Abstract
The microstructural changes due to the aging process in steel pipeline weldments as a function of the thermoelectric power (TEP) were studied. In general, the thermoelectric methods are based on the well-known Seebeck effect. The thermoelectric methods monitor the TEP via an electron flux induced by a temperature gradient in metallic materials, which is affected by the different types of defects that are present in the atomic lattice, such as atoms in the solid solution, precipitates and dislocations. In this present study, the relationship among the TEP data, hardness and the microstructure of steel pipeline weldments was investigated. In addition, the coarse and dendritic grain structure of the welding material is extremely and unpredictably anisotropic. Such microstructures are no longer direction-independent to the electron flux. Therefore, it has an opposite negative effect on the TEP and overlaps the precipitation effect due to the aging process. TEP and hardness measurements were obtained in each zone of the weldments. For each section of the weldment, the weld bead (WB), heat affected zone (HAZ) and base metal (BM) were found to correspond to particular values of TEP. The relationship between the TEP and the microstructure of a weldment of X60 and X65 micro-alloyed steel that was artificially aged was obtained using the conventional contact TEP technique (hot-tip) and scanning electron microscopy (SEM). It was found that thermoelectric power is very sensitive to the aging process in the two-studied steel pipeline weldments.
Highlights
Steel pipelines with low carbon contents, welded by an electric arc, have been used for many years in the oil industry
The two micro-alloyed steels were aged at 300 ◦ C for different aging times that ranged from 1 min to 30 h for the X60 weldment and from 1 min to 45 h for the X65 weldment in order to promote the formation of precipitates
We found a reheat affected zone (RHAZ) between4 of the
Summary
Steel pipelines with low carbon contents, welded by an electric arc, have been used for many years in the oil industry. 25 and 70 ◦ C) and variations in the operating pressure These operating conditions can cause changes in the microstructure, mechanical properties (elastic limit, hardness, ductility and toughness), type of fracture and probability of failure after several years of service. This natural aging process is described by two factors: the peak of aging associated with the maximum resistance and precipitation of carbides that interact with the dislocations and causes hardening by precipitation; and the over-aging related to the decrease in resistance and thickening of carbides after a long aging time [1,2].
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