]. Initial ballistic results were encouraging and further testing and production of heavier cast armor was initiated. By the early summer of 1940, it
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became evident that this country should embark on production of large quantities of cast armor for use on armored vehicles. Cast armor has always been less resistant ballistically than rolled armor due mainly to the fundamental difference in mechanical and metal lurgical properties between rolled and cast steel. This is not meant as a condemnation of cast armor. It is possible to design a casting with smoother contours and higher obliquities than a corresponding structure fabricated from rolled plate and in many cases with equal or even improved ballistic protection. Cast homogeneous steel armor is still used on Army Combat Vehicles under MIL-S-11356 to produce such components as hulls, turrets, cupolas, hatch covers, etc. Another type of steel armor developed during World War II, primarily for aircraft protection, was face-hardened steel armor. Face-hardened armor was processed as to have one face (the impact surface) much harder than the remainder of the plate thickness. The harder depth extended from 15 to 40% of the total thickness. Face-hardened armor has been produced by several different pro cesses including pack carburizing and nitriding. Although facehardened steel armor provided improved ballistic protection against certain projectiles as compared to rolled homogeneous armor, its use was minimized due to inherent brittleness and manufacturing difficulty. Commercial availability of this type armor is virtually nonexistent at the present time. However, military specification MIL-A-00784, entitled "Steel, Face Hard" is still maintained. For a period of about twenty years following World War II, a large amount of empirical data obtained from a variety of tests indicated and continuously verified the fact that armor strength or hardness is a very important parameter in resisting penetration. This important property has been achieved primarily by thermal or thermomechanical processing. The data in Fig. 4 indicates that homogeneous steel armor should be made as hard as possible for defeating small arms, armor piercing (AP) ammunition. However, as homogeneous steel is made harder it becomes more brittle. As the material becomes brittle, its ballistic limit cannot be measured due to severe fracture of the armor. Thus, limits on homogeneous armor hardness have to be established to prevent shatter of the armor due to embrittlement, but not because of strength limitations on the ballistic limit [5]. This important fact has formed the basic guidance for improved steel armor development programs over the years. That is, to increase steel armor ballistic limits by increasing its hardness without increasing the tendency of brittle failure. During the early 1960s, an increased effort was initiated to develop improved armor in response to the conflict in Southeast Asia. An urgent requirement was established for the provision of
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Relationship between armor hardness and ballistic performance.
modular armor appliques for attachment to trucks and jeeps in ambush situations. The armor was designed principally to protect the vehicles from 0.30 caliber ball projectiles fired from hand carried rifles. One of the materials developed was a high-hardness steel, which was basically a low cost, low alloy steel made by the open hearth process, rolled straight-away on a strip mill and h e a t - t r e a t e d to a hardness level of about 500 Bhn. A typical application is shown in Fig. 5. At the time, it was recognized that the ductile-brittle fracture transition temperature was high; but since using it in modules was restricted to the tropical climate of Southeast Asia, the material was considered to be satisfactory for armor modules. As a modular armor, the high-hardness steel armor performed very well. Over 454 000 kg (1000 000 lbs) of high-hardness steel armor was shipped as armor kits to Vietnam [6]. This type of steel was then used in the unitized construction of an armored vehicle. Prototype vehicles were fabricated and field tested with no apparent difficulty. The vehicle was then put into production for use in Southeast Asia. Several vehicles developed large hull cracks during shipment. The problem was attributed to the inherent brittleness in the material and, since application was intended to be for modular armor, use of the material in a structural
METALLIC ARMOR MATERIALS
Fig. 5.
151
Modular armor p l a t e on 4.94 m e t r i c ton (5 ton) truck.
load carrying capacity was not recommended [7]. Since that time, more rigid controls have been implemented over the fabrication of high-hardness steel armor and its use has continued under closer scrutiny and upgrading of the high-hardness steel armor specifica tion (MIL-S-46100) continues periodically. As previously stated, limits on armor hardness had to be estab lished to prevent shatter of the armor due to embrittlement, but not because of strength limitations on the ballistic limit. In order to take advantage of the resistance to penetration associated with high hardness while circumventing the consequences of reduced tough ness, the two major courses of improving steel armor were to develop metal laminated composites and to improve the casting technology to yield ingots. As a consequence, the wrought materials from the ingots also were improved [8] . In the area of metal laminate armor, the development of dual hardness steel armor is considered the most significant. The concept of dual hardness armor is not new. Examination of armor produced during various periods as far back as the fourteenth century indicated metals of different hardnesses were interwoven by various methods to produce dual hardness armor. The current product is the result of work initiated in the early 1960s. Ballistic data obtained at that time indicated that the most effective method of defeating armor piercing projectiles was to induce shattering of the projectiles upon impact and in order to shatter the projectile effectively, an armor hardness of at least 58 to 62 Rockwell C was required. Monolithic steel armor at that level of hardness would break up or crack excessively upon ballistic impact; thus, the concept of bonding the hard layer of steel to a somewhat softer more ductile back layer was devised. The first test plate of dual hardness steel armor was produced in 1964. The research was sponsored by the Army Materials Research Agency (AMRA) present-
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ly called the Army Materials and Mechanics Research Agency (AMMRC). Philco Corporation Aeronutronic Division along with Republic Steel Corporation produced the plates which consisted of 9 Ni-4 Co alloy with 0.50% carbon in the front plate and 0.30% carbon in the rear plate. Soon after, the front face material was changed to H-ll steel and was marketed as DPSA-2 armor, which was produced by grinding the mating surfaces of the front and rear plates and welding the outside edges of the two materials together. The welded assembly was then metallurgieally bonded by hot-rolling at a temperature of 1149°C (2100°F), reheated to 1038°C (1900°F) (austenitizing temperature), air cooled to a temperature range between 760°C (1400°F) and 593°C (1100°F) and rolled again fol lowed by oil quenching and tempering. This type of dual hardness armor is commonly referred to as ausformed armor since the thermal mechanical strengthening or ausforming is the major part of the processing cycle, as shown graphically in Fig. 6. Ausformed dual hardness steel armor was used in the field during the war in Southeast Asia. Typical applications included critical aircraft component protection as shown in Fig. 7. The armor was also used on Navy Riverine Patrol Boats. Ausformed dual hardness armor, by the nature of its process, is in the fully hardened condition upon leaving the rolling mill. In terms of producing flat, hard dual property armor, this process is efficient. However, fabrication such as machining, forming, etc. is difficult due to the condition of the as-rolled material (flat and hard). Some
Fig. 6. Rolling sequence used to produce ausformed dual hardness armor compared to conventional armor plate rolling.
METALLIC ARMOR MATERIALS
Fig. 7.
153
Dual hardness armor application.
preliminary secondary fabrication can be performed on the ausformed armor, however, the need for more flexibility in this area had become apparent. Since 1964, research and development at the AMRA and the U.S. Steel Corporation determined the feasibility of producing a heattreatable type of dual hardness steel armor which could be produced in plate form in the annealed condition. The plate could then be fabricated to the desired configuration and then heat-treated to armor hardness levels, thus greatly increasing the flexibility of the armor. In 1967, AMRA sponsored a program to establish commercial availability of heat-treatable dual hardness steel armor. The program was successful, as demonstrated by Fig. 8, showing a hydraulic actuator component and a sump cover both made from
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DINO J. PAPETTI
Fig. 8. Heat-treatable dual hardness armor fabricability.
heat-treatable dual hardness armor. The material is presently produced under specification MIL-S-46099. It is the most efficient steel armor plate available commercially for protection against armor piercing projectiles. Figure 9 shows the desired behavior of dual hardness steel armor when impacted by a projectile. Note that the hard frontal face has defeated the projectile by inducing projectile shatter. The expected cracking of the hard impact surface is arrested by the softer more ductile back face and the sound metallurgical bond interfacing the two materials maintains structural integrity of the composite. In the early 1970s, research and development on dual hardness steel was focused on processes to produce the armor at lower cost. Conventional dual hardness steel armor, which is produced by roll bonding, must have clean interfaces, therefore, slab grinding is a standard procedure prior to welding the periphery of the front and rear components. This operation is expensive and efforts to eliminate it were initiated. AMMRC along with the Aeronutronic Division of Philco-Ford Corporation evaluated test plates of dual hardness steel armor made by the electroslag remelting (ESR) process [9]. The process was used not only to produce the front and rear materials themselves, but also to bond the two components metallurgically to each other by melt bonding using the ESR
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process, thereby eliminating the need for slab grinding, periphery welding, and roll bonding. Figure 10 shows a schematic view of the procedure followed. The ingots for the front and rear components were prepared by the ESR process (to be described in more detail in a subsequent paragraph). Although a 50/50 front-to-rear thickness ratio was to be achieved in the final composite slab, the individual ingots were not of equal thickness. The electrode used for ESR bonding would be of a composition similar to that of the thinner ingot so that when the bonding was completed, the ingot ratio of the composite was 50/50. The composite ingots were then rolled into plates using conventional methods. The plates were then heattreated to dual hardness armor strength levels. Subsequent ballistic testing showed results were reasonably compatible with commercial, roll bonded, dual hardness steel armor. Although the program was considered successful and feasibility was demonstrated, development scale-up was discontinued due to other priorities in the metal armor research and development program. In the early 1970s, the second major development of steel armor was to improve the casting technology in an effort to produce steel armor with high-hardness while circumventing the consequences of reduced toughness. Improved quality steels, when processed into armor plate, perform very well in terms of ballistic properties and ability to sustain multiple impacts with reduced cracking and spallation. It is recognized that armor hardness is most important and that generally speaking, steels of any quality will provide similar ballistic limits if they are heat-treated to the same hardness levels. The steel of higher quality, however, will have less tendency
Uilcmi
Fig. 9-
Transverse section through impacted area of a dual hardness p l a t e .
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W (12.7 cm) thick 6SR s i * X-27 steal
t / 2 " (1*27 cm) thick H*11 attctroctes g
■18" (45.72 cm)
, r„ Water cooled copper and pfatas 3" (7,62 cm) thick ESB slab H-11 steel Molten sla§ Solidifying melt
20" (50.8 cm
Fig. 10.
Stool, slabs sptctd 1 " > 12.54 oml apart
ESR bonded dual hardness armor process.
to crack and spall after multiple ballistic impacts. The reasons for this are varied depending on the processes used to produce the materials, yet the advantages can, in most cases, be traced to the metallurgical superiority of one material over another. The ballistic superiority of steels of higher metallurgical quality has been demonstrated often. An important example was the development of unidirectionally solidified wrought steel armor [10] • Work conducted at AMRA, Massachusetts Institute of Technology, and U.S. Steel Corporation showed that cast steels with superior ductility could be produced by unidirectional solidification, which is loosely defined as controlling solidification of the ingot by casting the metal in such a way that heat is extracted primarily from one surface to produce a cast structure in which columnar grains extend from the chill surface completely through the casting. The result ing solidified steel ingot has been found to be virtually free of gross porosity and with a much finer segregation pattern, factors which contribute to higher ductility. In addition, it was found that a homogenization heat treatment which consisted of holding the
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METALLIC ARMOR MATERIALS
casting at 1316°C (2400°F) for 64 hs would virtually eliminate alloy segregation in the as-cast structures. Figure 11 shows a chill mold used for casting unidirectionally solidified slab type ingots. Molten steel was poured into the mold. When the mold was filled, loose exothermic powder was placed on top of the exothermic slab and covered with insulation. Since the sides of the mold were lined with exothermic material, the liquid steel was in contact with the cast iron mold only at the bottom, which acted as a chill, thus assuring heat extraction in the downward direction only. Subsequent bottom chills were made of copper which produced smoother bottom surfaces. Figure 12 shows a typical unidirectionally solidified cast steel plate. Note the columnar grain structure, as compared to conventional casting. Steels of armor composition were produced by this process, homogenized, rolled, and h e a t - t r e a t e d to hardness levels ranging from 50 to 60 HRC (Hardness in the Rockwell-C scale). Plates of various thicknesses were ballistically evaluated at each hardness level. Excellent ballistic properties and ductility was exhibited by armor plates at hardness levels as high as 55-56 HRC. Cracking did occur at higher hardnesses. Nevertheless, the results of these tests signified an important breakthrough in homogeneous steel armor development. Figure 13 shows a ballistic test plate made by the unidirectional solidification process at a hardness in excess of 55 HRC. The excellent ductility after multiple ballistic impacts is apparent. Unfortunately, unidirectional solidification loses effec tiveness when the cast thickness increases to a point where the chill bottom does not extract heat efficiently during solidification, thus scale-up of the process to produce large ingots would be difficult, at best. Large ingots produced at a reasonable cost is an absolute requisite for the production of tonnage quantities of steel armor
mm mLU
ctmm Mote*
Fig, 11. Mold for producing unidirectionally solidified steel ingots.
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TOP
BOTTOM OR CHILL FACE
TOP
SLAB TYPE INGOT Fig. 12.
CONVENTIONAL VERTICAL INGOT
Comparison between unidirectionally and conventionally solidified ingots
Fig. 13. Multiple ballistic impact capability of armor plate made from an unidirectionally solidified ingot.
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plate. Because of this fact, the concept of producing armor plate from unidirectionally solidified ingots was abandoned. The decision to curtail work on unidirectional solidification for improved steel armor was made primarily for reasons stated above. An additional factor which contributed to that decision, however, was the emergence of an expanded electroslag remelt (ESR) capabil ity in this country. In the years between 1960 and 1975, the production of ESR ingots in the United States had grown from less than 9078 metric tons (10 000 tons) to over 113 475 metric tons (125 000 tons) per year [11]. The ESR process is a secondary steel melting process characterized by simple equipment, ease of opera tion, and good metallurgical results. A schematic view of the process is shown in Fig. 14. Also shown are some comparisons between steels of similar composition made by conventional prac tices and by the ESR process. In electroslag remelting, energy is introduced into a slag bath by a conventionally made consumable electrode and the circuit is closed through the ingot to be remelted and a water-cooled bottom plate. The liquid slag contained in the water-cooled mold acts as a resistance. The passage of current
Fig. 14.
ESR process s c h e m a t i c .
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through the slag causes it to heat, thereby melting the tip of the electrode. The metal, on dripping through the slag, is freed from impurities. The droplets of refined steel collect in a pool at the bottom of the mold, solidify and result in building up a new ingot, which is of uniform structure over its entire length and cross section. Reactions with the slag remove unwanted elements in the steel. Sulfur content is greatly reduced and oxygen and nonmetallic inclusions are substantially lowered. ESR ingots, unlike convention ally made ingots, are free of shrinkage and central porosity, and in many respects, have been shown to be superior to those made by other remelting processes, including vacuum arc remelting (VAR). Yet, the ESR process is more economical. In 1972, Lukens Steel Company reported the installation of an ESR furnace capable of making large slab-type ESR ingots. The ingots having a cross section of 76 cm (30 in.) by 203 cm (80 in.) and weighing 27 216 kg (60 000 lbs) were reputed to be the largest in the free world. Large slab ingots are most desirable for the production of tonnage quantities of armor plate and the prospect of producing tonnage quantities of high quality armor plate by the relatively economical ESR process could not be ignored. It was, therefore, with much anticipation that a program was initiated in 1972 by the United States Army to conduct an intensive evaluation of armor plate made by ESR steel. The initial program involved testing various alloys of ESR steel at different hardness levels in an effort to determine optimum properties for an advanced structural tank armor which would not only perform as good or better than highhardness armor (described earlier), but also possess substantial toughness so that structural integrity could be maintained in any climatic environment. Of all the material tested, 4340 ESR steel performed the best at a hardness level of 55-56 HRC. The performance of this material had already matched that obtained from unidirectionally solidified armor plate. In addition, the ESR plate could be produced from a 27.2 metric ton (30 ton) ingot. Subsequent testing of 4340 ESR steel produced results a t t r a c t i v e enough to utilize the material at the high strength level for producing a substantial amount of critical helicopter components in the first important application of the integral armor concept. Basically integral armor involves selecting critical components, which require ballistic tolerance, and fabricating these components using ballistically tolerant material, thus eliminating the need for additional armor protection. Since 4340 is a material commonly used to produce components of ordnance material and since excep tional toughness is maintained at higher strength levels of 4340 ESR steel when compared to conventionally made 4340, it was logical to produce and evaluate critical components made from the ESR
METALLIC ARMOR MATERIALS
Fig. 15.
161
Integral armor for critical aircraft components.
material. Hughes Helicopter Company, the prime contractor for the ArmyTs YAH-64 Advanced Attack Helicopter (AAH), included in the design and construction of the AAH over one hundred critical components made from 4340 ESR steel. Figure 15 shows a typical component made from ballistically tolerant 4340 ESR steel. Other parts included flight controls, engine mounts, bearing liners, etc. Survivability requirements of the aircraft were met or exceeded with little or no weight penalties, thus contributing to the superior performance of the AAH. The success of 4340 ESR armor, when used for critical aircraft components, could not be applied to structural tank armor. An important requirement of a structural tank armor is that it should maintain structural integrity at sub-zero temperatures when impac ted by overmatching artillery rounds. Test plates are inspected after proof testing for their ability to withstand fracture, spallation, and cracking. A long standing empirical materials specifica tion, which applies to structural tank armor and its ability to maintain integrity at low temperatures, requires that the material must have a minimum of 27.12 J (20 ft-lbs) Charpy V-Notch resist ance at a temperature of -40°C. In 1975, a program was initiated by AMMRC to optimize an ESR alloy which would combine the ballistic properties of high-hardness steel armor, yet possess ade quate toughness at low temperatures which would enable its use as a
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structural tank armor. Basically, modifications were made to nominal 4340 steel by lowering the carbon and raising the nickel content. Ingots weighing between 181 and 226 kg (400 and 500 lbs) were produced using AMMRCs Research and Development ESR furnace. A typical ingot is shown in Fig. 16, where each ingot is of a different composition. The optimum composition was determined, an additional quantity of ingots were made to that same composi tion, and then armor plate was rolled to various thicknesses from the ingots. They were then heat-treated and thoroughly evaluated for ballistic, mechanical, and low temperature ductility properties. The program successfully established the feasibility of using ESR for tank armor and the requirements were incorporated in specifica tion MIL-A-46173 (MR) Armor, Steel Plate Wrought (ESR), released in February, 1976.
Fig. 16.
Low alloy steel ingots from an AMMRC ESR furnace.
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B. Aluminum Armor Because of their low weight, aluminum alloys have been consid ered for potential armor applications for many years. Studies were initiated in the early 1940s on commercially available alloys by the Frankford Arsenal, Aberdeen Proving Ground, and the Army TankAutomotive Command. The initial objective was to achieve im proved protection against fragmentation by artillery shells. Alloys commercially available at the time were evaluated. The results were good enough to initiate a cooperative program between the United States Army and the aluminum industry to exploit fully existing alloys and to develop new aluminum alloys with improved ballistic and mechanical properties. Aluminum alloys were ulti mately designated for specific armor applications, and included 5083 and 5456 alloys, which were strengthened by strain hardening to increase their resistance to fragment penetration. Additionally, these alloys were characterized by good structural strength, formability, and weldability. Strain-hardened 5083 and 5456 aluminum armor has been used extensively under specification MIL-A-46027C for ballistic protection in many applications, the most notable of which is the M113 Personnel Carrier hull structure. The vehicle, shown in Fig. 17, is a major United States Army armored personnel
Fig. 17.
United States Army M113/M113A1 personnel carrier.
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carrier, currently being produced by the FMC Corporation, with the quantity produced to date well in excess of fifty thousand. The success of 5083 and 5456 aluminum alloy armor against artillery fragmentation prompted an increased awareness and effort to develop aluminum armor for threats other than fragmentation, specifically AP ammunition. The importance of increased armor strength for increased ballistic properties discussed earlier for steel armor applies to aluminum armor as well. Once again, a coopera tive effort was initiated between the United States Army and the aluminum industry to develop, in this instance, an alloy with high strength for AP protection [12]. The effort was in specific response to the requirements of the General Sheridan XM551 tank. Previous studies of the 5083 and 5456 alloys indicated the impracticality of achieving the desired higher strength levels by work or strain hardening. Therefore, attention was directed to the development of a h e a t - t r e a t a b l e alloy. A number of alloys were investigated, the optimum of which was determined to be the 7039 alloy subsequently made in conformance to MIL-A-46063A MR. The 7039 alloy provides significantly better protection per pound than the 5083 or 5456 alloys against AP projectiles due primarily to the increased strength level. Unfortunately, high strength aluminum alloys, like other metallic armor materials, become more brittle as the strength level increases. Thus, high strength alloys which provide improved protection against AP ammunition do so with an accompanying reduction in resistance to fragmentation. The loss of ductility results in severe spalling of the armor upon impact, as shown in Fig. 18. In addition, the higher strength alloys are more susceptible to stress corrosion, a problem which has generated much concern by the United States Army for many years. A strict test has been incorporated in the armor specification to guard against stress corrosion failures in a corrosive environment. Many studies have been conducted over the years in an attempt to improve the ductility of high strength aluminum alloys. Generally, these attempts were directed to improving the purity of the material to improve the ductility at the higher strength levels. Improvements in ingot making practice along with mechanical processing techniques and thermal treatments were modified to achieve higher ductility. During the past several years the purity of commercially available aluminum has increased significantly. In view of this, high purity grades of 2024 and 7075 alloys were studied comprehensively and compared to similar alloys of lower purity. Although some improvement was obtained, a significant increase was not achieved. Research aimed at developing aluminum armor alloys with im proved mechanical and ballistic properties is continuing. Controlled
METALLIC ARMOR MATERIALS
Fig. 18.
Back spalling of 7000 series aluminum armor.
solidification and newly developed thermal mechanical processing techniques are being emphasized with the hope of producing one aluminum armor alloy, which would combine efficient AP and fragment protection along with acceptable stress corrosion resist ance. C. Titanium Armor Titanium armor development began in the early 1950s. As was the case in aluminum armor development, the low weight of titanium compared to steel provided the inducement. A titanium armor alloy was developed in 1955 which offered excellent ballistic protection against fragment type ammunition. The alloy (Ti-6A1-4V) was considered superior to all other titanium alloys investigated up to that time. Subsequent work on titanium armor has focused on the
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evaluation of commercial and experimental alloys in an effort to obtain ballistic improvements over the 6A1-4V alloy. Chemical composition, heat treatment, and fabrication techniques were the main areas studied for optimization [13]. It became readily apparent that metallurgically sound ingots with a minimum amount of inhomogeneities were required which when rolled to plate would reduce the material weakness in the thickness plane. This weakness has been associated with spalling behind the plate upon ballistic impact shown clearly in Fig. 19. The designation Ti-6A1-4V ELI has been used to obtain titanium armor with extra low interstitial (ELI) content. The ELI grade has somewhat higher ductility along with ballistic advantage especially at lower thick nesses. Back spalling which was prevalent in earlier titanium armor has been substantially reduced. The titanium armor currently described under specification MIL-T-46077 (MR) represents the best current practice for producing 6A1-4V titanium armor alloy. Mater ial produced under this specification has ballistic superiority over standard steel and aluminum armor over the majority of fragmenta tion and small arms ammunition threats. Titanium armor has been used extensively on aircraft for ballistic protection however. Because of its high cost when compared to other metallic armor, its use on ground combat vehicles is virtually nonexistent. The severe weight limitations imposed on aircraft do not apply to ground combat vehicles. Thus, the trend has tradition ally been for tank designers and manufacturers to select steel and aluminum armor which best satisfies both cost and performance
Fig. 19.
Titanium alloy after ballistic a t t a c k with a 0.30 caliber AP M2 projectile.
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restrictions and requirements, with weight considerations occupying a somewhat lower priority in vehicles than in aircraft. In order for titanium armor to be more widely used, additional ballistic improvement must be achieved. This will best be realized when titanium armor can be used at higher strength levels without brittle plate failure.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13
W. Millis, Arms and Men—A Study of American Military History, G.P. Putnam Sons, NY, 1965, p. 75. T. Wintringham, The Story of Weapons and Tactics, Houghton Mifflin Co., Boston, MA, 1943, p. 173. The Vulnerability of Armored Vehicles II—Armor and Its Resistance to Penetration (U), Technical Information Rept., Office, Chief of Ordnance, Washington, DC, 1956, p. 1. Ordnance D e p a r t m e n t Journal, Manufacture of Cast Armor During World War II, Steel Founders Soc. of Amer., Cleveland, OH, 1946, p. VII Preface, 1,3,17. D.J. P a p e t t i and K.H. Abbott, Simulating Soviet Armor (U), Rept. No. MEDA-7, AMMRC, Watertown, MA, 1969, p. 2. K.H. Abbott and D.J. P a p e t t i , Evaluation of High Hardness Steel (U), Rept. No. ABI-109, AMMRC, Watertown, MA, 1968, p. 1. E.N. Hegge, Lightweight Armor (U), Rept. No. ABI-49, AMMRC, Watertown, MA, 1966, p. 4,5. H.P. George, Metallic Armor Materials (U), in Proc. Third Symp. Lightweight Armor Materials (U), M.M. Jacobson (Ed.), Rept. No. MS69-02, AMMRC, Watertown, MA, 1969, p. 189. B.N. Briggs, Development of Low Cost Heat Treated Electroslag Melt-Bonded Dual Hardness Steel Armor, Aeronutronic C o n t r a c t Rept. No. CTR 73-42, Philco-Ford Corp., AMMRC, Watertown, MA, 1973, pp. 1-3,7. C.E. Bieniosek, K.F. Skidmore, and L.F. P o r t e r , Unidirectionally Solidified Wrought Steel Armor, C o n t r a c t Rept. No. CR69-01 (F), U.S. Steel Corp., AMMRC, Watertown, MA, 1969, pp. 1,2,4. Electroslag Remelting and Plasma Arc Melting, Rept. No. NMAB-324, National Materials Advisory Board, Washington, DC, 1975, p. 1,3 (draft copy). W.A. Mannschreck and C.J. Porembski, Mechanical Properties of 2024 and 7075 Aluminum Alloy (U), Rept. No. R-1852, Frankford Arsenal, Philadelphia, PA, 1967, p. 1. J.L. Sliney, Metallurgical and Ballistic Study of Titanium for Armor Part III (U), Rept. No. TR 63.34, AMRA, Watertown, MA, 1963, p. 11,12.
METALLIC ARMOR MATERIALS
Dino 3. Papetti
I. INTRODUCTION The history of armor materials and the role of metals in that history is vast, and a complete chronological account of all signifi cant events is impossible. However, it is well documented that through thousands of years of warfare, armored men have defeated unarmored men. As metals became more common and men more skilled in working them, it became possible for small armies to be fully equipped with armor so effective that few projectiles could penetrate it. In those early times, however, it was impossible to equip large armies with complete kits of armor. It was only during the early years of the Industrial Revolution that improved methods of providing more armor were developed [1]. This was due mainly to improved production techniques and the need to overcome continu ously escalating threats, many of which, ironically, were also a product of the Industrial Revolution. For thousands of years, warfare was conducted on foot and horseback as suggested in Fig. 1. During the last sixty years, however, drastic changes in warfare have taken place. Mobility changed from feet to wheels and wings. Despite these changes, the keys to victory in warfare including mobility, firepower, and protection remain unchanged [2]. Mobility and protection have always imposed particular demands on armor. Mobility requires the lightest possible armor. However, protection generally increases with increased armor weight, thereby impeding mobility. This enigma has perplexed armor developers for centuries. On the other hand, it has encouraged innovation and ingenuity in armor materials research and armor systems design. Metallic materials have always received special consideration in the develop ment of armor protective systems for many reasons including availability, fabricability, flexibility, and cost.
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II. CHARACTERISTICS OF METAL ARMOR Armor is intended to provide maximum possible resistance to penetration by all types of projectiles as well as by fragmentation, resulting from the detonation of high explosive (HE) shells, gren ades, mines, high velocity jets, kinetic energy penetrators, etc. In addition, the armor must resist cracking, spalling, and fracture upon multiple impacts while maintaining structural integrity if the armor is being used as a structural member. Metal armor falls into three general categories. These are steel, aluminum, and titanium, all of which through alloying, heat treat ing, or other processing techniques, result in armor materials of different characteristics to meet different threats [3]. The major metallic armor materials and systems which have been developed and incorporated to varied extent on material requiring ballistic protection are illustrated in Fig. 2.. Hardness and toughness, along with material soundness, greatly influence the ballistic performance of the armor. Armor resistance to penetration is also affected by
147
METALLIC ARMOR MATERIALS
I l _
DUAL PROPERTY
TITANIUM
Fig. 2.
FACE-HARDENED PLATE
Classification of metallic armor.
the angle at which a plate is mounted (obliquity) and the angle of impact of the projectile. The greater the armor obliquity, the greater will be the distance the projectile must travel through it in order to perforate it. Figure 3 illustrates the penetration required for various thicknesses of metal armor at obliquities from 0° to 60°. Selection of the optimum obliquity to provide maximum ballistic protection at minimum weight has been widely investigated along with the kind of armor and amount which can be used. The limits within which those factors can be varied are set by the kinds of armor available, the weight restrictions, the shape, and the ease of manufacture. Metallic armor provides greater flexibility and a wider range of limits to the designer who wants to build maximum protection into a combat vehicle or other armored system.
20" Obliquity
45 Obliquity
T.54cm) 0 Obliquity
30 Obliquity
60 Obliquity
Fig. 3. Variation in the distance of p e n e t r a t i o n necessary for perforating a Z.54 cm (1 in.) plate at various obliquities.
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DINO J. PAPETTI
III. ARMOR TYPES Metallic armor is either ferrous or nonferrous and is produced by rolling, forging, or by casting. Rolling and forging are employed to produce plates and casting is used to produce special shapes such as tank turrets, e t c . In many instances wrought armor, after having been rolled or forged is further treated to achieve additional ballistic characteristics, such as face-hardened steel armor. Homo geneous armor plate is used alone as monolithic armor, or several plates can be used in spaced or layered configurations. Homogene ous metal armors are also used to produce metallurgically bonded dual property armor. Homogeneous plate can be fitted with spikes attached to the impact surface to defeat particular types of threats. Because of the wide variety of threats to be considered by armor developers and users, no one metal armor can be expected to cope with all of them efficiently. A. Steel Armor When discussing metal armor, the appropriate introductory discus sion would have to begin with homogeneous wrought steel armor plate. Such a material was introduced over sixty years ago as armor plate on the first tanks developed by the United States Army. Various modifications to the original homogeneous steel armor were developed and evaluated during the next twenty years, but with little or no improvement in ballistic protection. During World War II, rolled homogeneous steel armor underwent an important change due mainly to alloy conservation priorities brought about by the conflict. The resulting armor was predicated on low alloy material with low carbon content and adequate toughness characterized by military specification MIL-S-12560. The current rolled armor specification (MIL-S-12560B) is written for two types of armor, i.e. Class I and Class II. Each Class is heat treated to provide maximum resistance to penetration (Class I) and maximum resistance to shock (Class II). Rolled Homogeneous Armor (MIL-S-12560B) has become the standard steel armor material which is used for comparison to determine performance improvements (merit ratings) of new candi date armors. Prior to World War II only limited experience was available in the production of cast armor. In 1935, limited production of cast armor was started for the Watertown Arsenal. By the l a t e 1930s, both the United States Army and Navy began to show greater interest in armor castings, and work on heavier sections was started [4]. Initial ballistic results were encouraging and further testing and production of heavier cast armor was initiated. By the early summer of 1940, it
METALLIC ARMOR MATERIALS
149
became evident that this country should embark on production of large quantities of cast armor for use on armored vehicles. Cast armor has always been less resistant ballistically than rolled armor due mainly to the fundamental difference in mechanical and metal lurgical properties between rolled and cast steel. This is not meant as a condemnation of cast armor. It is possible to design a casting with smoother contours and higher obliquities than a corresponding structure fabricated from rolled plate and in many cases with equal or even improved ballistic protection. Cast homogeneous steel armor is still used on Army Combat Vehicles under MIL-S-11356 to produce such components as hulls, turrets, cupolas, hatch covers, etc. Another type of steel armor developed during World War II, primarily for aircraft protection, was face-hardened steel armor. Face-hardened armor was processed as to have one face (the impact surface) much harder than the remainder of the plate thickness. The harder depth extended from 15 to 40% of the total thickness. Face-hardened armor has been produced by several different pro cesses including pack carburizing and nitriding. Although facehardened steel armor provided improved ballistic protection against certain projectiles as compared to rolled homogeneous armor, its use was minimized due to inherent brittleness and manufacturing difficulty. Commercial availability of this type armor is virtually nonexistent at the present time. However, military specification MIL-A-00784, entitled "Steel, Face Hard" is still maintained. For a period of about twenty years following World War II, a large amount of empirical data obtained from a variety of tests indicated and continuously verified the fact that armor strength or hardness is a very important parameter in resisting penetration. This important property has been achieved primarily by thermal or thermomechanical processing. The data in Fig. 4 indicates that homogeneous steel armor should be made as hard as possible for defeating small arms, armor piercing (AP) ammunition. However, as homogeneous steel is made harder it becomes more brittle. As the material becomes brittle, its ballistic limit cannot be measured due to severe fracture of the armor. Thus, limits on homogeneous armor hardness have to be established to prevent shatter of the armor due to embrittlement, but not because of strength limitations on the ballistic limit [5]. This important fact has formed the basic guidance for improved steel armor development programs over the years. That is, to increase steel armor ballistic limits by increasing its hardness without increasing the tendency of brittle failure. During the early 1960s, an increased effort was initiated to develop improved armor in response to the conflict in Southeast Asia. An urgent requirement was established for the provision of
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DINO J. PAPETTI
ESR High Hardness
-Ca c+ M
*j
*"
Rolled Homogeneous
O
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1 - 7" - 12" (17.78-30.48 cm) 3 - V - I V (0.64-3.18 cm) 4 - 9" - 12" (22.85-30.48 cm)
r—
5 - 2" - 4" (5.08-10.15 cm) 6 - V - H" (0.64-1.27 cm) 7 -
——Rolled Homogeneous—*■
ARMOR HARDNESS Fig. 4.
^— Uni-Directionally Cast
2 - 2V - 3V (5.72-8.25 cm)
Std. Cast
|*—
Dual Hardness
ARMOR THICKNESS
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/ 8 " - 1" (0.32-2.54 cm)
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/ 8 " (0.64-1.59 cm)
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Relationship between armor hardness and ballistic performance.
modular armor appliques for attachment to trucks and jeeps in ambush situations. The armor was designed principally to protect the vehicles from 0.30 caliber ball projectiles fired from hand carried rifles. One of the materials developed was a high-hardness steel, which was basically a low cost, low alloy steel made by the open hearth process, rolled straight-away on a strip mill and h e a t - t r e a t e d to a hardness level of about 500 Bhn. A typical application is shown in Fig. 5. At the time, it was recognized that the ductile-brittle fracture transition temperature was high; but since using it in modules was restricted to the tropical climate of Southeast Asia, the material was considered to be satisfactory for armor modules. As a modular armor, the high-hardness steel armor performed very well. Over 454 000 kg (1000 000 lbs) of high-hardness steel armor was shipped as armor kits to Vietnam [6]. This type of steel was then used in the unitized construction of an armored vehicle. Prototype vehicles were fabricated and field tested with no apparent difficulty. The vehicle was then put into production for use in Southeast Asia. Several vehicles developed large hull cracks during shipment. The problem was attributed to the inherent brittleness in the material and, since application was intended to be for modular armor, use of the material in a structural
METALLIC ARMOR MATERIALS
Fig. 5.
151
Modular armor p l a t e on 4.94 m e t r i c ton (5 ton) truck.
load carrying capacity was not recommended [7]. Since that time, more rigid controls have been implemented over the fabrication of high-hardness steel armor and its use has continued under closer scrutiny and upgrading of the high-hardness steel armor specifica tion (MIL-S-46100) continues periodically. As previously stated, limits on armor hardness had to be estab lished to prevent shatter of the armor due to embrittlement, but not because of strength limitations on the ballistic limit. In order to take advantage of the resistance to penetration associated with high hardness while circumventing the consequences of reduced tough ness, the two major courses of improving steel armor were to develop metal laminated composites and to improve the casting technology to yield ingots. As a consequence, the wrought materials from the ingots also were improved [8] . In the area of metal laminate armor, the development of dual hardness steel armor is considered the most significant. The concept of dual hardness armor is not new. Examination of armor produced during various periods as far back as the fourteenth century indicated metals of different hardnesses were interwoven by various methods to produce dual hardness armor. The current product is the result of work initiated in the early 1960s. Ballistic data obtained at that time indicated that the most effective method of defeating armor piercing projectiles was to induce shattering of the projectiles upon impact and in order to shatter the projectile effectively, an armor hardness of at least 58 to 62 Rockwell C was required. Monolithic steel armor at that level of hardness would break up or crack excessively upon ballistic impact; thus, the concept of bonding the hard layer of steel to a somewhat softer more ductile back layer was devised. The first test plate of dual hardness steel armor was produced in 1964. The research was sponsored by the Army Materials Research Agency (AMRA) present-
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DINO J. PAPETTI
ly called the Army Materials and Mechanics Research Agency (AMMRC). Philco Corporation Aeronutronic Division along with Republic Steel Corporation produced the plates which consisted of 9 Ni-4 Co alloy with 0.50% carbon in the front plate and 0.30% carbon in the rear plate. Soon after, the front face material was changed to H-ll steel and was marketed as DPSA-2 armor, which was produced by grinding the mating surfaces of the front and rear plates and welding the outside edges of the two materials together. The welded assembly was then metallurgieally bonded by hot-rolling at a temperature of 1149°C (2100°F), reheated to 1038°C (1900°F) (austenitizing temperature), air cooled to a temperature range between 760°C (1400°F) and 593°C (1100°F) and rolled again fol lowed by oil quenching and tempering. This type of dual hardness armor is commonly referred to as ausformed armor since the thermal mechanical strengthening or ausforming is the major part of the processing cycle, as shown graphically in Fig. 6. Ausformed dual hardness steel armor was used in the field during the war in Southeast Asia. Typical applications included critical aircraft component protection as shown in Fig. 7. The armor was also used on Navy Riverine Patrol Boats. Ausformed dual hardness armor, by the nature of its process, is in the fully hardened condition upon leaving the rolling mill. In terms of producing flat, hard dual property armor, this process is efficient. However, fabrication such as machining, forming, etc. is difficult due to the condition of the as-rolled material (flat and hard). Some
Fig. 6. Rolling sequence used to produce ausformed dual hardness armor compared to conventional armor plate rolling.
METALLIC ARMOR MATERIALS
Fig. 7.
153
Dual hardness armor application.
preliminary secondary fabrication can be performed on the ausformed armor, however, the need for more flexibility in this area had become apparent. Since 1964, research and development at the AMRA and the U.S. Steel Corporation determined the feasibility of producing a heattreatable type of dual hardness steel armor which could be produced in plate form in the annealed condition. The plate could then be fabricated to the desired configuration and then heat-treated to armor hardness levels, thus greatly increasing the flexibility of the armor. In 1967, AMRA sponsored a program to establish commercial availability of heat-treatable dual hardness steel armor. The program was successful, as demonstrated by Fig. 8, showing a hydraulic actuator component and a sump cover both made from
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DINO J. PAPETTI
Fig. 8. Heat-treatable dual hardness armor fabricability.
heat-treatable dual hardness armor. The material is presently produced under specification MIL-S-46099. It is the most efficient steel armor plate available commercially for protection against armor piercing projectiles. Figure 9 shows the desired behavior of dual hardness steel armor when impacted by a projectile. Note that the hard frontal face has defeated the projectile by inducing projectile shatter. The expected cracking of the hard impact surface is arrested by the softer more ductile back face and the sound metallurgical bond interfacing the two materials maintains structural integrity of the composite. In the early 1970s, research and development on dual hardness steel was focused on processes to produce the armor at lower cost. Conventional dual hardness steel armor, which is produced by roll bonding, must have clean interfaces, therefore, slab grinding is a standard procedure prior to welding the periphery of the front and rear components. This operation is expensive and efforts to eliminate it were initiated. AMMRC along with the Aeronutronic Division of Philco-Ford Corporation evaluated test plates of dual hardness steel armor made by the electroslag remelting (ESR) process [9]. The process was used not only to produce the front and rear materials themselves, but also to bond the two components metallurgically to each other by melt bonding using the ESR
METALLIC ARMOR MATERIALS
155
process, thereby eliminating the need for slab grinding, periphery welding, and roll bonding. Figure 10 shows a schematic view of the procedure followed. The ingots for the front and rear components were prepared by the ESR process (to be described in more detail in a subsequent paragraph). Although a 50/50 front-to-rear thickness ratio was to be achieved in the final composite slab, the individual ingots were not of equal thickness. The electrode used for ESR bonding would be of a composition similar to that of the thinner ingot so that when the bonding was completed, the ingot ratio of the composite was 50/50. The composite ingots were then rolled into plates using conventional methods. The plates were then heattreated to dual hardness armor strength levels. Subsequent ballistic testing showed results were reasonably compatible with commercial, roll bonded, dual hardness steel armor. Although the program was considered successful and feasibility was demonstrated, development scale-up was discontinued due to other priorities in the metal armor research and development program. In the early 1970s, the second major development of steel armor was to improve the casting technology in an effort to produce steel armor with high-hardness while circumventing the consequences of reduced toughness. Improved quality steels, when processed into armor plate, perform very well in terms of ballistic properties and ability to sustain multiple impacts with reduced cracking and spallation. It is recognized that armor hardness is most important and that generally speaking, steels of any quality will provide similar ballistic limits if they are heat-treated to the same hardness levels. The steel of higher quality, however, will have less tendency
Uilcmi
Fig. 9-
Transverse section through impacted area of a dual hardness p l a t e .
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DINO J. PAPETTI
W (12.7 cm) thick 6SR s i * X-27 steal
t / 2 " (1*27 cm) thick H*11 attctroctes g
■18" (45.72 cm)
, r„ Water cooled copper and pfatas 3" (7,62 cm) thick ESB slab H-11 steel Molten sla§ Solidifying melt
20" (50.8 cm
Fig. 10.
Stool, slabs sptctd 1 " > 12.54 oml apart
ESR bonded dual hardness armor process.
to crack and spall after multiple ballistic impacts. The reasons for this are varied depending on the processes used to produce the materials, yet the advantages can, in most cases, be traced to the metallurgical superiority of one material over another. The ballistic superiority of steels of higher metallurgical quality has been demonstrated often. An important example was the development of unidirectionally solidified wrought steel armor [10] • Work conducted at AMRA, Massachusetts Institute of Technology, and U.S. Steel Corporation showed that cast steels with superior ductility could be produced by unidirectional solidification, which is loosely defined as controlling solidification of the ingot by casting the metal in such a way that heat is extracted primarily from one surface to produce a cast structure in which columnar grains extend from the chill surface completely through the casting. The result ing solidified steel ingot has been found to be virtually free of gross porosity and with a much finer segregation pattern, factors which contribute to higher ductility. In addition, it was found that a homogenization heat treatment which consisted of holding the
157
METALLIC ARMOR MATERIALS
casting at 1316°C (2400°F) for 64 hs would virtually eliminate alloy segregation in the as-cast structures. Figure 11 shows a chill mold used for casting unidirectionally solidified slab type ingots. Molten steel was poured into the mold. When the mold was filled, loose exothermic powder was placed on top of the exothermic slab and covered with insulation. Since the sides of the mold were lined with exothermic material, the liquid steel was in contact with the cast iron mold only at the bottom, which acted as a chill, thus assuring heat extraction in the downward direction only. Subsequent bottom chills were made of copper which produced smoother bottom surfaces. Figure 12 shows a typical unidirectionally solidified cast steel plate. Note the columnar grain structure, as compared to conventional casting. Steels of armor composition were produced by this process, homogenized, rolled, and h e a t - t r e a t e d to hardness levels ranging from 50 to 60 HRC (Hardness in the Rockwell-C scale). Plates of various thicknesses were ballistically evaluated at each hardness level. Excellent ballistic properties and ductility was exhibited by armor plates at hardness levels as high as 55-56 HRC. Cracking did occur at higher hardnesses. Nevertheless, the results of these tests signified an important breakthrough in homogeneous steel armor development. Figure 13 shows a ballistic test plate made by the unidirectional solidification process at a hardness in excess of 55 HRC. The excellent ductility after multiple ballistic impacts is apparent. Unfortunately, unidirectional solidification loses effec tiveness when the cast thickness increases to a point where the chill bottom does not extract heat efficiently during solidification, thus scale-up of the process to produce large ingots would be difficult, at best. Large ingots produced at a reasonable cost is an absolute requisite for the production of tonnage quantities of steel armor
mm mLU
ctmm Mote*
Fig, 11. Mold for producing unidirectionally solidified steel ingots.
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DINO J. PAPETTI
TOP
BOTTOM OR CHILL FACE
TOP
SLAB TYPE INGOT Fig. 12.
CONVENTIONAL VERTICAL INGOT
Comparison between unidirectionally and conventionally solidified ingots
Fig. 13. Multiple ballistic impact capability of armor plate made from an unidirectionally solidified ingot.
METALLIC ARMOR MATERIALS
159
plate. Because of this fact, the concept of producing armor plate from unidirectionally solidified ingots was abandoned. The decision to curtail work on unidirectional solidification for improved steel armor was made primarily for reasons stated above. An additional factor which contributed to that decision, however, was the emergence of an expanded electroslag remelt (ESR) capabil ity in this country. In the years between 1960 and 1975, the production of ESR ingots in the United States had grown from less than 9078 metric tons (10 000 tons) to over 113 475 metric tons (125 000 tons) per year [11]. The ESR process is a secondary steel melting process characterized by simple equipment, ease of opera tion, and good metallurgical results. A schematic view of the process is shown in Fig. 14. Also shown are some comparisons between steels of similar composition made by conventional prac tices and by the ESR process. In electroslag remelting, energy is introduced into a slag bath by a conventionally made consumable electrode and the circuit is closed through the ingot to be remelted and a water-cooled bottom plate. The liquid slag contained in the water-cooled mold acts as a resistance. The passage of current
Fig. 14.
ESR process s c h e m a t i c .
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DINO J. PAPETTI
through the slag causes it to heat, thereby melting the tip of the electrode. The metal, on dripping through the slag, is freed from impurities. The droplets of refined steel collect in a pool at the bottom of the mold, solidify and result in building up a new ingot, which is of uniform structure over its entire length and cross section. Reactions with the slag remove unwanted elements in the steel. Sulfur content is greatly reduced and oxygen and nonmetallic inclusions are substantially lowered. ESR ingots, unlike convention ally made ingots, are free of shrinkage and central porosity, and in many respects, have been shown to be superior to those made by other remelting processes, including vacuum arc remelting (VAR). Yet, the ESR process is more economical. In 1972, Lukens Steel Company reported the installation of an ESR furnace capable of making large slab-type ESR ingots. The ingots having a cross section of 76 cm (30 in.) by 203 cm (80 in.) and weighing 27 216 kg (60 000 lbs) were reputed to be the largest in the free world. Large slab ingots are most desirable for the production of tonnage quantities of armor plate and the prospect of producing tonnage quantities of high quality armor plate by the relatively economical ESR process could not be ignored. It was, therefore, with much anticipation that a program was initiated in 1972 by the United States Army to conduct an intensive evaluation of armor plate made by ESR steel. The initial program involved testing various alloys of ESR steel at different hardness levels in an effort to determine optimum properties for an advanced structural tank armor which would not only perform as good or better than highhardness armor (described earlier), but also possess substantial toughness so that structural integrity could be maintained in any climatic environment. Of all the material tested, 4340 ESR steel performed the best at a hardness level of 55-56 HRC. The performance of this material had already matched that obtained from unidirectionally solidified armor plate. In addition, the ESR plate could be produced from a 27.2 metric ton (30 ton) ingot. Subsequent testing of 4340 ESR steel produced results a t t r a c t i v e enough to utilize the material at the high strength level for producing a substantial amount of critical helicopter components in the first important application of the integral armor concept. Basically integral armor involves selecting critical components, which require ballistic tolerance, and fabricating these components using ballistically tolerant material, thus eliminating the need for additional armor protection. Since 4340 is a material commonly used to produce components of ordnance material and since excep tional toughness is maintained at higher strength levels of 4340 ESR steel when compared to conventionally made 4340, it was logical to produce and evaluate critical components made from the ESR
METALLIC ARMOR MATERIALS
Fig. 15.
161
Integral armor for critical aircraft components.
material. Hughes Helicopter Company, the prime contractor for the ArmyTs YAH-64 Advanced Attack Helicopter (AAH), included in the design and construction of the AAH over one hundred critical components made from 4340 ESR steel. Figure 15 shows a typical component made from ballistically tolerant 4340 ESR steel. Other parts included flight controls, engine mounts, bearing liners, etc. Survivability requirements of the aircraft were met or exceeded with little or no weight penalties, thus contributing to the superior performance of the AAH. The success of 4340 ESR armor, when used for critical aircraft components, could not be applied to structural tank armor. An important requirement of a structural tank armor is that it should maintain structural integrity at sub-zero temperatures when impac ted by overmatching artillery rounds. Test plates are inspected after proof testing for their ability to withstand fracture, spallation, and cracking. A long standing empirical materials specifica tion, which applies to structural tank armor and its ability to maintain integrity at low temperatures, requires that the material must have a minimum of 27.12 J (20 ft-lbs) Charpy V-Notch resist ance at a temperature of -40°C. In 1975, a program was initiated by AMMRC to optimize an ESR alloy which would combine the ballistic properties of high-hardness steel armor, yet possess ade quate toughness at low temperatures which would enable its use as a
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structural tank armor. Basically, modifications were made to nominal 4340 steel by lowering the carbon and raising the nickel content. Ingots weighing between 181 and 226 kg (400 and 500 lbs) were produced using AMMRCs Research and Development ESR furnace. A typical ingot is shown in Fig. 16, where each ingot is of a different composition. The optimum composition was determined, an additional quantity of ingots were made to that same composi tion, and then armor plate was rolled to various thicknesses from the ingots. They were then heat-treated and thoroughly evaluated for ballistic, mechanical, and low temperature ductility properties. The program successfully established the feasibility of using ESR for tank armor and the requirements were incorporated in specifica tion MIL-A-46173 (MR) Armor, Steel Plate Wrought (ESR), released in February, 1976.
Fig. 16.
Low alloy steel ingots from an AMMRC ESR furnace.
METALLIC ARMOR MATERIALS
163
B. Aluminum Armor Because of their low weight, aluminum alloys have been consid ered for potential armor applications for many years. Studies were initiated in the early 1940s on commercially available alloys by the Frankford Arsenal, Aberdeen Proving Ground, and the Army TankAutomotive Command. The initial objective was to achieve im proved protection against fragmentation by artillery shells. Alloys commercially available at the time were evaluated. The results were good enough to initiate a cooperative program between the United States Army and the aluminum industry to exploit fully existing alloys and to develop new aluminum alloys with improved ballistic and mechanical properties. Aluminum alloys were ulti mately designated for specific armor applications, and included 5083 and 5456 alloys, which were strengthened by strain hardening to increase their resistance to fragment penetration. Additionally, these alloys were characterized by good structural strength, formability, and weldability. Strain-hardened 5083 and 5456 aluminum armor has been used extensively under specification MIL-A-46027C for ballistic protection in many applications, the most notable of which is the M113 Personnel Carrier hull structure. The vehicle, shown in Fig. 17, is a major United States Army armored personnel
Fig. 17.
United States Army M113/M113A1 personnel carrier.
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carrier, currently being produced by the FMC Corporation, with the quantity produced to date well in excess of fifty thousand. The success of 5083 and 5456 aluminum alloy armor against artillery fragmentation prompted an increased awareness and effort to develop aluminum armor for threats other than fragmentation, specifically AP ammunition. The importance of increased armor strength for increased ballistic properties discussed earlier for steel armor applies to aluminum armor as well. Once again, a coopera tive effort was initiated between the United States Army and the aluminum industry to develop, in this instance, an alloy with high strength for AP protection [12]. The effort was in specific response to the requirements of the General Sheridan XM551 tank. Previous studies of the 5083 and 5456 alloys indicated the impracticality of achieving the desired higher strength levels by work or strain hardening. Therefore, attention was directed to the development of a h e a t - t r e a t a b l e alloy. A number of alloys were investigated, the optimum of which was determined to be the 7039 alloy subsequently made in conformance to MIL-A-46063A MR. The 7039 alloy provides significantly better protection per pound than the 5083 or 5456 alloys against AP projectiles due primarily to the increased strength level. Unfortunately, high strength aluminum alloys, like other metallic armor materials, become more brittle as the strength level increases. Thus, high strength alloys which provide improved protection against AP ammunition do so with an accompanying reduction in resistance to fragmentation. The loss of ductility results in severe spalling of the armor upon impact, as shown in Fig. 18. In addition, the higher strength alloys are more susceptible to stress corrosion, a problem which has generated much concern by the United States Army for many years. A strict test has been incorporated in the armor specification to guard against stress corrosion failures in a corrosive environment. Many studies have been conducted over the years in an attempt to improve the ductility of high strength aluminum alloys. Generally, these attempts were directed to improving the purity of the material to improve the ductility at the higher strength levels. Improvements in ingot making practice along with mechanical processing techniques and thermal treatments were modified to achieve higher ductility. During the past several years the purity of commercially available aluminum has increased significantly. In view of this, high purity grades of 2024 and 7075 alloys were studied comprehensively and compared to similar alloys of lower purity. Although some improvement was obtained, a significant increase was not achieved. Research aimed at developing aluminum armor alloys with im proved mechanical and ballistic properties is continuing. Controlled
METALLIC ARMOR MATERIALS
Fig. 18.
Back spalling of 7000 series aluminum armor.
solidification and newly developed thermal mechanical processing techniques are being emphasized with the hope of producing one aluminum armor alloy, which would combine efficient AP and fragment protection along with acceptable stress corrosion resist ance. C. Titanium Armor Titanium armor development began in the early 1950s. As was the case in aluminum armor development, the low weight of titanium compared to steel provided the inducement. A titanium armor alloy was developed in 1955 which offered excellent ballistic protection against fragment type ammunition. The alloy (Ti-6A1-4V) was considered superior to all other titanium alloys investigated up to that time. Subsequent work on titanium armor has focused on the
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evaluation of commercial and experimental alloys in an effort to obtain ballistic improvements over the 6A1-4V alloy. Chemical composition, heat treatment, and fabrication techniques were the main areas studied for optimization [13]. It became readily apparent that metallurgically sound ingots with a minimum amount of inhomogeneities were required which when rolled to plate would reduce the material weakness in the thickness plane. This weakness has been associated with spalling behind the plate upon ballistic impact shown clearly in Fig. 19. The designation Ti-6A1-4V ELI has been used to obtain titanium armor with extra low interstitial (ELI) content. The ELI grade has somewhat higher ductility along with ballistic advantage especially at lower thick nesses. Back spalling which was prevalent in earlier titanium armor has been substantially reduced. The titanium armor currently described under specification MIL-T-46077 (MR) represents the best current practice for producing 6A1-4V titanium armor alloy. Mater ial produced under this specification has ballistic superiority over standard steel and aluminum armor over the majority of fragmenta tion and small arms ammunition threats. Titanium armor has been used extensively on aircraft for ballistic protection however. Because of its high cost when compared to other metallic armor, its use on ground combat vehicles is virtually nonexistent. The severe weight limitations imposed on aircraft do not apply to ground combat vehicles. Thus, the trend has tradition ally been for tank designers and manufacturers to select steel and aluminum armor which best satisfies both cost and performance
Fig. 19.
Titanium alloy after ballistic a t t a c k with a 0.30 caliber AP M2 projectile.
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restrictions and requirements, with weight considerations occupying a somewhat lower priority in vehicles than in aircraft. In order for titanium armor to be more widely used, additional ballistic improvement must be achieved. This will best be realized when titanium armor can be used at higher strength levels without brittle plate failure.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13
W. Millis, Arms and Men—A Study of American Military History, G.P. Putnam Sons, NY, 1965, p. 75. T. Wintringham, The Story of Weapons and Tactics, Houghton Mifflin Co., Boston, MA, 1943, p. 173. The Vulnerability of Armored Vehicles II—Armor and Its Resistance to Penetration (U), Technical Information Rept., Office, Chief of Ordnance, Washington, DC, 1956, p. 1. Ordnance D e p a r t m e n t Journal, Manufacture of Cast Armor During World War II, Steel Founders Soc. of Amer., Cleveland, OH, 1946, p. VII Preface, 1,3,17. D.J. P a p e t t i and K.H. Abbott, Simulating Soviet Armor (U), Rept. No. MEDA-7, AMMRC, Watertown, MA, 1969, p. 2. K.H. Abbott and D.J. P a p e t t i , Evaluation of High Hardness Steel (U), Rept. No. ABI-109, AMMRC, Watertown, MA, 1968, p. 1. E.N. Hegge, Lightweight Armor (U), Rept. No. ABI-49, AMMRC, Watertown, MA, 1966, p. 4,5. H.P. George, Metallic Armor Materials (U), in Proc. Third Symp. Lightweight Armor Materials (U), M.M. Jacobson (Ed.), Rept. No. MS69-02, AMMRC, Watertown, MA, 1969, p. 189. B.N. Briggs, Development of Low Cost Heat Treated Electroslag Melt-Bonded Dual Hardness Steel Armor, Aeronutronic C o n t r a c t Rept. No. CTR 73-42, Philco-Ford Corp., AMMRC, Watertown, MA, 1973, pp. 1-3,7. C.E. Bieniosek, K.F. Skidmore, and L.F. P o r t e r , Unidirectionally Solidified Wrought Steel Armor, C o n t r a c t Rept. No. CR69-01 (F), U.S. Steel Corp., AMMRC, Watertown, MA, 1969, pp. 1,2,4. Electroslag Remelting and Plasma Arc Melting, Rept. No. NMAB-324, National Materials Advisory Board, Washington, DC, 1975, p. 1,3 (draft copy). W.A. Mannschreck and C.J. Porembski, Mechanical Properties of 2024 and 7075 Aluminum Alloy (U), Rept. No. R-1852, Frankford Arsenal, Philadelphia, PA, 1967, p. 1. J.L. Sliney, Metallurgical and Ballistic Study of Titanium for Armor Part III (U), Rept. No. TR 63.34, AMRA, Watertown, MA, 1963, p. 11,12.