The Difference Between Partially Annealed and Fully Annealed
(1) Uncomplete Annealing, also known as Incomplete Crystalline Annealing, involves heating steel to a temperature between Ac1 and Ac3 or Ac1 and Acm, holding it at that temperature, and then cooling it slowly to induce an incomplete recrystallization of the steel structure.
(2) Partial annealing involves heating the workpiece to a semi-austenitic state before annealing, while full annealing is achieved by heating the workpiece to a fully austenitic state for annealing.
(3) Spheroidizing annealing is typically used for hypoeutectoid steel, as well as hypereutectoid steel, whereas complete annealing is generally used only for hypoeutectoid steel.
(4) Full annealing typically results in lamellar pearlite, whereas incomplete annealing yields spheroidal pearlite.
Application Range of Partially Annealed Products
Uncomplete annealing can be used for hypoeutectoid steel as well as hypereutectoid steel. Its characteristic is that the cementite in the pearlite becomes spherical after annealing, and this type of uncomplete annealing is also known as spheroidizing annealing.
For hypoeutectoid steel, incomplete annealing often occurs due to the following situations:
(1) Enhance the machining performance of these components, which are primarily used for forgings, especially those with high carbon content. These forgings often exhibit excessively fine pearlite and high hardness after forging, making them difficult to machine.
(2) Spheroidizing treatment for improving cold deformation properties, used for hypoeutectoid steel requiring cold deformation.
What is the Difference Between Isothermal Annealing and Normal Annealing?
Isothermal annealing differs from conventional annealing in the process and cooling methods. Conventional annealing typically involves slowly cooling the workpiece after the heat preservation process is complete, with the workpiece being removed from the furnace and air-cooled once it reaches below 500 degrees Celsius. In contrast, isothermal annealing involves cooling the workpiece to a certain temperature below Ac1 after heating and heat preservation, holding it there for a period to allow the Austenite to transform into Pearlite, and then rapidly cooling it (air-cooling) to room temperature. The advantage of isothermal annealing lies in its shorter processing time and superior quality.
Isothermal Annealing Cooling Method
(1) The cooling rate from the self-aging temperature to the isothermal temperature of the workpiece can be arbitrary, and it is commonly done using two furnaces in production.
(2) The isothermal temperature is generally below Ar1 by 10-30 degrees Celsius, which is within the pearlite transformation temperature range.
(3) After transformation, the cooling can be arbitrary.
Five: Method of ball return
(1) Normal Ball Retraction: Heat the steel parts to slightly above Ac1 (usually Ac1 + 20-30 degrees Celsius), hold the temperature, then cool slowly to below 500 degrees Celsius at a rate of 20-50 degrees Celsius per hour, and air cool after removal from the furnace.
(2) Isothermal Spheroid Annealing: After being held at the same heating temperature as the conventional spheroid annealing, the furnace is cooled slightly below the Ar1 temperature for isothermal treatment, followed by air cooling after the isothermal process is completed.
(3) Cycle cooling and reheating: It involves alternating heating and cooling above and below Ac1, which is equivalent to multiple isothermal cooling cycles.
