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Incomplete Annealing & Spheroidizing Annealing of Metal Castings

Incomplete annealing of Metal Castings

Definition:

Incomplete annealing is a heat treatment process for steel castings, which involves heating the steel to a temperature range between Ac1 and Ac3 (for hypoeutectoid steel) or Ac1 and Accm (for hypereutectoid steel), cooling it at a slow cooling rate of less than 50 ℃/h after holding it until the temperature drops below 500 ℃ and then taking it out of the furnace for air cooling. This process aims to obtain a structure close to the equilibrium state.

Austenitization characteristics:

Since the heating temperature is in the two-phase region, the structure of the steel is not completely transformed into austenite. Only the pearlite part undergoes phase transformation and recrystallization, transforming into austenite, while the morphology and distribution of proeutectoid ferrite or cementite remain basically unchanged.

Application:

Incomplete annealing is mainly used as a pretreatment method before quenching of hypereutectoid steel, and its application in hypoeutectoid steel is relatively rare.

 

Spheroidizing annealing of Metal Castings

Definition:

Spheroidizing annealing is a specific heat treatment process that aims to make carbides in steel form spherical particles, thereby obtaining granular pearlite structure. In fact, it is a kind of incomplete annealing.

Application:

This process is mainly used for eutectoid steel, hypereutectoid steel and alloy tool steel castings, aiming to reduce the hardness of the material, improve the cutting performance, ensure uniform structure, optimize the heat treatment process performance, and lay a good organizational foundation for the subsequent quenching process.

Process details:

The heating temperature needs to be precisely controlled, usually slightly higher than the Ac1 temperature by 20 to 30 °C, and the furnace heating method is adopted.

The holding time should not be too long, generally maintained between 2 and 4 hours.

There are various cooling methods, but furnace cooling or long-term isothermal treatment at about 20 °C below Ar1 is a common method.

The key to spheroidizing annealing is to ensure that a sufficient number of undissolved carbide points are retained in the austenite, resulting in uneven distribution of carbon concentration to promote the spheroidization of carbides.

Heating at too high a temperature or holding for too long will cause a large amount of carbides to dissolve, forming uniform austenite, reducing the spheroidization core and affecting the spheroidization effect.

The shape and size of cementite particles are affected by the cooling rate or isothermal temperature. Rapid cooling or low isothermal temperature is prone to form flaky carbides, resulting in increased hardness.

 

 

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Commonly used spheroidizing annealing processes:

Single spheroidizing annealing:

heating to 20-30 ℃ above Ac1, slowly cooling to below 600 ℃ after keeping warm, and then air cooling out of the furnace.
Isothermal spheroidizing annealing:

heating to 20-30 ℃ above Ac1, quickly cooling to about 20 ℃ below Ar1 after keeping warm, and then cooling to below 600 ℃ with the furnace and air cooling out of the furnace. This process is widely used.
Reciprocating spheroidizing annealing:

heating to a temperature slightly higher than Ac1 for multiple times, cooling to a temperature slightly lower than Ar1 after keeping warm, and then air cooling until room temperature to obtain a better spheroidizing effect. It is suitable for difficult-to-spheroidize steel castings, but the operation is complicated.


Post time: Jan-03-2025