Effect of heat treatment on mechanical and ballistic properties of a high strength armour steel P.K. Jena * , Bidyapati Mishra, M. RameshBabu, Arvindha Babu, A.K. Singh, K. SivaKumar, T. Balakrishna Bhat Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad 500 058, India article info Article history: Received 11 February 2009 Received in revised form 8 September 2009 Accepted 16 September 2009 Available online 6 October 2009 Keywords: Ultra high strength steel Heat treatment Ballistic property abstract In the present study an ultra high strength armour steel was austenatised at 910  C followed by tempering at 200, 300, 400, 500 and 600  C. After heat treatment the properties of tensile strength, ductility, charpy impact strength, hardness and microstructure were evaluated from the mechanical tests and metallographic analysis respectively. The ballistic behavior of the heat-treated plates was evaluated impacting against non-deformable hard steel core projectiles at 840  15 m/s at normal angle of attack. The changes in the microstructure and mechanical properties with heat treatment have been correlated with ballistic performance of the steel. Experimental results showed that 200  C tempering gives the best ballistic performance. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction The armour on combat vehicles has always been constrained by its weight, which with rising threat levels has become an increas- ingly serious problem. Much effort is therefore being devoted on the development of armour materials that would provide greater ballistic protection with little increase in weight. Amongst all materials, ultra high strength (UHS) steels are the most extensively used metallic armour today. They possess a unique combination of high strength, high hardness with good toughness, weldability and ease of heat treatment making them attractive for ballistic applications. Generally, quenching and tempering are well established means to produce strengthening in UHS steels, while at the same time retaining or even increasing its impact toughness. Of all the structures produced by heat treatment, martensite forms the highest level of strength in steels. However, because of large internal stresses associated with the martensitic transformation, martensite phase is rarely used in an untempered condition. Tempering increases both the ductility and the toughness, which are essential for enhancing impact energy absorption. Tempered martensite lath structure also provides best dynamic strength in steel [1,2]. Considerable knowledge exists on how the alloy compositions and the heat treatment processes affect the mechanical properties [3–7]. However, the understanding of the effects of heat treatment on ballistic performance is rather limited [8,9]. The present work thus describes the effect of tempering temperature on the micro- structure, mechanical properties and impact toughness of a high hardness heat-treated armour steel. An attempt has also been made to correlate these factors with ballistic properties. 2. Material and experimental details The steel used is a medium carbon UHS steel that can be heat- treated to provide a wide range of hardness values. This steel was made by vacuum arc melting in Steel Authority of India Limited, India. It was supplied in the form of 25 mm thickness rolled plates. The nominal chemical composition of the steel is given in Table 1 . For determining the equilibrium austenite start (Ac 1 ) and finish (Ac 3 ) temperatures, differential thermal analysis (DTA) was carried out at a heating rate of 10  C/min Ac 1 temperature gives the start of formation of austenite from the ferrite-cementite mixture and Ac 3 temperature indicates the completion of austenite transformation [10]. Solid cylindrical samples with a diameter of 1 mm and a length of 1 mm were cut from the as-received plates and were heated to 1400  C under vacuum for DTA experiment. Phase transformation causes a net change in the latent heat of the material at a given temperature. The change in heat flow in the DTA curve indicates the occurrence of a phase change at that particular temperature. From * Corresponding author. Tel.: þ91 040 24586332; fax: þ91 040 24342252. E-mail address: pradipta_rrlb@rediffmail.com (P.K. Jena). Contents lists available at ScienceDirect International Journal of Impact Engineering journal homepage: www.elsevier.com/locate/ijimpeng 0734-743X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijimpeng.2009.09.003 International Journal of Impact Engineering 37 (2010) 242–249