Effect of Strain Rate on the Yield Stress of Ferritic Stainless Steels

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INTRODUCTION AND BACKGROUND

THE excellent corrosion resistance and reasonable cost of ferritic stainless steel alloys make this class of materials important to the forming industry. Specific application of these materials includes automotive exhaust components, from the exhaust manifold to the tailpipe. Common stamped parts include catalytic converter housings, heat shields, stamped mufflers, and hydroformed exhaust tubing. Generally, AISI 409 (Cr ~10 wt pct) is primarily used for exhaust applications, while AISI 439 and 18 Cr-Cb (Cr ~18 wt pct) are generally used on some demanding ‘‘hot’’ end applications (exhaust manifold to catalytic converter), where improved oxidation resistance is desired. These exhaust applications are becoming more challenging from a forming standpoint, as limited space in the undercarriage of modern vehicles requires exhaust components to be routed in indirect and complex ways. The lack of KESTER D. CLARKE, Graduate Research Assistant, MARTIN C. MATAYA, Physical Metallurgy Lecturer, CHESTER J. VAN TYNE, FIERF Professor, and DAVID K. MATLOCK, Charles F. Fogarty Professor, are with Advanced Steel Processing and Products Research Center, Colorado School of Mines, Golden, CO 80401. Contact e-mail: [email protected] ROBERT J. COMSTOCK, Jr.,Senior Research Engineer, is with the Research and Technical Services Division, AK Steel Corporation, Middletown, OH 45043. This article is based on a presentation made in the symposium entitled ‘‘Dynamic Behavior of Materials,’’ which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ASM Mechanical Behavior of Materials Committee. Article published onlined February 22, 2008 752—VOLUME 39A, APRIL 2008

published mechanical property data with respect to strain rate and temperature in these higher alloy materials results in increased challenges during process development and production. Under certain conditions, the ductile-to-brittle transition temperature in ferritic stainless steels is at temperatures as high or higher than ambient.[2,3] In addition, the yield stress varies significantly with temperature and strain rate, which also affects formability. Thus, it has been recommended that ferritic stainless steels should not be formed in conditions under 10 C (283 K), especially in higher Cr compositions or thicker sheets.[3] If ambient or outside (if material is delivered on a just-in-time basis) temperatures are less than 16 C (289 K), the material should be allowed to warm (temperature effect) prior to forming and the forming speed should be decreased (strain rate effect) to prevent cracking and achieve the desired formability.[3] In addition, it is recommended that thinner and lower alloyed material be formed first, allowing the tooling and thicker high alloy material to warm, thereby increasing the probability of forming good parts with the thicker or higher alloyed materials.[3] Developing correlation

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