There are many factors that affect the properties of the sintered body, mainly the properties of the powder, the forming conditions and the sintering conditions. The factors of sintering conditions include heating speed, sintering temperature and time, cooling rate, sintering atmosphere and sintering pressure.
(1) the sintering temperature and time sintering temperature and the length of time affect the porosity, density, strength and hardness of the sinter. If sintering temperature is too high and time is too long, the product performance will be reduced, and even the product over burning defects will occur; if the sintering temperature is too low or the time is too short, the product will be reduced due to under firing.
(2) the sintering atmosphere commonly used in powder metallurgy is reduction atmosphere, vacuum, hydrogen atmosphere, etc. The sintering atmosphere also directly affects the properties of the sinter. Sintering in reducing atmosphere can prevent burning of blank pressing and reduce oxide on surface. For example, the iron-based and copper based products often use producer gas or ammonia decomposition, and the cemented carbide and stainless steel often use pure hydrogen. Active metal or refractory metal (such as beryllium, titanium, zirconium, tantalum), cemented carbide with tic and stainless steel can be sintered in vacuum. Vacuum sintering can avoid the adverse effects of harmful components (H2O, O2, H2) in the atmosphere, and also reduce sintering temperature (generally, it can reduce 100-150 ℃). 2.5 post treatment refers to further treatment after sintering of pressed billet, and it is determined whether post treatment is needed according to the specific requirements of the product. The common post-treatment methods include pressure recovery, impregnation, heat treatment, surface treatment and cutting.
1. pressure treatment, including finishing and shaping, is carried out to improve the physical and mechanical properties of sintering body. Finishing is a complex pressing to achieve the required size. The precision of the sinter is improved by pressing the fine finishing die. Shaping is a kind of complex pressure to achieve a specific surface shape. The deformation can be corrected and the surface roughness value is reduced by pressing the plastic die on the product. The compound pressure is suitable for products with high requirements and good plasticity, such as iron base and copper based products.
2. impregnation method of filling the pore of sinter with nonmetallic substances (such as oil, paraffin and resin, etc.). The common impregnation methods include oil immersion, plastic immersion, molten metal, etc. Oil immersion is to immerse lubricating oil in sintering body to improve its self-lubricating performance and rust prevention, and is commonly used in iron and copper based oil bearing. The impregnated plastic is a kind of Teflon dispersion solution, which is used to reduce friction of metal plastics after curing, and it can realize oil-free lubrication. The strength and wear resistance of molten metal can be improved by immersion. Copper or lead is often used in iron-based materials.
3. the heat treatment is used to heat the sinter to a certain temperature, and then the cooling method is controlled to improve the product performance. The common heat treatment methods are quenching, chemical heat treatment, thermal mechanical treatment, etc. the process methods are similar to dense materials. For iron base parts which are not subjected to impact and wear-resistant, the whole quenching can be adopted. Because of the existence of pores, the internal stress can be reduced, and tempering is not allowed. But the iron base parts which are hard and tough can be quenched or carburized. Hot forging is a common method to obtain compact parts. The hot forging products have fine grains and high strength and toughness.
4. the common surface treatment methods for surface treatment include steam treatment, electroplating, zinc immersion, etc. Steam treatment is a surface process for heating and maintaining the workpiece in the hot steam of 500-560 ℃ for a certain time, forming a layer of dense oxide film on its surface and pore, which is used for iron base parts requiring rust prevention, wear resistance or high pressure penetration. The electroplating process is the same as that of compact materials. Electroplating is used for parts requiring rust prevention, wear resistance and decoration. In addition, the shape of sintered body can be further changed or the precision can be improved by forging, welding, cutting and special processing to meet the final requirements of parts. The special processing methods such as EDM, electron beam processing and laser processing, as well as surface engineering technologies such as ion nitriding, ion implantation, gas deposition and thermal spraying, have been used in the post-treatment of PM products, which further improve the production efficiency and product quality.