Cemented carbide is one or more refractory metal compounds with high hardness and high elastic modulus (WC,TiC,TaC,NbC, etc.) as the matrix, with transition metals (Co,Ni,Fe, etc.) or alloys as binders, usually prepared by powder metallurgy method of multiphase composite materials, is one of the most typical and most important material products in the field of powder metallurgy.

Cemented carbide integrates the excellent properties of the hard phase and the adhesive phase, which has a series of advantages, with a high hardness (80 ~ 94HRA) and wear resistance, especially at higher temperatures can still maintain a high hardness and strength, 600℃ hardness more than the normal temperature hardness of high-speed steel, 1,000℃ hardness higher than the normal temperature hardness of carbon steel, The strength can also be maintained near 300MPa; It has a high elastic modulus, usually 400 ~ 700MPa; Cemented carbide has a high compressive strength, can withstand large loads and keep the shape unchanged, has a low coefficient of thermal expansion, generally 50% of steel, has good chemical stability, and has better oxidation and corrosion resistance than steel. Cemented carbide has become an indispensable mold material and structural material in almost all industrial sectors and new technology fields.

Heat treatment is an important means to improve the organizational properties of materials, and cryogenic treatment, as an extension and extension of traditional heat treatment processes, has been widely used in the material heat treatment industry since the middle of the 20th century. For traditional steel materials, cryogenic treatment can transform residual austenite, improve the hardness of the workpiece, and stabilize the size of the workpiece; Ultrafine carbide can be precipitated to improve the wear resistance of the workpiece. It can refine the grain and improve the impact toughness of the die. It can improve the corrosion resistance of martensitic stainless steel and improve the polishing performance of the workpiece. With the further development and maturity of liquid nitrogen cooling technology and insulation technology, cryogenic treatment of cemented carbide has also attracted the attention of some industrial enterprises at home and abroad.

Status of cryogenic treatment technology

Cryogenic treatment usually uses liquid nitrogen cooling, which can cool the workpiece to below -190 ° C. The microstructure of the treated material changes at low temperature, and some properties are improved. Cryogenic treatment was originally proposed by the former Soviet Union in 1939, until the 1960s, the United States will be cryogenic treatment technology industrial practical, began to be mainly used in the field of aviation, and expanded to the field of machinery manufacturing in the 1970s.

According to the different cooling methods can be divided into liquid method and gas method. The liquid method is that the material or workpiece is directly immersed in liquid nitrogen, so that the workpiece is quenched to the liquid nitrogen temperature, and held at this temperature for a certain time, and then taken out and heated to a certain temperature. It is difficult to control the rising and cooling speed in this way, and it has a large thermal shock to the workpiece, and it is generally believed that it is likely to damage the workpiece. Cryogenic equipment is relatively simple, such as liquid nitrogen tanks. The gas rule is refrigeration by the latent heat of vaporization of liquid nitrogen (about 199.54kJ/kg) and the heat absorption of low temperature nitrogen. The gas method can make the cryogenic temperature reach -190℃, so that the low temperature nitrogen and the material contact, through convection heat transfer, the nitrogen is sprayed by the nozzle in the deep cold box gasification, the use of gasification latent heat and low temperature nitrogen heat absorption effect to cool the workpiece. By controlling the input of liquid nitrogen to control the cooling rate, the cryogenic treatment temperature can be automatically adjusted and accurately controlled, and the thermal shock effect is small, and the possibility of cracking is also small. At present, the gas method is more recognized by researchers in the application, and its cooling equipment is mainly temperature-controlled programmed deep cold box. Cryogenic treatment can significantly improve the service life, wear resistance and dimensional stability of ferrous metals, non-ferrous metals, metal alloys and other materials, and has considerable economic benefits and market prospects.

The cryogenic technology of cemented carbide was reported in the 1980s and 1990s. Japan's "Mechanical Technology" in 1981 and the United States' "modernmachine shop" in 1992 all reported that cemented carbide cryogenic treatment significantly improved the performance. Since the 1970s, foreign research on cryogenic treatment has been effective, and the former Soviet Union, the United States, Japan and other countries have successfully used cryogenic treatment to improve the service life of the die, the wear resistance of the workpiece and the dimensional stability. The actual application results of a mold company in the United States show that the service life of the cemented carbide blade is increased by 2 to 8 times after treatment, and the dressing cycle of the cemented carbide wire drawing die is extended from a few weeks to a few months after treatment. In the 1990s of the 20th century, China carried out research work on the cryogenic technology of cemented carbide, and achieved certain research results. In general, the current research on cemented carbide cryogenic treatment technology is less, and it is not systematic enough, and the conclusions are more inconsistent, which needs to be further explored by researchers. According to the existing research data, cryogenic treatment mainly improves the wear resistance and service life of cemented carbide, but the effect on physical properties is not obvious.

Strengthening mechanism of cryogenic treatment

Wear and early fracture are the main failure forms of tooling. The cryogenic treatment of cemented carbide is mainly used in the mold. The wear of die is mainly adhesive wear and abrasive wear. The early fracture is mainly due to lack of toughness. Therefore, the research on its strengthening mechanism mainly focuses on its wear resistance and service life, but the impact on physical properties is not obvious.

(1) Phase transformation strengthening.

There are two crystal structures of Co in cemented carbide: α phase (fcc) with face-centered cubic crystal structure and ε phase (hcp) with close-packed hexagonal crystal structure. ε-Co has a smaller coefficient of friction than α-Co and is more resistant to wear. Above 417℃, the free energy of α phase is low, so Co exists in the form of α phase. Below 417℃, the ε phase has a lower free energy, and the high temperature stable phase α transforms into the ε phase with lower free energy. However, due to the presence of WC particles and solid soluble heterogeneous atoms in the α phase, there is a greater binding force on the phase transition, which increases the resistance of α→ε phase transition, so that the α phase cannot be completely transformed into ε phase when the temperature falls below 417℃. Cryogenic treatment can increase the free energy difference between α and ε, thus increasing the driving force of phase transition and increasing the ε phase transition. After cryogenically treated cemented carbide, some atoms dissolved in Co, due to the reduction of solubility and precipitation in the form of a compound, can increase the hard phase in the Co matrix, hinder the dislocation movement, and play a second phase particle strengthening role.

(2) Strengthening of surface residual stress.

After cryogenic treatment, the surface residual compressive stress increased. Many researchers believe that a certain value of residual compressive stress in the surface layer can greatly improve its service life. In the cooling process of cemented carbide after sintering, the bonding phase Co is subjected to tensile stress, and the WC particles are subjected to compressive stress. Therefore, some researchers believe that the increase of surface compressive stress caused by deep cooling can slow down or partially offset the tensile stress generated by the bonding phase during the cooling process after sintering, and even adjust to the compressive stress to reduce the generation of micro-cracks.

(3) Other strengthening mechanisms.

Some people believe that after cryogenic treatment, the η phase particles formed in the matrix together with WC particles make the matrix become tighter and stronger, and because of the formation of the η phase, the Co in the matrix is consumed. The decrease of Co content increases the thermal conductivity of the whole material, and the increase of carbide particle size and adjacency also increases the thermal conductivity of the matrix. Due to the increase of thermal conductivity, the heat dissipation of the die tip is faster; The wear resistance and high temperature hardness of the die are improved. It is also believed that due to the shrinkage and densification of Co after cryogenization, the firm holding of Co on WC particles is strengthened. Physicists think that deep cooling changes the structure of the atoms and molecules of metals.