Effect of Chemistry on Nonisothermal Tempering and Softening of Dual-Phase Steels
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important issue with DP steel, which hampers its use in practical applications, is heat-affected zone (HAZ) softening that occurs in welding because of the tempering of the martensite phase in the base metal (BM).[2–8] Tempering of martensite is well documented in the literature, but it has been addressed mostly to fully martensitic steels subjected to isothermal tempering treatment.[9–11] In the context of explaining the tempering of martensite that causes softening in DP steel, we recently compared the characteristics of martensite tempering in DP steels subjected to isothermal and nonisothermal tempering, and the consequent effect on softening behavior.[1] Nonisothermal tempering cycle was achieved in resistance spot welding (RSW). We observed fine cementite in nonisothermally tempered DP steel unlike the coarser and spheroidized cementite in isothermal tempering. In addition, the extent of softening was observed to vary with steel chemistry and martensite structure, which likely indicates a dependence of tempering behavior on chemistry. Thus, the objective of the current article was to support our recent study by investigating the effect of chemistry on the nonisotherS.S. NAYAK, Postdoctoral Fellow, and Y. ZHOU, Director and Professor, are with the Center for Advanced Materials Joining, Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada. Contact e-mails: [email protected] and [email protected] V.H. BALTAZAR HERNANDEZ, formerly Graduate Student, with the Department of Mechanical Engineering, University of Waterloo, is now Professor, with the MPyM-EPMM Academic Unit of Engineering, Autonomous University of Zacatecas, C.P. 98000 Zacatecas, Mexico. Manuscript submitted April 13, 2011. Article published online August 31, 2011 3242—VOLUME 42A, NOVEMBER 2011
mal tempering of martensite in DP steels and its effect on softening. The starting materials were three zinc-coated 1.2-mm thick DP steel sheets. DP steels used in the study were designated, according to their alloying level and, in turn, carbon equivalent (CE) calculated using Yurioka formula,[12] as follows: lean (DPL), moderate (DPM), and rich (DPR), which are listed in Table I. All chemistries listed are in wt pct. The mechanical properties and volume fraction of martensite of the DP steels are also included in the table. A standard metallographic analysis was used for calculating the volume fraction ( fm ). Figure 1 illustrates the nonisothermal tempering thermal cycle (RSW) used in the study; the details of this are reported elsewhere.[1,5] Figure 2 illustrates a schematic cross section of resistance spot-welded DP steel showing different zones of a weldment. It is to be noted that HAZ is subdivided at the location of Ac1 as upper-critical and subcritical regions[5]; however, the study was focused on subcritical HAZ (location ‘‘a’’ in Figure 2) where maximum softening occurs in the DP steel welds[5] and BM (location ‘‘b’’ in Figure 2), where martensite does not get affected in nonisothermal temp
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