EP0349446B1 - Process for directly forming and for optimizing the characteristics of armour-piercing projectiles made of high-density tungsten alloys - Google Patents

Abstract

This invention relates to a process for directly forming and for optimising the mechanical characteristics of armour-piercing projectiles. <??>This process involves using a blank of ductile heavy metal having an axis of revolution and mass per unit volume at least equal to 17000 kg/m<3> and is characterised in that the said roughly prepared blank is subjected to a finishing treatment at a temperature between ambient temperature and 500 DEG C and at a rate variable in a direction parallel to the axis of the blank. <??>This process is used in military munitions. <IMAGE>

Description

This invention relates to a process for direct shaping and optimization of the mechanical characteristics of perforating projectiles made of high density tungsten alloy, in particular for military munitions.

The armor-piercing projectiles used in military weapons have undergone significant development in recent years.
The use of increasingly dense alloys on which an optimization of the mechanical characteristics has been sought, combined with an increase in the firing speed, has made it possible to obtain increasingly effective projectiles.

• les alliages à base d'uranium appauvri avec lesquels on peut obtenir une masse volumique voisine de 19000 kg/m³ et une bonne ductilité et dont l'utilisation est rendue attractive par la nécessité de trouver des débouchés aux stocks d'uranium appauvri générés par l'industrie nucléaire;
• le carbure de tungstène avec un ajout d'environ 13% à 15% de Cobalt qui a cependant l'inconvénient d'avoir une masse volumique de 14000 kg/m³ insuffisante pour certaines applications. Par ailleurs, sa faible ductilité peut représenter un handicap pour traverser des cibles multiples;
• les alliages à base de tungstène élaborés par métallurgie des poudres, c'est-à-dire le tungstène avec ses impuretés habituelles dont la faible ductilité et l'usinage toujours très délicat constituent un handicap pour sa mise en oeuvre, mais aussi le tungstène avec des ajouts volontaires par exemple de nickel, cuivre, fer, conduisant aux alliages de type W-Ni-Cu et W-Ni-Fe dont les propriétés peuvent être relativement bien réglées en fonction de leur utilisation. C'est le cas pour les alliages W-Ni-Cu de masse volumique comprise entre 17500 et 18500 kg/m³ environ qui ont une ductilité moyenne intéressante lorsqu'une fragmentation du projectile est recherchée mais aussi et surtout pour les alliages W-Ni-Fe dont la masse volumique peut être ajustée également entre 17500 et 18500 kg/m³ en agissant sur la teneur en tungstène (93% à 97% en poids) et la ductilité modifiée en fonction du rapport Fe/Ni.

Among the alloys, there may be mentioned:

• alloys based on depleted uranium with which one can obtain a density close to 19000 kg / m³ and good ductility and the use of which is made attractive by the need to find outlets for the stocks of depleted uranium generated by the nuclear industry;
• tungsten carbide with an addition of about 13% to 15% of Cobalt which however has the disadvantage of having a density of 14000 kg / m³ insufficient for certain applications. In addition, its low ductility can represent a handicap for crossing multiple targets;
• alloys based on tungsten produced by powder metallurgy, that is to say tungsten with its usual impurities whose low ductility and always very delicate machining constitute a handicap for its implementation, but also tungsten with voluntary additions, for example of nickel, copper, iron, leading to alloys of the W-Ni-Cu and W-Ni-Fe type, the properties of which can be relatively well regulated according to their use. This is the case for W-Ni-Cu alloys with a density between 17,500 and 18,500 kg / m³ approximately which have an interesting average ductility when fragmentation of the projectile is sought but also and above all for W-Ni alloys. Fe whose density can also be adjusted between 17,500 and 18,500 kg / m³ by acting on the tungsten content (93% to 97% in weight) and the ductility modified as a function of the Fe / Ni ratio.

Obtaining W-Ni-Cu and W-Ni-Fe alloys also known as "heavy metals" calls for powder metallurgy. The raw materials used consist of powders of each of the metals with a particle size of between 2 and 10 μm approximately. These are mixed in rotary devices, in particular, so as to obtain a homogeneous product whose analysis responds to the intended composition.

This mixture is then put into the form of blanks suitable for the desired application by an operation either of compression in a steel-shaped matrix, or of isostatic compression during which the powder placed in a rubber mold is subjected to the action of a compression fluid in a high pressure enclosure.

These blanks are porous, of low density and fragile and it is necessary to subject them to densification which is carried out by sintering between 1400 and 1600 ° C. approximately in ovens under a hydrogen atmosphere. During this operation a ternary phase constituted by the three metals in presence is formed by diffusion and becomes liquid. This liquid coats the tungsten grains and allows complete densification of the alloy by a notable dimensional shrinkage of the blank.

• masse volumique : 17 500 kg/m³
• résistance à 0,2% d'allongement R_p 0,2 : 750 MPa
• résistance à la rupture Rm : 950 MPa
• allongement en % : 25

présente, après un corroyage homogène sous un taux de réduction de la section de 18% environ, les valeurs de résistance suivantes :

Rp 0,2

: 1100 MPa

Rm

: 1250 MPa

The alloys based on tungsten metal whose production process has just been described are likely to have good ductility: thanks to this property, it is possible to improve their elastic limit and their breaking load by working.
For example, a blank made of an alloy containing by weight 93% W, 4.5% Ni and 2.5% Fe which after sintering at 1450 ° C has the following characteristics:

• density: 17,500 kg / m³
• resistance to 0.2% elongation R _p 0.2: 750 MPa
• breaking strength Rm: 950 MPa
• elongation in%: 25

has the following resistance values after homogeneous working under a cross-section reduction rate of around 18%:

Rp 0.2

: 1100 MPa

Rm

: 1250 MPa

Such a hardened material is used to make sub-calibrated projectiles intended for the perforation of armorings, because it has a high elastic limit capable of withstanding the stresses due to acceleration in the barrel where the exit speeds can reach 1400 to 1600 m /dry.

In this type of application, the blank is generally of cylindrical shape and the working obtained by hammering in the process. To give the blank the final profile of the projectile, it is then subjected to suitable machining.

Such a process has been described in US 3,979,234. It is said therein that the W-Ni-Fe projectiles of composition by weight: 85-90% W and such that Ni / Fe is between 5.5 and 8.2, are obtained by powder compression, sintering, wrought with a reduction rate of 20% then final machining of the wrought blank. The invention indicates that it is thus possible to obtain a Rockwell ^C hardness of 42 uniform to within one unit.

• d'une part, les opérations d'usinage de l'ébauche après frittage et après corroyage conduisent à une perte relativement importante de matière chère ce qui grève fortement le prix de revient des projectiles sans parler du coût de la main-d'oeuvre qu'elle nécessite;
• d'autre part, l'homogénéité des propriétés des projectiles n'est pas toujours souhaitable. En effet, ces derniers sont soumis au cours de leur utilisation à des sollicitations différentes qui sont :
  • . des contraintes mécaniques par choc lors du chargement à cadence rapide dans la culasse du canon
  • . des contraintes élastiques très élevées pendant la phase d'accélération dans le canon
  • . des contraintes diverses lors de l'impact sur la cible qui peut être composée de couches de matériaux différents et qui provoquent des phénomènes de compression, d'écrouissage et d'élévation de températures.

However, it should be noted that such a process has two major drawbacks:

• on the one hand, the operations for machining the preform after sintering and after wrinkling lead to a relatively large loss of expensive material, which greatly increases the cost price of projectiles, not to mention the cost of labor that 'it requires;
• on the other hand, the homogeneity of the properties of the projectiles is not always desirable. Indeed, the latter are subjected during their use to different stresses which are:
  • . mechanical stresses by shock during rapid loading into the breech of the barrel
  • . very high elastic stresses during the acceleration phase in the barrel
  • . various constraints during the impact on the target which can be composed of layers of different materials and which cause compression, hardening and temperature rise phenomena.

Furthermore, it is desirable that in the final phase of perforation, projectiles can fragment to increase their destructive capacity.

It is therefore advantageous to have projectiles having zones with different metallurgical characteristics and optimized so as to best respond to the specific stresses to which they will be subjected locally.

Patent EP-A-143775 provides a satisfactory solution to the latter problem by producing, by cold forging, a tungsten alloy blank of suitable shape, a projectile whose mechanical characteristics vary locally according to the reduction rate of section which can vary between 0 and 30% in a direction parallel to the axis of the blank. However, this document does not provide a solution to the problem of the direct shaping of the projectile into its final shape during forging in order to get rid of any subsequent machining or even even any machining of the blank in shape before forging.

This is why the applicant has sought and developed a method enabling these 2 problems to be resolved simultaneously.

The aim of this process is therefore to form perforating projectiles, in particular for military munitions, by work hardening, at a temperature between ambient and 500 ° C., of a compressed and sintered blank of a tungsten alloy comprising additions of metallic elements such as Fe, Ni, Cu having an axis of revolution and density at least equal to 17000 kg / m³ said work hardening being carried out according to a variable rate of reduction of section in a direction parallel to the axis of the blank of appropriate shape, characterized in that in order to simultaneously obtain the projectile in its final shape as well as variable characteristics and locally adapted to the constraints of use, said blank of suitable shape is subjected to a treatment of work hardening, with a reduction rate of section which can vary from 5% to 60%, by a shaping tool comprising hammers whose profile is determined by the f elm of the finished projectile.

For this purpose the stroke of the hammers can be adjusted with a tolerance of ± 0.05 mm relative to the diameter of said projectile.

Thus, according to the invention, use is made of a tungsten alloy preferably chosen from alloys such as W-Ni-Cu and W-Ni-Fe. These metals are put in the form of blanks having an axis of revolution, that is to say that they are most often cylindrical or cylindro-conical.
These blanks have a density at least equal to 17000 kg / cm³ and are obtained by powder metallurgy from tungsten, nickel, iron, copper powders which have been mixed, compacted in the form of blanks and sintered under hydrogen atmosphere between 1400 and 1600 ° C, that is to say under conditions which joined to the nature of the alloy make it possible to obtain ductile products which are not likely to degrade during work hardening.

Another object of the invention is that the blanks of suitable shape obtained roughly produced after sintering, without any prior machining intended to give them the final profile of the projectile, are subjected to a work hardening treatment.

This treatment is carried out on blanks either cold or after moderate preheating and not exceeding 500 ° C. This reheating depends on the nature of the alloy and makes it possible to reduce for some of them the effort to be applied in order to obtain the desired work hardening rate.

Under these conditions, the material which constitutes them being relatively ductile lends itself well to deformation and it is thus possible to give the final profile to the projectile without initially resorting to machining and to confer at the same time a much higher mechanical resistance.

However, unlike the prior art, this work hardening is adjusted in the different sections of the blank perpendicular to its axis of revolution according to a particular rate depending on the shape of the blank so as to obtain throughout the projectile adapted mechanical characteristics, that is to say optimized to the heterogeneous stresses undergone by the projectile during its active phases. Thus, the reduction rate from the initial section S to the final section s of the blank defined by the ratio (S-s) / S x 100 can vary from 5% to 60%.

If an object of the invention is to directly subject the rough rough workmanship and suitable shape to a work hardening treatment to obtain the final profile of the projectile, the method according to the invention applies in the same way to a blank of suitable shape obtained by machining a rough blank of development, generally of simple geometric shape, cylinder, parallelepiped ... according to the prior art. In this case, part of the economic interest of the process which constitutes the elimination of the machining of the sintered blank before wrought is lost but without calling into question the essential object of the process and the advantages, in particular technological, who as a result.

As for the fact of not machining after work hardening, in addition to the advantage which it presents of avoiding labor and maintenance costs for equipment and losses of relatively expensive material, it makes it possible to maintain the surface of the projectile of the surface layers in compression this which greatly improves its resistance to different elastic stresses.

The hardening operation is obtained by any suitable process, preferably by rotary hammering of the blank so as to develop mechanical characteristics exhibiting axial symmetry.
This hammering can be carried out by means of various devices such as for example a rotary and alternating hammering machine equipped with a shaping tool comprising at least two hammers.

Thus, one can use, for example, a tool with 4 hammers whose profile is defined by the shape of the desired projectile.
The hammer striking rate is around 2000 to 2500 rounds per minute.
The hammers are made of high-speed steel but for large series, their development in tungsten carbide proves more judicious to control the phenomena of wear and the dimensional tolerances targeted on the projectile.
To limit the force of the machine, the blanks are preheated before hammering to a temperature between 250 ° C and 500 ° C depending on the materials involved and the hardening rates exerted.
The blank is introduced into the tooling by a pusher system allowing it to be kept between centers and ensuring, using a jack, the translation of the projectile in the axis of the tooling with a variable speed compatible with the constraints of shrinking exerted.

The stroke of the hammers can be precisely adjusted in order to obtain the target hardening rates and the dimensional tolerances required on the different parts of the projectile. The dimensions on the diameter can be easily mastered with a tolerance of ± 0.05mm.

In order to appreciate the variations obtained in the mechanical characteristics according to the rate of work hardening, one gives below in Table I the results, obtained on test specimens of 15mm of diameter corresponding to 3 types of alloys of tungsten, Vickers HV30 hardness measurement as a function of the measurement points relative to the axis of the bar.

• l'évolution de la dureté est une fonction directe de la concentration en tungstène de l'alliage d'une part, et du taux d'écrouissage appliqué, d'autre part.
• à l'intérieur du matériau, la dureté respecte une fonction croissante allant du centre de l'éprouvette aux couches extérieures superficielles.
• cette évolution du centre vers le bord n'est pas linéaire mais devient plus rapide en périphérie et ceci d'autant plus que le taux de corroyage appliqué est élevé. Pour les trois types d'alliages considérés, on constate que :
  • . Pour un taux de corroyage de 6%, l'écart moyen HV30 de 0 à 5 mm est supérieur à celui de 5 à 7 mm,
  • . alors qu'il lui est équivalent pour un taux de corroyage de 10%,
  • . et qu'il lui est inférieur pour un taux de corroyage de 15%

ce que confirme l'intérêt de ne pas éliminer ou détériorer, par usinage, les couches superficielles du matériau obtenues après écrouissage.We observe that :

• the change in hardness is a direct function of the tungsten concentration of the alloy on the one hand, and of the applied work hardening rate, on the other hand.
• inside the material, the hardness complies with an increasing function going from the center of the test piece to the outer surface layers.
• this evolution from the center towards the edge is not linear but becomes faster at the periphery and this all the more so as the applied rate of wrought is high. For the three types of alloys considered, it can be seen that:
  • . For a wrought rate of 6%, the average deviation HV30 of 0 to 5 mm is greater than that of 5 to 7 mm,
  • . whereas it is equivalent to it for a rate of wrought of 10%,
  • . and that it is lower for a 15% wrought rate

which confirms the advantage of not eliminating or damaging, by machining, the surface layers of the material obtained after work hardening.

The invention can be illustrated with the aid of the following 3 application examples which will be better understood with the aid of the 9 attached figures.

These figures represent axial sections of the blanks before and after hammering on which the measured hardness values are indicated. at different points as well as the profile of the tools used for hammering.

Figures 1-2-3 correspond to Example 1, Figures 4-5-6 to Example 2 and Figures 7-8-9 to Example 3.

EXAMPLE 1 Tungsten-nickel-iron alloy with 93% of Tungsten

• 93 % de tungstène pur
• 4,5 % de nickel pur
• 2,5 % de fer pur.

A mixture of powders is produced containing by weight:

• 93% pure tungsten
• 4.5% pure nickel
• 2.5% pure iron.

• Rp 0,2 = 750 MPa environ
• Rm = 950 MPa environ
• A % = 25 environ
• masse volumique = 17600 kg/m³ environ.

The blanks are obtained by isostatic compression at 2000 bars of the powder mixture in molds of homothetic shape to that shown in FIG. 2. They are then placed on alumina trays and sintered in a passage oven under a hydrogen atmosphere at 1460 ° C.
After treating the blanks under vacuum at 1100 ° C., the following characteristics are noted on test pieces:

• Rp 0.2 = around 750 MPa
• Rm = 950 MPa approximately
• A% = about 25
• density = 17600 kg / m³ approximately.

We then proceed to formatting in a hammer machine with 4 hammers whose profile is shown in fig.1.

In this example, we sought to obtain a high hardness at the front of the projectile (tip), good ductility in the middle part of the projectile and an ability to fragment in the rear part of the projectile.
The striking hammers were made of high-speed steel.
The blanks were preheated to approximately 350 ° C before hammering.
To limit the hardening constraints, the operation was carried out in two successive passages between the hammers.
The tools were adjusted, during the first pass, to a reduction rate of about 25% on the most hardened sections.
After the second pass, a heat treatment was carried out, under argon, at about 550 ° C.
The evolution of the projectile shapes and of the HV30 hardness before and after hammering, is given in figs. 2 and 3.

EXAMPLE 2 Tungsten-nickel-iron alloy with 95% W

• 95% de tungstène pur
• 3,2 % de nickel pur
• 1,8 % de fer pur.

A mixture of powders is produced containing by weight:

• 95% pure tungsten
• 3.2% pure nickel
• 1.8% pure iron.

• Rp 0,2 = 720 MPa environ
• Rm = 940 MPa environ
• A % = 25 % environ
• masse volumique = 18000 kg/m³ environ.

The blanks are compressed in an isostatic enclosure at 2000 bars in molds of rubber shape homothetic to the shape of the blank shown in fig.4.
They are then sintered in a hydrogen oven at 1510 ° C. After treating the blanks under vacuum at 1100 ° C., the following characteristics are obtained on test pieces:

• Rp 0.2 = 720 MPa approximately
• Rm = 940 MPa approximately
• A% = about 25%
• density = 18000 kg / m³ approximately.

The hammering operation is then carried out using the machine cited in Example I. The profile of the hammers adapted to this type of projectile is defined in FIG. 4.

In this example, a high hardness was sought in the point of the projectile, a high elasticity in its middle part and a high ductility at the rear.
The striking hammers were made of high-speed steel.
The blanks were preheated to about 400 ° C before hammering.
The hammering operation was carried out in a single pass.
A heat treatment was then carried out, under argon, at approximately 860 ° C.
The evolution of the projectile shapes and the HV30 hardness, before and after hammering, is given in figs. 5 and 6.

EXAMPLE 3 Tungsten-nickel-iron alloy with 98% W

• 96,85% de tungstène pur
• 2,15% de nickel pur
• 1,00 % de fer pur.

A mixture of powders is produced containing by weight:

• 96.85% pure tungsten
• 2.15% pure nickel
• 1.00% pure iron.

• Rp 0,2 = 740 MPa environ
• Rm = 960 MPa environ
• A % = 17 environ
• masse volumique = 18500 kg/m³ environ.

The blanks are compressed in an isostatic enclosure at 2000 bars in molds of rubber shape whose shape is homothetic to the blank shown in fig.7.
They are then sintered in an oven passing under hydrogen at 1600 ° C. After a vacuum treatment at 1100 ° C., the following characteristics are obtained on test pieces:

• Rp 0.2 = approximately 740 MPa
• Rm = 960 MPa approximately
• A% = about 17
• density = around 18500 kg / m³.

The hammering operation is then carried out using the machine cited in Example I. The profile of the hammers adapted to this type of core is defined in FIG. 7.

In this example, a maximum hardness was sought in the point of the projectile, high hardness combined with significant ductility in its middle part, maximum ductility at the rear.
The hammers were made of tungsten carbide.
The blanks were preheated to about 450 ° C.
The hammering operation was carried out in two successive passes.
A heat treatment was then carried out under argon at about 450 ° C. The evolution of the projectile shapes and the HV30 hardness, before and after hammering, is given in figs. 8 and 9.

It can be seen that the hammering operation made it possible to increase the hardness values and to make them heterogeneous in particular along the projectile.

Claims (4)

1. A process for shaping penetrating projectiles, in particular for military ammunition, by work-hardening at a temperature between ambient temperature and 500°C of a compressed and sintered blank of an alloy of tungsten comprising additions of metallic elements such as Fe, Ni and Cu, having an axis of revolution, of a density which is at least equal to 17,000 kg/m³, the work-hardening operation being effected in accordance with a variable degree of reduction in section in a direction parallel to the axis of the blank of suitable shape, characterised in that in order simultaneously to produce the projectile in its definitive shape and characteristics which are variable and adapted locally to the stresses involved in use, said blank of suitable shape is subjected to a work-hardening treatment with a degree of reduction in section which can vary from 5% to 60%, with a shaping tool means comprising hammers whose profile is determined by the shape of the finished projectile.
2. A process according to claim 1 characterised in that the travel of the hammers whose profile is determined by the shape of the finished projectile can be regulated with a tolerance of ± 0.05 mm with respect to the diameter of said projectile.
3. A process according to claim 1 or claim 2 characterised in that the blank of suitable shape is a rough-produced blank obtained from a mixture of powders belonging to the group formed by powders of W-Ni-Fe and W-Ni-Cu, which was compressed in a shaping mould and then sintered in hydrogen at between 1400°C and 1600°C.
4. A process according to claim 1 or claim 2 characterised in that the blank of suitable shape is a blank which is obtained from a mixture of powders belonging to the group formed by powders of W-Ni-Fe and W-Ni-Cu which was compressed in a mould in accordance with a simple geometrical shape such as a cylindrical or parallelepipedic shape and then machined.

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