Fig. 12 — Secondary electron images of an interfacially failed spot weld in DP20 steel (12 kA current, 10-cycle weld time, 1200 lb weld force).
Table 1 — Measured Chemical Composition of the Two DP600 Steels (wt-%) with CE, as Defined by Equation 1
identical coupons. On each, three spot welds, separated by about 35 mm, were produced. Weld current and weld time were varied between 7 and 40 kA and be- tween 2 and 40 cycles, respectively. The weld force (i.e., the compressive force in- duced by the electrodes) was applied 90 cycles before the current, and lasted 5 cy- cles longer. Although several force levels were selected, primarily to differentiate the two DP600 steels, the weld force was preferentially held at 1200 lb (5.3 kN).
identify the predominant type of fracture (Refs. 9, 10). Other welded samples were tensile-sheared at a low-crosshead veloc- ity of 4 mm/min to precisely capture and quantify weld static characteristics. This test was preferred over cross-tension and coach-peel tests because of greater data reproducibility, reliability, and accuracy; all necessary to compare the two DP600 steels. For tensile-shear testing, the welds were centered in the overlap section, which like the coupon width was 38-mm long. The same test configuration is de- scribed in other studies (Refs. 5, 6).
reagent and examined by optical and scan- ning electron microscopy (SEM). Chemi- cal compositions, including high- resolution mapping, were measured using electron/wavelength dispersive spec- troscopy (EDS/WDS). The Vickers mi- crohardness under a 200-g load was also measured, and linked to the observed microstructures.
Results and Discussion
Base Material Characteristics
The overwhelming majority of the welded samples was either peeled to force the formation of weld buttons and mea- sure their diameters, or chisel-tested to
Several welds were cross-sectioned, cold-mounted, ground, and polished with carbide paper, diamond paste, and fine colloidal silica. The microstructures were then revealed by etching with a 2% nital
Measured chemical compositions, summarized in Table 1, show that the DP18 steel had slightly more carbon than the DP20 steel. This greater carbon content of DP18 appeared to counterbalance a rel-
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