In the production process of fasteners, in order to facilitate cold heading, the disk elements are usually drawn and spheroidized. The quality of spheroidizing annealing will directly affect the quality of the final product. So, as a fastener manufacturer, how to choose a suitable spheroidizing annealing furnace?

First, we need to understand the purpose of spheroidizing annealing:

1. Improve mechanical properties, reduce material hardness and strength, and improve cold workability;

2. The workpiece is forged and formed with lower energy, which is easy to cut;

3. Reduce mold loss;

4. Avoid cracking and twisting of the workpiece.


Secondly, we need to understand the relationship between cold heading performance and spheroidization quality

1. Deformation resistance:

It is related to the hardness value and tensile strength of the material.

Hardness value Deformation resistance

lower lower

2. Deformation energy:

It is related to the metallographic structure of the material, such as complete annealing, the deformation energy of the normalized structure is worse than that of the spheroidized structure. As far as the spheroidization group is concerned, the higher the spheroidization ratio of cementite (Fe3C), the better the deformation energy; and the cementite (Fe3C) is uniformly dispersed in the ferrite structure in the form of fine particles, and its deformation energy is the best. good.


Third, we need to understand several indicators that are usually concerned after the disk element is spheroidized and annealed:

1. Surface oxidation

2. Surface decarburization;

3. Hardness and hardness uniformity;

4. Spheroidization rate

5. Metallographic organization

Next, let's analyze the impact of the above indicators on the production of fasteners one by one:

Surface Oxidation and Decarburization:

Oxidation of the surface of the material will cause material loss (machining allowance enlargement) and increase production costs (material loss, rust removal time is prolonged). When the surface decarburization is serious, the strength level of the product will be reduced, and even waste products will be generated.

The key to solving surface oxidation and decarburization is precise control of the furnace atmosphere! Only by accurately controlling the oxygen potential value in the furnace atmosphere, can no oxidation and micro-decarburization be achieved; if only some gases are simply introduced without precise control, it will also not help to solve the problem of surface decarburization and oxidation.

The national standard requirement for surface decarburization is 15‰ D, generally, the surface decarburization can be controlled within 8‰ D, and the decarburization should be controlled within 0.05mm or even 0.03mm when the requirement is high.


Hardness and hardness uniformity:

When the metal material is cold heading or cold extruded, a suitable hardness is required in order to obtain higher yield and lower die loss. Of course, we also need the hardness uniformity (hardness dispersion) of the same batch of materials to be controlled within the range as small as possible; otherwise, the sudden high and low hardness will seriously affect the production efficiency, resulting in a decrease in the yield. low, the service life of the mold is shortened.

Appropriate hardness and hardness uniformity mainly depend on the uniformity of temperature; and the uniformity of temperature first depends on the structure of the spheroidizing furnace: the optimized design of the furnace cover, furnace gallbladder, convection barrel, guide seat, and convection fan, It can meet the requirements of fluid mechanics; secondly, the selection of protective atmosphere, such as the use of highly active reducing gas, accelerates the heat conduction of the metal surface.

Usually, the hardness dispersion can be controlled within 3-5HRB to meet the production requirements. When the requirements are high, the hardness dispersion should be controlled within 3HRB.

Nodularity and Metallographic Structure

The steel is air-cooled after rolling and forging, and the resulting structure is lamellar pearlite and reticulated cementite. This structure is hard and brittle, which is not only difficult to cut, but also easy to deform and crack in the subsequent quenching process. The spherical pearlite structure obtained by spheroidizing annealing, in which the cementite is spherical particles, is dispersed on the ferrite matrix. When heated, the austenite grains are not easy to grow, and the workpiece deformation and cracking tendency are small when cooling. Therefore, when the formed part has a large amount of deformation, it is necessary for the material to have a higher level of spheroidization and a good metallographic structure. Otherwise, the parts will be difficult to form, and the qualification rate will be low.

The pros and cons of the spheroidization rate and metallographic structure are related to the structure of the spheroidizing furnace and the annealing process; in terms of structure, there must be a good atmosphere circulation system, a forced cooling device that can be programmed, and a high-efficiency heating rate that can be controlled. In terms of annealing process, it should be designed according to the material of the material, the manufacturer of the material, the use, etc., such as the heating rate and the cooling rate should be controlled.

Usually the spheroidization rate is controlled at 4.5-6 grades (according to the Japanese JIS G3539 standard, grade 6 is the highest)

In addition to the above technical indicators, when choosing a spheroidizing annealing furnace, we also put forward some requirements for the spheroidizing annealing furnace itself:

1. Sealing performance (how much pressure can be maintained in 48 hours of production?)

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, automatic adjustment of air-fuel ratio)

4. The quality and brand of the materials used in the manufacture of the 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 reasons for the above analysis are not generated independently, but will interact. Therefore, when choosing a spheroidizing annealing furnace, you can refer to the above points for comparison and selection.