Stainless steels weld-decay zones

An example of weld effects is shown in Fig. 6. Four different 18Cr-8Ni stainless steel panels were joined by welding and were then immersed in a solution of nitric-hydrofluoric acids. This solution produces rapid attack on grain boundaries containing chromium-rich carbide precipitates, which are surrounded by chromium-depleted zones. The attack is similar to that shown in Fig. 3. Dislodgement of grains in the weld-decay zones leads to the formation of grooves on both sides of the panel of Type 304 (about 0.06 % C) steel that are parallel, but not adjacent to the weld bead. If immersion had been continued, the panel would have dropped off from the assembly. 

As shown in Fig. 6, the weld decay zone which contains chromium carbide precipitate is not adjacent to the cast metal, but at some distance from it, in austenitic stainless steels. The reason for this is that the temperature of the metal in the zone adjacent to the molten zones has been... 

Figure 7.1 Weld decay zone as a function of the welding temperature of stainless steel.

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. 

Weld Decay localised attack of austenitic stainless steels at zones near a weld, which results from precipitation of chromium carbides. 

Figure 16.7 Areas susceptible to weld decay (heat affected zones or HAZ) in a welded stainless steel article.

In stainless steels containing chromium and carbon, carbon segregates as chromium carbide at the grain boundaries, leading to the depletion of chromium around them. As a result, corrosion resistance decreases when the steel is heated. Corrosion of these heat-affected zones next to the welds is called weld decay. ... 

Austenitic stainless steels. These are non-magnetic and contain 18% chromium, 8% nickel and less than 0.15% carbon. Carbides may form in these steels if they are allowed to cool slowly from high temperature, or if they are reheated in the range 500-700 °C (heat-affected zones adjacent to welds). Small stabihzing additions of titanium or niobium prevent the intercrystaUine corrosion, weld decay. They are widely used in chemical engineering plant. 

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