Abstract
A characterization study was done to analyze how microstructural regions affect the mechanical properties, corrosion and fractography of the Heat Affected Zone (HAZ), weld bead and base metal for pipe naturally aged for 21 years at 30 °C. Results showed that microstructures exhibited damage and consequently decrease in properties, resulting in over-aged due to service. SEM analysis showed that base metal presented coarse ferrite grain. Tensile test indicated that microstructures showed discontinuous yield. Higher tensile strength was obtained for weld bead, which exhibited a lower impact energy in comparison to that of HAZ and base metal associated with brittle fracture by trans-granular cleavage. The degradation of properties was associated with the coarsening of nano-carbides observed through TEM images analysis, which was confirmed by SEM fractography of tensile and impact fracture surfaces. The weld bead reached the largest void density and highest susceptibility to corrosion in H2S media when compared to those of the HAZ and base metal.
Highlights
API 5L steel pipeline is extensively used for gas, hydrocarbons and crude oil transportation in the petroleum industry, which are generally designed for a service life of about 20 years
The weld bead (Fig. 1b) exhibited a solidification of the microstructure resulting from the welding cycle, which was composed of large columnar ferrite grains oriented in the direction of the heat flow, acicular ferrite between columnar grains (Hamada and Yamauchi, 2002) and an important percentage of non-metallic inclusions having an average diameter of 1.5 μm within the columnar grains
The analysis showed that the base metal had the largest carbide density, followed by weld bead and Heat Affected Zone (HAZ) due to the particles generated by the Thermo-Mechanically Controlled rolling Process (TMCP) rolling process contributing to this density (Vargas et al, 2006)
Summary
API 5L steel pipeline is extensively used for gas, hydrocarbons and crude oil transportation in the petroleum industry, which are generally designed for a service life of about 20 years. The Thermo-Mechanically Controlled rolling Process (TMCP) is commonly employed to control strain induced precipitation, dislocation networks and strengthening through the precipitation of niobium carbonitrides (Vargas et al, 2011). This kind of pipeline is fabricated with a longitudinal welding which is performed using a double pass submerged arc welding process. The aging process is described by a peak-aged condition associated with strengthening precipitation from particles; and afterward an over-aging state that is related to the decrease in strength and the coarsening of precipitates after extended periods of time (Vargas et al, 2009)
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