Annealing of Steel
① Adjust hardness for machining operations. If the workpiece is too hard, it cannot be cut; if it is too soft, chip breaking becomes difficult during cutting. Typically, a hardness range of
170–250 HB is suitable for general machining.
② Eliminate residual internal stress to prevent deformation or cracking of steel parts during subsequent processing or heat treatment. Residual internal stress is generated on the surface and inside the workpiece during blank forming processes (such as casting, forging, welding) or machining. This stress will redistribute during subsequent processing of the workpiece, leading to deformation or cracking.
③ Refine grain size, improve microstructure, enhance mechanical properties, and prepare the structure for final heat treatment.
2. Classification of Annealing Processes
- Annealing above the critical temperature (Ac1, Ac3)
This category includes full annealing, incomplete annealing, spheroidizing annealing, and diffusion annealing.
- Annealing below the critical temperature
Examples include recrystallization annealing and stress relief annealing.
3. Common Annealing Processes
A type of annealing process that involves heating the workpiece slowly to a temperature 30–50°C above Ac3, holding it at that temperature for a specified period (soaking), and then cooling it slowly. It is also known as conventional annealing or recrystallization annealing.
It is called "full" because the structure of the steel can undergo a complete austenitization transformation through recrystallization (nucleation and grain growth).
Limitations: Full annealing uses slow furnace cooling, which results in a long process cycle and prolonged equipment occupancy. To improve equipment utilization, isothermal annealing is often used as a replacement.
2) Isothermal Annealing
The isothermal annealing process is as follows: heat hypoeutectoid steel to a temperature 30–50°C above Ac3, and heat eutectoid steel and hypereutectoid steel to a temperature 30–50°C above Ac1. After holding at the respective temperatures for an appropriate period, stop heating and open the furnace door to rapidly cool the workpiece to a specific temperature below Ar1. Hold the workpiece at this temperature until all austenite is transformed into lamellar pearlite—hypoeutectoid steel also forms proeutectoid ferrite, while hypereutectoid steel also forms proeutectoid cementite. Finally, cool the workpiece at any rate, usually by removing it from the furnace and cooling it in air.
The isothermal temperature must not be too low or too high. If it is too low, the hardness after annealing will be relatively high; if it is too high, the isothermal holding time needs to be extended.
Isothermal annealing has the same purpose as full annealing. It can shorten the workpiece's residence time in the furnace and achieve a more uniform microstructure and hardness.
Application: Isothermal annealing is mainly used for high-carbon steels and alloy steels with a long incubation period. The supercooled austenite of these steels transforms quite slowly in the pearlitic transformation temperature range. If full annealing is adopted, it often takes dozens of hours, which is very uneconomical.
A spheroidizing annealing process involves heating eutectoid or hypereutectoid steel to a temperature 10–20°C above Ac1, allowing more undissolved carbide particles to spontaneously spheroidize during long-term soaking. After holding for a certain period, the steel is slowly cooled to below 600°C and then removed from the furnace for air cooling, thereby spheroidizing the cementite in pearlite.
The microstructure obtained from spheroidizing annealing is characterized by granular cementite particles dispersedly distributed in a ferrite matrix, which is referred to as spheroidal pearlite (or nodular pearlite).
Note: If there is severe network carbide present in the steel before spheroidizing annealing, normalizing should be performed first to eliminate the network cementite, followed by spheroidizing annealing. Failure to do so will affect the spheroidization effect.
It refers to a heat treatment process where the workpiece is slowly heated with the furnace to 500–600°C, held at this temperature for a certain period, then slowly cooled with the furnace to below 200–300°C before being removed from the furnace. Notably, no microstructural transformation occurs in the workpiece during this process.
Purpose: It is mainly applicable to blank workpieces and parts that have undergone machining. The goal is to eliminate residual stress in blanks and parts, stabilize the dimensions and shape of the workpiece, and reduce the tendency of deformation and cracking of parts during machining and service.
Note: It should be noted that stress relief annealing cannot completely remove internal stress, but only partially eliminates it, thereby neutralizing its harmful effects.
It refers to a heat treatment process that involves heating steel to a relatively high temperature below the solidus temperature of the alloy, holding it for an extended period (10–15 hours), and then cooling it slowly.
It is an annealing method applied to ingots or castings of steel and non-ferrous alloys (such as tin bronze, silicon bronze, cupronickel, etc.).
Purpose: Homogenizing annealing promotes the solid-state diffusion of elements in the alloy to reduce chemical composition segregation and microstructural inhomogeneity within grain sizes (intragranular segregation, also known as dendritic segregation) that occur in steel ingots, steel castings, or cast billets during solidification.
The reason why the homogenizing annealing temperature is so high is to accelerate the diffusion of alloying elements and shorten the holding time as much as possible. The homogenizing annealing temperature for alloy steels is much higher than Ac3, typically ranging from 1050°C to 1200°C.
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