Stainless steels weld decay

The corrosion of stainless steel welds has probably been studied more fully than any other form of joint corrosion and the field has been well reviewed by Pinnow and Moskowitz , whilst extensive interest is currently being shown by workers at The Welding Institute. Satisfactory corrosion resistance for a well-defined application is not impossible when the austenitic and other types of stainless steels are fusion or resistance welded in fact, tolerable properties are more regularly obtained than might be envisaged. The main problems that might be encountered are weld decay, knifeline attack and stress-corrosion cracking (Fig. 9.29). 

Note that sensitization will not result in weld decay in all environments. Stainless steels may be used in environments that do not require the full corrosion resistance of the alloy. In these cases, weld decay may not occur even though sensitization has taken place. 

Locations. Weld decay may affect welded stainless steels that have normal carbon contents and are not specifically inhibited for sensitization. Weld decay affects only the immediate weld area. 

Critical factors. The critical factors governing weld decay include the use of a sensitized stainless steel and the exposure of this metal to an environment that is sufficiently aggressive to cause degradation in the sensitized region. 

Note that low carbon or stabilized grades of stainless steel do not possess intrinsically greater corrosion resistance than their unadjusted counterparts. Their sole value in typical cooling water systems results from their resistance to sensitization and potential weld decay that can result when the metals are welded. It is therefore not economically justifiable to specify low carbon or stabilized grades of stainless steel for typical cooling water system components that are not to be welded. 

When stress-relief-annealing 300 series stainless steel components, care must be taken to avoid slow cooling through the sensitization range (see Weld Decay in this chapter). 

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. 

Fig. 1.9(a) Curves showing the variation of temperature with time at various points adjacent to a fusion weld in an austenitic stainless steel and (b) weld decay in an unstabilised austenitic... 

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

Type 304 (the so-called 18/8 stainless steels) the most generally used stainless steel. It contains the minimum Cr and Ni that give a stable austenitic structure. The carbon content is low enough for heat treatment not to be normally needed with thin sections to prevent weld decay (see Section 7.4.4). 

Intergranular corrosion (weld decay) and stress corrosion cracking are problems associated with the use of stainless steels, and must be considered when selecting types suitable for use in a particular environment. Stress corrosion cracking in stainless steels can be caused by a few ppm of chloride ions (see Section 7.4.5). 

Aluminum does not have the mechanical reliability of stainless steel, especially at higher temperatures. However, it is not nearly as susceptible to weed decay or localized attack around the welds. 

Figure 16.6 Creation of microscopic anodic areas in the weld decay of a typical stainless steel.

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

The best-known weld-related corrosion problem in stainless steels is weld decay (sensitization) caused by carbide precipitation in the weld HAZ. This sensitized... 

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. ... 

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