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About Annealing
The final iron product is comprised of various stages of the manufacturing process, such as the raw material stage, for which said material is supplied from giant steelmakers, and further stages. During the manufacturing process, it can become deformed through the stages. While becoming deformed, the internal strain is occurred which causes to break the iron. Thereby, in order to reduce such strains, annealing is required to respond to the next stage of work hardening. Annealing is a heat treatment to prevent such factors.
There are different methods depending on usage, i.e. "Full Annealing","Spheroidizing Annealing", and "Stress-Relief Annealing". The heat treatment of "Quenching" at a later stage is for hardening iron, The heat treatment of annealing raw material at an early stage, however, is for softening iron.
Hardness of Major Steel Grades after the treatment
| Steel Grade | Hardness (HRB) after the heat treatment |
|---|---|
| S15C | less than 60 |
| S20C | less than 60 |
| S25C | less than 70 |
| S35C | less than 80 |
| S45C | less than 82 |
| SCM415 | less than 75 |
| SCM435 | less than 85 |
| SUJ2 | less than 89 |
About Full Annealing
Full annealing is the process of slowly raising the temperature about 50℃(122℉) above line A3 in the case of Hypoeutectoid steels and 50℃(122℉) above line A1
in the case of Hypereutectoid steels. It is then slowly cooled down in a furnace.
The grain structure has coarse Pearlite with ferrite or cementite depending
on whether hypo or hypereutectoid.
The steel becomes soft and ductile.
About Spheroidizing Annealing
Spheroidize Annealing is the process used for high carbon steels that will
be machined or cold formed subsequently.
This method results in a structure in which all cementite is in the form of
small globules(spheroids) dispersed throughout the ferrite matrix.
This resulting product has improved ductility and toughness with reduced
hardness and strength. Spheroidize Annealing is normally carried out under
a protective(endothermic) atmosphere to prevent oxidation and
decarburization.
There are several heat patterns for this annealing and fairly slow cooling is required.
About Stress-Relief Annealing
Stress-Relief Annealing is used to reduce residual stresses in large castings, welded parts and cold-formed parts. Such parts tent to have stress due to thermal cycling or work hardening.
This is done below the 727℃（1340℉）(A1)line.
About the other Heat Treatment
Normalizing
Normalizing is the process of raising the temperature to over 60℃（140℉）,, above line A3 or line ACM fully into the Austenite range and cooled down at room temperature under natural convention.
Normalizing is used to refine the grain structure and create a more homogeneous structure when a steel is to be reheated for hardening and quenching or full annealing, or to unify the microstructure of the heat effected zone of welding and also to provide moderate hardening.
Features of Heat Treatment
| Atmosphere Gas | Applicable Steel Grade and Purpose of Use |
|---|---|
| N2 | Steel Material less than C=15 It is applicable to the ones with various plating after post-process |
| RX | S25C-S45C Steel Material over C=15 SCR、SCM、SNC、SNCM、SAE1018, and other hard/special steel |
| Atmosphere Annealing | Steel Material less than C-6 such as annealed iron wire etc. |