Hubei Fotma Machinery Co., Ltd.
Tungsten, Molybdenum, Titanium, Tungsten Carbide and Investment Casting Products
September 13, 2018
Cemented carbide sintering is the most basic and critical process in the production process, and it is also the last process. Sintering methods and equipment have a huge impact on the product. Conventional sintering methods include hydrogen sintering, vacuum sintering, hot isostatic pressing, vacuum subsequent hot isostatic pressing, sintering hot isostatic pressing, etc., followed by microwave sintering, and microwave sintering has become a new development in cemented carbide sintering. A comprehensive introduction to these several methods of cemented carbide sintering is now available.
The compact is placed in a graphite boat, and then filled with a certain amount of carbonaceous alumina filler or graphite particulate filler, and placed in a continuous propelling molybdenum wire furnace and sintered under hydrogen protection. Hydrogen sintering provides a reducing atmosphere and requires pre-sintering to remove the forming agent added during pressing. There are many deficiencies in hydrogen sintering. The advantage of the molybdenum wire corundum tube furnace is that the structure is simple, the furnace power is small, and the furnace tube has a long service life, but the furnace temperature is not controlled, the furnace atmosphere changes greatly, and the product is easy to carburize and decarburize. In addition, the sintering process is carried out under positive pressure, and the pores inside the product cannot be sufficiently eliminated, leaving residual pores, and the oxide impurities cannot be volatilized and eliminated.
Vacuum sintering is the process of sintering and pressing in a negative pressure gas medium. Compared with hydrogen sintering, vacuum sintering can improve the purity of the furnace gas, while the negative pressure improves the wettability of the bonded relative hard phase. Advantages of vacuum sintering: (1) better eliminate trace oxide impurities such as Si, Mg, Ca, etc. in the sintered body, improve the purity of the cemented carbide; (2) reduce the carburization and decarburization of the gas phase under vacuum, and ensure the final alloy Carbon content, control the microstructure of the alloy; (3) reduce the sintering temperature or holding time to prevent uneven growth of carbide grains; (4) the residual porosity of the sintered product is less than that of hydrogen, increasing the density and mechanical properties of the alloy; The product is not separated by filler during sintering, and the operation is simple, and the surface of the product has no adherend and white metallic aluminum deposit. The disadvantage is that there are small pores and defects inside the product.
Hot isostatic pressing
The hard alloy is prepared by vacuum sintering, and there are residual pores inside the product, and hot isostatic pressing solves this problem. Putting the powder compact and the powder body (ie, the powder wrap) contained in the special container into a high-pressure container of a hot isostatic press, applying high temperature and high pressure, and pressing and sintering the powder into a dense part or material, the process It is called powder hot isostatic pressing sintering process. The principle of powder hot isostatic pressing is that the powder body (powder in the powder compact or the sheath) is subjected to high temperature and high pressure in the isostatic high pressure vessel, strengthens the pressing and sintering process, reduces the sintering temperature of the product, and improves the product. The grain structure eliminates defects and pores between particles inside the material, and increases the density and strength of the material.
Vacuum subsequent hot isostatic pressing
After the cemented carbide product is sintered by vacuum (or hydrogen), the holes in the compact can be eliminated, and the densification process is basically completed. In order to further increase the density and bending strength of the cemented carbide, a subsequent hot isostatic pressing treatment may be performed to eliminate the micropores and dissolve the residual graphite in the liquid phase, and the graphite phase is eliminated by diffusion. The vacuum subsequent hot isostatic pressing process is a hot isostatic pressing of the product after conventional vacuum sintering, and the sintered product is sintered to a product having a density higher than 92% theoretical density, and the pressure is 80-150 MPa, and the inert gas is The pressurized medium is treated in a hot isostatic press at a temperature of 1,320 to 1,400 °C. The products produced by such methods have unrestricted shape and hard alloy type, and have good surface finish, can reduce or eliminate pores, have uniform distribution of components and hardness, and improve bending strength.
Such technical defects: (1) Although the hot isostatic pressing temperature is slightly lower than the vacuum sintering temperature, the product is sintered again, and the tungsten carbide is recrystallized by liquid phase at a liquidus temperature or higher than the liquidus temperature. Large, causing uneven distribution of tungsten carbide grain size inside the alloy, coarse tungsten carbide grains have the role of fracture source, alloy strength is reduced; (2) high pressure (100MPa or more) used during sintering, equipment design is complex, cost Expensive, difficult to maintain, and complicated to operate; (3) Due to high pressure, it is easy to cause movement and migration of liquid phase Co, which is extruded into the voids or pores of the alloy to form a “cobalt pool”, causing the binder phase to be in the alloy. Uneven distribution; (4) When hot isostatic pressing, the product is placed on a graphite plate or a refractory metal grid and is in contact with pressurized gas, so the surface layer of the product is inevitably associated with O2 and N2 in graphite and inert gases. The composition and structure change occur due to the action of gases such as H2O, CO2, CO, and CH4.
Sintering hot isostatic pressing
Sintering hot isostatic pressing, also known as over-pressure sintering or low-pressure hot isostatic pressing, is a process of simultaneously performing hot isostatic pressing and sintering on a workpiece at a pressure lower than a conventional hot isostatic pressure (about 6 MPa). The forming agent is removed, sintered and hot isostatically pressed in the same equipment, and the workpiece is placed in a vacuum sintering isostatic pressing furnace, and degassed at a lower temperature under a low pressure carrier gas (such as hydrogen) at 1350~ Vacuum sintering was carried out at 1450 ° C, followed by hot isostatic pressing in the same furnace, using argon gas as a pressure medium, pressing pressure of about 6 MPa, and cooling for a certain period of time.
Microwave sintering is a rapid sintering in which the dielectric loss of a material in a microwave electromagnetic field is used to heat the whole to a sintering temperature to achieve densification. Conventional sintering relies on the heating element to transmit heat through convection, conduction and radiation. The material is heated from the outside to the inside, the sintering time is relatively long, and the crystal grains are easy to grow. Microwave sintering relies on the kinetic energy and potential energy of the material itself to absorb the microwave energy into the internal molecules of the material. The internal and external heating of the material can minimize the thermal stress inside the material. Under the action of microwave electromagnetic energy, the kinetic energy of the molecules or ions inside the material increases. The sintering activation is lowered, the diffusion coefficient is increased, and rapid sintering at a low temperature can be performed, so that the fine powder can be sintered without being grown. Microwave sintering is one of the effective means to prepare fine-grained materials.