What is Annealing? Why Steel Needs to be Annealed?

Overview

Annealing is a metal heat treatment process that involves slowly heating the metal to a certain temperature and preserving for a sufficient period of time and then cooling it off at a suitable speed. The purpose is to reduce hardness, improve machinability, eliminate residual stress, stabilize size, reduce deformation and crack tendency, refine grains, adjust microstructure, eliminate tissue defects and so on. Accurately, annealing is a heat treatment process for both metallic materials and non-metallic materials.

Purpose of Annealing

• (1) Reduce hardness and improve machinability;
• (2) Eliminate residual stress, stabilize the size, and reduce the tendency of deformation and crack;
• (3) Refine the grains, adjust the structure, and eliminate tissue defects;
• (4) Uniform material organization and composition, improve material properties or prepare for later heat treatment.

Annealing processes are widely used in production. Annealing specification varies with the purpose of the workpiece to be annealed, such as full annealing, spheroidizing annealing, stress relief annealing and etc.

Annealing Processes

Maximum heating temperature (annealing temperature) is one of the most important parameters for annealing process. The annealing temperature of most alloys is selected based on the phase diagram of the alloy system, like carbon steel which is based on the iron-carbon balance diagram. Due to distinct annealing purposes, the annealing temperature of different steels (including carbon steel and alloy steel) fall above Ac3, above Ac1 or below Ac1.

Recrystallization Annealing

Recrystallization annealing is used in alloys where solid phase transformation (recrystallization) occurs during equilibrium heating and cooling. The annealing temperature falls above or within the phase transition temperature range. Heating and cooling are both 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 quite commonly applied to steel.

Recrystallization Annealing Process & Classification

Recrystallization annealing process involves slowly heating the steel 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 off. The pearlite (or pre-eutectoid ferrite or cementite) occurring during heating is transformed into austenite (first phase change recrystallization); in contrast, the second phase change recrystallizes occurring during cooling, forms into pearlite (or proeutectoid ferrite or cementite) with finer grains, thicker layers, and uniform microstructure. Annealing temperature above Ac3 (hypoeutectoid steel) allows for complete recrystallization of the steel, called full annealing. Annealing temperature falling between Ac1 and Ac3 (hypegmatitic steel) or between Ac1 and Acm (hyper-eutectoid steel), leads to partial recrystallization of the steel, called incomplete annealing. It is an annealing process in which an iron-carbon alloy is heated to the temperature between Ac1-Ac3 to achieve incomplete austenitization followed by slow cooling-off. Incomplete annealing is mainly applied to medium and high carbon steel and low alloy steel forgings. Its purpose is to refine the structure and reduce the hardness. The heating temperature is Ac1+(40-60) °C, and it is slowly cooled off after heat preservation.

Isothermal Annealing

Isothermal annealing is a controlled cooling annealing method applied to steel and certain non-ferrous alloys such as titanium alloys. In terms of steel, it is slowly heated to the temperature a bit above Ac3 (hypoeutectoid steel) or above Ac1 (eutectoid steel and hypereutectoid steel). After a period time of heat preservation, the steel is austenitized and then rapidly moved into another furnace which temperature is a bit below A1. The isothermal temperature is maintained until the austenite is completely transformed into lamellar pearlite (sub-eutectoid steel and pro-eutectoid ferrite; hypereutectoid steel and pro-eutectoid cementite). Finally, it is cooled down at any speed (usually the furnace is cooled off in the air).

Homogenization Annealing

Homogenization annealing, also known as diffusion annealing, is an annealing method for ingots or castings of steel and non-ferrous alloys, such as tin bronze, silicon bronze, white copper, magnesium alloy and etc. The ingot or casting is heated to a higher temperature below the solidus temperature of the alloy, preserved for a long time, and then slowly cooled off. Homogenization annealing leads to a solid diffusion of elements in the alloy to reduce chemical composition inhomogeneity (segregation), mainly to reduce chemical composition inhomogeneities (intragranular segregation or dendrite segregation) within the grain size. The homogenization annealing temperature is so high in order to accelerate the diffusion of alloying elements and to minimize the heat preservation time. The homogenization annealing temperature of alloy steel is much higher than Ac3, usually 1050°C ~ 1200 °C. The temperature at which the non-ferrous alloy ingot is homogenized and annealed is generally "0.95 × solidus temperature (K)". The homogenization annealing features a high heating temperature and a long heat preservation time; hence, the heat energy consumption is large.

Spheroidizing Annealing

Spheroidizing annealing is an annealing method that is only applied to steel. The steel is heated to a temperature slightly lower or slightly higher than Ac1 or the temperature is periodically changed above and below A1, and then slowly cooled off. The purpose is to change the flaky cementite and the pro-eutectoid cementite in the pearlite into spherulites evenly distributed in the ferrite matrix (this kind of structure is called spheroidized pearlite). The medium carbon steel and the high carbon steel with such a structure feature low hardness, good machinability, and large cold deformation ability. For tool steel, this structure is ideally ready for quenching.

Stress Relief Annealing

Stress relief annealing is heating the workpiece to a suitable temperature below Ac1 (non-alloy steel at 500~600 °C). The heat treatment process involving cooling off with furnace after heat preservation is called stress relief annealing. The stress-free heating temperature is low, eliminating structure transformation during the annealing process. It is mainly applied to the blanks and machined parts. The purpose is to eliminate the residual stress in the blanks and parts, stabilize the size and shape of the workpiece, and reduce deformation and cracking tendency during the cutting process and use.