Study of Solidification Cracking in a Transformation-Induced Plasticity-Aided Steel
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trength steels (AHSS) are rapidly being implemented in the automotive industry. The increased use of AHSS in car bodies is driven by both regulative guidelines and concerted effort. Transformation-induced plasticity steels (TRIP) belong to the AHSS family. These steels, with strength over 600 MPa, possess increased formability due to the TRIP effect.[1] The TRIP effect is achieved when meta-stable austenite transforms to martensite upon deformation. Matsumura[2,3] proposed the TRIP effect
G. AGARWAL, A. KUMAR, H. GAO, I.M. RICHARDSON, and M.J.M. HERMANS are with the Department of Materials Science and Engineering, Faculty of 3mE, Delft University of Technology, Mekelweg 2, 2628 CD, Delft, The Netherlands. Contact e-mail: [email protected] M. AMIRTHALINGAM is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai, 600036, India. S.C. MOON and R.J. DIPPENAAR are with the School of Mechanical, Materials and Mechatronic Engineering, Faculty of Engineering and Information Sciences, University of Wollongong, Wollongong, Australia. Manuscript submitted November 2, 2017. Article published online February 12, 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A
in low alloyed steel with a composition of C 0.2, Mn 1 to 2, and Si 1 to 2 (in wt pct). Silicon aids the TRIP effect by inhibiting cementite formation. However, an increase in the Si content beyond 0.5 wt pct reduces the galvanizability by forming silicon oxide.[4] Both Al and P are known to retard cementite formation and partially substitute Si. Phosphorus addition increases the amount of retained austenite and additions of 0.1 wt pct P stabilize small austenite grains, even at 100 K.[5] In addition, P is also an effective solution strengthening element[6,7] and addition of up to 0.25 wt pct is considered beneficial with regard to the TRIP effect. [7–9] Apart from formability requirements in these steels, welding is an important fabrication method for automotive applications. It involves the use of an appropriate heat source to form a weld pool, which solidifies during cooling. The weld thermal cycle alters the carefully designed microstructure of the base metal in the fusion zone and in the adjacent heat-affected zone. The term weldability refers to the resistance of a material to failure during or post welding and is often used to qualitatively describe the behavior of a material subjected to welding. From the standpoint of possible welding defects,[10] preventing solidification cracking is an essential prerequisite for the safety of welded components. Weld solidification of an alloy involves the coexistence of a solid and a liquid phase i.e., a mushy zone. The mushy zone generally comprises columnar dendrites separated by liquid. Due to a temperature gradient in the mushy zone, the solid deforms due to both solidification shrinkage and non-uniform thermal contraction; as a consequence, tensile strains are induced in the semi-solid region. During the initial stage of solidification, grains are isolated and can accommodate te
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