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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(b) (a) (c) (d) HTPRO 1200 13 Microstructural evolution Flash-processed microstructures depend on the initial microstructure and nonequilibrium conditions resulting from high heating rates, short hold times, and high cooling rates. A schematic of the microstructural evolution during different stages of flash processing of AISI 8620 steel tubing is shown in Fig. 3. Prior to rapid heating (Region I), the initial microstructure consists of ferrite grains with carbides. For FP to be successful, carbides must be chromium-enriched (Fe, Cr)3C carbides in the fully spheroidized condition. This initial microstructure leads to slow dissolution of carbides during the short time spent above the carbide solvus temperature. It is critical that the chromium-enriched carbides do not completely dissolve during processing. The steel is rapidly heated above the AC3 temperature (Region II), causing austenite to nucleate and begin consuming ferrite grains. While rapid heating may cause the A3 temperature to exceed the equilibrium value, the peak temperature is higher than the nonequilibrium AC3 transformation temperature. During the brief hold at peak temperature (Region III), growing austenite completely consumes previous ferrite grains and begins to dissolve carbides. However, carbides do not completely dissolve due to sluggish diffusivity of substitutional elements such as chromium. Further, the short hold time at peak temperature prevents carbon from the dissolving carbides to completely homogenize within austenite grains. These conditions lead to carbon concentration gradients; austenite with 800 Region III 600 400 Region IV Flash-processed steel exhibits very minimal shape distortion. Temperature, °C 1000 200 Region I Region II 0 16 18 20 22 Time, s 24 26 28 30 Fig. 3 — Schematic of the microstructural evolution during flash processing: (a) initial microstructure in Region I, (b) beginning of austenitization in Region II, (c) austenitization with incomplete carbide dissolution at peak temperature in Region III, and (d) final microstructure after quenching consisting of martensite, bainite, and chromium-enriched carbides in Region IV. Martensite Bainite / Fig. 4 — Transmission electron micrograph of flash processed AISI 8620 shows both martensite laths and bainite sheaves perpendicular to a prior grain boundary. regions of high carbon near partially dissolved carbides and regions of low carbon far from the carbides. The material is rapidly quenched (Region IV). Due to the carbon concentration gradients, regions of high carbon undergo a martensitic transformation, while regions of low carbon undergo a bainitic transformation. Martensite is the primary strengthening constituent and bainite provides ductility, resulting in a balance of strength and ductility unique to flash processing. The final microstructure after FP was characterized using transmission electron microscopy, single-sensor differential thermal analysis (SSDTA), and microindentation hardness. A transmission electron micrograph of FP AISI 8620 (Fig. 4) shows both martensite and bainite. However, it is difficult to quantify the fraction of martensite and bai- ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER 2013 49

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