A B dissolving the weld metal in a 25% HCl and 25% HNO3 mixture, followed by collection of the dissolved metal residue on filter paper. The residue as well as the bulk weld metal was examined by XRD analysis. The weld metal solidification was also simulated by calculating the Scheil diagram using ThermoCalc version S with the TCFE6 database. The ThermoCalc investigations examined the chemistry shown in Table 1 for only elements >0.1 wt-% as well as oxygen and carbon, and the stability of all phases within the TCFE6 database. The algorithm for this is included within ThermoCalc, where the equilibrium composition of solid phases are calculated, assuming negligible diffusion in the solid and perfect mixing in the liquid. Results Optical Microscopy The macroscopic section of the joint in the 0.75-in. plate is shown in Fig. 1A, and the typical microstructures observed in the as-deposited and reheated regions of the weld are shown in Fig. 1B and C. The root and fill passes of the weld metal contain a significant fraction of reheated weld metal, while the upper region capping passes comprise mainly as-deposited material. The weld metal mainly consisted of upper bainite, referred to as FS(A) microstructures in the as-deposited material, and bainitic ferrite or an FS(NA) microstructure in the reheated zones containing fine-grained material. These microstructures were identified using the modified IIW classification scheme (Ref. 29) as either ferrite with aligned second phase (FS(A)), ferrite with nonaligned second phases (FS(NA)), and polygonal ferrite (PF). Both the as-deposited and reheated weld metals were examined, and the area fractions of each of the ferrite morphologies or microconstituents were quantified by image analysis, summarized in Table 2. Within the fill passes, the asdeposited regions had an average hardness of about 285 (±6.7) HV1kgf, which was comparable to the reheated material with a hardness of 281 (±6.0) HV1kgf. It should be noted that the capping pass weld metal had a higher hardness of 332 (±2.0) HV1kgf as a result of the higher cooling rates. Weld metal testing indicated that the yield point was 763 MPa with an ultimate tensile strength of 866 MPa, and elongation to failure of 17.8%, which is consistent with the expected minimum ultimate tensile strength (UTS) of 825 MPa. Electron Microscopy and XRD Results The as-deposited and reheated regions of the weld metal are shown in Fig. 2. There is clearly no basket-weave structure or acicular ferrite present. Instead, bainitic ferrite dominates with a fine packet size in the as-deposited microstructure. The as-deposited regions containing predominantly aligned ferrite were organized into packets that comprised ferrite laths, with an average length of 7.4 ± 2.3 μm (n = 35) and width of 0.49 ± 0.18 μm (n = 54). The XRD pattern of the bulk weld metal is shown in Fig. 3, and the only peaks that could be indexed consisted of ferrite and retained austenite. Based on the relative intensities of the (220) ferrite peak, I1, and the (111) austenite peak, I2, the volume fraction of retained austenite RA% can be estimated (Ref. 29) using the following equation: (1) which indicated that the weld metal contained approximately 2.9% retained austenite. = + ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ RA I I % 1 1 0.65 1 2 WELDING JOURNAL 17-s WELDING RESEARCH Fig. 2— SEM micrographs. A — As-deposited; B — reheated weld metal. Fig. 3 — XRD spectrum of weld metal indicating presence of ferrite (α) and retained austenite (γ). Table 2 — Quantification of Ferrite Microstructures Region, % Area Fraction FS(A) FS(NA) PF As deposited 82.4 17.2 0.4 Reheated 4.6 92.5 2.9
Welding Journal | January 2014
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