Choosing the proper cooling rate is the key to formulate the cooling standard for forgings. Generally, the appropriate cooling rate is determined according to the chemical composition, structural characteristics, original state, section size and other factors of the blank.
Generally, the cooling rate of forgings forged with steel is greater than that of forgings forged with steel ingots. The cooling rate of small section forgings is higher than that of large section forgings.
Cooling specification for medium and small carbon steel and low alloy steel forgings: generally speaking, the simpler the chemical composition of the billet, the faster the cooling rate after forging; on the contrary, the slower the cooling rate. Therefore, for medium and small carbon steel and low alloy steel forgings, air cooling is used after forging.
Cooling specification of alloy steel forgings: for alloy steel forgings with complex alloy components, pit cooling or furnace cooling shall be adopted after forging. Cooling specification of tool steel and bearing steel: For steel with high carbon content (such as carbon family engineer steel, alloy tool steel and bearing steel), in order to prevent network carbide from settling in the crystal boundary, after forging, air cooling, air blast or spray cooling shall be carried out rapidly to 700 ℃, and then the forgings shall be put into the pit or furnace for slow cooling.
Cooling specification of non-phase change steel: for phase change steel (such as austenitic steel, ferritic steel, etc.), rapid cooling can be used because there is no phase change during the cooling process of forgings. At the same time, in order to obtain the single-phase structure after forging, and to prevent the brittleness of ferrite at about 475 ℃, rapid cooling is also required, so air cooling is usually used after forging.
Cooling specification of air-cooled self-quenched steel: for this kind of steel, cooling shall be carried out according to certain cooling specifications to prevent white spots during cooling.
Forgings shall not only be cooled according to standards after final forging, but also be cooled during forging sometimes, which is called intermediate cooling. Intermediate cooling is used for forgings that have not been forged after heating (such as multi-fire forging large swing crankshaft). The forgings need to be locally heated, and blank flaw detection or defect cleaning shall be carried out during the forging process. The determination of the intermediate cooling standard of forgings is the same as the final cooling standard of waste.
After the forging is formed, the most appropriate cooling rate must be selected. The size of the forging and the chemical composition of the material need to be considered when determining the cooling rate.
The commonly used methods of cooling force, stack cooling, pit (box) cooling, ash and sand cooling, furnace cooling, etc. depend on the cooling speed of forgings.
Air cooling: fast cooling in air. After the forging, it shall be cooled directly on the workshop floor in single or in piles, but it shall not be placed on the wetland or metal plate, nor on the place with penetrating wind perception, to avoid cracks caused by uneven cooling of the forging or local cooling.
Stack cooling: the forgings are stacked in the bin and cooled in the air. Commonly used for small carbon steel and low alloy steel forgings.
Pit (box) cooling: after forging, it is closed and cooled in the pit or steel plate fire layer containing insulating asbestos.
Cooling in thermal insulation material: fine sand buried in the pit of forgings. Lime or asbestos material. Internal cooling of slag. The temperature of the forging personnel shall not be lower than 500 ℃, and the thickness of the surrounding sand shall not be less than 80mm. The cooling rate of forgings in the pit can be adjusted by different thermal insulation materials and insulating media. In order to effectively prevent the fine forged parts from being oxidized during the cooling process, they can be cooled in the atmospheric protection device.
Furnace cooling: the forging directly enters the furnace and slowly cools according to certain cooling specifications. Suitable for high alloy steel. Special steel forging and cooling after various large forgings. Generally, the cooling rate can be controlled by controlling the furnace temperature. In general, the furnace temperature of forgings shall not be lower than 600 ℃~650 ℃. The furnace temperature shall rise to the same temperature as that of forgings in advance. After the installation of all furnace cooled parts, the cooling rate shall be controlled. Generally speaking, the outlet temperature shall not be higher than 100 ℃~150 ℃. Common cooling specifications include isothermal cooling and fluctuating isothermal cooling.
Cooling control: for some forgings, such as T12.GCrls.9Crsi and other steel forgings, cooling control must be carried out. Use spray to cool to 600 ℃~650 ℃, and then put them into the pit for slow cooling to avoid coarse network carbide and crack.
Isothermal annealing: It is very important to cool the white spot-sensitive steel ingot after forging. After the forging is stopped, the hydrogen expansion isothermal annealing is carried out directly to prevent the occurrence of white spots.