The Purpose of Annealing Treatment for Precipitation Hardened Stainless Steel:
(1) Reduce the hardness of steel, improve plasticity, and facilitate machining and cold deformation processing;
(2) Uniform the chemical composition and structure of steel, refine the grains, improve the properties of steel, or prepare the structure for quenching;
(3) Eliminate internal stress and work hardening to prevent deformation and cracking.
Annealing and normalizing are mainly used for preliminary heat treatment. For parts with low stress and low-performance requirements, annealing, and normalizing can also be used as final heat treatment.
Classification of Annealing Methods for Precipitation Hardening Stainless Steel
Commonly used annealing methods are divided: according to heating temperature:
- Phase change recrystallization annealing above the critical temperature (Ac1 or Ac3): complete annealing, diffusion annealing, incomplete annealing, spheroidizing annealing.
- Annealing below the critical temperature (Ac1 or Ac3): recrystallization annealing, stress relief annealing.
Annealing Treatment of Precipitation Hardening Stainless steel Seven Types of Annealing Methods
1. Fully Annealed
Process: Heat the steel to 20~30C above Ac3, keep it warm for a period of time, and then slowly cool it (with the furnace) to obtain a heat treatment process of a balanced structure (complete austenitization).
Complete annealing is mainly used for hypereutectoid steel (c=0.3~0. 6%), generally medium carbon steel and low and medium carbon alloy steel castings, forgings, and hot-rolled profiles, and sometimes used for their weldments. The hardness of low carbon steel is low after complete annealing, which is not conducive to cutting; when hypereutectoid steel is heated to an austenite state above Accm and slowly cooled and annealed, Fe3C11 will precipitate in a network along the grain boundaries, reducing the strength, hardness, and plasticity of the steel. And the toughness is significantly reduced, leaving hidden dangers for the final heat treatment.
Purpose: to refine grain, uniform structure, eliminate internal stress, reduce hardness, and improve machinability of steel. The structure of hypoeutectoid steel after complete annealing is F+P. In actual production, in order to improve productivity, annealing is cooled to about 500C and then air-cooled
2. Isothermal Annealing
It takes a long time to complete annealing, especially for alloy steels that are relatively stable in supercooled austenitization. For example, if the austenitized steel is cooled quickly to an isothermal temperature slightly lower than Ar1, A is transformed into P, and then air-cooled to room temperature, the annealing time can be greatly shortened. This annealing method is called isothermal annealing.
Process: Heating the steel to a temperature higher than AC3 (or Ac1), holding it for a suitable period of time, quickly cooling it to a certain temperature in the pearlite zone, and maintaining it isothermally to transform austenite into pearlite, and then air cooling to room temperature. heat treatment process
Purpose: Same as complete annealing, the transformation is easier to control.
Applicable to A relatively stable steel: high carbon steel (wc>0.6%), alloy tool steel, high alloy steel (total amount of alloying elements>10%). Isothermal annealing is also beneficial to obtain uniform structure and properties. However, it is not suitable for large cross-section steel parts and large batches of charge, because isothermal annealing is not easy to make the inside of the workpiece or the batch of workpieces reach the isothermal temperature.
3. Incomplete Annealing
Process: Heat the steel to ACI~Ac3 (hypereutectoid steel) or AcI~Accm (hypereutectoid steel) after heat preservation and then cool slowly to obtain a heat treatment process close to the equilibrium structure.
It is mainly used in hypereutectoid steel to obtain spherical pearlite structure to eliminate internal stress, reduce hardness and improve machinability. Spheroidizing annealing is a type of incomplete annealing.
4. Spheroidizing Annealing
A heat treatment process that spheroidizes carbides in steel to obtain granular pearlite.
Process: Heating to a temperature of 2030C above AC1, the holding time should not be too long, generally 2 to 4 hours. The cooling method is usually furnace cooling or comparison at around 20C below Ar1 Isothermal for a long time.
Mainly used for eutectoid steel and hypereutectoid steel, such as carbon tool steel, alloy tool steel, bearing steel, etc. The structure of hypereutectoid steel air-cooled after rolling and forging is lamellar pearlite and reticular cementite. This structure is hard and brittle, which is not only difficult to cut but also prone to deformation and cracking during the subsequent quenching process. Spherical annealing results in spherical pearlite. In the spherical pearlite, cementite is in the form of spherical fine particles dispersed on the ferrite matrix.
Compared with flaky pearlite, spherical pearlite is not only less convenient but also easier to process. However, during quenching and heating, the austenite grains are less likely to become coarse and have less tendency to deform and crack during cooling. If reticular cementite exists in hypereutectoid steel, it must be eliminated by the normalizing process before spheroidizing annealing to ensure normal progress of spheroidizing annealing.
Purpose: Reduce hardness, and uniform structure, improve cutting processability, and prepare the structure for quenching. There are many spheroidizing annealing process methods, the main ones are:
- a) One-time spheroidizing annealing process: Heat the steel to 20″30C above Ac1, keep it warm for an appropriate time, and then slowly cool it in the furnace. It is required that the original structure before annealing is fine lamellar pearlite and no cementite network is allowed to exist.
- b) Isothermal spheroidizing annealing process: After heating the steel and keeping it warm, it is then cooled in the furnace to a temperature slightly lower than Ar1 for isothermal treatment (usually 1030C below Ar1). After isothermal completion, it is slowly cooled in the furnace to about 500″C and then comes out of the furnace for air cooling. It has the advantages of short cycle time, uniform spheroidization structure, and easy quality control.
- c) Reciprocating spheroidizing annealing process.
5. Diffusion Annealing (homogenization annealing)
Process: A heat treatment process in which steel ingots, castings, or forged parts are heated to a temperature slightly lower than the solidus line and kept warm for a long time, and then slowly cooled to eliminate uneven chemical composition.
Purpose: To eliminate dendrite segregation and regional segregation produced during the solidification process of the cast chain, and to homogenize the composition and structure. The heating temperature of diffusion annealing is very high, usually Ac3 or Accm or above 100-200C. The specific temperature depends on the degree of segregation and the steel type. The holding time is generally 10 to 15 hours. After diffusion annealing, complete annealing and normalizing are required to refine the structure.
It is used in some alloy steels alloy steel castings and steel ingots with serious segregation.
6. Stress Relief Annealing
Process: The steel parts are heated to a certain temperature lower than Ac1 (usually 500~650C), kept warm, and then cooled in the furnace.
The stress relief annealing temperature is lower than A1, so stress relief annealing does not cause structural changes.
Purpose: Eliminate residual internal stress.
7. Recrystallization Annealing
Recrystallization annealing, also known as intermediate annealing, is a heat treatment process that heats the cold-deformed metal to above the recrystallization temperature for an appropriate time to transform the deformed grains into uniform equiaxed grains and eliminate work hardening and residual stress.
For the recrystallization phenomenon to occur, first of all, there must be a certain amount of cold plastic deformation, and secondly, it must be heated to above a certain temperature. The lowest temperature at which recrystallization occurs is called the lowest recrystallization temperature. The minimum recrystallization temperature of general metal materials is:
T re=0.4T melting
The heating temperature for recrystallization annealing should be 100~200C higher than the low recrystallization temperature (the minimum recrystallization temperature of steel is about 450C), and it should be slowly cooled after proper heat preservation.
Selection of Annealing Method
The selection of annealing methods generally follows the following principles:
(1) Various steels with a hypoeutectoid structure are generally fully annealed. In order to shorten the annealing time, isothermal annealing can be used;
(2) Hypereutectoid steel generally adopts spheroidizing annealing. When the requirements are not high, incomplete annealing can be used. Spheroidizing annealing is often used for tool steel and bearing steel. Cold extruded parts and cold paid parts of low carbon steel or medium carbon steel are sometimes spheroidized annealed;
(3) In order to eliminate work hardening, recrystallization annealing can be used;
(4) In order to eliminate the internal stress caused by various processing processes, stress relief annealing can be used: For some large steel castings of high-quality alloy steel, in order to improve the inhomogeneity of the organizational structure and chemical composition, diffusion is often used. annealing.