Basic knowledge of the steel pipe annealing process in heat treatment production
First, about the annealing process of steel pipes
The heat treatment process of heating the steel pipe to an appropriate temperature, keeping it warm for a certain period of time, and then slowly cooling it (generally cooling with the furnace) is called annealing. The essence of annealing is to heat the steel to austenitization and then transform it into pe...
First, about the annealing process of steel pipes
The heat treatment process of heating the steel pipe to an appropriate temperature, keeping it warm for a certain period of time, and then slowly cooling it (generally cooling with the furnace) is called annealing. The essence of annealing is to heat the steel to austenitization and then transform it into pearlite. The structure after annealing is close to the equilibrium structure.
1. Purpose of annealing:
(1) Reduce the hardness of steel, improve plasticity, and facilitate machining and cold deformation processing;
(2) Even the chemical composition and structure of steel, refine the grains, improve the performance 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 preparatory heat treatment. For parts with low stress and low-performance requirements, annealing and normalizing can also be used as final heat treatment.
2. Classification of annealing methods
Common annealing methods are classified according to heating temperature:
(1) Phase transformation recrystallization annealing above the critical temperature (Ac1 or Ac3): complete annealing, diffusion annealing, incomplete annealing, spheroidizing annealing.
(2) Annealing below the critical temperature (Ac1 or Ac3): recrystallization annealing, stress relief annealing.
Second, seven types of annealing methods for steel pipes
1. Complete annealing
- Process: Heat the steel to 20~30℃ above Ac3, keep it warm for a period of time, and then slowly cool it (with the furnace) to obtain a heat treatment process close to the equilibrium structure (complete austenitization). Complete annealing is mainly used for hypereutectoid steel (wc=0.3~0.6%), generally medium carbon steel and low and medium carbon alloy steel castings, forgings, and hot rolled profiles, and sometimes also for their welded parts. After complete annealing, the hardness of low-carbon steel is low, which is not conducive to cutting. When hypereutectoid steel is heated to an austenite state above Accm and slowly cooled for annealing, Fe3CⅡ will precipitate along the grain boundary in a network, which significantly reduces the strength, hardness, plasticity, and toughness of the steel, leaving hidden dangers for the final heat treatment.
- Purpose: to refine the grains, make the structure uniform, eliminate internal stress, reduce hardness, and improve the cutting processability 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 500℃ and then air-cooled.
2. Isothermal annealing
Complete annealing takes a long time, especially for alloy steels with relatively stable supercooled austenitization. If the austenitized steel is cooled to a temperature slightly lower than Ar1 isothermally, 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: Heat the steel to a temperature higher than Ac3 (or Ac1), keep it warm for a suitable time, cool it quickly to a certain temperature in the pearlite zone, and keep it isothermally to transform austenite into pearlite, and then air cool it to room temperature.
- Purpose: Same as complete annealing, the transformation is easier to control.
Applicable to steels with relatively stable A: high carbon steel (wc>0.6%), alloy tool steel, high alloy steel (total amount of alloy elements>10%). Isothermal annealing is also conducive to obtaining uniform structure and performance. However, it is not suitable for large-section steel parts and large batches of furnace materials, because isothermal annealing is not easy to make the workpiece inside or batch workpieces reach isothermal temperature.
3. Incomplete annealing
- Process: Heat the steel to Ac1~Ac3 (hypereutectoid steel) or Ac1~Accm (hypereutectoid steel), keep it warm, and then slowly cool it to obtain a heat treatment process close to the equilibrium structure. It is mainly used to obtain spherical pearlite structures in hypereutectoid steel to eliminate internal stress, reduce hardness, and improve cutting processability. Spheroidizing annealing is a kind of incomplete annealing.
4. Spheroidizing annealing
A heat treatment process that spheroidizes carbides in steel and obtains granular pearlite.
- Process: Heating to a temperature of 20~30℃ above Ac1, the holding time should not be too long, generally 2~4h is appropriate, and the cooling method usually adopts furnace cooling, or isothermal for a long time at about 20℃ below Ar1. It is mainly used for eutectoid steel and hypereutectoid steel, such as carbon tool steel, alloy tool steel, bearing steel, etc. The structure of hypereutectoid steel after rolling forging and air cooling is lamellar pearlite and network cementite. This structure is hard and brittle, not only difficult to cut but also easy to deform and crack during the subsequent quenching process. Spheroidizing annealing obtains spherical pearlite. In the spherical pearlite, the cementite is spherical fine particles, dispersed on the ferrite matrix. Compared with lamellar pearlite, spheroidal pearlite not only has lower hardness and is easier to cut, but also, during quenching and heating, austenite grains are not easy to coarse, and the tendency to deform and crack during cooling is small. If there is network cementite in hypereutectoid steel, it must be eliminated by the normalizing process before spheroidizing annealing to ensure the normal progress of spheroidizing annealing.
- Purpose: To reduce hardness, make the structure uniform, and improve cutting processability to prepare the structure for quenching. There are many spheroidizing annealing process methods, mainly:
a) One-time spheroidizing annealing process: Heat the steel to 20~30℃ above Ac1, keep it warm for a suitable time, and then slowly cool it with 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 and keeping the steel warm, cool it with the furnace to a temperature slightly lower than Ar1 for isothermal (generally 10~30℃ below Ar1). After the isothermal is finished, slowly cool it with the furnace to about 500℃ and then air cool it out of the furnace. It has the advantages of short cycles, a uniform spheroidization structure, and easy quality control.
c) Reciprocating spheroidization annealing process.
5. Diffusion annealing (homogenization annealing)
- Process: A heat treatment process in which the ingot, casting, or forging is heated to a temperature slightly below the solidus line for a long time, and then slowly cooled to eliminate the chemical composition unevenness.
- Purpose: To eliminate the dendrite segregation and regional segregation generated during the solidification of the ingot, so as to homogenize the composition and structure. The heating temperature of diffusion annealing is very high, usually 100~200℃ above Ac3 or Accm. The specific temperature depends on the degree of segregation and the type of steel, and the holding time is generally 10~15 hours. After diffusion annealing, complete annealing and normalizing treatment are required to refine the structure.
Applied to some high-quality alloy steels and alloy steel castings and ingots with severe segregation.
6. Stress relief annealing
- Process: Heat the steel to a temperature lower than Ac1 (generally 500~650℃), keep warm, and then cool with 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 and keeps it for an appropriate time to transform the deformed grains into uniform equiaxed grains to eliminate work hardening and residual stress. For the recrystallization phenomenon to occur, first there must be a certain amount of cold plastic deformation, and secondly, it must be heated to a certain temperature. The lowest temperature at which recrystallization occurs is called the minimum recrystallization temperature. The minimum recrystallization temperature of general metal materials is: Tre = 0.4Tmelt.
The heating temperature of recrystallization annealing should be 100~200℃ higher than the minimum recrystallization temperature (the minimum recrystallization temperature of steel is about 450℃), and it should be cooled slowly after appropriate heat preservation.
