# Study of the Precipitation Process in Aging Steel Pipeline Weldments by Thermoelectric Power Means
## 4 Results and Discussion  
Knowledge of the weldment microstructure is essential for understanding the TEP data. Figure 2a,b and Figure 3a,b clearly show the different microstructures obtained in the API X60 and X65 micro-alloyed weldments. We clearly observed a columnar structure of welding metal in the outer \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Steel** & **C** & **Mn** & **Si** & **P** & **S** & **Al** & **Nb** & **Cu** & **Cr** & **Ni** & **V** & **Ti** \\ \hline X60 & 0.025 & 1.57 & 0.14 & 0.012 & 0.002 & 0.044 & 0.097 & 0.31 & 0.29 & 0.17 & 0.002 & 0.014 \\ \hline X65 & 0.04 & 1.48 & 0.25 & 0.12 & 0.002 & 0.041 & 0.047 & 0.1 & 0.02 & 0.08 & 0.07 & 0.017 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of X-60 and X-65 micro-alloyed steels (wt%). portion of the weldment. Furthermore, we also found a reheat affected zone (RHAZ) between the two weld beads. The HAZ from the first weld bead (WB 1) is affected by the HAZ of the second weld bead (WB 2), producing a microstructure with a smaller grain size. Furthermore, the fusion zone (FZ) was also observed. Figures 2c and 3c show that the HAZ of the weld bead is mainly made up by polygonal and coarse acicular ferrite. This microstructure optimizes the strength and toughness in the weld beads. Finally, Figures 2d and 3d show the microstructure of the BM in API X60 and API X65 steel, which consists of polygonal ferrite (light areas) and pearlite (dark areas). In the X-65 steel (Figure 3d), there is a greater quantity of pearlite. The weldments are the main examples of structural components where the final mechanical properties depend on the features of the microstructure obtained after the welding process [7]. On the other hand, the SEM analysis performed on the aged samples revealed the thin dispersed precipitation of particles that were trans-granular, semi-round and fine (100-900 nm). Furthermore, these participles contained Fe-C as detected by EDS microanalysis. These nanoprecipitates showed preferential nucleation and interactions with the dislocations in ferritic grains, which is shown in  
Figure 3: Microstructures of API X65 micro-alloyed steel weldment: (**a**) Weldment zones; (**b**) Microstructure showing the FZ, HAZ and WB 1; (**c**) interface between HAZ and WB; and (**d**) Base Metal (BM), ferrite (light areas) and pearlite (dark areas). Figure 2: Microstructures of API X60 micro-alloyed steel weldment: (**a**) Weldment zones; (**b**) Microstructure showing the FZ, HAZ and WB 1; (**c**) interface between HAZ and WB; and (**d**) Base Metal (BM), ferrite (light areas) and pearlite (dark areas). Figures 4b-d and 5b-d. Fine Fe\({}_{3}\)C and Fe\({}_{2}\)C precipitates that are 50-200 nm in size are homogeneously distributed in the ferritic grains of the API X-65 and API X-65 micro-alloyed steels after different aging times. The precipitation of these particles induces the hardening of the weld joint. The dispersion of fine particles with heterogeneous nucleation and precipitation in the dislocations of the surroundings causes more obstacles and strong interactions. This blocks and delays the movement of the dislocations, resulting in an increase in mechanical properties, such as resistance. The quantitative analysis of this increase in the density of nanoparticles is represented as a function of aging time. The Vickers microhardness values for the API-5L X-60 and X-65 steels (see Figure 6a,b, respectively) show a tendency to increase in the early aging times at the three different zones of the weldment. This phenomenon is attributed to the precipitation of cementie (Fe\({}_{3}\)C) and the precipitation of carbide-\(\epsilon\) (Fe\({}_{2}\)C). The precipitation of carbides occurs mainly in the dislocations within the grain, resulting in the increase in hardness. After this, a considerable decrease in hardness can be observed due to the thickening of the cementie at the expense of the precipitates of smaller cementie and the carbide-\(\epsilon\).