Due to its good physical and chemical properties, graphite has become the preferred material for making hot pressing molds in the manufacture of diamond tools.
As a hot pressing mold, it has excellent characteristics:
1. Excellent thermal conductivity and electrical conductivity
2. Low coefficient of linear expansion, good thermal stability and thermal shock resistance
3. Chemical resistance and most metals are not easy to react
4. At high temperature (the sintering temperature of most copper matrix bodies is above 800 ℃), the strength increases with the increase of temperature.
5. It has good lubrication and anti-wear properties.
6. It is easy to process and has good machining performance. It can be made into hot pressing molds with complex shapes and high precision. In view of the above characteristics, graphite materials have been widely used in diamond sintering molds.
However, the use of graphite molds is small and the service life is short, which has always plagued diamond tool manufacturers. Such as the mold made of G4 graphite, when sintering iron-based tungsten carbide-containing matrix powder; the number of times the indenter is used is 2-4 times. The length of service life directly affects the production efficiency, production cost and product quality of the enterprise; therefore, improving the service life of graphite molds is a problem that diamond tool manufacturers are very concerned about.
2. Reasons for graphite mold damage
There are two modes of hot pressing sintering, resistance heating method and medium frequency induction heating. The northern diamond tool manufacturers mostly use resistance heating for hot pressing sintering. We know that the temperature of the outer mold is low and the temperature of the cutter head and the inner mold is high when the resistance heating is sintered. The uneven temperature distribution determines the successive parts of the mold damage. Now, taking the molds used in the sintering of diamond saw blades and grinding wheels as an example, combined with my actual field test and experience, the analysis of graphite mold damage is divided into the following categories.
1. Surface oxidation and cracks, because there is no reducing atmosphere protection during hot pressing sintering, graphite is in direct contact with air; surface oxidation will inevitably occur under high temperature conditions, and the higher the temperature, the faster the oxidation speed; with the progress of oxidation, graphite The carbon element of the mold is gradually consumed, leaving only part of the carbon and ash. In the actual production site, it can be found that the surface of the graphite mold will produce pores and the carbon powder will fall off after a period of use.
2. When crushed, the graphite mold is directly in contact with the diamond-containing matrix powder, and the contact surface is in a high-pressure state. The exposed diamond is extruded with the graphite mold. Due to the large difference in hardness, the mold surface in contact with the diamond cutter head will produce pits.
3. Fracturing, this damage method is because the side plate of the diamond cutter head is not clamped tightly, which causes the pressure drop on the pressure head to be unbalanced, resulting in lateral shearing force.
4. The wear size of the graphite mold is out of tolerance, and part of the carcass powder will be extruded to the surface of the mold in the molten state in a film form and adhere to the graphite material. Before using the mold next time, the adhered metal residue must be manually scraped off. This results in out-of-tolerance mold shapes and sizes.
5. Knock damage type, because the carcass powder infiltrates into the oxidation gap of the graphite mold in the molten state during sintering, causing the cutter head and the mold to stick together, and it is difficult to demold; and metal pits.
In production, these types of damage do not exist independently, but interact with each other, and are finally reflected in one form of expression.
The production engineer needs to analyze the actual situation. For the above types of damage, I propose the following according to on-site practice. several countermeasures. 1. Oxidation of graphite material is the primary cause of damage to hot pressing molds, so oxidation resistance directly affects the service life of graphite molds. The oxidation starting temperature of different graphite materials is different, and the oxidation temperature of ordinary graphite molds for hot pressing is about 450 °C. The oxidation process of graphite material is related to the diffusion rate of oxidizing gas, and the oxidation rate is controlled by the diffusivity of oxidizing gas. Therefore, the smaller the impurity content of the mold, the better, the metal elements in the material impurities have a catalytic effect on the oxidation of graphite; the less ash in the graphite, the lower the porosity of the material, the better, especially the open porosity.
Graphite products on the market currently include electrochemical graphite and calcined carbon graphite products, and hot pressing molds generally use calcined graphite products. The porosity of calcined graphite is related to the number of calcinations. The more times of calcination, the lower the porosity and the higher the bulk density.
It is recommended to use three-immersed and four-baked graphite products, which are more cost-effective.
In addition, the sintering humidity of the carcass powder is also a key factor affecting the oxidation rate of the mold; the use of the tool determines the composition of the carcass powder, which cannot be changed, but some molding agents and granulating agents in the carcass powder, they are Organic substances such as acetone generate droplets and organic vapors at the exhaust time of 300°C to 400°C during hot pressing; these organic droplets and vapors have a strong ability to dissolve graphite, and the steam and droplets dissolve the graphite mold. Carbon, ash fall off on the surface of the mold, resulting in pits and holes, causing damage to the mold.
Therefore, reducing the content of organics such as molding agents in the carcass powder plays a vital role in increasing the life of the mold, and also has a great impact on the relative density and strength of the carcass. 2. The crushing of graphite molds is a common mold damage method in the process of hot pressing and sintering.
This damage method is directly related to the compressive strength of the graphite material. The mechanical properties of graphite molds are generally characterized by compressive strength Se, flexural strength Sb and tensile strength St. The ratio of the three is St/Sb=0.47-0.68, Se/Sb=1.61-2.9; we are used to expressing the compressive strength Se; it has obvious anisotropy, and is closely related to the bulk density of graphite materials.
The greater the density, the greater the compressive strength; it has an approximate linear growth relationship. Generally, the Se of graphite with a bulk density above 1.6g/cm3 is not less than 20MPa, and the graphite of three-immersed and four-baked graphite can be larger than 45 MPa. Therefore, using graphite materials with high bulk density to make hot pressing molds is a wise choice to solve the problem of low compressive strength and easy crushing of molds.
The parameters of the hot-pressing process are also one of the factors of die crushing, which affects the service life of the graphite die.
For example, the pressure during smoke exhaust is inappropriate, which can easily cause the crushing of the mold.
It is recommended to adjust the hot pressing process parameters appropriately according to the strength of the mold, increase the number of times the mold is used, and reduce the production cost of the product. 3. The fracturing of the graphite mold is related to the poor clamping of the hot-pressing mold and the uneven pressure drop. Due to the uneven professional level and professional ethics of hot-pressing sintering workers, there is a large gap in the level of mold installation; it is recommended to regularly train workers on the precautions for mold installation every month, and the mold cost is multiplied by the coefficient to assess the wages of the workers. In addition, the fracturing damage of the mold is also related to the resistivity of the graphite material. The resistivity of the mold also characterizes the degree of graphitization of the material, which is proportional to the thermal conductivity. The lower the resistivity, the higher the thermal conductivity; the faster the temperature rises; because the strength of the graphite material increases with the increase of temperature, 1000 At ℃, the strength of graphite increases by about 20% compared with room temperature, so graphite materials with low resistivity are selected, and the hot pressing time is short and the temperature rises quickly; the strength of the graphite mold in contact with the cutter head increases rapidly, and it is not easy to produce fracturing.
At the same time, the hot-pressing sintering time is shortened, and the sintering cost is saved; the oxidation time is reduced, the oxidation damage rate is reduced, and the number of molds used is increased. 4. The wear of the graphite mold causes the size to be out of tolerance. This damage method is caused by the production process and sintering process of the mold itself. The mold making should be processed by grinding to improve the surface finish of the mold and reduce the degree of adhesion between the molten metal and the graphite. The composition of the matrix powder is also one of the factors that cause the metal film to adhere to the graphite.
Since iron is a strong decarburizer; if the carcass is iron-based, a strong reaction and dissolution phenomenon will occur between the carcass and the mold, resulting in the adhesion of the carcass melt and the graphite, which will damage the graphite mold. It is recommended that when sintering the iron-based matrix, a strong release agent should be applied to the surface of the cutter head and the mold to reduce the adhesion of the mold during mold removal, reduce the degree of damage, and improve the service life of the mold. 5. Knock damage, this type of damage is mostly caused by unreasonable design, inconvenient demoulding, and poor thermal shock resistance of the mold.
The thermal shock resistance of the material is usually characterized by its thermal shock resistance factor F = • /P where ∂–tensile strength, α-thermal expansion coefficient, K-thermal conductivity E-Young’s modulus, C -Specific heat, P-body density It can be seen from the above formula that the higher the thermal conductivity, the better the thermal shock resistance; the smaller the thermal expansion coefficient, the better the seismic performance; the higher the tensile strength, the better the seismic performance. The above three parameters are adjusted to the best, the thermal shock performance of the material is greatly improved, and the thermal stability of the material is high.
