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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HTPRO FLASH PROCESSING PRODUCES UNIQUE COMBINATION OF PROPERTIES IN STEELS 12 FLASH PROCESSING IS A NEW HEAT TREATMENT CAPABLE OF PRODUCING A COMBINATION OF EXTREMELY HIGH STRENGTH AND HIGH DUCTILITY FOR A GIVEN STRENGTH. D.J. Tung and T. Lolla The Ohio State University, Columbus Flash processing [FP] is an innovative steel heat treatment based on the concept of rapid thermal cycling to create a mixed microstructure consisting of martensite, bainite, and carbides. It is generally understood that the ductility of a material decreases as strength increases. However, the unique microstructure produced by FP results in S.S. Babu The University of Tennessee, Knoxville G.M. Cola, Jr. SFP Works LLC Washington Township, Mich. 70 TWIP/TRIP (high Mn) 60 Elongation, % 50 IF 40 30 Mild-C and C-Mn steels This article discusses the specifics of flash processing, microstructural evolution during processing, the industrial impact of flash processed materials, and ongoing research to further extend the capabilities of FP steels. TRIP 20 HSLA Flash processed 10 Dual phase 0 0 500 Martensitic 1000 Tensile strength, MPa Fig. 1 — Comparison of flash processed steels and other classes of steels; FP steels have high strength and high ductility for their given strength. 1500 2000 Steel sheet Fig. 2 — Schematic of flash processing; total process time including heating and quenching is less than 10 seconds. Feed rollers Flame/ Induction heating unit Quench bath Feed rollers 48 ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER 2013 extremely high strength while maintaining high ductility for the given strength. These properties vary for different alloys, reaching an ultimate tensile strength of 1.99 GPa [288.6 ksi] while maintaining 10.2% total elongation for flash processed AISI 4140. Figure 1 compares this unusual combination of strength and ductility with those of common advanced highstrength steels. A wide range of steels have been flash processed including AISI 1010, 1020, 1050, 4130, 4140, 6150, 8620, and DP800. Plate dimensions up to 6.35 mm 609 mm 3657 mm (0.25 24 144 in.) and tubular dimensions up to 50.8 mm in diameter and 3.05 mm wall thickness (2 and 0.12 in.) have been processed. Processing specifics Flash processing is an extremely rapid heat treatment with a total process time of less than 10 seconds. A schematic of the process is shown in Fig. 2. Steel enters an induction heating unit, which heats the material at a rate of approximately 400°C/s to a temperature between 1000° and 1100°C (1830° and 2010°F), depending on the initial microstructure and steel composition. Unlike traditional heat treatments, the material is quenched after a short hold (about 2 s) above the AC3 temperature. Water quenching in either a constantly agitated bath or by multiple jet spray produces cooling rates as high as 3000°C/s. A low-temperature, short-duration tempering treatment is often used to increase ductility. The process produces a wellcontrolled, mixed microstructure consisting of martensite, bainite, and carbides.

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