Evaluation of the Proportion of Phases and Mechanical Strength of Two-Phase Steels Using Barkhausen Noise Measurements:
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TRODUCTION
NEW-GENERATION steels are a combination of different phases, or constituents. By judiciously choosing phases with different physical or mechanical properties, the properties of the steels can be improved significantly. Among all the phases or constituents that can be obtained using thermomechanical treatment, two are frequently chosen: ferrite and martensite. The first one has lower yield strength but shows significant ductility. The second one presents higher mechanical strength but is more brittle. By combining these two phases, the final mechanical properties of the steel are strongly enhanced. This is the case with dual-phase (DP) steels, which are composed of a matrix of ferrite reinforced by small islands of martensite. The composite-like steels obtained have the advantage of presenting a low yielding ratio, a high work-hardening rate, continuous yielding, and significant elongation.[1] These properties make them suitable for automotive applications. These steels have a better deformability than other low-alloy steels with similar strength.[2] Under quasistatic loading, they are also characterized as having a continuous yielding during plastic deformation, where strength increases with increasing martensite volume fraction, in accordance with a rule of mixture as in composite materials.[3] Similarly, the benefit of combining different phases is also used for other iron-based materials such as the cast iron family. In this case, the formation of a hard phase in the ferritic matrix prevents the formation of fatigue X. KLEBER, Doctor (PhD), A. HUG-AMALRIC, PhD student, and J. MERLIN, Professor, are with Laboratoire Mateis, Batiment Blaise Pascal, INSA-Lyon, F-69621 Villeurbanne, France. Contact e-mail: [email protected] Manuscript submitted November 12, 2007. Article published online April 1, 2008 1308—VOLUME 39A, JUNE 2008
cracks due to the undesirable porosity. Spheroidal graphite cast iron can be improved by adding a small percentage of martensite. In addition to the characteristics of each phase (composition, distribution, etc.), the relative proportion of ferrite and martensite gives rise to the final properties of the steel and of the cast iron. The knowledge of these quantities is thus critical. Among all the techniques available, rare are the techniques that can give information on each phase in a multiphase environment. X-ray diffraction techniques generally fail in that task due to the similarity in the crystal structures of these two phases. Recently, using the neutron diffraction technique, Filippone et al. have succeeded in determining the martensite volume fraction in DP steels using ferrite peak broadening.[4] However, the complexity of the experimental setup needed makes this technique impractical for industrial purposes. The Barkhausen noise measurement is a magnetic nondestructive technique, which can be used to characterize ferromagnetic materials. The signal obtained is sensitive not only to the microstructural state of material but also to the stress state within.[5] Because this tec
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