Hydrogen Effects in Prestrained Transformation Induced Plasticity Steel
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TRODUCTION
HIGH-strength multiphase steels such as transformation induced plasticity (TRIP) steels, which consist of ferrite, bainite, austenite, and martensite, are being used more prevalently in the automobile industry as a means to increase strength and thereby decrease the gage of steel, enabling reduced fuel consumption. As strength levels increase, hydrogen embrittlement receives increased attention and an understanding of fundamental mechanisms involved is of interest. Hydrogen diffusivities and solubilities vary greatly between ferrite and austenite and need to be considered in multiphase microstructures such as TRIP steels.[1–3] Specifically, the room temperature hydrogen diffusivity in ferrite is high, approximately 105 cm2/s,[1] while the corresponding diffusivity in austenite is 1012 cm2/s.[2] As a consequence of the low solubility coupled with the high mobility in ferrite, hydrogen is expected to be distributed inhomogeneously in TRIP steel microstructures and may reside at traps, either reversible or irreversible. Hydrogen traps are defects in steel that provide lower energy sites.[4] Reversible hydrogen traps, which may include dislocations, microvoids, or grain boundaries, J.A. RONEVICH, Graduate Student, J.G. SPEER, Professor, E. De MOOR, Research Professor, and D.K. MATLOCK, Director, are with Advanced Steel Processing and Products Research Center, Department of Metallurgical and Materials Engineering, Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401. Contact e-mail: [email protected] B.C. De COOMAN, Professor, is with the Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 790-784, South Korea. Manuscript submitted June 20, 2011. Article published online March 8, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
have been characterized as having binding energies less than 60 kJ/mol, whereas irreversible hydrogen traps have binding energies in excess of 60 kJ/mol.[4] Reversible traps are weaker hydrogen traps that provide short residence times for hydrogen, but inevitably hydrogen surmounts the activation energy barrier. It has been well documented[1,5] that diffusible hydrogen, i.e., hydrogen not irreversibly trapped, is the cause for most hydrogen embrittlement failures. Thermal desorption analysis (TDA) is a method that uses thermal activation as a means to determine the location of atomic hydrogen with respect to microstructural features. Depending on the strength of a specific type of trap, heat is required to provide the thermal energy necessary to allow hydrogen to surmount an activation energy barrier and diffuse out of the steel. Extensive research on hydrogen in iron and steel was performed,[4,6–10] and activation energies associated with hydrogen traps, such as grain boundaries, dislocations, and microvoids, were reported to be 17.2, 26.8, and 35.2 kJ/mol, respectively. More recently, Escobar et al.[11] investigated the effect of deformation on TRIP steel with 700 MPa UTS on hydrogen trapping capabilities. I
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