Greenhouse Window Gear PM Sintered Part

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Heat Treatment Method

Powder metallurgy gear is an important part of transmission parts and the core component of power transmission. Therefore, powder metallurgy gears must have the characteristics of high hardness, high strength, and high density. How to improve the hardness and strength of powder metallurgy gears by heat treatment is a necessary link in the production and processing of powder metallurgy gears.

Powder metallurgy gears, like other metal materials, can improve their mechanical properties through heat treatment. The heat treatment methods used in powder metallurgy gears include annealing, normalizing, quenching, tempering, and carburizing, nitriding, and carbonitriding. These methods can undoubtedly greatly improve the mechanical properties of powder gears, but due to the uniqueness of powder metallurgy gears, when selecting heat treatment methods and process conditions, dense materials cannot be completely referred to, and reasonable adjustments must be made to suit the powder metallurgy gears. Otherwise, the effect of heat treatment will not be obtained, and even destructive results will be caused. The materials that use heat treatment to improve the performance of powder metallurgy gears are mainly iron-based alloys (sintered steel).

The following points should be paid attention to in heat treatment of powder metallurgy gear sintered steel:

1. The voids of sintered steel have the function of heat insulation. Therefore, compared with dense steel, sintered steel has low thermal conductivity, and it is difficult to dissipate heat, resulting in poor hardenability.

2. The influence of microstructure uniformity on austenitization, the microstructure uniformity of sintered steel is deteriorated due to the influence of factors such as uneven distribution of carbon. The temperature and time for homogenization of its austenite are much higher than those of dense steel, and under the same conditions, the time to achieve complete homogenization is 50% higher. If alloying elements are added to the sintered steel, the homogenization temperature will be higher and the time will be longer.

3. The effect of voids on carbon content. Due to the existence of pores in sintered steel, if it is treated in the same way as dense steel, oxidation and decarburization are prone to occur during the treatment process. Therefore, the heat treatment of sintered steel containing 6% pores should be carried out under a protective atmosphere or embedded in solid fillers (such as decomposed ammonia, natural gas conversion gas, charcoal, cast iron chips, etc.). In addition, due to the existence of pores and uneven density, it is easy to cause quenching cracks and deformation.

Several commonly used heat treatment methods for powder metallurgy gears:

1. Annealing and normalizing, annealing and normalizing are the preparatory heat treatment processes applied in the production of sintered steel. The purpose of annealing and normalizing is to eliminate internal stress, adjust the structure of the material, thereby adjusting the mechanical properties and process properties of the steel, and prepare the structure and properties for the next process, such as repressing, shaping, cutting, etc. Annealed. For mechanical parts with less demanding use, annealed and normalized products can also be used as finished products.

2. Quenching, the heat treatment process of heating the sintered steel to a temperature above the critical point, cooling to the martensitic structure at a cooling rate greater than the critical point after heat preservation is called quenching. Quenching is the most widely used heat treatment method for sintered steel. The martensitic structure obtained by quenching can improve the strength, hardness and wear resistance of sintered steel. The quenching principle and process of sintered steel are basically similar to those of dense steel. The difference is that the quenching process of sintered steel needs to be carried out in a neutral or carburizing atmosphere to prevent oxidation of the pore surface. Due to the pore characteristics of sintered steel, oil quenching is usually used, and the quenching process includes heating austenitization, quenching and tempering.

3. Tempering, tempering must be done after quenching. Tempering is a heat treatment process in which quenched steel is heated to a temperature above 780°C and then cooled to room temperature in an appropriate manner after heat preservation. There are two purposes of tempering, one is to eliminate internal stress and reduce the brittleness of the material. Tempering is divided into low temperature tempering, medium temperature tempering and high temperature tempering.

Several surface hardening treatments of powder metallurgy gears:

1. Carburizing the surface of powder metallurgy gear can further improve the hardness of its surface. Carburizing is to use carbon-containing gas, liquid or solid as a carburizing agent to diffuse carbon atoms to the surface of the part and react with iron at high temperature to form more cementite Fe3C. The higher the amount of carburizing, the more the amount of cementite formed, and the higher the depth and surface hardness of the carburized layer. The carburizing concern is the depth and hardness of the carburized layer. The depth of the carburized layer is generally 0.5-2.5mm. The main problem in carburizing powder metallurgy gear parts is the hardness of the surface carburized layer. Due to the presence of pores in iron-based powder parts, carbon atoms may diffuse to the inside of the part through the pores, and a clear carburized layer cannot be formed, and excessive carbon diffusion into the interior will increase the brittleness of the part, and cannot exert high surface hardness and Internal strong, high toughness characteristics. Therefore, parts with high porosity are not suitable for carburizing.

Carburizing is generally carried out at temperatures higher than 740°C. For iron-based parts with porosity less than 10%, the optimum carburizing temperature is 920-940°C. The lower the carburizing temperature, the less deflection of the part. Therefore, in the case of high precision requirements, low temperature carburizing at 860°C should be used. Quenching is generally performed after carburizing to obtain a higher hardness martensitic structure on the surface. There are two ways to quench carburizing and quenching. One is direct quenching, that is, direct oil quenching after cooling to 750-850°C. The structure obtained by this method is relatively coarse, because the austenite grains have been coarsened in carburizing, and the mechanical properties are reduced; the other method is to first The carburized gear is cooled, and then quenched by the quenching process of sintered steel. This method can overcome the determination of direct quenching and obtain powder metallurgy gears with better performance.

2. Gear surface nitriding, nitriding is the process in which nitrogen-containing gas contacts sintered steel, nitrogen atoms diffuse to the surface of sintered steel, and react with alloying elements chromium, aluminum, molybdenum, nickel, and tungsten in the steel to form nitrides. After nitriding, the surface hardness of the parts is further improved. Nitriding can be carried out alone or carbonitriding. The method of nitriding is to raise the gear to a temperature of 495-565°C, pass through ammonia gas, and the highly active nitrogen atoms decomposed from the ammonia gas will desalinate the surface of the parts. The biggest difficulty for powder metallurgy gear nitriding is porosity. Too many pores cannot form a nitrided layer, and the formation of nitrides inside the gear will make the part brittle.

3. Carbonitriding, that is, carbon and nitrogen are deeply penetrated into the surface of powder metallurgy gears at the same time, which further improves the hardness and wear resistance of the surface of the parts. The method of carbonitriding is to add ammonia during the carburizing process, so that nitrogen is also infiltrated when the carbon is deeper. The temperature of carbonitriding is lower than that of individual carburizing (about 55°C lower), and the time is shorter. Part density for carbonitriding should be kept at 6.85g/cm³, which is very effective for brazing and high density (7.2g/cm³).

4. High-frequency quenching, powder metallurgy gear high-frequency quenching is a method of quenching the surface of the workpiece, that is, the workpiece is placed in the coil, and the high-frequency current is passed through. Under the action of the alternating magnetic field generated by the high-frequency current, the surface of the workpiece will Generate induced electromotive force and eddy current. Due to the skin effect, the induced eddy current is mainly concentrated on the surface of the workpiece, which generates high temperature on the surface. High-frequency quenching is to use this heating principle to rapidly heat the surface of the workpiece to a high temperature, and then quench it to obtain a surface quenched structure. A considerable part of the powder metallurgy gears that require wear resistance adopt the heat treatment method of high frequency quenching. When the powder metallurgy gear adopts the heat treatment method of high-frequency quenching, it is necessary to pay attention to the density of the gear itself. The density must reach 6.85g/cm³, so that the strength of the gear itself can be achieved, and stress will be generated between local heating and unheating, so as not to damage the parts cracking. ​​

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