Table 1 — Chemical Composition of TRIP800 Steel (wt%) Elements C Si Mn P S Cr Mo Al Fe TRIP800 0.2 1.66 1.69 0.015 0.0002 0.006 0.011 0.43 Balance Table 2 — Welding Parameters Welding Welding Force Squeeze Hold Current Time (kN) Time Time (kA) (cycles) (cycles) (cycles) 1 5 6 15 15 3 10 5 15 6 20 7 25 8 9 10 Note: 1 cycle = 0.02 s A B C D E Fig. 1 — Microstructure of TRIP800 steel. A — Asreceived base metal; B — colddeformed base metal; C — SEM microstructure of asreceived TRIP800 steel; D — SEM microstructure of colddeformed base metal; E — SEM microstructure of colddeformed base metal at high magnification (M: martensite, A: austenite, : ferrite, b: bainite). given in Table 2. Mechanical Test and Metallographic Evaluation All the welded joints were exposed to a tensile-shear test for determining joint strength. For this purpose, five test samples were prepared for each weld variable. Samples were tested with a crosshead speed of 10 mm/min. A sample was cross sectioned through the center of the button, mounted, ground, polished, and then etched for 5 s with 2% nital. The weldment structure was evaluated by optical and scanning electron microscope (SEM). The hardness measurements were carried out by using a Shimadzu microhardness tester with a load of 500 g. Weld Button Geometry Evaluation The weld button geometry — button diameter (dn), button height (hn), and electrode indentation depth (ei) — was measured on the metallographic cross section of the weldment. The common criterion for the average weld button diameter should be equal to or larger than 4√t (t: material thickness in mm) for desired pullout failure (PF) mode for steels. However, there is no information on average weld button diameter, weld button height, and weld button size ratio (hn/dn) for colddeformed and welded samples for producing the desired PF mode. Results Effect of Deformation on the Structure of Weldment The microstructure of the asreceived TRIP800 steel contains bainite, martensite, and retained austenite phases embedded in a continuous ferrite matrix, as seen in Fig. 1A and C. The microstructure of the asreceived samples was taken from the vertical cross section in the rolling direction. However, the cold-deformed samples exhibited elongated grains along the strain direction, as seen in Fig. 1B. Most of the retained austenite in the structure was transformed into martensite (Fig. 1D) due to cold deformation process. A small amount of retained austenite grains were observed in the structure, only under high magnification — Fig. 1E. The as-received and cold-deformed TRIP steels were also analyzed by using x-ray diffractometer (XRD) to clarify the present phases in the structure — Fig. 2. As-received TRIP800 steel analysis revealed peaks for austenite, ferrite, and martensite, which are the main constituents also observed using metallographic examinations. However, deformed sample analysis revealed peaks for ferrite and martensite. The XRD result for cold-deformed TRIP800 steel indicated no evidence of austenite peaks, hence, it is thought that the majority of retained austenite WELDING RESEARCH 78-s WELDING JOURNAL / MARCH 2016, VOL. 95
Welding Journal | March 2016
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