Annealing Processes:

The maximum heating temperature Annealing temperature is one of the most important parameters for the annealing process. The annealing temperature of most alloys is chosen based on the phase diagram of the alloying system, such as carbon steel which is based on the iron-carbon equilibrium diagram. Due to different annealing purposes, the annealing temperature of different steels (including carbon steel and alloy steel) drops above Ac3, above Ac1, or below Ac1.

#1. Recrystallization Annealing

Recrystallization annealing is used in alloys where the solid phase change recrystallization occurs during equilibrium heating and cooling. The annealing temperature falls above or within the phase transition temperature range. Both heating and cooling are slow.

The alloy undergoes a phase change recrystallization in the processes of heating and cooling, respectively, so it is called recrystallization annealing, often referred to as annealing. This annealing method is commonly applied to steel.

#2. Recrystallization Annealing Process and Classification

The recrystallization annealing process involves heating the steel slowly to 30 °C ~ 50 °C above AC3 eutectic steel or AC1 eutectoid steel or hypereutectoid steel, preserving for a suitable time, and then slowly – cooling down slowly.

Perlite or pre-eutectoid ferrite or cementite, occurring during heating, is transformed into recrystallization of the first phase transformation of austenite; In contrast, the second phase transformation recrystallizes during cooling, turning into pearlite or pro eutectoid ferrite or cementite with finer grains, thicker layers, and uniform microstructure.

Annealing temperatures above Ac3 hypereutectoid steel allow complete recrystallization of the steel, which is referred to as complete annealing. The falling temperature between Ac1 and Ac3 pegmatitic steel or between Ac1 and Acm hyper-eutectoid steel causes partial recrystallization of the steel, which is referred to as incomplete annealing.

It is an annealing process in which an iron-carbon alloy is heated to a temperature between Ac1–Ac3 to achieve incomplete austenitization and then slowly cooled. Incomplete annealing is mainly applied to forging medium and high carbon steel and low alloy steel.

Its purpose is to refine the structure and reduce stiffness. The heating temperature is Ac1+(40-60) °C, and it is slowly cooled down after heat preservation.

#3. Isothermal Annealing

Isothermal annealing is the controller cooling annealing method applied to steel and some non-ferrous alloy such as titanium alloy. In terms of steel, it is slowly heated to temperatures slightly above Ac3 hypo eutectoid steel or slightly above Ac1 eutectoid steel and hyper eutectoid steel.

After a period of heat preservation, the steel is austenitized and then rapidly moved to another furnace with a temperature slightly below A1. The isothermal temperatures are maintained until the austenites are completely transformed into lamellar pearlite sub-eutectoid steel & pro-eutectoid ferrite; hypereutectoid steel & pro-eutectoid cementite. Finally, it is cooled at any speed, usually, furnace air-cooled.

#4. Homogenization Annealing

Homogenization annealing, also known as diffusions annealing, is an annealings method for ingots or castings of steel & non-ferrous alloys such as tin bronze, silicon bronze, white copper, magnesium alloys and etc. The ingots or casting is heated to a high degree.

The temperature below the solidification temperature of the alloy was preserved for a long time and then gradually cooled. Homogenization annealing results in a solid diffusion of elements in the alloy to reduce chemical composition inhomogeneity separation, primarily to reduce chemical composition inhomogeneity intragranular segregation or dendrite segregation within the grain size.

The homogenization annealing temperature is so high as to accelerate the diffusion of the alloying elements and shorten the heat preservation time. The homogenization annealing temperatures of alloy steel are much higher than that of Ac3, generally 1050 °C ~ 1200 °C.

The temperature at which the non-ferrous alloy ingots are homogenized & annealed is generally “0.95 × solids temperature (K)”. Homogenization annealing has a high heating temperature and a long heat preservation time; Therefore, the consumption of heat energy is large.

#5. Spheroidizing Annealing

Spheroidizing annealing is an annealing method only applicable to steels. Slightly higher than Ac1, or the temperature is changed from time to time above and below A1 and then slowly cooled.

Its purpose is to convert flaky cementite in pearlite and pro-eutectoid cementite into spheroids evenly distributed in the ferrite matrix; such a structure is called a spherical pearl.

Medium carbon steel and high carbon steel with this kind of structure have low hardness, good machinability, and large cold deformation ability. For tool steel, this structure is ideally prepared for quenching.

#6. Stress Relief Annealing

Stress relief annealing is heating the workpieces to a suitable temperature below AC1 non-alloy steel at 500~600°C. The furnace-cooled heat treatment process followed by heat preservation is called stress reliefs annealing. The stress-free heating temperature is low, eliminating structure changes during the annealing process.

It is mainly applied to blanks and machined parts. Its purpose is to eliminate residual stresses in blanks and parts, to stabilize the shape and size of the workpiece, and to reduce the deformation and crack tendency during the cutting process and use.