What is spheroidization annealing?

Spheroidal annealing is a specific heat treatment process mainly used for eutectoid steel, hypereutectoid steel, and alloy tool steel. Its purpose is to reduce hardness, improve plasticity and toughness, improve cutting performance, and prepare the microstructure for subsequent quenching. The following is a detailed explanation about spheroidization annealing:

Definition of spheroidization annealing
Spheroidization annealing is an annealing process performed to spheroidize carbides (mainly carbides such as Fe-C) in steel. By heating steel to a specific temperature, holding it for a period of time, and then slowly cooling it, the carbides in the steel transform from flakes or networks to spherical or granular shapes, evenly distributed on the ferrite matrix, forming a structure similar to granular pearlite, which is called spheroidized structure.

The process of spheroidization annealing is roughly as follows:

Heating: Heat the steel to a temperature range of 20-30 ℃ above Ac1 (the temperature at which austenite begins to transform) (also known as a temperature slightly higher than AC1). The selection of this temperature range is to ensure that carbides can fully dissolve in austenite without causing excessive grain growth.
Insulation: Keep at the heating temperature for a period of time to allow sufficient time for carbides to dissolve and redistribute.
Cooling: Then cool at a slow rate (such as not exceeding 50 ℃ per hour) to a temperature slightly lower than Ac1, and stay for a period of time to complete the tissue transformation. Finally, continue to slowly cool to a lower temperature (such as 500-600 ℃), and then remove from the furnace and cool in the air.

The main types of spheroidization annealing
According to the steel grade and annealing purpose, spheroidization annealing can be divided into the following types:

Ordinary spheroidization annealing: suitable for carbon tool steel near eutectic composition, heated to 730-740 ℃ for sufficient insulation time, and then slowly cooled to 650 ℃ at a rate of less than 20 ℃/h before being removed from the furnace.
Periodic spheroidization annealing (cyclic annealing): Heating and cooling are repeatedly carried out at a temperature near point A, usually for 3-4 cycles, to spheroidize carbides in the process of repeated dissolution and precipitation of pearlite flakes. Suitable for steels with severe flaky pearlite.
Isothermal spheroidization annealing: Heat to 800 ± 10 ℃, hold for a while, then quickly cool to 700 ± 10 ℃ (near A1) and hold for a longer period of time. Afterwards, cool to 600 ℃ at a rate of 30-50 ℃/h and remove from the furnace. This process is commonly used for bearing steel.
Deformation spheroidization annealing: Combining plastic deformation with spheroidization annealing process to accelerate the spheroidization process of carbides through the action of plastic deformation.

The spheroidized structure obtained after spheroidization annealing has the following characteristics and effects:

Hardness reduction: Spheroidal tissue has lower hardness than sheet-like tissue, which is beneficial for cutting processing.
Improvement in plasticity and toughness: Spheroidal structure has better plasticity and toughness than sheet-like structure, which is beneficial for cold deformation processing such as cold drawing and cold stamping.
Improvement of cutting performance: When cutting workpieces with spheroidized structure, the tool can avoid cutting hard and brittle carbides, and pass through soft ferrites, thereby extending the service life of the tool.
Preparation for quenching: The microstructure after spheroidization annealing is more conducive to subsequent quenching treatment, which can reduce the tendency of quenching deformation and cracking.

In summary, spheroidization annealing is an important heat treatment process that aims to reduce hardness, improve plasticity and toughness, and enhance cutting performance by altering the morphology and distribution of carbides in steel.