Solidification and Liquation Cracking Behavior of Dual-Phase Steel DP600
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INTRODUCTION
THERE is a need for reducing the weight of automobiles to minimize fuel consumption and CO2 emissions.[1] The use of advanced high-strength steels (AHSS) in place of conventional automotive-grade steels in automotive vehicles can help achieve significant weight savings. Automotive applications require materials to have good strength and ductility. Advanced high-strength steels (AHSS) fulfill this requirement.[2,3] In the event of a crash, AHSS perform far better and offer higher resistance to intrusion than conventional automotive-grade steels.[4] However, at present, the use of AHSS in the automotive industry is limited because of some concerns about their formability and weldability.[5,6] Among the various types of AHSS, dual-phase (DP) steels offer the greatest potential for industrial use.[7,8] DP steels have a microstructure of ferrite matrix with martensite islands dispersed in it. Because of their unique microstructure, DP steels exhibit no yield point phenomenon and a high ratio of ultimate tensile strength (UTS) to yield strength (YS) as well as a high hole expansion ratio, which gives them good formability.[9–11] Weld metal solidification cracking and heat-affected zone (HAZ) liquation cracking are problems associated with fusion welding of some grades of AHSS. Weld hot cracking is a typical occurrence governed by the
metallurgical and thermo-mechanical phenomena happening in the semi-liquid zone surrounding the weld pool.[12] As per Kou,[13] the tensile strain was induced in the semi-solid weld metal because of thermal contraction and obstruction of solidification shrinkage of the semi-solid weld metal, which leads to cracking around the grain boundaries. Solidification cracking susceptibility is a function of many mechanical as well as metallurgical factors.[14] The mechanical factors such as high thermal contraction, high solidification shrinkage and high external restraint lead to higher stresses and more susceptibility to hot cracking. Metallurgical factors such as a larger solidification temperature range, mode of solidification, solute redistribution and amount of liquid solidifying in the end influence the weld cracking susceptibility.[15] In steel, during the final stage of solidification, segregation of alloying elements and/or impurity in the inter-dendritic liquid leads to the formation of low melting eutectics, which can lead to cracking when stresses due to shrinkage and/or external restraint cannot be accommodated by these terminal liquid films.[16] Currently, no work has been done on the hot cracking or liquation cracking aspects of DP steels. In view of the above, in the present study, the hot cracking behavior (fusion zone hot cracking and HAZ liquation cracking) of a dual-phase steel DP600 was examined using Varestraint tests and hot ductility tests.
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EXPERIMENTAL DETAILS
A. Materials AMIT A. KURIL, G.D. JANAKI RAM, and SRINIVASA R. BAKSHI are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai, Tamil Nadu 600036, India. Con
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