Reason of PWHT after welding of 321 stainless steel

 

 Numerous failures of 18-8 austenitic stainless steels have occurred because of inter-granular corrosion. When this steels are heated in approximately the temperature range 510 to 790 Celsius, they become sensitized or susceptible to intergranular corrosion. The almost universally accepted theory for intergranular corrosion is based on impoverishment or depletion of Chromium in the grain- boundary areas. In this case the Cr is thereby removed from solid solution, and the result is metal with lowered Cr content in the area adjacent to the grain-boundaries. The Chromium carbide in the G. B. is not attacked. The Cr-depleted zone near the G. B. is corroded because it does not contain sufficient corrosion resistance to resist attack. This phenomenon is a common failure in 304 stainless steel and call it “weld decay”. It showed in fig. 1.

 

 

Fig. 1. Schematic of Chromium depletion region and weld decay.

 

 

In order to solve this problem the new generation of 304 series which are the 321 and 347 series have been developed as stabilized with Titanium and Niobium. In these series the extra carbon will create TiC or NiC and avoid to create Cr- carbide. But during welding these steels in a narrow band near to HAZ will take place in sensitization temperature range and subsequently in will corroded with same mechanism with weld decay. This kind of corrosion is named “Knife Line Corrosion” (KLC) or some other names which mentioned in references. In fig. 2 the position of weld decay in 304 and KLA in stainless steels is shown.

 

 

Fig. 2. The position of weld decay and KLA phenomena in 304, 321 and 347 St.

 

 

In fig. 3. the temperature range of Cr-carbide precipitation and dissolving is shown. Based on this temperature range when after welding in a narrow area near to HAZ the Cr-carbide precipitated the metal have to be heat treated in range of more than 790 Celsius which is called Stabilization Heat Treatment.

 

 

Fig. 3. Schematic Chart showing solution and precipitation of Ti and Cr in types 304 and 321.

 

 

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