The Heat Affected Zone

As said in the previous section, heat affected zone or HAZ plays an important role on the soundness and robustness of a weld. It develops at the interface between weld and base material. This zone, at variance with the weld pool, does not reach 

The cooling and sohdification of the weld pool is accompanied by a parallel shrinkage of the weld metal itself that is prevented by the adjacent base metal that 

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As in the case of many metal—ahoy systems, weld ductihty is not as good as that of the base metal. Satisfactory welds can be made in vanadium ahoys provided the fusion zone and the heat-affected zone (HAZ) are protected from contamination during welding. Satisfactory welds can be made by a variety of weld methods, including electron-beam and tungsten-inert-gas (TIG) methods. It is also likely that satisfactory welds can be made by advanced methods, eg, laser and plasma techniques (see Lasers Plasma technology). 

Zirconium is completely resistant to sulfuric acid up to Foiling temperatures, at concentrations up to 70 wt %, except that the heat-affected zones at welds have lower resistance in >55 wt % concentration acid (Fig. 1). Fluoride ions must be excluded from the sulfuric acid. Cupric, ferric, or nitrate ions significantly increase the corrosion rate of zirconium in 65—75 wt % sulfuric acid. 

Corrosive chemicals and external exposure can cause tank failure. Materials of construction should be chosen so that they are compatible with the chemicals and exposure involved. Welding reduces the corrosion resistance of many alloys, leading to localized attack at the heat-affected zones. This may be prevented by the use of the proper alloys and weld materials, in some cases combined with annealing heat treatment. 

There is one major pitfall which must be avoided in using stainless-steel components joined by welding it is known as weld decay. It is sometimes found that the heat-affected zone - the metal next to the weld which got hot but did not melt - corrodes badly. 

Selective corrosion in the heat-affected zone of a weld occurs most commonly when unstabilised stainless steels are used in certain environments. The obvious answer is to use an extra-low-carbon grade of stainless steel, e.g. types 304L, 316L or a stabilised grade of steel, e.g. types 321 and 347. Knifeline attack at the edge of a weld is not commonly encountered and is seldom predictable, and it must be hoped that it is revealed during preliminary corrosion testing. 

The heat-affected zone may become susceptible to stress-corrosion cracking, particularly the high-strength alloys, and expert advice is necessary... 

Weld Zone the zone containing the weld metal and the heat-affected zone. 

For comparison, the following relationship has been found for an X70 pipeline steel (slow strain rate test using compact tension specimens with the precrack located in the heat-affected zone of a weld) ... 

Wetted parts subject to welding, including fabrication and tack welding (for example, removable wear rings), shall be stress relieved, if required, so that both the welds and the heat-affected zones meet the yield strength and hardness requirements of this paragraph. 

Welds will be rejected and removed (including the heat affected zone) under the following conditions ... 

Joining is another problem. Fusion welding in inert atmospheres often develops recrystallized structures in the heat-affected zones, so that welded pans lose strength and are embrittled. Special joining techniques being used In help overcome these deficiencies include electron-beam and solid-phase welding, and the development of special brazing materials. 

Dealuminification occurs in aluminum bronzes containing the y-2 phase microstructure and the process is more severe when the y-2 phase forms a continuous grain boundary network. Dealuminification can be averted by rapid cooling from >600°C, or by addition of 1-2% iron or more than 4.5% nickel. Microstmctural changes can still occur during welding and lead to corrosion in the heat-affected zone. 

Welded microstructures can be extremely complex and often change drastically over a very short distance. The fusion zone or weld metal is a dendritic structure that has solidified from a molten state. Bordering the fusion zone are transition, unmixed and partially melted zones, and the heat-affected zone (HAZ). These zones can be reheated and altered by subsequent weld passes, in multipass welding. For alloys with structures that depend strongly on thermal history, such as steels, the final microstructure can be extremely complex. Since welded structures are often quite susceptible to corrosion, overalloyed filler metals are often used to enhance the weld corrosion resistance. For stainless steels with sufficiently high carbon content, sensitization in the HAZ is another major problem. (Frankel)5... 

Carbon steels. The corrosion behavior of carbon steel weldments produced by fusion welding can be due to metallurgical effects, such as preferential corrosion of the heat-affected zone (HAZ) or weld metal, or it can be associated with geometric aspects, such as stress concentration at the weld toe, or creation of crevices due to joint design. 

It has been shown that welds provide unique environments for the colonization of SRB with the subsequent production of sulfides that affect the weld seam surface of the heat-affected zone. Exposure of sulfide-derived surfaces to fresh, aerated seawater resulted in rapid spalling on the downstream side of weld seams. The bared surfaces became anodic to the sulfide-coated weld root, initiating and accelerating localized corrosion. (Dexter)5... 

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