Microstructure and Properties of Different Preforms for Cartridge Case Production

The aim of this work was to evaluate properties of two different preforms for production of 23mm steel cartridge case. Full scale testing, including chemical composition, macrostructure, microstructure and hardness were used. It was assumed that both metallurgical properties, presence of slag and coarse inclusions, as well as process parameters affected those defects that appeared during production of cartridge case from ČR2. In the macrostructure and microstructure of the C10E steel bars, different defects were observed which may have decrease formability during deformation.


Introduction
HE most of the steel cartridge cases are produced of low carbon steel.Plate or rod materials as a preform are used [1].Low carbon steel shave moderate strength and hardness in the as-rolled or extruded condition and good formability in the annealed condition.It can be hardened and strengthened by cold work.The formability of these steels allows high reduction of the wall thickness during manufacture of a cartridge case.It also has fair machinability and good weldability [2].
During production of cartridge cases various kinds of defects can occur, related either to material (microstructure, impurities) or technology parameters (unadequate load, temperature, cooling rate, lubricants etc.).These defects can be deleterious to the manufacture of a quality cartridge case [3][4][5][6].
The main objective of this paper is to characterize two steels for the manufacture of 23mm cartridge case and reveal the cause of defects during theirs production.

Material and technology
The materials used in this study were C10E and ČR2 steels, used for the manufacture of the cartridge case of 23mm.Steel C10E was supplied as bars (φ40 mm) and steel ČR2 was supplied as cups.During manufacturing process, when a preform (blank φ58.2x13 mm) was formed into a cup shape with wall thinning (1 st drawing), in some specimens defects were observed.For this investigation two bars and two cups with surface defects supplied.
The nominal chemical composition of the used steels C10E and ČR2 are given in Table 1.

Methods
Visual observation performed by naked eye and stereo microscope Leica 205A.
Dimension measurement was performed using a comparator.
The chemical composition was analyzed by the Optical Energy Spectrometer (OES) Belec Compact Port.
A transverse cross section of the bars and cups was taken and prepared for microstructure examination.
Macrostructure was revealed after etching in 50 % solution of HCl at 70-80°C after 60 min, according to SORS 1710.
The content of the inclusions estimated according to SRPS EN 10247 (Annex A).
Microstructure was examined on the cross section of the bars and longitudinal section of the cups, using light microscopy (LM) and 3% nitalas etchant.
Energy dispersive X-ray spectroscopy (EDS) was used in conjunction with SEM to characterize the composition of' the chosen area.

Visual examination and dimension measurement
The specimens of the cups after 1 st drawing are shown in Fig. 1.The surface defects, such as wavelength on the top of the wall, as well as notch in circumference in the inner side of the cups, were observed.At higher magnification these defect are shown in Fig. 2. The results of dimension measurements of the cups revealed the difference in the wall thickness on the circumference, as designated in Fig. 1 (tolerance ±0.1 mm).
Larger dimension deviation was found on the cup 2.
All these defects suggest non-regular material flow during drawing.This can be attributed to larger load, due to contact between tool and workpiece or poor lubrication, as well as unsupported or noncentric tools.

Chemical composition
The chemical composition of the bars and cups tested in this work are given in Table 2. Results showed that chemical composition of the bars is according to chemical composition C10E steel (SRPS EN 10084).However, the composition of the cup 2 does not correspond to the requirements of the standard for the steel ČR2 (SORS 1648).The content of carbon of 0.55 mas.% is lower than required (0.09-0.13 mas.%).

Macrostructure
In the macrostructure of the bar 1 defects type D, intensity D1, observed (Fig. 3a), while in cup 2 defects type A, i.e.A1.The macrostructure of the cups is shown in Fig. 4. The regular, continuous material flow in the cup wall observed in cup 1 (Fig. 3a), while in the inner side of the cup 2, in transition zone between bottom and wall side, the notch in circumferential is visible.Also, at the same specimen, along the wall, close to outside, cracks are observed.At higher magnification the notch, where stretching of the material occurred, is shown in Fig. 5a and cracks and voids in bottom in Fig. 5b.

Microstructure
Microstructure of the bars is ferrite-perlite with small grains.Grains are poligonal in the center and towards outside of the bar (Fig. 6a), but between these area banded microstructure was observed.(Fig. 6b).Microstructure of the cup material is ferrite-perlite with spheroidized perlite.This microstructure is expected after applied thermo mechanical treatment of the ČR2 steel.On the other hand, a large longitudinal voids in the vicinity of outside surface of the cup 2 (app.600 μm) were visible.The voids extend from the top to the bottom.Typical appearance of the voids is shown in Figures 7a-c.
The origin of these voids can be related to orientation of grains in the vicinity of the voids and material flow in the tools during plastic deformation.During plastic deformation, on the top of cup wall, large "open" voids form, as shown in Fig. 7a.As deformation proceeded the voids become "closed" and elongated.This is supported by local microstructure, i.e. similar material flow around the voids (Fig. 7b).These defects, observed in the macrostructure (Fig. 5b) are caused by initial defects which were present in the preform.

EDS analysis
Light optical microscopy revealed the presence of coarse inclusions in the microstructure of the bars (Fig. 8).It was found that they are mostly type globular oxide, group ED, type δ.Some of them were over 40 μm.EDS analysis gave their composition (Fig. 9).Also, large content of different size inclusions, as shown in Fig. 10, is observed.It was found that their composition (Fig. 11) correspond to slug composition [7].The presences of such nonmetallic oxides inclusions, as alumina and calciaalumina, or slug, have negative effect on formability, mechanical properties and generate many defects in the steel products [7][8].

Conclusion
The chemical composition and hardness of the tested bars correspond to annealed C10E steel according to SRPS EN 10084 standard.The chemical composition of one of the tested cups (cup 2) does not correspond to the requirements of the standard for the steel ČR2 (SORS 1648).
Macrostructure examination revealed different defects in the tested specimens.In the bars, inclusions series D, D1 intensity, as well as central defects series A, A1 intensity, according to SORS 1710, were observed.In the cup 2 voids and cracks are present, and unregular material flow during plastic deformation occured.
In the microstructure of bars, large content of coarse inclusions, most globular oxides and slugs were found.

Figure 1 .Figure 2 .
The wall thickness: a) cup 1; b) cup 2. Defects of the cup 2: a) on the top of the wall; b) notch in the inner side.

Figure 6 .
Microstructure of the bar: a) in the center; b) 2 mm from the outside surface.

Figure 7 .
Defects in the cup 2: a) at top of the wall; b), c) on the bottom.

Figure 9 .
Figure 9. EDS analysis of the oxide inclusions: oxide inclusion and corresponding spectra.

Figure 11 .
Figure 11.EDS analysis of the slag inclusions: slag inclusion and corresponding spectra.

Table 1 .
Nominal chemical composition of the C10E steel and ČR2

Table 2 .
Chemical composition of the tested C10E steel and ČR2

Table 3 .
Hardness of the bars and cups, HB