What is annealing? Why does stainless steel need to be annealed?
Overview
Annealing is a metal heat treatment process that involves slowly heating the metal to a certain temperature, maintaining it for a sufficient time, and then cooling it at an appropriate rate. Its purpose is to reduce hardness, improve workability, eliminate residual stress, stabilize dimensions, reduce deformation and crack tendencies, refine grains, adjust microstructure, eliminate structural defects, etc. To be precise, annealing is a heat treatment process for metallic and non-metallic materials. Stainless steel products will also require an annealing process.
1. The purpose of annealing
(1) Reduce hardness and improve workability;
(2) Eliminate residual stress, stabilize dimensions, and reduce deformation and crack tendencies;
(3) Refine the grains, adjust the structure, and eliminate tissue defects;
(4) Uniform material structure and composition, improve material properties or prepare for later heat treatment.
The annealing process is widely used in production. Annealing specifications vary depending on the use of the workpiece to be annealed, such as complete annealing, spheroidizing annealing, stress relief annealing, etc.
2. Annealing process
The maximum heating temperature (annealing temperature) is one of the most important parameters in the annealing process. The annealing temperature for most alloys is selected based on the phase diagram of the alloy system, just as carbon steel is selected based on the iron-carbon balance diagram. Due to different annealing purposes, the annealing temperature of different steel materials (including carbon steel and alloy steel) is lower than above Ac3, above Ac1 or below Ac1.
3. Recrystallization annealing
Recrystallization annealing is used to equilibrate alloys that undergo solid phase transformation (recrystallization) during heating and cooling. The annealing temperature is below or above the phase transition temperature range. Heating and cooling are slow. The alloy undergoes phase transformation and recrystallization respectively during heating and cooling, so it is called recrystallization annealing, usually called annealing. This annealing method is commonly used for steel.
4. Recrystallization annealing process and classification
The recrystallization annealing process involves slowly heating the steel to 30°C~50°C higher than Ac3 (eutectic steel) or Ac1 (eutectoid steel or hypereutectoid steel), maintaining it for an appropriate time, and then slowly cooling it. Pearlite (or proeutectoid ferrite or cementite) produced during the heating process transforms into austenite (first phase transformation recrystallization); conversely, the second phase transformation recrystallizes during cooling to form Pearlite (or proeutectoid ferrite or cementite), has finer grains, thicker layers and a uniform microstructure. Annealing temperatures above Ac3 (hypoeutectoid steel) allow complete recrystallization of the steel and are called complete annealing. The annealing temperature is between Ac1 and Ac3 (semi-homogeneous steel) or between Ac1 and Acm (hypereutectoid steel), resulting in partial recrystallization of the steel, which is called incomplete annealing. This is an annealing process in which an iron-carbon alloy is heated to a temperature between Ac1-Ac3 to achieve incomplete austenitization and then cooled slowly. Incomplete annealing is mainly used for medium and high carbon steel and low alloy steel forgings. Its purpose is to refine the structure and reduce hardness. The heating temperature is Ac1+(40-60)°C, and then cool slowly after keeping warm.
5. Isothermal annealing
Isothermal annealing is a controlled cooling annealing method suitable for steel and some non-ferrous metal alloys, such as titanium alloys. In the case of steel, slowly heat to a temperature slightly above Ac3 (hypoeutectoid steels) or slightly above Ac1 (eutectoid and hypereutectoid steels). After a period of heat preservation, the steel is austenitized and then quickly moved into another furnace with a temperature slightly lower than A1. The isothermal temperature is maintained until austenite is completely transformed into lamellar pearlite (hypoeutectoid steel and pro-eutectoid ferrite; hypereutectoid steel and pro-eutectoid cementite). Finally, it cools at any rate (usually furnaces cool in air).
6. Homogenization annealing
Homogenization annealing, also called diffusion annealing, is an annealing method for ingots or castings of stainless steel and non-ferrous metal alloys (such as tin bronze, silicon bronze, cupronickel, magnesium alloys, etc.). The ingot or casting is heated to a higher temperature below the solidus temperature of the alloy, stored for a long period of time, and then slowly cooled down. Homogenization annealing causes solid-state diffusion of elements in the alloy to reduce chemical composition inhomogeneities (segregation), primarily within the grain size (intragranular segregation or dendrite segregation). In order to accelerate the diffusion of alloy elements and shorten the holding time, the homogenization annealing temperature is very high. The homogenization annealing temperature of alloy steel is much higher than Ac3, generally 1050℃~1200℃. The temperature for homogenization and annealing of non-ferrous alloy ingots is usually "0.95×solidus temperature (K)". The heating temperature of homogenization annealing is high and the holding time is long; therefore, the heat energy consumption is very large.
7. Spheroidizing annealing
Spheroidizing annealing is an annealing method suitable only for steel. The steel is heated to a temperature slightly below or slightly above Ac1, or the temperature is periodically above or below A1, and then cooled slowly. The purpose is to transform the lamellar cementite and proeutectoid cementite in pearlite into spherulites evenly distributed in the ferrite matrix (this structure is called spheroidized pearlite). Medium carbon steel and high carbon steel with this structure have low hardness, good machining properties and large cold deformation ability. For tool steels, this structure is best suited for quenching.
8. Stress relief annealing
Stress relief annealing is to heat the workpiece to a suitable temperature below Ac1 (500~600°C for non-alloy steel). The heat treatment process of furnace cooling after insulation is called stress relief annealing. Stress-free heating at low temperatures eliminates structural transformations during annealing. Mainly used for blanks and machined parts. Its purpose is to eliminate residual stress in blanks and parts, stabilize the size and shape of the workpiece, and reduce the tendency of deformation and cracking during the cutting process and use.
