materials Article Microstructure Effect of Heat Input on Ballistic Performance of Welded High Strength Armor Steel Elson Renato Santos Souza 1 , Ricardo Pondé Weber 2 , Sergio Neves Monteiro 2, * and Suzane de Sant’Ana Oliveira 3   Citation: Souza, E.R.S.; Weber, R.P.; Monteiro, S.N.; Oliveira, S.d.S. Microstructure Effect of Heat Input on Ballistic Performance of Welded High Strength Armor Steel. Materials 2021, 14, 5789. https://doi.org/ 10.3390/ma14195789 Academic Editor: Daniel Casellas Received: 7 August 2021 Accepted: 27 September 2021 Published: 3 October 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Brazilian Army Assessment Center—CAEx, Rio de Janeiro 23020-240, Brazil; elsonqmb@yahoo.com.br 2 Department of Materials Science, Military Institute of Engineering—IME, Rio de Janeiro 22290-270, Brazil; rpweber@ime.eb.br 3 Department of Inorganic Chemistry, Federal University of Rio de Janeiro—UFRJ, Rio de Janeiro 21941-909, Brazil; susan.oliver@hotmail.com * Correspondence: snevesmonteiro@gmail.com or sergio.neves@ime.eb.br Abstract: The effect of two different heat inputs, 1.2 and 0.8 kJ/ mg, on the microstructure associated with a welded high hardness armor (HHA) steel was investigated by ballistic tests. A novel way of comparing the ballistic performance between fusion zone (FZ), heat-affected zone (HAZ), and base metal (BM) of the HHA joint plate was applied by using results of the limit velocity V 50 . These results of V 50 were combined with those of ballistic absorbed impact energy, microhardness, and Charpy and tensile strength revealing that the higher ballistic performance was attained for the lower heat input. Indeed, the lower heat input was associated with a superior performance of the HAZ, by reaching a V 50 projectile limit velocity of 668 m/s, as compared to V 50 of 622 m/s for higher heat input as well as to both FZ and BM, with 556 and 567 m/s, respectively. Another relevant result, which is for the first time disclosed, refers to the comparative lower microhardness of the HAZ (445 HV) vs. BM (503 HV), in spite of the HAZ superior ballistic performance. This apparent contradiction is attributed to the HAZ bainitic microstructure with a relatively greater toughness, which was found more determinant for the ballistic resistance than the harder microstructure of the BM tempered martensite. Keywords: ballistic armor; low alloy high strength steel; shielded metal arc welding (SMAW); fusion zone (FZ) and heat affected zone (HAZ); microstructure; V 50 ballistic resistance 1. Introduction Steels used in ballistic armor need both a high hardness to stop an armor-piercing bullet by shattering its tip as well as enough toughness to prevent armor fragmentation after the projectile impact [1–4]. The way to simultaneously achieve these properties, which in principle are antagonists, is by combining a heat treatment, usually quench and temper (Q&T), and adding alloying elements that increase the hardenability of the material [5–8]. In the case of high hardness steel for armor (HHA), the main alloying additives are chromium (Cr), nickel (Ni), and molybdenum (Mo). In the fabrication of combat vehicles, HHA are usually welded by shield metal arc welding (SMAW) [9,10], flux core arc welding (FCAW) [5], gas metal arc welding (GMAW) [11,12], or laser-arc hybrid welding (LAHW) [13]. The welded joint, comprising the fusion zone (FZ) and heat affected zone (HAZ), has been considered over the years a relatively softer region, which should have lower ballistic resistance, in comparison to the base metal (BM). Indeed, in an earlier report, Wilson [14] stated that hardness is a major requirement for ballistic performance. Ade [15] as well as Reddy and Mohandas [16] corroborated hardness as a main factor in the ballistic qualification of armor steel weldment. In a general work on metallic armors, Crouch [17] emphasized the relevance of hardness on the ballistic performance of materials from aluminum to high strength steels. The special case of welded HHA is of common interest regarding ballistic protection. According to Ramana et al. [18], the harder the Materials 2021, 14, 5789. https://doi.org/10.3390/ma14195789 https://www.mdpi.com/journal/materials