What kind of annealing furnace should be used for spheroidizing annealing of fasteners?
In order to ensure the quality of fastener spheroidizing annealing, the appropriate annealing furnace should be selected. In order to facilitate the cold heading forming, the drawing and spheroidizing annealing are usually carried out on the plate element. The quality of spheroidizing annealing will directly affect the quality of the final product.
First, we need to understand the purpose of spheroidizing annealing
1. Improve mechanical properties, reduce material hardness and strength, improve cold workability;
2. The workpiece is formed by forging with low energy and easy to cut;
3. Reduce die loss;
4. Prevent the workpiece from cracking and twisting.
Secondly, we need to understand the relationship between cold heading properties and spheroidizing quality
1. Deformation resistance:
It is related to the hardness and tensile strength of the material.
Hardness value deformation resistance
The lower, the lower
2. Deformation energy:
The deformation energy of normalized structure is worse than that of spheroidized structure. As for the spheroidizing group, the higher the spheroidizing ratio of cementite (Fe3C), the better the deformation energy; and the better the deformation energy is when the cementite (Fe3C) is uniformly dispersed in the ferrite structure as fine particles. Steel tube annealing furnace
Thirdly, we need to understand the following indicators that we usually pay attention to after spheroidizing annealing
1. Surface oxidation
2. Surface decarburization;
3. Hardness and hardness uniformity;
4. Spheroidization rate
5. Metallographic structure
Next, we will analyze the impact of the above indicators on fastener production one by one
Surface oxidation and decarburization:
The oxidation of material surface will cause material loss (machining allowance amplification) and increase production cost (material loss and rust removal time extension). When the surface decarburization is serious, the strength grade of the product will decline, and even produce waste products.
The key to solve the problem of surface oxidation and decarbonization is to accurately control the atmosphere in the furnace! Only by accurately controlling the oxygen potential in the furnace atmosphere, can we achieve no oxidation and micro decarburization; if we simply inject some gas and can not accurately control, it will not help to solve the problem of surface decarburization and oxidation.
The requirement of national standard for surface decarburization is 15 ‰ D. generally, the surface decarburization can be controlled within 8 ‰ D. when the requirement is high, the surface decarburization should be controlled within 0.05mm or even 0.03mm.
Hardness and hardness uniformity:
When the metal material is cold heading or cold extrusion, a suitable hardness is needed to obtain higher yield and lower die loss. Of course, at the same time, we need to control the hardness uniformity (hardness dispersion) of the same batch of materials within a small range as far as possible; otherwise, the high and low hardness will seriously affect the production efficiency, resulting in low yield and shortened service life of the mold.
The suitable hardness and hardness uniformity mainly depend on the temperature uniformity; and the temperature uniformity first depends on the structure of the spheroidizing furnace: the optimization design of the furnace cover, furnace bladder, convection bucket, guide seat and convection fan, which can meet the requirements of fluid mechanics; secondly, the selection of protective atmosphere, such as the use of highly active reducing gas, can accelerate the heat conduction on the metal surface.
Generally, it can meet the production requirements to control the hardness dispersion within 3-5hrb. When the requirements are high, the hardness dispersion should be controlled within 3hrb.
Spheroidization rate and microstructure
After rolling, forging and air cooling, the microstructure of steel is lamellar pearlite and network cementite, which is hard and brittle. It is not only difficult to cut, but also easy to deform and crack in the later quenching process. The spheroidal pearlite structure is obtained by spheroidizing annealing, in which the cementite is spherical particles, which are dispersed on the ferrite matrix. Compared with the flake pearlite, the hardness is low and it is easy to cut. Moreover, the austenite grains are not easy to grow up during quenching and heating, and the deformation and cracking tendency of the workpiece are small during cooling. Therefore, when the formed parts have a large amount of deformation, the material needs to have a higher level of spheroidization rate and good microstructure. Otherwise, it will cause the parts are not easy to form and low qualified rate.
The advantages and disadvantages of spheroidizing rate and metallographic structure are related to the structure of spheroidizing furnace and annealing process; in terms of structure, it is necessary to have a good atmosphere circulation system, a forced cooling device capable of program control, and a high-efficiency heating device capable of controlling heating speed; in terms of annealing process, it is necessary to design according to the material quality, manufacturer and application of materials, such as Shengli Temperature rate and cooling rate should be controlled.
Generally, the spheroidization rate is controlled at 4.5-6 grade (according to Japanese jisg3539 standard, grade 6 is high)
In addition to the above technical indexes, when selecting the spheroidizing annealing furnace, we have to put forward some requirements for the spheroidizing annealing furnace itself
1. Sealing performance (how much pressure can 48 hours of production maintain?)
2. Energy saving measures (surface temperature rise, heating efficiency, waste heat utilization, energy consumption)
3. Intelligent (heat treatment expert system, remote control, process curve recording, fault alarm, atmosphere control, air-fuel ratio automatic adjustment)
4. Quality and brand of materials used in manufacturing spheroidizing furnace
In a word, the quality of spheroidizing annealing is closely related to the furnace structure, atmosphere control, automatic control, annealing process design, etc. in actual production, the causes of the above analysis are not produced alone, but interact with each other. Therefore, in the selection of spheroidizing annealing furnace, we can refer to the above points for comparison.
