The most commonly used seamless steel tubes, precision network management heat treatment process is divided into two categories:
Preparation of heat treatment Objective: To eliminate some defects in the blanks and semi-finished products and prepare them for subsequent cold processing and final heat treatment.
 
Final Heat Treatment Purpose: To achieve the desired performance of the workpiece.
Annealing and normalizing purposes: Elimination of certain defects caused by hot working of the steel, or preparation for subsequent cutting and final heat treatment.
 
First, the annealing of steel
1. Concept: The steel is heated to a suitable temperature (above or below Ac 1 ) for a certain period of time, then slowly cooled to obtain a near-equilibrium state. The heat treatment process is called annealing.
 
2, purpose:
(1) reduce hardness and increase plasticity,
(2) Refine grain and eliminate tissue defects
(3) Eliminate internal stress
(4) Prepare for quenching the organization
 
3, type: (according to the heating temperature can be divided into critical temperature (Ac1 or Ac3) above or below the annealing, the former also known as phase change recrystallization annealing, including complete annealing, diffusion annealing uniform annealing, incomplete annealing, ball Annealing; the latter includes recrystallization annealing and stress relief annealing.)
 
(1) Completely Annealed:
1) Concept: Heat the hypoeutectoid steel (Wc=0.3%～0.6%) to AC3+(30～50)°C, after complete austenitizing, slowly heat insulation (in furnace, buried sand, lime), The heat treatment process to obtain a near-equilibrium tissue is called full annealing.
2) Purpose: To refine grains, uniform structure, eliminate internal stress, reduce hardness, and improve cutting performance.
3) Process: The use of slow annealing with furnace can ensure the precipitation of pre-eutectoid ferrite and the transformation of undercooled austenite into pearlite below Ar1 below the main temperature range. The holding time of the workpiece at the annealing temperature must not only allow the workpiece to burn through, ie, the core temperature of the workpiece reaches the required heating temperature, but also ensures that all homogenized austenite is seen to achieve complete recrystallization. The annealing time for complete annealing is related to factors such as steel composition, workpiece thickness, charging amount, and charging method. In actual production, in order to increase the productivity, the furnace is air cooled by annealing and cooling to about 600°C.
 
4) Scope of application: Casting, welding, forging and rolling parts of medium carbon steel and medium carbon alloy steel.
Matters needing attention: Low carbon steel and hypereutectoid steel should not be used for complete annealing. The low hardness of low carbon steel after complete annealing is not conducive to cutting. When the eutectoid steel is heated to the austenite state above Accm and slow annealing is performed, secondary network cementite precipitates and the strength, plasticity, and impact toughness of the steel are significantly reduced. Article from the network by the "Changzhou precision steel blog" finishing publication, reproduced please indicate the source.

 
(2) Spheroidizing annealing
1) Concept: The annealing process for spheroidizing carbides in steel is called spheroidizing annealing.
2) Process: The general spheroidizing annealing process Ac 1 + (10-20) °C is cooled with the furnace to 500-600 °C air cooling.
3) Purpose: To reduce the hardness, improve the organization, improve the plasticity and cutting performance.
4) Scope of application: It is mainly used for eutectoid steel, cutting tools, measuring tools, molds and so on.
When mesh-like secondary cementite is present in the hypereutectoid steel, not only is it high in hardness, it is difficult to perform cutting processing, but also the brittleness of the steel is increased, and quench deformation and cracking are likely to occur. For this reason, after the hot working of steel, a ball annealing must be added to spheroidize the reticulated secondary cementite and pearlite in the pearlite to obtain granular pearlite. The cooling rate and the isothermal temperature also affect the effect of carbides being spheroidized. The cooling rate is rapid or the isothermal temperature is low. The pearlite forms at a relatively low temperature, the carbide particles are too fine, the aggregation is small, and carbides are easily formed. So that the hardness is high. If the cooling rate is too slow or isothermal temperature is too high, the formation of carbide particles is more coarse, the aggregation is also very strong, easy to form coarse and fine granular carbides, so that the hardness is low.
 
(3) Homogenization annealing (diffusion annealing)
1) Process: Heat the alloy steel ingots or castings to a temperature of 150-200°C above Ac 3, slowly cool for 10-15h, and eliminate the uneven heat treatment process.
2) Objective: To eliminate dendrite segregation during crystallization and homogenize the components. Because the heating temperature is high and the time is long, austenite grains may be seriously coarsened. Therefore, a complete annealing or normalizing is generally required to refine grains and eliminate overheating defects.
3) Scope of application: It is mainly used for alloy steel ingots, castings and forgings with high quality requirements.
4) Note: The high temperature diffusion annealing has a long production cycle, consumes large amounts of energy, and the workpiece is oxidized and decarburized seriously. The cost is high. Only some high-quality alloy steels and severely segregated alloy steel castings and ingots use this process. For castings with a generally small size or carbon steel castings, since the degree of segregation is light, full annealing can be used to refine the grains and eliminate the casting stress. Article from the network by the "Changzhou precision steel blog" finishing publication, reproduced please indicate the source

(4) Stress relief annealing
1) Concept: Annealing to remove stresses due to plastic deformation, welding, etc., and residual stresses present in the casting is called stress relief annealing. (Tension annealing does not occur in phase change)
2) Process: Slowly heat the workpiece to Ac 1 below 100 ~ 200 °C (500 ~ 600 °C) for a certain period of time (1 ~ 3h) and slowly cool to 200 °C with the furnace, and then out of the furnace cooling.
Steel is generally at 500～600°C
Cast iron generally tends to cause the graphitization of pearlite when it exceeds 550 at 500 to 550°C.
Welding parts are generally 500 to 600 °C.
3) Scope of application: Eliminate residual stresses in cast, forged, welded parts, cold stamped parts, and machined parts to stabilize the size of steel parts, reduce distortion, and prevent cracking.
 
 
Second, the normalization of steel
1, the concept:
The steel heated to Ac 3 (or Ac cm) above 30 ~ 50 °C, after insulation for an appropriate time; cooling process in still air is called steel normalizing.
 
2. Purpose:
To refine the grain, uniform the structure, adjust the hardness and so on.
 
3. Organization:
Eutectoid steel S, subeutectoid steel F+S, hypereutectoid steel
 
4. Process:
The normalizing holding time is the same as that of complete annealing. The burning of the workpiece, ie, the heating temperature of the core, shall be considered. The factors such as steel, original structure, charging capacity and heating equipment shall also be considered. The most common way of normalizing cooling is to take the steel out of the furnace and let it cool naturally in the air. For large parts, it is also possible to control the cooling rate of the steel by using methods such as blowing, spraying, and adjusting the stacking distance of the steel to achieve the required structure and performance. Article from the network by the "Changzhou precision steel blog" finishing publication, reproduced please indicate the source.

5, the scope of application:
1) Improve the cutting performance of steel. Carbon steels and low-alloy steels with a carbon content of less than 0.25% have a lower hardness after annealing and are easier to “stick a knife” during cutting. By normalizing, they can reduce free ferrite and obtain fine-sheet pearlite. Increasing the hardness can improve the cutting workability of the steel and increase the life of the tool and the smoothness of the surface of the workpiece.
2) Elimination of thermal processing defects. Middle carbon structural steel castings, forgings, rolling pieces and welding parts are prone to hot defects such as coarse grains and banded structures after heat processing. Through the normalizing treatment, these defects can be eliminated, and the purpose of grain refinement, uniform organization, and elimination of internal stress can be achieved.
3) Elimination of the eutectoid steel mesh carbide, easy to spheroidize annealing. Hypereutectoid steels are spheroidized annealed prior to quenching to facilitate machining and preparation for quenching. However, when there is severe network carbide in the eutectoid steel, good spheroidizing effect will not be achieved. The network carbide can be eliminated by normalizing.
4) Improve the mechanical properties of common structural parts. Some carbon steel and alloy steel parts with low stress and low performance requirements are treated with normalizing to achieve a certain degree of comprehensive mechanical properties, which can be used as a final heat treatment for parts instead of tempering treatment.
 
Third, the choice of annealing and normalizing
The main difference between annealing and normalizing:
1, the normalized cooling rate is slightly faster than the annealing, the greater the degree of supercooling
2. The microstructure obtained after normalizing is relatively fine, and the strength and hardness are higher than those of annealing.
 
Annealing and normalizing options:
1. Low-carbon steel with carbon content <0.25%, normally normalizing instead of annealing. Because the faster cooling rate can prevent the low carbon steel from depositing free cubic cementite along the grain boundary, thus improving the cold deformation performance of stamping parts; using normalizing can improve the hardness of steel, and the machining performance of low carbon steel; In the heat treatment process, normalizing can refine the grains and increase the strength of the low carbon steel.
 
2. The medium carbon steel with carbon content between 0.25 and 0.5% can also be normalized instead of annealed. Although the hardness of medium carbon steel near the upper limit of carbon content is high after normalizing, it can still be processed and the normalizing cost can be realized. Low and high productivity.
 
3. For steels with carbon contents between 0.5 and 0.75%, due to the high carbon content, the hardness after normalizing is significantly higher than that during annealing, and it is difficult to perform the cutting process. Therefore, the complete annealing is generally used to reduce the hardness and improve the cutting. Processability.
 
4. High-carbon steel or tool steel with carbon content> 0.75% generally adopts spheroidizing annealing as pre-heat treatment. If there is mesh-like secondary cementite, normalizing should be performed first. Annealing is a heat treatment process that heats the workpiece to a suitable temperature for a certain period of time and then slowly cools it. Slow cooling is the main feature of annealing. Annealing parts are usually cooled when the furnace is cooled below 550°C. Annealing is a very widely used heat treatment. In the manufacturing process of tooling or mechanical parts, etc., it is often arranged as a preliminary heat treatment after casting and forging, and before cutting (rough) processing, it is used to eliminate some of the previous processes. Defects, and prepare for the subsequent process.
 
 
Annealing purpose:
1 reduce the hardness of the material to facilitate cutting;
2 eliminate all kinds of stress and prevent the deformation of parts;
3 Refine the coarse grains and improve the internal structure to prepare for the final heat treatment.