Fundamentals of Annealing Process in Cryogenic Treatment 

Fundamentals of Annealing Process in Cryogenic Treatment

Xinfengli Technology Co., Ltd. is a domestic brand specializing in the research, development, and sales of ultra-low temperature cryogenic equipment and various liquid nitrogen cryogenic treatment devices in China. Its primary products include the XFL series liquid nitrogen cryogenic chambers, XFL series liquid nitrogen low-temperature installation equipment, XFL series cryogenic treatment equipment, XFL series high-low temperature test chambers, XFL series mold cryogenic treatment systems, XFL liquid nitrogen ultra-low temperature processing technologies, XFL liquid nitrogen ultra-low temperature treatment skills, and XFL liquid nitrogen ultra-low temperature deep processing machinery. With years of experience in liquid nitrogen low-temperature equipment and liquid nitrogen quick-freezing food machinery, Xinfengli offers comprehensive products and services.

Fundamentals of Annealing Process

Annealing is a heat treatment process that involves heating a metal or alloy to an appropriate temperature, holding it at that temperature for a certain period, and then slowly cooling it (typically within the furnace).

The essence of annealing lies in heating the steel to a temperature above its Austenite transformation range, followed by a Pearlite transformation, resulting in a nearly equilibrium structure.

Objectives of Annealing:

Reduce the hardness of steel and increase its plasticity, facilitating machining and cold deformation processes.
Homogenize the chemical composition and structure of steel, refine grain size, improve steel properties, or prepare the structure for subsequent quenching processes.
Eliminate internal stresses and work hardening effects, preventing deformation and cracking.
Annealing and normalizing are primarily used as preliminary heat treatments. For components subjected to low stresses or with modest performance requirements, they can also serve as final heat treatments.

Classification of Annealing Methods

Common annealing methods, categorized by heating temperature, include:

Phase transformation recrystallization annealing above the critical temperature (Ac1 or Ac3): This encompasses full annealing, diffusion annealing, incomplete annealing, and spheroidizing annealing.
Annealing below the critical temperature (Ac1 or Ac3): This includes recrystallization annealing and stress-relief annealing.
Seven Types of Annealing Processes

Full Annealing
Process: The steel is heated to 20-30°C above Ac3, held for a period, and then slowly cooled (within the furnace) to obtain a balanced structure (complete Austenitization).
Application: Primarily used for hypoeutectoid steels (wc=0.3-0.6%), typically medium-carbon steels, low and medium-carbon alloy steel castings, forgings, and hot-rolled sections. It is occasionally applied to their welded components. Low-carbon steel may become too soft after full annealing, complicating machining. Hypereutectoid steels, when slowly cooled from above Accm, may form network-like Fe3CⅡ along grain boundaries, significantly reducing strength, hardness, ductility, and toughness, potentially compromising subsequent heat treatments.
Objective: Refine grain size, homogenize structure, eliminate internal stresses, reduce hardness, and improve machinability. The resulting structure in hypoeutectoid steel is Ferrite + Pearlite.
Practice: To enhance productivity, components are often air-cooled after annealing to approximately 500°C.
Isothermal Annealing
Process: The steel is heated above Ac3 (or Ac1), held for a suitable duration, rapidly cooled to a temperature slightly below Ar1 for isothermal holding to transform Austenite to Pearlite, and then air-cooled to room temperature.
Objective: Achieves the same outcomes as full annealing but with more controlled transformation and significantly reduced processing time, particularly for alloy steels with stable Austenite.
Application: Suitable for steels with stable Austenite, such as high-carbon steels (wc>0.6%), alloy tool steels, and high-alloy steels (total alloy content >10%). Isothermal annealing facilitates uniform structure and properties but may be impractical for large sections or batches due to challenges in achieving consistent isothermal temperatures throughout.
Incomplete Annealing
Process: The steel is heated to Ac1-Ac3 (for hypoeutectoid steels) or Ac1-Accm (for hypereutectoid steels), held, and then slowly cooled to attain a near-equilibrium structure.
Application: Primarily used for hypereutectoid steels to obtain a spheroidal Pearlite structure, eliminating internal stresses, reducing hardness, and improving machinability. Spheroidizing annealing is a variant of incomplete annealing.
Spheroidizing Annealing
Process: The steel is heated to 20-30°C above Ac1, held for 2-4 hours (not excessively long), and then furnace-cooled or isothermally held at approximately 20°C below Ar1 for an extended period.
Application: Primarily used for eutectoid and hypereutectoid steels, such as carbon tool steels, alloy tool steels, and bearing steels. Rolled or forged hypereutectoid steels typically exhibit a lamellar Pearlite and network cementite structure, which is hard, brittle, and difficult to machine. Spheroidizing annealing produces a granular Pearlite structure with spheroidal cementite particles dispersed in a Ferrite matrix. This structure is softer, easier to machine, and less prone to distortion or cracking during quenching due to refined Austenite grains. Network cementite must be eliminated via normalizing before spheroidizing annealing.
Objective: Reduce hardness, homogenize structure, improve machinability, and prepare the structure for quenching. Spheroidizing annealing methods include:
(a) Single-stage spheroidizing annealing: Heating to 20-30°C above Ac1, holding, and then slowly cooling.
(b) Isothermal spheroidizing annealing: Heating, holding, rapidly cooling to just below Ar1 for isothermal holding, and then slowly cooling.
(c) Cyclic spheroidizing annealing.
Diffusion Annealing (Homogenization Annealing)
Process: Steel ingots, castings, or forgings are heated to just below the solidus temperature, held for an extended period, and then slowly cooled to mitigate chemical inhomogeneity.
Objective: Eliminate dendritic and regional segregation from solidification, achieving chemical and structural uniformity.
Practice: Typically conducted at 100-200°C above Ac3 or Accm, with 10-15 hours of holding. Subsequent full annealing or normalizing is required for grain refinement. Applied to high-quality alloy steels and severely segregated alloy steel castings or ingots.
Stress-Relief Annealing
Process: The steel component is heated to 500-650°C (below Ac1), held, and then furnace-cooled.
Objective: Eliminate residual internal stresses without altering the structure.
Recrystallization Annealing
Process: Cold-worked metal is heated above its recrystallization temperature, held for a suitable duration, and then slowly cooled to restore equiaxed grains, eliminating work hardening and residual stresses.
Objective: Restore ductility by forming new, strain-free grains.
Practice: The recrystallization temperature (Trec) is approximately 0.4 times the melting point (Tmel). For steel, Trec ≈ 450°C. Annealing is conducted 100-200°C above Trec, followed by slow cooling.
Selection of Annealing Methods

The choice of annealing method depends on:

Hypoeutectoid steels: Typically full annealing; isothermal annealing for shorter cycles.
Hypereutectoid steels: Generally spheroidizing annealing; incomplete annealing for less demanding applications. Tool steels and bearing steels often undergo spheroidizing annealing. Low-carbon or medium-carbon steel cold-extruded and cold-headed parts may also be spheroidizing annealed.
Work hardening removal: Recrystallization annealing.
Internal stress elimination: Stress-relief annealing.
Chemical inhomogeneity correction: Diffusion annealing for high-quality alloy steel castings.
Xinfengli Technology Co., Ltd. is a domestic brand specializing in the research, development, and sales of ultra-low temperature cryogenic equipment and various liquid nitrogen cryogenic treatment devices in China. Its primary products include the XFL series liquid nitrogen cryogenic chambers, XFL series liquid nitrogen low-temperature installation equipment, XFL series cryogenic treatment equipment, XFL series high-low temperature test chambers, XFL series mold cryogenic treatment systems, XFL liquid nitrogen ultra-low temperature processing technologies, XFL liquid nitrogen ultra-low temperature treatment skills, and XFL liquid nitrogen ultra-low temperature deep processing machinery. With years of experience in liquid nitrogen low-temperature equipment and liquid nitrogen quick-freezing food machinery, Xinfengli offers comprehensive products and services.