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Fig. 1 Amplitude of changes in hardness of WC-Co cemented carbides with different Co contents after deep-cooling treatment

Fig. 2 Variation amplitude of flexural strength of WC-Co cemented carbide with different Co content after deep-cooling treatment

As can be seen from Figs. 1 and 2, the hardness of low-cobalt cemented carbide changes slightly more than that of high-cobalt cemented carbide after deep-cooling treatment, but in general the effect of deep-cooling treatment on the hardness of cemented carbide is not obvious (most of the difference in hardness before and after deep-cooling is within 1%). However, with the increase of Co content, there is a tendency for the improvement of flexural strength of Cemented Carbide after deep cooling treatment to increase gradually. It is noteworthy that when the Co content is the same, there are differences in the changes of mechanical properties of cemented carbide after deep-cooling treatment, which can be attributed to two reasons: first, the grain size affects the effect of deep-cooling treatment of cemented carbide. Zhang et al [20] carried out deep-cooling treatment on YG10C coarse-grained cemented carbide and found that the changes in hardness and flexural strength were not obvious, while Jiao Penghe [29] increased the flexural strength of ultra-fine YG10 cemented carbide by 7.5% after deep-cooling treatment. This phenomenon was found in the deep-cooling treatment studies of YG6 [20,23], YG11 [21,22], YG20 [22,30] and other cemented carbides; secondly, different deep-cooling treatment processes cause different mechanical property changes. The deep-cooling treatment process of cemented carbide follows the traditional deep-cooling treatment process of “cooling-insulation-retempering”, so the deep-cooling treatment temperature, holding time, number of deep-cooling cycles, rate of temperature increase and decrease, and other process parameters have a significant effect on the effect of deep-cooling treatment of cemented carbide. The following combines the changes of mechanical properties of cemented carbide after deep cooling to analyze the influence of each process parameter on mechanical properties and the potential law.

1.1 Deep-cooling temperature

The deep-cooling treatment temperature is the core process parameter in the deep-cooling treatment of cemented carbide. Different deep-cooling treatment temperatures cause different changes in mechanical properties, and Reddy et al. [31,32] compared the hardness changes of P30 Cemented Carbide after deep-cooling treatment at -110 ℃ and -176 ℃, respectively, and the results are shown in Table 1. It can be seen that although the hardness of P30 Cemented Carbide was reduced after deep-cooling treatment at room temperature, the hardness after deep-cooling treatment was higher than that without deep-cooling in the temperature range from 100 to 600 ℃, which indicates that the deep-cooling treatment improves the thermal hardness of Cemented Carbide. At the same time, the lower the deep-cooling temperature, the higher the relative hardness of cemented carbide at high temperature. However, it is not the case that the lower the deep-cooling temperature, the better the improvement of mechanical properties.Gill [33] et al. performed deep-cooling treatment on P25 cemented carbide at -110 °C and -196 °C, respectively, and the results showed that the hardness of cemented carbide was increased by 4.75% by the deep-cooling treatment at -110 °C, and continuing to reduce the deep-cooling treatment temperature (-196 °C) did not further improve the hardness. This work shows that the optimization effect of deep-cooling treatment is not better with further decrease of deep-cooling treatment temperature after dropping to a certain temperature. Our team carried out ordinary cold treatment at - 80 °C and deep cold treatment at -140 °C and -196 °C for YG20 cemented carbide, respectively. The results show that the ordinary cold treatment has little effect on the flexural strength of YG20 cemented carbide, while the deep cold treatment increases the flexural strength by 9.2% (-140 °C) and 6.2% (-196 °C), respectively, which suggests that the deep cold treatment effectively increases the cemented carbide's Flexural strength, however, the flexural strength is not with the deep-cooling treatment temperature shows a linear law of change, deep-cooling treatment temperature of -140 ℃ when the flexural strength of YG20 Cemented Carbide to obtain the best optimization effect.

Table 1: Hardness of P30 Cemented Carbide at different temperatures before and after deep-cooling treatment [31,32]

1.2 Holding time of deep cooling

Insulation for a certain period of time in a very low temperature environment is a necessary condition for deep-cooling treatment to improve the mechanical properties of cemented carbide, and the mechanical properties of cemented carbide will change with the change of insulation time.Jiang et al[18] placed YG8 cemented carbide in a low temperature environment of 77K and insulated it for 2 h, 4 h, 8 h, 24 h and 72 h respectively, and found that after deep-cooling treatment for 2h, the hardness and compressive strength reached the maximum value respectively after 2h of deep-cooling treatment. It can be seen that the deep-cooling treatment is not usually considered the longer the insulation time, the better the optimization effect. Therefore, the selection of accurate deep cooling holding time can ensure a good optimization effect on the one hand, and can achieve the purpose of cost saving and efficiency improvement on the other hand. Chen Zhenhua et al [19] carried out deep cooling treatment of YL20.3 cemented carbide for 2~72 h. It was found that the Vickers hardness reached the maximum value after 2 h of holding time in liquid nitrogen, and the hardness began to decrease with the prolongation of the holding time and the hardness change tended to be flat from 30 h onwards, as shown in Fig. 3. Therefore, a short time of deep cooling treatment can effectively improve the hardness of this cemented carbide. This result also confirms that in the process of deep-cooling treatment, continuing to extend the necessary holding time will not improve the optimization effect of deep-cooling treatment. It can be seen that at a certain deep-cooling temperature, the mechanical properties of cemented carbide do not change linearly with the prolongation of the holding time, and for cemented carbides of specific compositions, there is also an optimal holding time during deep-cooling treatment.Gao et al. [34] carried out a deep-cooling treatment at -196 °C on WC-Fe-Ni (with a bonded-phase Fe/Ni content of 20%) cemented carbide for the holding times of 2 h, 12 h and 24 h. The results showed that both hardness and fracture toughness reached the maximum and minimum values respectively after 12 h of deep cooling, in which the maximum hardness increased by 20%, while the fracture toughness decreased from 25.7 MPam-1/2 to 19.6 MPam-1/2. The results indicated that, at the suitable temperature, when the hardness has the optimal effect, the other properties may not reach the expected effect, and thus the different properties should be further developed. Therefore, for different properties must be further developed corresponding deep cooling holding time and other process parameters. Most of the foreign scholars used long holding time: Reddy [31] used 24 h of deep-cooling treatment for P30 cemented carbide; Yong [35] placed cemented carbide inserts in a deep-cooling environment at -184 ℃ and held them for 24 h; and Özbeka [36] used deep-cooling treatment of uncoated cemented carbide inserts for more than 12 h. The results indicate that the hardness of the inserts may not be optimal while other properties may not be as effective as expected.

1.3 Number of deep-cooling cycles

Cyclic deep-cooling treatment has been attempted in the deep-cooling treatment research of titanium alloys [37], and the cyclic deep-cooling treatment of cemented carbide has also been explored. Chen Hongwei [21] carried out 0~4 times deep-cooling treatment on YG8 cemented carbide respectively, and the results showed that: with the increase of the number of deep-cooling times, the life coefficient of YG8 cemented carbide was gradually increased and reached the highest value after 3 times of deep-cooling, which indicated that multiple repeated deep-cooling treatment has obvious strengthening effect on YG8 cemented carbide. Wu Liangqin [38] et al. compared the effect of 1 to 3 deep cooling treatments on the hardness of YT15 turning inserts and found that the average hardness value of YT15 carbide increased with the increase of the number of deep cooling. This study showed that compared with single deep-cooling treatment, the deep-cooling cycle treatment could lead to further optimization of some of the cemented carbide properties. Reduction of mechanical properties of cemented carbide due to repeated deep cooling treatment has also been reported. Fan Lian [39] carried out 2 h deep-cooling treatment on YL30.4 cemented carbide for 1, 2, 3, and 5 times, respectively, and found that the cyclic deep-cooling treatment did not improve the compressive strength of the material, but rather caused it to decrease with the increase in the number of cycles. At present, the mechanism of the effect of cyclic deep-cooling treatment on the performance of cemented carbide is not clear, and further in-depth study is required.

1.4 Temperature rate

In the early stage, due to the imperfect function of the deep cooling equipment, most researchers [21,30] used the method of “direct immersion in liquid nitrogen” to carry out deep cooling treatment, which could not realize the function of controlling the temperature and the cooling rate. Generally speaking, directly immersing cemented carbide materials into liquid nitrogen for deep-cooling treatment is likely to cause the cooling speed is too fast, so that the difference between the different phases of the material due to the difference in the coefficient of thermal expansion and produce a large thermal stress, resulting in cracking or deformation of the workpiece, which adversely affects the mechanical properties. Therefore, it is necessary to slowly cool down the material in the process of deep-cooling treatment. With the increasing improvement of deep-cooling equipment [40], most of the studies [18,32,33,36,41] placed cemented carbide in a programmable deep-cooling chamber, using low-temperature nitrogen and convective heat transfer of the material and its latent heat of vaporization to carry out precise and slow cooling, in which the rate of temperature rise and fall is controlled to be within 0.5~2 ℃/min.

In summary, deep cooling treatment can effectively improve the strength and toughness of cemented carbide, but the effect on hardness is not obvious. The degree of improvement of mechanical properties after deep-cooling treatment is closely related to the deep-cooling treatment process. The traditional concept of deep-cooling treatment is that the slower the rate of temperature rise and fall (0.5~2 ℃/min), the lower the temperature of deep-cooling treatment (-196 ℃), and the longer the holding time (more than 12 h), the more complete the transformation of the organization of the cemented carbide material in the extremely low-temperature environment, and the more uniform the distribution of stress, which results in a more significant improvement of the mechanical properties. However, the mechanical properties of Cemented Carbide do not change linearly with the decrease of deep-cooling treatment temperature and the prolongation of holding time, i.e., for Cemented Carbide of specific composition, there exists an optimal deep-cooling treatment temperature and holding time. Deep-cooling treatment temperature is too low, the holding time is too long will increase the process cost and energy consumption. Therefore, it is of great significance to master the influence of deep-cooling treatment process parameters on the mechanical properties of cemented carbide and optimize the deep-cooling treatment process of cemented carbide in order to promote the application and development of deep-cooling treatment technology in cemented carbide-related industries.