Studies on the role of microstructure on performance of a high strength armour steel

doi:10.1016/j.engfailanal.2007.11.011

Engineering Failure Analysis

Volume 15, Issue 8, December 2008, Pages 1088-1096

https://doi.org/10.1016/j.engfailanal.2007.11.011 Get rights and content

Abstract

This investigation describes and analyses the experimental results pertinent to the ballistic performance of two apparently identical low alloy high strength steel plates against deformable lead projectiles at a velocity about 840 m/s. All the tests are carried out at normal impact angle, i.e. zero obliquity. One plate stopped all projectiles fired at it. However, the other plate failed to stop the projectiles at some locations. Both the plates were subjected to detailed analysis using standard metallurgical techniques to identify the cause of failure in one plate. The experimental results presented include the variation in the microstructure, hardness and retained austenite of the two target plates. The study concludes that the failure is caused by the decrease in resistance of the plate possibly due to higher retained austenite and coarser martensitic structure.

Introduction

Armour is used for providing protection in different applications. Low alloy high strength steels have long attracted attention, due to their high strength and energy absorbing properties, offering many armour applications for personnel, military and non-military vehicles. Before being put to use, these armour steel plates are assessed mechanically and ballistically to confirm their ability to withstand the resistance of armour-piercing ammunition [1], [2], [3], [4]. Mechanical evaluation includes determination of strength and toughness of the steel plates. For ballistic evaluation the plates are fired by the projectiles, they are required to withstand. However, a microstructural observation that is an essential aspect in connection with the performance of armour generally does not form part of the specification.

In the present study, two apparently identically processed armour steel plates of identical composition were brought from the production batch for ballistic clearance. The ballistic performance of the two plates designated 1-A and 5-E was studied in relation to their ability to stop the penetration of the specified projectiles fired at them at high velocities. The 1-A plate passed the test while the 5-E plate failed in the ballistic testing. The cause of failure was analyzed through comparative optical microscopy, XRD analysis, mechanical properties and hardness measurements of the two plates.

Section snippets

Experimental details

The nominal chemical composition of the steel plates is given in Table 1. Rolled and heat treated steel plates of 300 mm × 300 mm × 6.5 mm have been impacted with deformable cylindro–conical lead projectiles of 90 Vickers hardness. The testing arrangement is shown in Fig. 1. The nominal diameter of the projectiles is 7.62 mm whereas the actual diameter is 6.4 mm (Fig. 2). The mass of the projectile is 9.6 g. The projectiles were fired through a rifled gun from a distance of 10 m. The angle of attack is

Result of ballistic test

After the ballistic test, each impact site on the target surface was subjected to detailed examination. First, all the impact sites on both the target plates were photographed from both front and rear. Damage patterns were investigated at the front and rear of the target plates. Table 2 shows velocity of projectiles and the nature of damage caused on the two plates.

The visual comparison of the damage in targets is shown in Fig. 3. It was observed that no projectile was able to penetrate the 1-A

Discussion

Microstructure and XRD studies of both target plates exhibit almost similar microstructural features consisting of martensite and austenite phases. The grain size and lath size of 5-E plate is slightly higher than 1-A plates. However, the volume fraction of retained austenite is considerably high in 5-E plate as compared to that of the 1-A plate. Presence of retained austenite points towards an imperfection in the heat treatment procedure adopted for 5-E plate, where complete transformation of

Conclusion

The importance of microstructural observations in the characterization and quality control of the armour plates is noteworthy. Incorporation of microstructural observation, along with the mechanical and ballistic properties gives a better assurance of the performance of the armour plates.

Acknowledgements

The authors express their gratitude to the Director, DMRL for granting permission to publish this paper. The authors wish to thank Sri. K.K. Papu Kutty and Small arms Range team of DMRL for their help in carrying out ballistic trials.

References (8)

Specification No. CQA(M)/ 51, Controller of quality assurance, Ministry of Defence, Government of India,...
Speccification No. – I.T. 100 E,...
Specification No. – I.T. 80 F,...
GOST B –23958-80, Committee of USSR on Standards....

There are more references available in the full text version of this article.

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