Fusion boundary metals

Rowe, M.D., Nelson, T.W., and Lippold, J.C., Hydrogen-induced cracking along the fusion boundary of dissimilar metal welds, Welding Res., February, 1999, p. 31. 

Unmixed zones. All methods of welding stainless steel with a filler metal produce a weld fusion boundary consisting of base metal that has been melted, but not mechanically mixed with filler metal and a partially melted zone in the base metal. An unmixed zone has the composition of base metal, but the microstructure of an autogenous weld. The microsegregation and precipitation phenomena characteristic of autogenous weldments decrease the corrosion resistance of an unmixed zone relative to the parent metal. Unmixed zones bordering welds made from overalloyed filler metals can be preferentially attacked when exposed on the weldment surface.14,61... 

If PWHT cannot be employed, it is possible, in principle, to soften the uppermost parent metal HAZ with a temper bead. This method should not be used unless careful control can be exercised. The temper bead is an extra weld run deposited so that its toe is a small, fixed distance (typically 3 mm) from the fusion boundary and it tempers the uppermost parent metal HAZ without creating a new hardened region. 

The identification of hydrogen cracks is not simple, as they can occur in a variety of locations and orientations, and can be trans-granular or intergranular, the latter being more likely if the steel or weld metal is of the alloyed type, or exceptionally hard. In the HAZ, hydrogen cracks are usually longitudinal to the weld (unless they are extensions of transverse weld metal cracks) and usually have a portion close to the fusion boundary. However, the cracks may divert into the fine-grained HAZ, as in Fig. 1.2(b), or into the weld metal. 

Solders are cathodic to steel, zinc and cadmium, and anodic to Monel metal. Although tin or tin-coated metals may be used safely in contact with aluminium when they are not fused with it, a joint in aluminium made with a tin-lead solder is liable to destructive corrosion. The formation, on fusion, of the grain-boundary state, which, as already mentioned, makes aluminium so dangerous an impurity in tin, is responsible. Tin-zinc solders may be used the zinc gives a useful degree of protection. 

Structure A is the simplest structure formed by fusion sphcing a section of a coreless fiber to two pieces of continuous multimode fibers. In this sensor, different modes propagating in the multimode fiber expand to the surface of the coreless fiber in the central region and after reflection from the coreless fiber-environment boundary, get coupled back into the second multimode fiber fused to the coreless fiber. The evanescent waves at the coreless fiber-environment boundary excite plasmon resonance is the metallic nanoparticles formed on the surface of the coreless fiber section, as shown in Figure 3 A. This structure is not very effective for sensing purposes as there are a lot of... 

Metallographic studies showed a continuous net of eutectic precipitations in the weld metal along the grain boundaries of the a-solid solution of copper in aluminum. Their coarse accumulations were also discovered in the heat-affected zone (HAZ) near the fusion line [ ]. 

Separate intermetallic inclusions were absent in the weld metal structure. In the fusion zone, the eutectic consisted of isolated clusters and interlayers along the grain boundaries (Figs. 4a and 4b) in the central areas of the weld, interlayers predominated (Figs. 4c and 4d). The structure of the eutectics formed in the HAZ, both within the fusion zone and in the weld, was practically the same. The areas of the basic solid solution appear to be homogeneous, while the intermetallic phase, as in the initial material, is distinguished by its own substructure (Figs. 3b, 4b, and 4d). The nature of the uniformly distributed spherical formations in the structure of the intermetallic inclusions is not known. 

Knife-line attack, immediately adjacent to the weld metal, is a special form of sensitization in stabilized austenitic stainless steels. Stabilizing elements (notably Ti and Nb) are added to stainless steels to prevent intergranular corrosion by restricting the formation of Cr-rich grain boundary precipitates. Basically, these elements form carbides in preference to Cr in the austenitic alloys. However, at the high temperatures experienced immediately adjacent to the weld fusion zone, the stabilizer carbides dissolve and remain in solution during the subsequent rapid... 

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