Tác giả :
Hà Xuân Hùng - Đại học Sư phạm Kỹ thuật Nam Định.

ABSTRACT: Residual stresses are those stresses that would exist in a body if all external loads were removed. Three different sources of residual stresses due to welding can be identified according to Macherauch and Wohlfahrt [1], all contributing to the inelastic strain. One is the difference in shrinkage of differently heated and cooled areas of a weld joint. This caused a formation of high longitudinal stresses, sL, in the weld metal. Similar tensile stresses,sT, arise in the transverse direction too, but of smaller magnitude. A second source is the uneven cooling in the thickness direction of the weld, which can lead to heterogenous plastic deformations and hence to residual stresses (normal residual stresses,sN,). The final source of residual stresses will come from the phase transformations of austenite to ferrite, bainite, or martensite occurring during cooling. These transformations are accompanied by an increase in specific volume, resulting in compressive stresses of the transformed material and tensile in the other regions.The total residual stresses will thus be a combination of the above. These stresses can affect the structure life in service as they may add to applied loads causing fatigue and failure.

In this study, the automtic GMAW was used to create multi-pass weld of API 5L X70 pipeline steel of thickness 14.7mm. Experimental measurements on the residual stress distributions were carried out using neutron diffraction technique, which is non-destructive method and enables in-depth measurements. Therefore, the residual stress distributions at near-surface plane, middle-plane and near-bottom plane of welded specimen were completely investigated. The residual stress distributions of longitudinal direction (welding direction), transverse direction (perpendicular to welding direction) and through-thickness direction were measured.

Key words: Neutron diffaction method; Residual stress; GMAW; API 5L X70 pipeline steel; Longitudinal, Transverse and Through-thickness directions.
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