# Metallurgical Aspects in the Welding of Clad Pipelines--A Global Outlook
## 3 Microstructure Evolution
### Solidification  
#### 3.1.1 Fundamentals of Solidification Theory  
Welding of clad pipes is most often performed with an Inconel 625 or an Alloy 59 filler wire. Sometimes, AISI 309 wire is employed for root and hot pass deposition. Stainless steel solidifies in different modes according to its chemical composition. In addition, the cooling rate may induce a shift in solidification [24]. The solidification mode of stainless steel can be divided into four types according to the ratio between ferrite-(\(\delta\)) and austenite-(\(\gamma\)) stabilizing elements, i.e., Cr\({}_{\text{eq}}\)/Ni\({}_{\text{eq}}\)[25]:  
\[\text{F (ferritic) mode: $L$ (liquid phase)}\to L+\delta\rightarrow\delta \rightarrow\delta+\gamma\qquad\text{for ${\text{Cr}_{\text{eq}}}$/Ni${}_{\text{eq}}$ $>$ 2.0} \tag{1a}\] \[\text{FA (ferritic-austenitic) mode: $L$ \to L+\delta\to L+\delta+\gamma\rightarrow\delta+\gamma \rightarrow\gamma$}\qquad\text{for $1.5<\text{Cr}_{\text{eq}}$/Ni${}_{\text{eq}}$ $<$ 2.0}\] (1b) \[\text{AF (austenitic-ferritic) mode: $L$ \to L+\gamma\to L+\delta+\gamma\rightarrow\gamma$}\qquad\text{for $1.37<\text{Cr}_{\text{eq}}$/Ni${}_{\text{eq}}$ $<$ 1.5}\] (1c) \[\text{A (austenitic) mode: $L$ \to L+\gamma\rightarrow\gamma$}\qquad\text{for ${\text{Cr}_{\text{eq}}}$/Ni${}_{\text{eq}}$ $<$ 1.37} \tag{1d}\]  
The values of Cr\({}_{\text{eq}}\) and Ni\({}_{\text{eq}}\) can be calculated by a variety of equations. [PERSON] [26] presented more than 15 different equations for different purposes (welding, casting) and including/excluding some of the microalloying elements, such as Cu and Co for Ni\({}_{\text{eq}}\) and Nb, Mo, Ti, Al, V, W, and Ta for Cr\({}_{\text{eq}}\). However, the authors [27] had previously used equations by [PERSON] and [PERSON] [28] to determine the solidification of stainless steel weld metals. The Cr and Ni equivalents can be calculated from Equation (2a) and Equation (2b), respectively. Values are represented by the wt % of the elements. Based on these equations, the solidified weld metal can be visualized (see Figure 4). \[\text{Cr}_{\text{eq}}=\text{Cr}+1.37\text{ Mo}+1.5\text{ Si}+2\text{ Nb}+3\text{ Ti} \tag{2a}\] \[\text{Ni}_{\text{eq}}=\text{Ni}+22\text{ C}+14.2\text{ N}+0.31\text{ Mn}+\text{Cu} \tag{2b}\]  
Thus, it is indicated by extrapolation in Figure 4 that the classical austenitic stainless steels like AISI 309 SS and AISI 316 SS primarily solidify as ferrite. For Alloy 59, very limited information is available in the literature, while there are numerous publications on other Ni-based alloys. However, those are mainly precipitation-hardened alloys in the 7 xx series of Inconel alloys, frequently used in aerospace applications. Figure 4: Possible solidification modes and nomenclature according to Equations (1a)–(1d). Ni-based alloy is Inconel 625, and the nominal chemical compositions of Alloy 59 and Inconel 625 are outlined in Table 1. The main difference between them is the Nb content of Inconel 625 and the much higher Mo addition in Alloy 59. The mechanical properties are also comparable, with yield strengths of \(\geq\)340 and 350 MPa for Alloy 59 and Inconel 625, respectively. Their corresponding tensile strengths are \(\geq\)690 and 770 MPa. Several experimental studies have been conducted to investigate the behavior of the solidification in superalloys containing niobium (Nb). Nb-bearing superalloys (like the popular Inconel 625/718) terminate the solidification by a eutectic-type reaction between \(\gamma\)- and various Nb-rich phases, such as NbC and Laves phases [29]. The creation of a crystal from an alloy melt causes a local change in the composition [30]. When the dendrites solidify, elements such as Mo and Nb, as well as impurities, segregate to interdendritic regions [31]. It was shown [32] that the presence of iron in Ni-based superalloys lowers niobium solubility in the austenite phase. When iron is in a solid solution in the \(\gamma\) Ni-Cr phase, the ability of Nb to remain in solution is limited [33] and partitioning of Nb to interdendritic regions in the WM is increased.