PSI - Issue 14

Rajesh Sharma et al. / Procedia Structural Integrity 14 (2019) 738–745 Author name / Structural Integrity Procedia 00 (2018) 000–000

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Generally, cartridge cases are coated with Gun Kote (MoS 2 )to improve the life from corrosion. To check the efficacy of the Gun Kote coating, cartridge cases were fired. The trials were found unsatisfactory with observations: (a) hard extraction of cartridge cases probably due to excessive expansion in plastic zone and (b) circumferential head separation / shearing. The nature of shearing is identical in all the cartridge cases coated with Gun Kote. One sheared cartridge case was received for failure investigation. Present work is thus concerned with the failure investigation of sheared cartridge case. Attempt has been made to identify the failure mechanism of the cartridge case based on experiment observations. 2. Experimental Visual observations of the damaged cartridge case were carried out and few photographs of the failed component were taken to capture the surface information. In addition, close views of the damaged cartridge case were also photographed. Photography was followed by sectioning of the cartridge case. The fracture surfaces were separated by sectioning and preserved during cutting of samples. Small pieces were sectioned near to the fracture surface and away from fracture surface for microstructural evaluation. Samples at equidistance intervals were sectioned from head to body of cartridge case for microstructural characterisation. The sample containing fracture surface was ultrasonically cleaned. The fracture surface was completely covered with oxide layer which was removed by cleaning with oxide remover reagent followed by acetone cleaning. Subsequently, fracture surface was captured using stereo microscopy. Fracture surfaces were also examined in scanning electron microscope (SEM). Microstructure of the sample was examined in optical and SEM (secondary and back scattered electron modes (SE and BSE)). The polished surface was etched using potassium dichromate solution to reveal microstructure. The chemical composition of cartridge case has been determined by using energy dispersive spectroscopy (EDS) facility attached with SEM. 3. Results and discussion 3.1. Visual observation and stereo microscopy The photograph of as-received sheared cartridge case is shown in Fig. 1. The shearing has occurred near to head of the cartridge case. The head region containing some part of body has been completely separated from the remaining part of body of cartridge case. The outer surface is smooth with coated layer. The inner surface near to head region contains some white deposits with large number of damaged regions looks like pitting. The close view of fracture surface obtained by stereo microscope is depicted in Fig. 2. The fracture surface has slant feature with intermittent projections. The fracture surface is similar at all locations with intermittent projections i.e. some pointing towards inner radial direction while some pointing toward outer radial direction.

Fig. 1. Photograph of as-received cartridge case. Numbers marked from 1 to 6 at different locations for microstructural characterisation.