Effect of post-forging cooling on forgings
The process of cooling a forging from the final forging temperature to room temperature is called post-forging cooling. Improper selection of the post-forging cooling method may result in cracks, deformation, and even abnormalities in the forging. Therefore, post-forging cooling is an important part of the forging process.
When the metal is heated to a high temperature and cooled by uneven plastic deformation, it will inevitably cause changes in structure and properties. These changes are mainly as follows.
(1) Internal stress generated by the forging The internal stress is generated by the cooling and heating of the forging. The internal stress generated by cooling is much more dangerous than the internal stress generated by heating. It is a superposition of cooling temperature stress, tissue stress and deformation residual stress.
1 Cooling temperature stress It is caused by the temperature difference between inside and outside when the forging is cooled. In the initial stage of forging cooling, the surface layer cools quickly, the volume shrinks greatly, the core cools slowly, the volume shrinkage is small, the shrinkage of the surface layer is hindered by the core, and the temperature stress is generated inside the forging. At this time, the surface layer is tensile stress and the core is compressive. As the forging continues to cool, this stress state will vary with the forging material; for materials with low deformation resistance and deformation, the surface temperature is very low, the volume shrinkage stops, and the volume of the core stops. The shrinkage is restricted by the surface layer, and the core becomes the tensile stress, and the surface layer becomes the compressive stress. For the material with large deformation resistance and difficult to deform, the tensile stress generated by the surface layer at the initial stage of cooling is not relaxed, and the stress is reduced to the late stage of cooling. The status has not changed.
2 Tissue stress For a metal with isomeric transformation, a phase change occurs during cooling, and the specific volume of the structure changes before and after the phase change, and the phase transition time in the forging table is different, thus causing the tissue stress. For example, when the metal has martensite transformation during cooling, the surface layer first undergoes martensite transformation, because the specific volume of martensite is larger than the specific volume of austenite (the specific volume of martensite is 0.127?0.131cm3/g, The specific volume of austenite is 0.120?0.125cm3/g), the tissue stress caused at this time, the surface layer is compressive stress, and the core is tensile stress. As the forging continues to cool, the core also undergoes martensite transformation, and the resulting tissue stress becomes a compressive stress in the core and a tensile stress in the surface.
3 Deformation residual stress It is caused by uneven deformation and work hardening of forgings during forging forming. If recrystallization softening is not obtained in time, it is eliminated, and after forging, it becomes residual stress and remains. The distribution of residual stress in the forging is different depending on the uneven deformation. It may be that the surface layer is tensile stress and the core is compressive stress, or vice versa.
The result of the above three stress superpositions forms the internal stress inside the forging. If the internal stress exceeds the tensile strength of the material, it will cause cracks in the forging; if the internal stress does not cause cracking, it will remain as the residual stress of the forging after the forging is cooled. Therefore, the forgings should be annealed as soon as possible after forging to remove stress and avoid cracks.
(2) Cooling deformation of forgings When the forgings are cooled, if the relative cooling speeds of the two sides are inconsistent (such as one side being blown by cold air or touching the wet ground), the forgings may be warped due to the difference in temperature and the order of the structural transformation.
(3) Abnormal structure of forgings Forgings forged with steel ingots, improper cooling will produce white spots inside the forgings. The generation of white spots is not only related to cooling, but also related to the chemical composition, structure, inclusions, segregation, stress and hydrogen content of steel, mainly due to the combination of hydrogen and tissue stress in steel. White spots are common defects in hot rolled billets and large forgings, and are more common in martensite and semi-martensitic steels.
If austenitic stainless steel (lCrl8Ni9, lCrl8Ni9Ti, etc.) is slowly cooled in the range of 800 to 550 °C, a large amount of chromium-containing carbides will precipitate along the grain boundary, which will cause chromium deficiency in the grain boundary and reduce the resistance of the steel to intergranular corrosion. .
According to the different cooling rates, there are three cooling methods for forgings: cooling in air, faster cooling; cooling in lime sand, slower cooling; cooling in the furnace, the cooling rate is the slowest.
(1) Cooling forgings in the air after forging one piece or piles directly on the floor of the workshop to cool, but not on wet ground or metal plates, and not in the place where there is wind, so as to avoid uneven cooling or local Quenching causes cracks.
(2) Cooling in dry ash and sand pit (box) Generally, the temperature of human steel should not be lower than 5001, and the thickness of surrounding ash and sand should be no less than 80 mm.
(3) Cooling in the furnace After the forging is forged, it is directly placed in the furnace for cooling. The temperature of the steel furnace should not be lower than 600?650X; the furnace temperature is equivalent to the temperature of the human forging. Since the cooling rate of forgings can be controlled by furnace temperature adjustment, it is suitable for post-forging cooling of high alloy steels, special alloy steel forgings and large forgings.
Forgings commonly used in forging workshops, according to the order of cooling speed, there are seven kinds of water cooling, air cooling, pile air cooling, pit (box) cold, ash (sand) cold, furnace cooling and hydrogen expansion treatment.
1 Water cooling The forgings are cooled in cold water.
2 Dispersing air-cooled Forgings are placed on the working floor at regular intervals and cooled in air at room temperature. However, it should be noted that it should not be placed on a damp ground, or placed in a place where there is a wind in the house, in order to prevent the forging from being too cold to cause deformation and other defects.
3 piles of air-cooled forgings are piled up on the ground with iron plates or in an empty iron box.
4 pit (box) cold forgings are placed in a pit or incubator for cooling.
5 Ash (sand) cold Forgings are cooled in lime, slag or sand. The lime, furnace or sand used must be dry. Generally, the temperature of the steel forgings should not be lower than 500?700 °C, and the thickness of the surrounding cover ash should not be less than 80 mm. The ashing temperature should not be higher than 150 °C.
6 furnace cold forgings are directly forged after the forging, and slowly cooled in the furnace with the furnace. The temperature of the steel forgings is generally not lower than 500?700 °C. The furnace should be raised to the same temperature as the final forging of the forgings. After all the forgings are installed in the furnace, the cooling rate is controlled according to specific requirements. The forging temperature of the forgings is generally not higher than 100?150 °C.
7 Hydrogen-expanding treatment For large-scale steel forgings sensitive to white spots, hydrogen expansion treatment should be carried out directly after forging. The specific requirements are shown in the heat treatment section.
The choice of forging cooling method depends on the forging material, size, throughput and specific conditions of the shop.
Cooling specification for forgings
The key to developing a forging cooling specification is the cooling rate. The appropriate cooling rate should be determined based on factors such as the chemical composition of the forging material, the structural characteristics, the section size of the forging, and the forging deformation. In general, forgings with a low degree of alloying, a small cross-sectional size, and a relatively simple shape allow for a fast cooling rate, which can be cooled in air after forging; otherwise, it must be slowly cooled (ash cold or furnace cold) or Stage cooling.
For steels with high carbon content (such as carbon tool steel, alloy tool steel and bearing steel), in order to avoid the precipitation of reticulated carbides along the grain boundary during the initial cooling stage after forging, air cooling or blasting and spraying should be used first. Cool to 7001, then place the forgings in ash, sand or slowly in the furnace.
For steels without phase change (such as austenitic steel, ferritic steel, etc.), it should be rapidly cooled in the temperature range of 800 ° 550 ° C to avoid the precipitation of network carbides. Ferritic steels have temper brittleness at 475 ° C and require rapid cooling. Usually both types of steel use air cooling.
For steels that are prone to martensite transformation in air cooling (such as high speed steel W18Cr4V, W9Cr4V; stainless steel lCrl3, 2Crl3, 4Crl3, Crl8, Crl7Ni2, 13Crl2WMoVA; high alloy tool steel 3Cr2W8V, CrMn, Crl2, etc.), in order to avoid cracks After forging, it must be slowly cooled.
For white point sensitive steel (such as chrome-nickel steel 34CrNiMo? 34CrNi4Mo, etc.), in order to prevent white spots in the cooling process, the furnace should be cooled according to certain cooling specifications?
For high-temperature alloys, recrystallization is only possible at the same time as the deformation at the higher temperature and appropriate degree of deformation due to the slow recrystallization rate; therefore, the residual heat after forging is often used to slowly cool. For some small and medium forgings. The stacking air cooling method is often used; the nickel-based superalloy has a higher recrystallization temperature and a slower recrystallization rate. In order to obtain a forged piece with a completely recrystallized structure, the forged forging piece can be released in time higher than the alloy recrystallization temperature [T +(50?100) °C] in the furnace for 5 ~ 7min, then take out the air cooling.
For aluminum alloys, it has good thermal conductivity, and is usually cooled in air after forging, sometimes directly with water.