Quantitative Analysis of Microstructural Constituents in Welded Transformation-Induced-Plasticity Steels
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TRANSFORMATION-INDUCED-PLASTICITY (TRIP) steels are one class of advanced high-strength steels that offer a combination of high strength (tensile strengths of 500 to 1000 MPa), high formability (uniform elongation 20 to 40 pct), and high dynamic energy absorption during high-strain-rate crash deformation (350 MJ/m3 at 1000 s1). The use of high-strength thinner-gage TRIP steels reduces the weight of the automobiles and thus can potentially reduce fuel consumption.[1] Moreover, the higher dynamic energy absorption during a crash improves the passenger safety and the crashworthiness of the vehicle. These combinations of properties are achieved mainly by the deformation-induced transformation of austenite to martensite. Thus, TRIP steels ought to have suitable alloying elements and heat treatment to develop multiphase microstructures with a mixture of ferrite, bainite, and austenite retained at room temperature. Unfortunately, the commercial applicability of TRIP steels is limited by their poor weldability, which is due to the higher alloying content. The thermal cycle of a welding process destroys the carefully designed microstructure, which results in inferior mechanical properties of the weld. A previous microstructural analysis during the welding M. AMIRTHALINGAM, Ph.D. Researcher, and L. ZHAO, Research Fellow, are with the Materials Innovation Institute, 2628 CD, Delft, The Netherlands. Contact e-mail: [email protected] M.J.M. HERMANS, Assistant Professor, and I.M. RICHARDSON, Professor and Head, are with Joining and Mechanical Behaviour, Department of Material Science and Engineering, Delft University of Technology, 2628 CD, Delft, The Netherlands. Manuscript submitted June 8, 2009. Article published online December 2, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
of silicon and aluminum-based TRIP steels showed the formation of complex inclusions in the fusion zones of high-Si and high-Al TRIP steels. These inclusions were found to be comprised of several substructural features with different compositions. The formation of allotriomorphic ferrite was found at the fusions line and the grain boundaries of high-Al steel welds. The partitioning of aluminum to the solidified d ferrite led to the stabilization of ferrite at the fusion lines and at columnar grain boundaries.[2,3] In TRIP steel welds, the formation of hard intermetallic inclusions in the fusion zones and the presence of soft ferritic grains at the fusion boundaries invariably change the partitioning behavior of alloying elements during the phase transformations and affect the stabilization behavior of retained austenite across the weld zone. Thus, in order to improve the weldability of these steels, it is necessary to understand the influence of weld thermal cycles on the microstructural evolution during welding, with a special emphasis on the retained austenite distribution in the weld microstructure. The magnetic saturation technique has developed into a reliable method for accurately quantifying austenite contents in steels. It measures t
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