Heat Treatment
Introduction
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.
Heat treatment is a stage in the fabrication of structures and is often forgotten, but it has perhaps more wide-reaching and important ramifications than many of the other stages in the fabrication of structures or components.
Definition
Heat treatment may be defined as an operation or combination of operations involving heating and cooling of a metal/alloy in solid state to obtain desirable conditions and properties.
Various heat treatment processes can be classified as:
Classification of Heat Treatment Processes
1. Annealing
2. Normalising
3. Hardening
4. Tempering
5. Surface hardening
Classification of Heat Treatment Processes
Purpose of Heat Treatment
In general all heat treatment processes are carried out in order to:
(i) Relieving internal stresses developed during cold working, welding, casting, forging etc.
(ii) Harden and strengthen metals.
(iii) Improve machinability.
(iv) Change in grain size.
(v) Soften metals for further (cold) working as in wire drawing or cold rolling.
(vi) Improve ductility and toughness.
(vii) Increase, heat, wear and corrosion resistance of materials.
(viii) Improve electrical and magnetic properties.
Applications
Heat treatment of forgings of shafts and axles, drills, cutting tools, taps, dies, measuring instruments etc.
Annealing
DEFINITION
Primarily is the process of heating a metal which is in a metastable or distorted structural state, to a temperature which will remove the instability or distortion and then cooling is (usually at a slow rate) so that the room temperature structure is stable and/or strain free.
Purpose of Annealing
(i) Inducing a completely stable structure.
(ii) Refining and homogenizing the structure.
(iii) Reducing hardness.
(iv) Improving machinability.
(v) Improving cold working, characteristics for facilitating further cold work.
(vi) Producing desired microstructure.
(vii) Removing residual stresses and gases
(viii) Improving mechanical, physical, electrical and magnetic properties.
Applications of Annealing
General applications of annealing are given below but choice of application is depend upon specific type which are discussed further,
i)Steels used in sheet and wire drawing
ii) Castings of carbon and alloy steels.
ii) High carbon tool steels
iv) Ball bearing steels.
Types of Annealing
The various methods of annealing are as follows:
A) Stress relieving
B) Process annealing
C) Spheroidise annealing
D) Full annealing
A) Process annealing
It is also called interstate annealing or subcritical annealing. In this process, the steel is heated below the lower critical temperature usually 500°C to 200°C, holding at this temperature for a prolonged period followed by slow cooling i.e. air cooling. The holding time is usually 2 to 4 hours.
Process annealing is generally carried out in either batch-type or continuous furnaces, usually with an inert atmosphere of burnt coal gas.
The process is applied to low carbon steel, cold rolled steel used in wire drawing and deep drawing operations.
B) Spheroldise annealing
Heat treatment used to produce spheroidal or globular form of cementite from plates of cementite in steel called Spheroidise annealing or simply spheroidizing.
The process applies more to the high carbon steels.
The process involves heating the steel just below the lower critical temperature i.e. between 650°C to 700°C, and holding there for prolonged period.
This resulting steel has improved machinability, ductility and toughness compared to the original steel with reduced hardness and strength.
Advantages
(i) Improves machinability
(ii) Facilitates a subsequent cold working operation.
(iii) Obtains a desired structure for subsequent heat treatment
(iv) Softens tool steels and some of the air hardening alloy steels.
(v) Improves surface finish during machining, the steels can machined relatively freely.
(vi) Prevents cracking of steel during cold forming operations.
C) Full annealing
When full annealing is applied to steel, it is called as annealing or conventional annealing.
It involves heating the steel to a temperature within the range. 30°C-50 °C above the upper critical temperatures, held at this temperature for definite period and then allowing the steel to cool slowly within the furnace.
The rate of cooling 30°C-200 °C per hour depends on the composition.
This process is used mainly to remove internal stresses built up as a result of cold working and fabrication processes.
It is applied on castings carbon and alloy steels as well as hypereutectoid hot worked steel in the form of sheets and forgings.
Advantages
(i) Refines grains.
(ii)Removes strains (from forgings and castings)
(iii) Induces softness
(iv) In some cases improves machinability.
(v) Improves formability.
(vi) Improves electrical and magnetic properties.
D) Stress relief annealing
i) Stress relief annealing relieves or eliminates stresses induced by casting, machining, cold working, welding, etc.
ii) It is a special type of annealing applied for the sole purpose of stress relief. It is classified as finish annealing process.
iii) The process consists of heating cold worked steel about 500°C usually below its recrystallisation temperature, held at this temperature for about 1 to 2 hours, followed by cooling in air to room temperature.
iv) It results in relieving internal stresses without changing microstructure.
v) Tool steels are often subjected to this type of annealing..
