Full Description

The half-bead/temper-bead/controlled deposition repair welding techniques, which utilize the thermal cycles of the second and later weld layers to temper and refine the HAZ of the first layer, have been applied in accordance with ASME Boiler and Pressure Vessel Nuclear Code Section III for new construction since the late 1960?s and Section XI for in-service repair welding of nuclear power plant components. Thus Post Weld Heat Treatment (PWHT) may be omitted without causing degraded properties of the component; especially the base metal HAZ. The extensive ASME Nuclear Code studies of SA533 and SA508 materials clearly show the efficacy of non-PWHT technique on the C-Mn and C-Mo steels.

The University of Tennessee, Knoxville (UTK) joined hands with the Ontario Hydro Company to conduct research on the Temper-Bead welding techniques employed primarily in Cr-Mo and also a low alloy steel with the Shielded Metal Arc Welding (SMAW) process. Two layer temper-bead refining techniques were applied in this study. Different temper-bead welding parameters were utilized for obtaining complete CGHAZ refinement, in terms of the energy input of first buttering layers and fill layers. The energy ratio between the second butter layer to the first layer is the controlling entity. Conventional stringer bead welds with and without PWHT were made for the purpose of comparison.

This program was sponsored by the Pressure Vessel Research Council (PVRC) and spanned for a total of 4 years. The materials used in study were SA387-11 (1 1/4Cr-1/2Mo), SA387-22 (2 1/4Cr-1Mo) and A516-70. Ontario Hydro supplied the weld coupons and the examination and testing were conducted at The University of Tennessee, Knoxville (UTK). The goals of the program lay in the evaluation of the temper-bead welding techniques and thus the determination of the welding procedures pertinent to the refinement of the base metal HAZ.

To evaluate the weldments, a series of tests were conducted. Hardness traverses across the weld metal through HAZ to base metal were taken, macrostructural and microstructural examination was conducted using optical light microscopy. The reheat cracking tendency of the weld HAZ for each of the three heats of material was evaluated using spiral notched transverse weld specimens with both small (0.125" dia.) and large (0.350" dia.) diameter samples. Gleeble thermal simulation was applied for evaluation of the HAZ refining procedures. HAZ Charpy V-notched impact tests were conducted for the temper-bead, conventional with/without PWHT welds, as well as for Gleeble simulated and UTK fabricated welds. Creep rupture testing of cross weld HAZ specimens was also carried out for the different procedure conditions, in which both small and large diameter samples were utilized for testing. Ontario Hydro temper-bead and conventional, UTK weave bead and conventional welds in the as-welded and PWHT conditions were tested for sensitivity to hydrogen cracking by a hydrogen charging-bend test method. Stress rupture testing of longitudinal smooth bar specimens, a new test method for the evaluation of the creep ductility in different weld regions, was developed during this investigation. A singular and straight CGHAZ produced by a weave bead welding technique and the overlapped CGHAZ induced by conventional deposition sequences made at UTK were also evaluated and compared to the Ontario Hydro weld coupons in terms of Charpy V-notched impact, large diameter creep rupture, spiral notched stress rupture and hydrogen sensitivity tests.

The results showed a general superiority for the temper-bead welds over the conventional and weave bead welds, as regard to the tests conducted in the program. The Ontario Hydro welding procedures were found to achieve a high degree of CGHAZ refinement. Gleeble simulated samples showed lower properties than the actual welds. The temper-bead welding procedures can be used in practice if more attention is paid to root passes and the final layer of fill passes.

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Published: 2005 Number of Pages: 263 File Size: 1 file , 11 MB