Metallurgy of Stainless Steel and Cobalt – Chromium alloys

STAINLESS STEEL

 

When 12-30% chromium is added to steel the alloy is commonly called stainless steel. These steels resist tarnish and corrosion primarily because of the passivating effect of chromium. A typical composition of the alloy consists of 18% chromium, 8% Nickel, 71% iron, 0.2% carbon and other metals are also there like Titanium, Manganese (2%) Silicon (1%) Sulfur (0.15%), molybdenum, phosphorus, niobium and tantalum.

IRON : Fe, derived from Latin word ferrum. Atomic number-26; melting point 1535°C. Iron is the 4th common element in the earth’s crust.

The ores of iron are mainly Haematite, Magnetite, Limonite and siderite. Blast furnace is used for the primary reduction of iron ore to iron.

 

CHROMIUM: Cr, 24, M.P-1857°C

Its principal ores are (a) Chromite or Chrome iron (b) chrome ochre,
(c) chromelite.

Chromium is extracted by a process known as Goldschmidt’s alumino thermic process.

NICKEL : Ni, 28,M.P-1450°C

Its principal ores are

(a)          Garnierite

(b)         Nickel Glance

 

Nickel is extracted by a process known as Monds Process.

 

EFFECT OF ALLOY CONSTITUENT

 

CHROMIUM:

 

The corrosion resistance of stainless steel is largely due to the passivating effect of chromium. About 11% of chromium is needed to produce corrosion resistance in pure iron. Chromium resists corrosion well because of the formation of a strongly adherent coating of Cr2O3 on the surface. Cr favours the stability of the [BCC] unit cells.

 

NICKEL:

Nickel stabilizes a homogenous mass and corrosion resistant austenitic phase at low temperature. Alloying with nickel improves the corrosion resistance to oxidizing acids. Ni. Cu, Mn and N favour the FCC structure.

The alloys resistance to pitting corrosion is based on the content of chromium, Mo, and Ni. The higher the PRE value the more resistant the alloy to pitting corrosion.

 

CARBON:  Provides strength and hardness and it increases corrosion.

 

SILICON: Improves resistance to oxidation and carburization at higher temperature and to corrosion.

 

SULFUR: Allows easy machining of the alloy parts.

 

PHOSPHOROUS: Allows for a lower temperature for Sintering, a process in powder metallurgy in which the particles are heated to coalesce just under their melting point.

 

MANGANESE: Stabilizes the austenitic phase, but it decreases the corrosion resistance.

 

MOLYBDENUM: improve the corrosion resistance to non oxidizing acid and salts.