During fastener production process, wire rod need to be drawn and spheroidized annealing treatment which will be propitious to cold forming. And spheroidizing annealing treatment quality will influence the finished products quality. So choose suitable spheroidizing annealing furnace is very important for fastener companies. However, how to choose?

First, we need to know the purpose of spheroidizing annealing treatment:
1. improve mechanical property ,low material hardness and strength, improve cold working property;
2. Workpiece can forging forming by less energy and cut easily.
3. Reduce die loss
4. Avoid workpiece split and twist

Second, we need to know the relationship between cold forging property and spheroidizing annealing quality.

1. Deformation resistance:
Deformation resistance is related to material hardness and tension intensity.
Less hardness, less deformation resistance.

2. deformation energy ：
The deformation energy is related to material metallurgical structure. e.g the normalization structure get from full annealing process, its deformation energy is worse than spheroizied structure as for spheroidized structure, if spheriodization ratio of cementite is higher ,the deformation energy will be better. And when cementite is evenly dispersed in ferrite structure in the form of micro-particle, the deformation energy will be the best.

Third, we need to know the below indexes of wire after spheroidizing annealing.
1. Surface oxidation
2. Surface decarburization
3. Hardness and it’s uniformity 
4. Spheroidization rate
5. Metallurgical structure

The influence of above indexes for fasteners production will be explained as follows:
1. Surface oxidation and decarburization
Material surface oxidation will made material loss and raise the production cost. Seriously surface decarburization will low the products strength grade even make waste products.

The key to solve surface oxidation and decarburization problem is precisely control the furnace inside atmosphere. Only precisely control the oxygen potential value of furnace inside atmosphere, no surface oxidation and little decarburization can be reached. If just simply feed some atmosphere but no precisely control, surface decarburization and oxidation can’t be solved.

Chinese standard stipulated the 15‰D surface decarburization, generally surface decarburization layer can be controller within 8‰D, and high quality material can be controlled within 0.05mm even within 0.03mm.

2. Hardness and its uniformity
When metal materials make cold forging or cold extrusion, suitable hardness should be qualified for getting higher rate of finished products and lowering the die loss. Also, we should control the hardness uniformity of same batch material within possibly small range in case hardness difference too much will influence the production efficiency and lower the rate of finished products and shorten the service life of die.

Suitable hardness and its uniformity are mainly decided by temperature uniformity; while temperature uniformity is decided by furnaces structure and protective atmosphere.

Generally hardness difference controlled within 3-5HRB can satisfy the production requirement, high quality required hardness difference controlled within 3HRB

3. Spheroidization rate and metallurgical structure
The structure of wire material after rolling and forging is laminated pearlite and network cementite structure which is hard, brittle and hard to cutting. During the subsequent quenching process, it is easy to deform and crack. While through spheroidizing annealing, the wires get spheroidal pearlite. And the cementite of spheroidal pearlite is dispersed on ferrite in the form of spherical particle. Compared to laminated pearlite, it has lower hardness and easier to cut, also during the quenching process, the possibility of deformation and crack is lowered. When the forming parts need more deformation energy, the wire material should have higher spheriodization rate and good metallurgical structure. Otherwise the parts will hard to forming.

Spheroidization rate and metallurgical structure is related to furnace structure and annealing process. Good spheroidization rate and metallurgical structure need good atmosphere circulation system, programmable forced cooling device and high efficiency and speed controllable heating device; also the annealing process such temperature heating rate, temperature cooling rate etc should be controllable.

Generally Spheroidization rate should be controlled at 4.5-6 grade (accord with JIS G3539, 6 Grade is the highest grade.)

Expect above technical indexes, we should have specific requirements on furnace while you choose the furnace:
1. Sealing property (how much pressure will be kept during 48 hours production.)
2. Energy-saving measures（furnace surface temperature rise , heating efficiency , heat recovery and energy consumption ）
3. Intelligence (heat treatment expert system. Remote control system, technology process curve, fault warning , atmosphere control , air and gas, ratio auto regulation )
4. Quality and brand of furnace material used

In a word, the furnace quality is related to furnace structure, atmosphere control, auto control and technology process design etc. these above reasons are interrelated and interact on each other. When you choose the furnace, you can consider above reasons and make a good choice.  

The above article is provided by Mr. Yu Yunfei, vice general manager of Haiyan Chnstar Industrial Furnace Co., Ltd. For more information about surface treatment, welcome to visit www.chnstar.cn.