# Metallurgical Aspects in the Welding of Clad Pipelines--A Global Outlook
## 2 Production of Clad Pipes  
The metallurgical behavior of clad pipes depends on their production route during fabrication. Diffusion bonding represents a frequently used technique. During production, a certain temperature range is required to achieve metallurgical bonding, usually from 0.5T\({}_{\text{m}}\) to 0.8T\({}_{\text{m}}\) (where T\({}_{\text{m}}\) is the melting temperature). The temperature-time cycle will thus allow the diffusion of alloying elements that may influence the structural integrity of the pipe. Thus, the interface between the two materials will attain a gradient of chemical composition and hence produce other microstructures compared to the two surrounding materials, as indicated in Figure 1. One example is shown in Figure 2, which shows carbon diffusion into the 316L stainless steel (SS) clad due to the higher solubility in austenite. The carbon content at the bimetallic or clad/pipe interface on the clad side is between 0.15 and 0.20 wt %. A consequence is that extensive precipitation of Cr\({}_{23}\)C\({}_{6}\) takes place [15]. It is also expected that other elements such as Ni, Cr, and Mo may diffuse to the interface, although to a lower extent than carbon due to the lower diffusivity of these large-atom elements. Due to diffusion of alloying elements, microsegregation may facilitate martensite formation at the interface. Local microhardness increases of up to 350 HV has been found, as shown in Figure 3, as an indication of martensite presence. Another influencing factor may be a deformation hardening during the hot-roll bonding process [16].  
Measurements made on welded samples gave evidence that martensite was formed. The maximum microhardness values of 362 and 392 HV in the clad close to the interface for two different macros was reported [17]. Because of the carbon depletion of the pipeline steel measured by an electron probe microanalyzer (EPMA), the hardness level on the steel side close to the interface is low (156 HV). As an example, Line 3 in Figure 2 shows a carbon reduction from 0.16 to 0.03 wt % C. Moreover, some grain growth of the carbon steel close to the interface was observed, which is also consistent with the lower hardness in this region. Therefore, remarkable gradients of hardness and microstructure occur across the interface, which may potentially influence the integrity of the pipe.  
In contrast to direct cladding with AISI 316, the application of a 30-35 \(\upmu\)m nickel interlayer between the clad and the carbon steel reduces the carbon diffusion into the clad substantially [19].  
With the Ni-based alloy 625 as the clad material, there is a higher potential for elemental segregation than with stainless steel. However, diffusion of Fe into the clad is regarded as a more critical issue. This is also the case in the cladding of carbon steel with Alloy 625. This alloy is frequently used in the overlay welding of carbon steels to improve the corrosion resistance, where the dilution with the substrate is kept low to preserve the initial corrosion resistance of the clad [20].  
Figure 1: Carbon steel–clad (316L) interface microstructure as observed by (**a**) optical microscopy and (**b**) SEM; 1—the interface layer and 2—the width of the carbon diffusion layer [18].  
It should be noted that the chemical composition of the weld clad/overlay must comply with the specification of the material. In the case of Alloy 625, the maximum allowable Fe content is 5% (according to API 5 LD [22] specifications) or 10% (according to the DNV GL [23] standard). To achieve such low dilution, particularly the 5% requirement, the cold metal transfer (CMT) variant of the gas metal arc welding (CMT-GMAW) is an advantageous process due to lower heat input.