ARMAD™, a new steel grade for small calibre gun barrels

Dominique THIERREEAubert & Duval

Metallurgical Department

Jacques BELLUSAubert & Duval

Grades R&D Department

Jean-Marc LARDONAubert & Duval

Applications Engineering Department

Abdelaziz ADDADAubert & Duval

Marketing Department

A subsidiary of the ERAMET Group in its Alloys division, Aubert & Duval designs cutting-edge metallurgical solutions in the form of parts or long products for the projects of the most demanding industries (aerospace, energy, defence, industrial tooling, motor racing, medical, etc.). Its core business is developing, melting and hot transformation (by open and closed die-forging, rolling or powder metallurgy) special steels, superalloys and aluminum or titanium alloys to meet its customers’ most stringent requirements.

For 70 years, Aubert & Duval has been serving defence industries, mainly by producing:

• Gun barrel blanks for small, medium and large calibres

• Missile casings,

• Critical parts for submarines, military aircraft engines, launchers and satellites.

Small calibre guns: requirements and issues

Properties in use of “small calibre” tubes

For the sake of safety and reliability, the gun barrel has to be able to withstand very high pressure on firing and also have great resistance to bursting without projecting material from the barrel. This strength must be guaranteed over a wide range of environmental conditions including temperatures down to – 40°C.

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Fig. 1: Gun barrel burst after shooting - no projections in this case

Fig. 2: Test in severe environmental conditions (ice)

Fig. 3: Test in severe environmental conditions (mud)

Barrel manufacture by hammering

The manufacturing process starts with the preparation of the tips and the outer diameter to ensure the hammering machine engages and strikes correctly. This operation is followed by drilling and reaming, then honing and polishing of the barrel’s inner surface. Once this important preparation has been completed, the cold hammering operation takes place to form the cartridge chamber and rifling. Finishing machining may be followed by mass or local heat treatment before highly precise straightening and straightness control.

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Hammering

The key parameter in obtaining good hammerability is consistency in terms of microstructures and mechanical strength, as well as relieving the stresses from straightening. This leads to great dimensional stability during production but also in service (when shooting). Surface quality is also primordial (no orange peel effect). This property depends on a fine grain microstructure.

Fig. 4: Steel gun barrel manufacturing process

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Fig. 5: Steel gun barrel manufacturing process

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Metallurgy for small calibre gun barrels: ARMAD

Small calibre gun barrels are mostly made from a Cr-Mo base (AISI 4140 and AISI 4150). The most commonly required mechanical properties are approximately: 800 - 1 000 MPa. Other grades based on the 32CrMoV12-10 grade have been developed for more severe applications. In military weapons in particular, 3% Cr-Mo-V grades have been developed. Strength varies relatively widely, from 1,000 to 1,500 MPa. At this level of strength, high toughness must be achieved, usually 40 Joules at - 40 °C. If toughness is too low, the barrel could burst. To give an optimum response to this demand, Aubert & Duval designed a new steel grade called ARMAD

CHEMICAL COMPOSITION

ARMAD’s chemical composition remains based on a 3% Cr, GKH® steel, i.e. 0.3% C – 3% Cr – 1% Mo – 0.3% V. (GKH®: steel developed by Aubert & Duval and used for the FAMAS assault rifle barrel).

The technical improvements in this grade are:

• Very high purity, due to furnace loading quality control,

• Very low phosphorous and sulfur content, due to melting process

Aubert & Duval has specific know-how in alloy melting. Maximum phosphorus content is far below 50 ppm and sulfur is less than 5 ppm. Phosphorus is removed during the electric furnace melting phase. Process control and relevant electric furnace technology enable Aubert & Duval to limit phosphorus pick-up during transfer to the refining stand. The choice of high-purity ferroalloys for final blending also limits phosphorus pick-up during the refining process. Sulfur is removed during the heated ladle refining stage + tank degassing (pressure <1 mBar). Aubert & Duval’s know-how enables it to reach sulfur content standards below 5 ppm,

• Higher molybdenum content, around 1%. This content is adjusted by the steelmaker during the refining operation using ferroalloys according to the specifications for the grade. Molybdenum improves mechanical properties after quenching and tempering heat treatment. It increases the steel’s hardenability. This element precipitates at temperatures close to 600° C,

• Lower silicon and manganese content than standard for alloyed steels. Si and Mn content result from specific know-how relating to the reduction process during the refining phase, due to control of oxidation in the electric furnace. The combination of the two elements significantly improves the balance between strength and toughness.

Our studies show that the combined limitation results in fine precipitated particles after quenching, particularly for (Fe3C, Cr23C6, VC, MoC2) type carbides.

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Hammerability

Fig. 6: Typical structure of the new, low-residual ARMAD grade after quenching and tempering

Fig. 7: Standard structure of a 32CrMoV12-10 type grade after quenching and hardening

Let’s go over the specific characteristics of steel metallurgy for gun barrels.

First the metal has to be softened by tempering. After this stage, strength is around 1,000 MPa. Cold formability does not raise any particular metallurgical problems when the microstructure is homogenous, with lower microstructure banding (figure 6). The hardness range along the diameter must be no more than 20 Hv.

Homogenous hardness depends on several factors. The first factor relates to chemical composition, in terms of both the basic component and its residual component elements. For the basic chemical composition, there are almost no levers, as the grade is strictly codified in customer specifications. For the residual component elements, the lowest level of impurities must be achieved. As described above, the alloy-making process now makes it possible to achieve sulphur content below 5 ppm and phosphorous below 50 ppm. To limit the risks of low temperature weakening, unwanted element content, e.g. arsenic, tin and antimony, must be very low, at under 100 ppm. The control of alloy-making processes leading to such low residual element content has a cost, but this is needed to obtain the exceptional quality demanded for the most critical applications.

Mechanical qualities after hammering

The cold forming resulting from the hammering operation leads to a slight strain hardening, depending on the distortion rate. A recent study (1) showed an approximately 150 MPa increase after a 30% length increase. In addition, 30% forming seems to be the maximum if good toughness is to be maintained at the raw hammering stage. Above 30% forming, a sharp fall in toughness is observed. Consequently, high quality thermal treatment will usually be needed, i.e. quenching and tempering, to give the steel all its metallurgical properties.

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Fig. 8: Change in mechanical proprieties of 32CrMoV12-10 according to cold distortion rate, Ref [1]

Fig. 9: Change in toughness of 32CrMoV12-10 according to cold distortion rate, Ref [1]

QuenchabilityTo obtain proprieties after high-quality heat treatment, achieving a microstructure with no free ferrites to obtain a fully martensitic structure is essential. The CCT (continuous-cooling transformation) process in figure 10 shows the quenchability of the standard grade 32CrMoV12-10. A cooling temperature speed of at least 8°C/s results in a 100% martensitic structure. The microstructure in this case is very fine and comprised of martensite laths. The resulting grain size is 10 ASTM, which corresponds to an average of 10 microns. For ARMAD, a cooling speed of 5°C/s is sufficient, which shows better quenchability, C. The martensitic microstructure is very similar to standard 32CrMoV12-10. The martensitic transformation temperature is the same for both grades.

Change in mechanical properties after high-quality heat treatment

Fig. 10: Standard 32CrMoV12-10 CCT diagram, austenizing temperature 920 °C

Fig. 11: ARMAD CCT diagram, austenizing temperature 920 °C

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Fig. 12: Standard 32CrMoV12-10 martensitic microstructure

Fig. 13: ARMAD martensitic microstructure

The new ARMAD grade offers a better strength/toughness balance than standard 32CrMoV12-10 grades. For example, at 1,300 MPa, the ARMAD grade has 160 Joules impact energy at (- 40 °C), compared with 100 Joules with GKH® and 70 joules for the standard 32CrMoV12-10 grade. The chart in figure 14 describes the change in toughness, Kv, at (- 40 °C) according to mechanical strength Rm (MPa). The ARMAD grade stands out from the other 3% chrome grades with a remarkable Kv (- 40 °C) /Rm (MPa) balance. The very good strength/toughness balance mainly results from the microstructural changes that occur during tempering. For tempering temperatures from 540°C to 600°C, we were able to show the morphology and nature precipitates. In general, precipitates are smaller in size than for standard 32CrMoV12-10 steels. Study of precipitation by transmission electron microscope shows a very fine state of precipitation in the form of very fine needles with average length around 300 nm. These needles are orthorhombic Fe3C type with Mo and Cr enrichment. In the case of standard 32CrMoV12-10, needled precipitates are larger by around 500 nm. Other precipitates with different morphology can also be observed. Rounded (V,Mo)C type precipitates, enriched with Vanadium and Molybdenum, as well as leaf-shaped (Fe,Cr)23C6 type precipitates, were also observed.

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Fig. 14: Change in toughness according to break resistance of the new grade ARMAD, GKH® and standard 32CrMoV12-10 grades at - 40 °C

Fig. 15: Carbon replica precipitation state of the ARMAD grade (photo credit © Centre de Recherche Perre-Chevenard - Aperam)

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Conclusion

The new ARMAD grade developed by Aubert & Duval, derived from 3% Cr steel metallurgy, has the major benefit of outstanding properties in terms of the strength/toughness balance, particularly at very low temperatures. This new development shows that steels in the 3% Cr family, which are already well known in the industrial world, can still be improved. The technical potential seen in this development offers interesting prospects in terms of safety control in use for small calibre guns, but also in applications that demand good strength/toughness balance.

The authors wish to thank the personnel of Aubert & Duval’s Les Ancizes for their valuable help, particularly, S. Avouac, C. Buvat, F.Legay, D. Becuwe and J. Rougier.

(1) The effect of cold forming on structure and properties of 32 CDV 13 steel by radial forging process, Mat. Res. vol. 17 no. 2 São Carlos Mar/Apr. 2014.

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