Advanced Materials & Processes

NOV-DEC 2013

Covers developments in engineering materials selection, processing, fabrication, testing/characterization, materials engineering trends, and emerging technologies, industrial and consumer applications, as well as business and management trends

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mand reports that FP 4130 potentially can reduce product cost and weight and enhance mechanical performance. Metallic armors typically work best against bullets and blast fragments. However, ballistic tests at the U.S. Army Aberdeen Test Center show that 6.3-mm thick FP 4130 with a 50 kg/m2 area density was the highest performing metallic armor against bullets and blast fragments at that mass. The material stopped higher velocity armor piercing bullets and blast fragments better than more costly aluminum, magnesium, and titanium armors now in use. 700 Transformation 1 (bainite) 650 HTPRO x: -3.222 y: 648.9 Temperature, °C x: 0.01079 y: 549.8 14 550 500 450 Transformation 2 (martensite) x: -0.1943 y: 459.4 400 350 x: 20.04 y: 358.8 300 0 5 10 T, °C 15 20 Fig. 5 — SSDTA curve shows two transformation events representing the formation of martensite and bainite. Martensite Frequency 1000 500 Bainite 0 400 450 500 550 600 Hardness, HV 650 700 750 Fig. 6 — Microindentation hardness histogram of flash processed AISI 4130 shows two hardness peaks indicative of approximately 82.5% martensite and 17.5% bainite. processing could have significant impact in various industrial applications. Those requiring high strength and toughness, such as armor, and automotive components requiring good crash resistance would benefit most. nite present throughout an entire FP sample via microscopy due to extremely small feature size and intermixing of phases. An SSDTA curve produced during rapid quenching in Region IV for FP AISI 4130[1] (Fig.5) shows two transformation events: the formation of martensite and bainite in FP due to the transformation temperatures and high cooling rate. A histogram of microindentation hardness values measured over the entire sample cross section (Fig. 6) shows two hardness peaks corresponding to the presence of approximately 82.5% of a harder phase and 17.5% of a softer phase. In the case of FP AISI 4130, phase fractions were determined to represent 82.5% martensite and 17.5% bainite. Research shows that martensite and bainite fractions can be adjusted with different processing conditions, initial microstructure, and steel composition. FP produces negligible values of retained austenite. Industrial impact The properties produced using flash 50 In the automotive industry, many OEMs and Tier 1 automotive suppliers find that FP steels reduce component weight without compromising performance. Hyundai Motor Group recently tested FP steels for use in applications such as vehicle door-side impact beams. In a drop weight test with a mass of 320 kg and an impact velocity of 5 m/s[2], FP tubing outperformed the industry standard boron-steel tubing by 20% in total bending energy absorbed, and by 15% in resisting bending force. Mathematical modeling shows FP steel in automotive bumpers and trailer hitches meet OEM performance requirements at 67% of the weight of current materials. In armor applications, the U.S. Army Research Development Engineering Com- ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER 2013 FP steel has good weldability because it does not require pre- or post-weld heat treating required for other armors with similar ballistic resistance. Softening occurs in the HAZ (also common in Al, Ti, and Mg armors), which is detrimental to ballistic resistance, but vehicles are often built with overlapping joints, which could negate softening effects. In addition, due to its reduced thickness, FP steel can be welded in a single pass compared with other lower density materials, which may necessitate multipass welding due to increased thicknesses required to provide the same strength. FP steels may be easier to fabricate and handle due to greater familiarity with the steel grades versus other materials. Current developments Research in flash processing is currently underway to fully exploit the potential of FP steels and their applications. Researchers at The Ohio State University are examining FP steel weldability with respect to strength and ballistic requirements, exploring strategies to mitigate softening at elevated temperatures, and creating a model with the ability to predict material properties for different applications. HTPRO References 1. T. Lolla, B. Alexandrov, S.S. Babu, and G. Cola, Towards Understanding the Microstructure Development During Flash Heating and Cooling of Steels, Materials Science and Technology, 2008. 2. G.M. Cola, Jr., Flash Bainite Process, Adventures in the Physical Metallurgy of Steels, 2013. For more information: S.S. Babu, Department of Mechanical, Aerospace & Biomedical Engineering, The University of Tennessee, Knoxville, sbabu@utk.edu, www.engr.utk.edu.

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