Changing Properties of Metals

Carbon steel is a versatile material whose properties within limits can be altered to improve hardness and toughness by the addition of carbon and heat treatment.

However carbon steels have a number of limitations which makes them unsuitable for certain applications. These are:

-Poor resistance to oxidation
-Limited tensile, hardness and ductility when untreated
-Drastic quenching causes cracking and distortion
-Poor resistance to acid
-Large sections cannot be hardened
-Steel can soften at high temperatures

Hardening
Hardening is only possible via heat treatment on medium to high carbon steels, the metals are heated to certain temperatures depending on their carbon content (780°C to 850°C) then cooled quickly usually by quenching the metals in water or oil, the reason the metals are heated to these temperatures called their ‘austenitic crystal phase’ is because the crystal structures of the metals can then start to alter, forming bonds to create cementite as the carbon diffuses which is a very hard and brittle material.

% Carbon	Annealing Temperature
0.2%	850°C
0.5%	810°C
1.0%	780°C

Annealing
When annealing the metal is heated just above the re-crystallization temperature process and is left to cool slowly, for what is called a full anneal the metal is left to cool in the furnace, this allows the metal to cool off more gradually. Annealing changes the metals properties to become more soft and ductile, this is caused by the grain structure within altering and re-aligning at heat.

Hot Rolled Grain	Annealed Grain

Normalising
Normalising removes stresses within the grain structure of the metal thus making the metal more stable and ready for other processes. This is done by heating the metal to a higher temperature than that of the annealing process then leaving it to air cool.

Tempering
This gives the metal more malleability but taking away a small amount of hardness in the process. Tempering is done by heating the metal to a pre determined temperature which will depend on the level of malleability required. The size and the arrangement of the metals grain structure will affect the metals properties such as; hardness, ductility, malleability ect.

Basic Iron/Carbon Phase Graph

This graph shows the basic relationship of carbon content in steel when hardening with heat treatment, as the graph shows high carbon steels can become very hard at the cost of ductility.

Ferrite Phase
-High ductility
-Poor tensile strength
-Low hardness

Pearlite Phase
-High tensile strength
-Increasing hardness
-Less ductility

Cementite Phase
-Brittle
-Low ductility
-Lower tensile strength

Low Carbon	Mild Steel	Medium Carbon	High Carbon
Applications: -Car body panels -Filing cabinets Properties: -Malleable -Very ductile -Soft	Applications: -Automotive parts -Brackets Properties: -Low hardness -Medium tensile strength -Good ductility	Applications: -Hammers -Chisels Properties: -Medium hardness -Good tensile strength -Medium ductility	Applications: -High speed tooling Properties: -Hard -Brittle -Low ductility

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