Stainless Steel Welding:D
The stainless properties of stainless steels are primarily due to the presence of chromium in quantities greater than roughly 12 weight percent. This level of chromium is the minimum level of chromium to ensure a continuous stable layer of protective chromium-rich oxide forms on the surface. The ability to form chromium oxide in the weld region must be maintained to ensure stainless properties of the weld region after welding. In commercial practice, however, some stainless steels are sold containing as little as 9 weight percent chromium and will rust at ambient temperatures.
Stainless steels are generally classified by their microstructure and are identified as ferritic, martensitic, austenitic, or duplex (austenitic and ferritic). The microstructure significantly affects the weld properties and the choice of welding procedure used for these stainless steel alloys. In addition, a number of precipitation-hardenable (PH) stainless steels exist. Precipitation-hardenable stainless steels have martensitic or austenitic microstructures.
Iron, carbon, chromium and nickel are the primary elements found in stainless steels and significantly affect microstructure and welding. Other alloying elements are added to control microstructure or enhance material properties. These other alloys affect welding properties by changing the chromium or nickel equivalents and thereby changing the microstructure of the weld metal. Generally, 200 and 300 series alloys are mostly austenitic and 400 series alloys are ferritic or martensitic, but exceptions exist.
Stainless steels are subject to several forms of localized corrosive attack. The prevention of localized corrosive attack is one of the concerns when selecting base metal, filler metal and welding procedures when fabricating components from stainless steels.
Stainless steels are subject to weld metal and heat affected zone cracking, the formation of embrittling second phases and concerns about ductile to brittle fracture transition. The prevention of cracking or the formation of embrittling microstructures is another main concern when welding or fabricating stainless steels.
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