Overview of cemented carbide sintering
We call it SINTERING: The process of heating at a temperature lower than the melting point of the main component of the cemented carbide to connect the adjacent particles in the compact. The purpose of sintering is to make the porous powder compact into an alloy with a certain structure and properties. The sintering process has both physical and chemical changes. The main changes are the densification of the sintered body and the composition change of the binder phase. The grain growth of carbides forms an alloy structure with certain physical and mechanical properties. The temperature uniformity in the sintering furnace is an important factor for producing cemented carbide.
In actual production, it gradually forms various sintering methods. The more traditional ones include hydrogen sintering, vacuum sintering, hot isostatic pressing sintering, vacuum subsequent hot isostatic pressing sintering, etc. The current mainstream cemented carbide sintering equipment is mainly based on vacuum sintering furnaces and air pressure sintering furnaces.
High-performance cemented carbide has excellent physical and chemical properties such as high strength, reasonable wear and toughness, high temperature, and corrosion resistance. It mainly manifests the product quality defects as under-burning, over-burning, and uneven carbon content. Studies have shown that the main reasons for under-burning, over-burning, and uneven carbon content are the uneven temperature in the sintering furnace and the unreasonable gas flow in the furnace. This paper discusses the furnace temperature uniformity of the sintering furnace.
Influence of sintering furnace body structure and furnace internals on temperature uniformity
The heating structure of the furnace body mainly has a square shape and a cylindrical shape according to the heating and heat preservation forms. The square heating is basically a squirrel cage type, and spaces the graphite tubes evenly. The cylindrical structure is the bottom, and the left and right sides are heated. The heating power at the bottom is high, and the heating power at both sides is slightly smaller, and the distance between the graphite tubes increases toward the top.
Due to the gas flow in the furnace, according to the gas characteristics, the temperature difference in the low-temperature section is relatively large, sometimes up to tens of degrees. Therefore, the structural design of the furnace has a great influence on its temperature uniformity. According to actual production experience and data comparison, the circular structure is better than the square structure. Especially better than the equidistant distribution of graphite tubes in a square structure.
Furnace internals mainly include graphite tube heating element, material box, material box door, thermal insulation layer (insulation cylinder), and insulation cylinder cover (muff door). The heating element requires good conductivity and uniform density. The material box is preferably a whole board, and the opening gap of the material box door is consistent. It is best to use the inner and outer carbon fiber surface layer for the insulation cylinder, which has been treated at a high temperature of 2000 ℃. Protect the opening, and cover the end face with CFC material. The muffle door adopts a V-shaped progressive positioning sealing method to ensure no heat leakage.
Influence of control mode on temperature uniformity in sintering furnace
Currently, the mainstream cemented carbide sintering furnace’s control system basically adopts the upper computer + PLC control method.
The configuration of the upper computer is used for man-machine dialogue, and PLC is used for logic and process control. Divide the temperature control into thermocouple + smart instrument, analog input module, cascade control module, and other methods. Smart meters have their controls and settings. Low cost, simple operation, and control. Generally, the PID regulation control of PLC realizes the analog input module. It is mainly realized by parameter debugging of artificial PID, and the change of external conditions greatly influences it. It often uses the hardware cascade control to improve accuracy and reduce temperature inertia, such as the cascade module FM355 in the Siemens S7 system.
Smart meters and analog input PID temperature control methods have relatively poor temperature control accuracy and large temperature fluctuations. It is suitable for small furnace bodies and temperature precision control occasions with general requirements. Cascade control includes software and hardware cascade control methods. The program is relatively complex, the cost is high, and the debugging process is long. However, the temperature control accuracy is high, which can reflect the temperature change trend in advance, reduce the temperature inertia, and improve the temperature control accuracy.
In addition, the position of the thermocouple in the furnace also greatly influences the temperature control accuracy.
Currently, the mainstream is to insert thermocouples outside the material box, and some users and manufacturers believe that inserting thermocouples in the material box can accurately reflect the actual sintering temperature of the green compact. We can see from the temperature control curve that there is almost no fluctuation in the temperature control curve (temperature setting value and actual value) of the thermocouple inserted outside the material box. However, the temperature control curve of the thermocouple inserted in the material box fluctuates greatly at each stage, the temperature inertia is large, and it isn’t easy to control the temperature accurately.
Effect of maintenance on temperature uniformity in Sintering Furnace
From the analysis of the heating principle, the heat conduction methods include convection, conduction, and radiation, and the sintering furnace can only rely on radiation heat transfer during the heating and heat preservation process. This shows that the material properties and production and assembly level of the furnace internals mainly determine the temperature uniformity of the sintering furnace. That is, the heating element, material box and material box door, thermal insulation layer (insulation tube), and insulation tube cover (muff door). and subsequent maintenance decisions.
After the sintering furnace is put into production and use, the performance of each part of the furnace will gradually change, and the temperature uniformity will also change accordingly. Therefore, the maintenance of the furnace is of great significance to the state of the furnace.
According to the long-term maintenance, the main reason for the poor temperature uniformity of the sintering furnace after a certain period of use is the excessive local heat leakage during the heating process or heat preservation process. The main reasons for the local heat leakage of the furnace are:
- It doesn’t close properly or seal tightly the two muffle doors at the front and rear of the insulation cylinder.
- The gaps at graphite electrode outlets, material box support rods, etc., are too large.
- After long-term use of the insulation cylinder, the airflow scours the insulation performance and causes partial damage.
Therefore, after using the sintering furnace for a period of time, it should carry out the furnace temperature uniformity test regularly to provide accurate analysis and guidance for the maintenance of the sintering furnace.
The coercive force is the performance index of cemented carbide, and the control value of the coercive force of different brands of alloys is different. The cemented carbide sintering temperature is the direct performance of coercive force. Therefore, the better the temperature uniformity of the sintering furnace, the more consistent the coercivity control value of the cemented carbide, and the better the performance of the cemented carbide. For this reason, it is also one of the pursued goals of the temperature uniformity control of the sintering furnace in producing cemented carbide.