Effect of Pre-strain and High Stresses on the Bainitic Transformation of Manganese-boron Steel 22MnB5
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UCTION
PARTIAL press hardening, also known as tailored tempering, is an important technology for the production of safety-related components within the automotive industry. Parts are hot formed in heated tools (in the range of 350 C to 550 C) and afterwards cooled down inside them.[1] Complex shapes can be achieved in parts made of ultrahigh strength steel, which, depending on the cooling conditions determined by the forming tools, may contain martensitic and bainitic microstructures throughout the components. Due to the intrinsic nature of this process, loads and deformations will affect the original austenitic microstructure and consequently the bainitic and martensitic transformation kinetics. The formation of bainite starts with pre-strained austenite, and, at lower temperatures, it is transformed under stress because of the applied load during the press-hardening process. These conditions will significantly affect the transformation; consequently, it is important to understand their role. Additionally, the transformation of bainite during press hardening is influenced by carbon and its diffusion.[2]
DIEGO SAID SCHICCHI and MARTIN HUNKEL are with the Leibniz Institut fu¨r Werkstofforientierte Technologien (IWT), Badgasteiner Str. 3, 28359 Bremen, Germany. Contact e-mail: [email protected] Manuscript submitted May 29, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Pre-deformation states influence the bainitic transformation because of the larger possibility for carbon to diffuse in the structure with higher density of defects, causing its acceleration. Similar behavior has been reported for transformations under the effect of externally applied stresses, where faster transformations occur as the load increases.[3] Hence, the strong coupling among mechanical loading, internal stresses, internal strains, chemical composition and phase transformation kinetics needs to be deeply studied to be able of understanding this phenomenon and developing close-to-reality models for simulation purposes. Moreover, transformation plasticity strains will play a main role in this manufacturing process because of the simultaneous bainitic transformations and high loads involved. It is worth remembering that Greenwood-Johnson[4] and Magee[5] mechanisms are usually considered to explain this phenomenon from the microstructural point of view. The former refers to the plastic strain arising in the mother phase because of the expansion of the newly formed one. The latter corresponds to the formation of selected martensitic/bainitic variants resulting from the applied stress. In the literature, it is very common to find linear models based on Greenwood-Johnson’s mechanism that give good results for low applied stresses (considerably lower than austenite yield strength).[6–9] These models however are not able to capture the nonlinearity of the transformation plasticity strain starting, for the bainite case, in the range of external loads close to the austenite yield strength.[3,10] Relevant contributionshave been made by Leblond and co-w
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