Carbon steel is
an alloy of iron and carbon. Low alloy steel includes carbon and small
additions of other alloying elements such as chromium, manganese, molybdenum,
etc. up to maximum of 5% total added alloying content.
What
happens when the carbon content is increased? Hardness is
increased. But the hardness of the metal has to be controlled
because it could become brittle. Depending on the application, brittleness may
be a critical factor. Think about a drill bit that you’ve been using and it
breaks while you’re in the middle of an operation. That faulty tool could have
broken because it had high carbon content and became quite brittle. In addition
to brittleness, yield point, tensile strength and rusting are all affected by
increased carbon concentration.
Increasing
carbon also reduces the weldability, especially above ~0.25% carbon.
Plasticity and ductility are similar. Think of a blacksmith, where he’s
hammering on a knife blade. If there’s too much carbon, the metal could break,
and won’t be able to be formed or wrought into the final product. If a product
doesn’t break, that doesn’t necessarily mean it’s of good
quality. Higher carbon also reduces air corrosion resistance, which
causes rusting. Rusting, of course, could cause problems later.
The
incorrect carbon level could also result in weld decay and creep stress
rupture. Here is a summary of the study.
On
increasing carbon content there is a increase in
1. Hardness
2. Brittleness
3. Yield
Point
4. Tensile
Strength
5. Rusting
On
increasing carbon content there is a decrease in
1. Weldability
2. Plasticity
3. Ductility
4. Air corrosion resistance.
Effect of Temperature on hardening of Steel
Hardening
and tempering of engineering steels is performed to provide components with
mechanical properties suitable for their intended service. Steels are heated to
their appropriate hardening temperature {usually between 800-900°C), held at
temperature, then "quenched" (rapidly cooled), often in oil or water.
This is followed by tempering (a soak at a lower temperature) which develops
the final mechanical properties and relieves stresses. The actual conditions
used for all three steps are determined by steel composition, component size
and the properties required.
Hardening
and tempering can be carried out in "open" furnaces (in air or
combustion products), or in a protective environment (gaseous atmosphere,
molten salt or vacuum) if a surface free from scale and decarburisation (carbon
loss) is required ("neutral hardening", also referred to as
"clean hardening").
Two
specialised quenching options can be applied in special circumstances:
Martempering
(also known as "marquenching") uses an elevated-temperature quench
(in molten salt or hot oil) which can substantially reduce component
distortion. This process is limited to selected alloy-containing steels and
suitable section sizes.
Austempering
can be applied to thin sections of certain medium- or high-carbon steels or to
alloy-containing steels of thicker section. It requires a high temperature
quench and hold, usually in molten salt, and results in low distortion combined
with a tough structure that requires no tempering. It is widely used for small springs
and presslngs.
What are the advantages?
Hardening
and tempering develops the optimum combination of hardness, strength and
toughness in an engineering steel and offers the component designer a route to
savings in weight and material. Components can be machined or formed in a soft
state and then hardened and tempered to a high level of mechanical properties.
