# Metallurgical Aspects in the Welding of Clad Pipelines--A Global Outlook
## 3 Microstructure Evolution
### Segregation in Welds  
Segregation is a defect in welded joints, where the alloying elements are enriched within specific areas due to motion of liquid and can decrease mechanical properties locally. Segregation is very common in stainless steels and Ni-based superalloys due to high alloy content, whereas low segregation tendency in low-carbon steel is observed, except for the segregation of impurities (S, P). The Scheil equations, or non-equilibrium level rule, describe the solute redistribution during solidification of an alloy [50]:  
\[C_{S}=kC_{0}(1-f_{S})^{(k-1)} \tag{10}\]  
and  
\[C_{L}=C_{0}f_{L}^{(k-1)} \tag{11}\]  
where \(C_{S}\) is the element concentration in the solid, \(C_{L}\) is the concentration in liquid, \(C_{0}\) represents the initial concentration in the liquid, \(k\) is the effective segregation coefficient (\(\leq\)1.0), and \(f_{S}\) and \(f_{L}\) denote the volume fraction of solid and liquid, respectively. The segregation coefficient can be estimated as follows [50]:  
\[k=\frac{C_{S}}{C_{L}} \tag{12}\]  
Figure 8: (**a**) SEM of cellular (columnar) dendritic structure in the WM of 304 SS [43] and (**b**) classification of dendritic arc spacing (modified from [49]). \(\lambda_{1}\), \(\lambda_{2}\), and \(\lambda_{3}\) indicate primary, secondary, and tertiary dendrite arm spacing, respectively. The segregation coefficient \(k\) can also be applied in determination of microsegregation in solidification, i.e., the distribution of elements between the core of primary dendrites and the interendritic areas. Based on Figure 9, it is shown that Mo and Nb segregate to the interendritic region, while Ni and to some extent Cr are enriched in the dendrites. [PERSON] et al. [51] reported that the constant \(k\) for Ni and Fe had values just above 1.0, which indicates a slight segregation into the dendrite core. A coefficient of 1.0 implies that no segregation occurs. In the case of Mo, the coefficient is \(\sim\)0.9, resulting in segregation to the liquid metal and hence enriching the interendritic region. Like Mo, Nb also segregates to the liquid, but with a greater intensity due to a lower \(k\) value of \(\sim\)0.5. Therefore, this strong segregation of Nb has been appointed as reason for the formation of the secondary phases such as the Laves phase in welding with Alloy 625 wire. For the use of Alloy 59 wire, this will not be the case due to the absence of Nb. The extensive segregation in high-alloy materials can cause residual stresses during solidification and therefore reduced mechanical properties [52]. Carbon diffusion to the interface area has already been shown to take place during production of the clad pipe. Transportation of the alloying elements may change the phase transformation behavior at the interface. A temperature for the martensite start transformation (\(M_{s}\)) is strongly influenced by the chemical composition of the alloy, according to the following, i.e., the Andrews equation [53]:  
\[M_{s}\;(^{\circ}\mathrm{C})=539-423\;\mathrm{C}-30.4\;\mathrm{Mn}-17.7\; \mathrm{Ni}-12.1\;\mathrm{Cr}-7.5\;\mathrm{Mo} \tag{13}\]  
Different \(M_{s}\) temperatures can be used for Fe-Mn-Ni-Cr austenitic weld metal, which can be found in [26]. With a carbon content increase from 0.03 to 0.20 wt % C, the transformation temperature is reduced by around 85 \({}^{\circ}\)C. With segregation of the alloying elements, being Ni and Cr in the present case, the \(M_{s}\) temperature will decrease by 37 \({}^{\circ}\)C per wt % Ni and 11 \({}^{\circ}\)C per wt % Cr. Note that for clad steels, this will be the segregated concentration, not that of the initial bulk. In addition to the influence from martensite transformation, the enhanced carbon diffusion to the clad side causes extensive precipitation of chromium carbides at austenite grain boundaries in the clad [54]. The precipitation kinetics of Cr\({}_{23}\)C\({}_{6}\) is shown in Figure 10, clearly showing how carbon raises the precipitation temperature, as indicated by the dotted red arrow.