Selecting the right vacuum heat treatment process for powder metallurgy

Powder metallurgy is a process that uses metal powders (or mixtures of metal and non-metal powders) of micron to nanometer scale as raw materials, and combines these particles tightly through molding and vacuum sintering to prepare various engineering materials and parts with complex shapes. In the powder metallurgy process, the vacuum heat treatment process plays a vital role. By precisely controlling the heating and cooling process, it can adjust the crystal structure of powder metallurgy products and increase the distance between molecules, thereby improving the defects of the previous process without changing the shape of the original material as much as possible, and improving its key properties such as hardness, toughness, strength and wear resistance, so that powder metallurgy products are widely used in aerospace, automobile manufacturing, medical equipment and electronics industries, providing unlimited possibilities for the development of innovative and high-performance materials.
Basic process of vacuum heat treatment
Vacuum heat treatment is to change the basic organizational structure of the surface or internal metal material by heating first and then cooling, so that the metal material can meet certain performance and structural requirements. Vacuum heat treatment methods vary depending on the type of material and the final performance required, but generally include three basic stages of heating, heat preservation and cooling. Each stage is connected to form a complete vacuum heat treatment process.
1. Vacuum annealing
Vacuum annealing is a metal heat treatment process that slowly heats the metal to a certain temperature (the temperature is set according to different annealing requirements or processes), keeps it for a sufficient time, and then slowly cools it. Vacuum annealing can improve or eliminate various structural defects and residual stresses caused by powder metallurgy pressing and sintering, prevent deformation and cracking of the workpiece, and soften the workpiece to facilitate processing. Its purpose is usually to prepare the organization for the final heat treatment (vacuum quenching, vacuum tempering).
2. Vacuum quenching
Vacuum quenching is a heat treatment process method that heats the material to above the critical temperature, keeps it warm for a certain period of time, and then cools it at a cooling rate greater than the critical cooling rate to obtain an unbalanced organization dominated by martensite. Hardenability and hardenability are important evaluation indicators of the vacuum quenching process, which respectively indicate the ability of the material to obtain the hardness and depth of the hardened layer when quenched under certain conditions. In order to improve these two indicators, alloy elements such as nickel, molybdenum, manganese, chromium, and vanadium can usually be added for chemical heat treatment.
3. Vacuum tempering
After vacuum quenching, the hardness of the material is significantly improved, but the material will also have great internal stress and brittleness. Usually, vacuum tempering process needs to be carried out in time to avoid deformation and cracking of the material. The specific operation is to heat the metal material or parts after vacuum quenching to a certain temperature (usually less than 650℃), keep it warm for a long time, and then cool it in a certain way. During the tempering heating process, since the atoms in the powder metallurgy product can diffuse and rearrange quickly, the unstable unbalanced structure is gradually transformed into a stable balanced structure, which ultimately improves the shaping of the material.