Mechanism of Secondary Hardening in Rapid Tempering of Dual-Phase Steel
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UCTION
DUAL-PHASE (DP) steel is composed of soft ferrite (a) matrix mixed with hard martensite (a¢) islands. This microstructure leads to its high strength and excellent ductility, which is why it has been so widely adopted for automotive construction resulting in reductions in vehicle weight and improvements in fuel economy. The strength of DP steel is determined by the volume fraction of martensite within its structure.[1] Although DP steels can be used to produce high-strength components with lower sheet thicknesses than conventional steels, it has also been reported that a decrease in hardness occurs in DP steels after welding, which is commonly termed as heat-affected zone (HAZ) softening.[2–4] Softening occurs when the martensite within the DP microstructure is tempered. In welding applications, softening has been reported to deteriorate weld mechanical properties compared to the base metal.[5–10] In the case of high-strength martensitic steels, this is a challenge as the softened zones may lead to failure in-service or during forming at stress levels far below the ultimate tensile strength (UTS) of the base material. Martensite tempering is a multistage process that is generally performed to enhance the ductility and toughness of quenched steels with martensitic microstructures. DULAL CHANDRA SAHA, Ph.D. Student, SASHANK S. NAYAK, Research Associate, ADRIAN P. GERLICH, Associate Professor, and Y. ZHOU, Professor, are with the Department of Mechanical and Mechatronics Engineering, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada. Contact e-mail: [email protected] ELLIOT BIRO, Manager Product Characterization, is with the ArcelorMittal Global Research, 1390 Burlington Street East, Hamilton, ON L8N 3J5, Canada. Manuscript submitted January 13, 2014. Article published online October 1, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
In the first stage of tempering, at temperatures ranging from 353 K to 453 K (80 C to 180 C), segregation and redistribution of carbon atoms occur at lattice defects—carbon clusters—and transitional carbides, especially e-carbide (Fe2.4C), are formed.[11–13] However, it should be noted that transitional carbides typically only precipitate when the C content of the steel exceeds 0.2 pct. Retained austenite, which is sometimes present at the interlath area of the martensite structure, decomposes into ferrite and cementite between the temperatures ranging from 473 K to 573 K (200 C to 300 C) during the second stage of tempering.[12] In the third stage of tempering, segregated carbon and transitional carbides transform into cementite (Fe3C). This occurs at higher temperatures ranging from 523 K to 623 K (250 C to 350 C).[12] Reduction of hardness in the tempered area of the HAZ is directly influenced by the phase fraction of martensite in the base material. It was reported that fully martensite-containing steel is prone to severe softening during tempering. Therefore, we attempted to modify microstructure of the DP steels by the addition of suitable m
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