Metallurgy of Stainless Steel and Cobalt – Chromium alloys

 

COBALT-CHROMIUM ALLOYS

Co-Cr alloys drawn into wire can be used successfully in orthodontic appliances.

 

COMPOSITION

A representative composition for such an alloy is cobalt- 40%, chromium- 20%, Nickel- 15%, molybdenum- 7%, manganese- 2% carbon 0.16%, Be- 0.04%, iron- 15.8%.

EFFECT OF ALLOY CONSTITUENT

In the cobalt-chromium system Cobalt is the basic element which can be considered as solid solutions.

 

Chromium by its passivating effect ensures corrosion resistance of the alloy. It also aids in solid solution hardening. 30% chromium is considered the upper limit for attaining maximum mechanical properties.

 

Cobalt and Ni are somewhat interchangeable. As Nickel replaces cobalt strength, hardness, modulus of elasticity and fusion temperature tend to decrease while the ductility increases.

 

Molybdenum and Tungsten are very effective solid solution hardeners. Molybdenum reduces the ductility also.

 

Iron, Copper, Beryllium and other elements also aid in solid solution hardening.

 

Manganese and Silicon are used as oxide scavengers to prevent oxidation of other elements during melting. They also act as hardeners.

 

Boron acts as deoxidizers and increases hardness and reduces ductility.

 

Beryllium is a hardener and grain structure refiner. It is generally added to decrease the fusion temp.

 

The Carbon content is the most critical and small variations have a pronounced effect on the strength hardness and ductility of the alloy. Carbon can form carbides with any of the metallic constituents. Carbide precipitation is a very important factor in strengthening this alloy but it increases the brittleness of the alloy.

 

MICROSTRUCTURE OF (Co-Cr ALLOY) (ELGILOY)

The microstructure of any substance is the basic parameter that controls the properties. A change in the physical properties of a material is a strong indication that there must have been some alteration in its microstructure.

 

The micro structure of Co-Cr alloys is inhomogeneous consisting of an austenitic matrix composed of a solid solution of cobalt and chromium in a cored dendritic structure. The dendritic regions are cobalt rich. Where as the interdendritic regions can be a quaternary mixture consisting of a cobalt rich n phase a chromium rich M23C6 phase where Ms is co, cr, or Mo an M7C3 phase and a chromium and molybdenum rich s phase.

 

Many elements present in the alloy such as chromium cobalt Molybdenum are carbide forming elements. If the carbides are continuous along the grain boundaries the alloy possesses low elongation values with a good clean surface. If the carbides are spherical and discontinuous it results in good elongation values and a good and clean surface.

 

The Co-Cr Mo alloy exhibits a eutectic point at approximately 1235oc. At temp. Above the eutectic point localized melting of the solute rich zones occurs cooling to below the eutectic point yields a microstructure consisting of grain boundarys, n and M23C6 which embrittle the alloy.

 

COBALT-CHROMIUM WIRES

Co-Cr alloys are available commercially as Elgiloy (Rocky Mountain Orthodontics), Azura (Ormco Corporation) and Multiphase (American Orthodontics Corporation).

 

Elgiloy is manufactured in four tempers.

 

(a)   Soft (Blue)

            Softest of the four wire tempers and can be bent easily with fingers or pliers. It is recommended for use when considerable bending soldering or welding is required. Heat treatment of blue Elgiloy increases its resistance to deformation.

(b)   Yellow (Ductile)

            More resilient than blue Elgiloy. It can also be bent with relative ease. We can increase its resistance and spring performance by heat treatment.

 

(c)    Green (Semi resilient)

It is more resilient than yellow and can be shaped with pliers before heat treatment.

 

(d)   Red (Resilient)

            This is the most resilient Elgiloy and provides high spring qualities. Careful manipulation with pliers is recommended, when using this wire because it withstands only minimal working. Heat treatment makes red Elgiloy extremely resilient. Since this wire fractures easily after heat treatment, all adjustments should be made before this precipitation hardening process.

 

 

MECHANICAL PROPERTIES ACCORDING TO ADA spn no 32

 

 

Alloy Modulus of Elasticity

Gpa% 0.2 yield strength

MpaUltimate Tensile strength

MpaNo. of 90o BendsCo-Cr Alloy184141316828