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Steel often requires heat treatment to obtain improved properties e.g increase hardness or strength, or to neutralise
negative effects resulting from previous manufacturing processes e.g.remove internal stresses generated by fabrication processes.
Normalising involves heating the steel to about 40oC above its upper
critical limit. The steel is then held at this temperature for a period of
time and is then cooled in air.. It is desireable that the temperature
of the steel shall be maintained for a time period more than 2 minutes per mm of
section thickness and shall not exceed the upper critical temperature
by more than 50oC.
Anealing is reheating steel followed by slow cooling. It is completed
Hardening involves heating a steel to its normalising temperature and cooling (Quenching )
rapidly in a suitable fluid e.g oil, water or air.
This process involves heating the metal to a temperature in the range 550oC to 650oC and held
at this temperature before being cooled at a controlled rate. This also reduces stresses
resulting from cold working and fabrication by allowing dislocations to rearrange to a lower energy
The process applies more to the hypereutectoid steels (above 0,8% C). The process involves
heating the metal to between 600oC and 650oC and holding it at at the
selected temperature for a period of time the cementite changes from a lamella formation
to a formation based on an alpha ferrite matrix with particles of spheroidal cementite (Fe3C)
are embedded. This resulting steel has improved ductility and toughness compared
to the original steel with reduced hardness and strength.
Tempering is the process of reheating the steel leading to precipitation and
spheroidisation of the carbides. The tempering temperature and time are
generally controlled to effect the final properties required of the steel.
The benefits resulting are the increase in the metal toughness and elongation.
The negative effects are the reduction of the martensite (BCT) structure and the
progression towards a spheroidal carbide + ferrite matrix structure.
The hardenability of a steel is broadly defined as the property which determines
the depth and distribution of hardness induced by quenching. Hardenability is a
characteristic determined by the following factors
The hardenability is the depth and evenness of hardness of a steel upon quenching from austenite.
The properties of heat treated steel are significantly affected by the thickness of the section. Hardening consist of heating the steel through and just above its critical range to obtain the condition of solid solution and quenching with sufficient rapidity to retain this condition. If a steel has a large thickness it is practically impossible to obtain an even temperature throughout and the middle of the section is always at a lower temperature compared to the outside surfaces. On quenching the heat is absorbed rapidly from the outside and it is impossible even with the most drastic quench processes to remove heat from the core region sufficient to obtain the desire structure. For thin sections it may be possible to obtain the desire structure throughout the section with a comparative mild quenching process.
There are a number of fluids used for quenching steels listed below in order of quenching severity
Note: Agitation of medium increases its quenching severity
Many of the heat treatment processes can be completed in vacuum furnaces at very low pressures (high vacuums). The advantages of using vacuum furnaces are listed below.
This process involves direct an oxy acetylene flame on the surface of the steel being hardened
and heating the surface above the upper critical temperature before quenching the steel in a spray
of water. This is also known as the shorter process.
Induction hardening provides a similar surface treatment regime to flame hardening .
The steel component is located inside a water cooled copper coil which has (AC)
alternating current through it. This causes the outer surface of the component to heat up.
Depending on the AC frequency and current, the rate of heating as well as the
depth of heating can be controlled. This process is well suited for surface heat treatment.
The primary purpose of case hardening is to produce a surface which is resistant to wear while
maintaining the overall toughness and strength of the steel core. This type of process
is normally used on a steel with a low carbon content and introduces carbon by diffusion (carburising)
into the local surfaces requiring treatment.. Subsequent heat treatment develops the desired combination
of high surface hardness and internal toughness. Another process called Nitriding consists of
the diffusion of nitrogen.
This process is the simplest and earliest carburising process based on placing the components
to be treated in metal containers with the caburising mixture, based on powdered
charcoal and 10% barium carbonate, packed around the components. The containers are then heated
to a constant temperature (850oC to 850oC )for a time period to ensure an even temperature throughout and
sufficient to enable the carbon to diffuse into the surface of the components to sufficient depth.
Gas caburising allos is accurate control of the process temperature and caburising
atmosphere. The components are brought to a uniform temperature in a neutral atmosphere.
The caburising atmosphere is introduced only for the required time to ensure the correct depth
of case. The carbon potential of the gas can be lowered to permit diffusion avoiding
excess carbon in the surface layer.
This process is mostly used for producing shallow case depths in thin sections.
The components are heated quickly in a bath containing a suitable sodium cyanide salts
and sodium carbonate. The proportion of NaCN being maintained 20% to 30% by controlled
feed strong NaCN.
Heat treatment following carburisation
The time of heat treatment post carburisation relates to the condition of the steel. If the steel is prepared as a fine grain steel it is possible to complete a single quench operation following case hardening. If the steel does not have a fine grain structure a normal process is to quench from about 870oC the quench again from about 790oC . This ensures reasonable mechanical properties in case and core.
Certain steel alloys can absorb nitrogen with a resulting extremely hard surface layer.
The process consists of maintaining the steel component at a carefullly controlled temperature
of 490oC to 530oC under the action of nascent of active nitrogen produced on the
surface of the component by the decomposition of gaseous ammonia. The resulting surface
is extremely hard and extremely thin but very brittle. An nitrides based on steel alloys
are less brittle and more stable than straight iron nitrides and therefor this process is only
used for certain alloy steels..
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Last Updated 12/03/2010