Vacuum oil quenching and vacuum gas quenching are two commonly used vacuum quenching processes. The latter utilizes N2 and other gases as cooling media. Compared to oil, using gas as the quenching medium can reduce the distortion of the workpiece due to the single-phase nature of gas convection cooling, thereby avoiding state changes during the process. Compared with workpiece quenched under atmospheric pressure (salt bath furnace, well furnace, mesh furnace, box furnace, controlled atmosphere furnace, etc.), the hardness of a vacuum gas quenching workpiece is more uniform and slightly higher, mainly because the workpiece surface is active during vacuum heating, without decarbonization, oxide film hindering cooling, etc. For a HSS workpiece with a diameter of 50 mm, the hardness after vacuum gas quenching is HRC64, but when the diameter is greater than 100 mm, the hardness of the workpiece is HRC60, which is lower than that of salt bath quenching. The hardness of die steel Cr12MoV after vacuum gas quenching is HRC59-61, and the hardness after oil quenching is HRC60-63, among which the hardness of a workpiece larger than 100 mm after gas quenching is below HRC60, and it is in the incomplete quenching state. The 200 mm forging die steel SKT4 oil quenching (heating temperature of 800 °C), which reaches hardness up to HRC58, can meet the requirements of the use of a die [30].
Lin et al. [31] conducted comparative studies involving oil quenching and high-pressure vacuum gas quenching techniques on Cr12MoV material. It was found that after two different heat treatments, the heat-treated steel possessed a structure comparing martensite, carbide, and residual austenite, with a significant improvement in hardness. Among them, the surface of the sample treated by vacuum gas quenching was clean and neat, and there was no need to clean the surface oil in the subsequent treatment, and it was very friendly to the environment.
Yu et al. [32] found that the grain growth rate of HSS was not obvious under vacuum heating. In the quenching temperature range, the grain size of W6Mo5Cr4V2 steel changes in the range of 11 grades with the difference of pre-hot rolling deformation. After comparison, there is basically no difference compared with that after salt bath quenching. However, at quenching temperatures exceeding 1220 °C and heating and holding coefficients beyond 40 s/mm, grain growth is noticeable with prolonged holding time. When the quenching temperature is 1220 °C and the insulation coefficient is >60 s/mm, or the quenching temperature is ≥1230 °C and the insulation coefficient is ≥30 s/mm, abnormal grain growth occurs in the vacuum gas quenching with a workpiece size of φ20 × 20 mm. Many fine carbide particles exist in the form of chains in samples that produce mixed crystals. The size of the particles varies greatly (as small as 0.1 μm and as large as 3.5 μm), and the distribution is uneven.
In the quenching temperature range of 1180~1240 °C, W6Mo5Cr4V2 steel exhibits high hardness after vacuum gas quenching. In addition, the hardness value after quenching gradually increases with an increase in quenching temperature, reaching a peak at 1220 °C. When quenching is conducted above 1220 °C, the quenching hardness decreases due to the increase in the residual austenite volume in the steel. The hardness of HSS after gas quenching and tempering also increases with the increase in the quenching temperature. After true air quenching at 1220 °C and salt bath tempering at 540 °C three times an hour, the hardness of HSS can reach HRC 64–65. However, when compared to the hardness of salt bath quenching and tempering under the same conditions, the hardness values of vacuum gas quenched and tempered steel are lower. The low tempering hardness and red hardness of vacuum gas quenched HSS are related to the low cooling rate during vacuum gas quenching. As shown in and , it is well-known that the cooling rate increases with the gas pressure. In order to avoid the precipitation of secondary carbides and achieve high hardness of HSS tools with large section sizes obtained after vacuum gas quenching and tempering, vacuum gas quenching can be adopted [32].
Wang et al. [33] studied the properties of 1Crl7Ni2 alloy steel after vacuum oil quenching and vacuum gas quenching, respectively, and compared their microstructures. After oil quenching and gas quenching with the same process parameters, the mechanical properties of 1Crl7Ni2 steel were improved. Increasing the quenching pressure during gas quenching led to an increase in the tensile strength of 1Crl7Ni2 steel to some extent.
Yan et al. [34] conducted vacuum quenching, tempering, and secondary tempering of 4Cr5MoSiV1 steel for die casting at different temperatures, and analyzed the microstructures of 4Cr5MoSiV1. It was indicated that 4Cr5MoSiV1 presented excellent mechanical properties after vacuum quenching at 1050 °C and twice tempering at 600 °C. The heat treatment procedures of the die casting materials commonly used in the world, especially 4Cr5MoSiV1, were described and compared by using various processes such as air quenching, tempering and isothermal spheroidizing annealing [35,36,37].
Syifa Luthfiyah et al. [38] using 4Cr5MoSiV1 steel subjected to Vacuum Heat Treatment at 1030 °C for 3 h and progressively quenched at different quenching rates. The results showed that the hardness, structure, and corrosion resistance of the material surface are affected by the quenching speed. The difference in the cooling speed of each sample resulted in variations in surface properties. The 4Cr5MoSiV1 steel corrosion resistance was determined in the entire vacuum heat treatment process since the final phase formed and air condition will be the determining factors of how resistant the material is to corrosion.
R. Sola et al. [39] studied the effects of laser, vacuum, and induction hardening on the mechanical properties, wear resistance, and corrosion resistance of stainless steel AISI420. The results showed that all three heat treatments greatly improved the wear resistance of the steel due to the increase in hardness. Laser and vacuum quenching also increased the corrosion resistance, while the corrosion resistance after induction quenching is lower than that of untreated steel. The effect of different kinds of quenching, i.e., laser, vacuum, and induction ones, on the mechanical properties and wear and corrosion resistance of stainless steel AISI 420 was studied. It was shown that all three kinds of heat treatment considerably increased the wear resistance of the steel due to growth in the hardness. Laser and vacuum quenching also increased corrosion resistance. After induction quenching the corrosion resistance was lower than in untreated steel.
In summary, vacuum gas quenching achieved no oxide skin on the surface, and the distortion degree was small. The microstructure was more uniform. For parts with low precision requirements, there can be reduced or no subsequent processing, and successful experiments have been conducted on different steel grades, and significant results have been obtained, as shown in .
Vacuum heating prevents particle oxidation through degassing, ensuring excellent surface quality without the need for post-quenching treatment. This process is environmentally friendly as it avoids the use of harmful gases, while steel subjected to vacuum quenching exhibits enhanced mechanical properties. Vacuum quenching can greatly reduce the hydrogen content and other gas content of high-strength steel prone to hydrogen embrittlement so that the ability of steel to resist brittle fracture has been improved. At the same time, the service life after vacuum quenching is longer than that of conventional heat treatment. In the heat treatment of different kinds of steel, the vacuum gas quenching process is very advantageous, and vacuum heat treatment is also the most appropriate heat treatment method at present, so it is worth focusing on the development [40].