Niobium additions prevents inter-granular corrosion, particularly in the heat effected zone after welding. Niobium helps prevent the formation of chrome carbides, that can rob the microstructure of the required amount of chromium for passivation. In “ferritic” stainless steels the addition of niobium is an effect way to improve thermal fatigue resistance.
Titanium is the main element used to stabilize stainless steel before the use of AOD (Argon-Oxygen Decarburization) vessels. When stainless steel is melted in air, it is difficult to reducing the carbon levels. 302, the most common grade before AOD’s, was allowed to have a maximum carbon level of 0.15%). At this high level, something was needed to stabilize the carbon and titanium was the most common way. Titanium will react with the carbon to form titanium carbides and prevent the formation of chrome carbides, that could affect the formation of the “passive” layer. Today all stainless steel are finished in an AOD vessel and the carbons levels are generally low due to the absence of oxygen. The most common grade today is 304 (with 0.08 max carbon, although in reality the levels are lower).
Sulfur is generally kept to low levels as it can form sulfide inclusions. It is used to improve machinability (where these inclusion act as “chip breakers). The addition of sulfur, however, does reduce the resistance to pitting corrosion.ductile iron china