Technical Information about Stainless Steel - Alloying Elements in Stainless Steel

Stainless stell is a steel to which a minimum 10.5% chromium and other alloying elements are added. The addition of different elements influences the mechanical, physical and corrosion resistant properties of stainless steel. Listed below is a summary of the effects of the most important alloying elements.

Carbon

In most stainless steels carbon is kept to low levels, typically 0.05% C, or 0.03% C in the low carbon "L" grades and other stainless steels used for fabricated components in thicker sections. However, in martensitic stainless steels C is the alloying element added, in amounts varying from 0.15% - 1.2% C, to render these steels heat treatable by quenching and tempering, to develop high strength and hardness levels.

Chromium

A minimum of 10.5% Chromium is needed in stainless steel, at which level is continuous, stable and inert (passive) chromium oxide film forms on the surface, producing resistance to corrosion, both wet(aqueous) and dry (gaseous). Higher levels of Cr (up to about 26% Cr) further increase corrosion resistance.

Nickel

Nickel, if added to stainless steel in sufficient quantity, develops a fully austenitic crystal structure, hence austenitic stainless steels, as typified by grade 304 (18% Cr 8% Ni). Lower levels of Ni, insufficient to develop a fully austenitic crystal structure, can result in a duplex (mixed) crystal structure of ferrite and austenite (found in duplex stainless steels).

Molybdenum

Molybdenum enhances the properties of the passive surface film, and renders stainless steels which contain it more corrosion resistant, particularly to pitting in chloride environments. The higher the Mo content the more aggressive the corrosive conditions that can be handled (eg grade 316 with 2% Mo, grade S31254 with 6% Mo).

Titanium

Titanium is added to produce stabilised grades such as 321. Titanium is a strong carbide former and preferentially forms Ti carbides obstructing the formation of Cr carbides. This activity prevents intergranular corrosion occuring in the region adjacent to welds.

Manganese

Manganes like Ni promotes the formation of an austenitic crystal structure. In the 200 series austenitic stainless steels, Mn is used to partially replace Ni, Mn is also increased to slightly higher levels than normal in the free machining grades of stainless steel to which sulphur or selenium have been added.

Sulphur

Sulphur is normally kept to very low levels, typically 0.03% maximum, but in practice much lower. If increased to around 0.2% S the machinability of the steel is improved, but the fabricational, mechanical and corrosion resitant properties are impaired.

 Niobium and Tantalum

Niobium and Tantalum are stabilising elements with effects similar to Ti. Grades incorporating Nb and Ta are seldom used, the Ti stabilised grades being preferred. However, Nb is used for the manufacture of welding consumables, as Ti tends to be lost in transfer across the arc.

Nitrogen

Nitrogen promotes an austenitic crystal structure, and is used to complement Ni in the "N" grades of austenitic stainless steels. The yield strength of such grades at subzero temperatures is vastly improved. N is also used in duplex stainless steels to increase the austenitic fraction of the crystal structure, improving weldability.

Silicon

Silicon is added to improve the scaling resistance of austenitic heat resistant stainless steels. In castings higher Si content increases the fluidity of the molten metal improving the "castability" and high Si GMAW welding wire is usual as the Si promotes washing and wetting behaviour.