Improving Ballistic Resistance of Armor Steel by FCAW with Hardfacing Alloys of Fe–Mo–Mn–B–C System

Abstract Armor steels can be strengthened against modern ballistic threats through surface hardfacing with advanced Fe-based alloys. This study develops a series of flux-cored arc welded (FCAW) hardfacing alloys in the Fe–Mo–Mn–B–C system to enhance the ballistic resistance of Armox® 440T steel. Single-layer (~3 mm) hardfacings were deposited on 6 mm armor plates using an automated hardfacing device, and subsequently characterized with respect to their microstructure, phase composition, and ballistic performance. Thermodynamic calculations and microscopy revealed an austenitic matrix reinforced by hard boride inclusions. By increasing the Mo and B concentrations, the alloy solidification shifted from a primary austenite + austenite boride mixture to a primary boride + austenite boride mixture structure. The compositions with high Mo:B ratio yielded a ~25% volume fraction of Mo2(Fe,Mn)B2 borides. Ballistic tests using 7.62 mm steel-core bullets (~830 m/s) showed that uncoated Armox 440T was completely perforated, whereas all hardfaced plates prevented penetration. Notably, coatings with higher boride content eliminated rear-side spalling, indicating superior energy absorption and projectile fragmentation. These findings demonstrate that FCAW-applied hypereutectic Fe–Mo–Mn–B–C hardfacing alloys can significantly improve the ballistic performance of armor plates.