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Problems that need attention in steel heating process
Posted: 02/20/2019 14:05:12 Hits: 47
Overheating
If steel materials stay for too long in the high-temperature furnace, their grain will gradually grow and become coarse crystal. This phenomenon is called overheating. Overheating is related to heating temperature and heating time. The higher the heating temperature, the longer the time, the coarser the crystal grains, and the lower the mechanical properties of the steel and the more brittle. Generally, the effect of overheating on steel is not great, but when the overheating is serious, cracks will occur at the corners of the forging. After some alloy steels are severely overheated, it is quite difficult to improve the superheated structure by heat treatment.
Overburning
A deep crack will be formed on the outer surface of the over-burnt forging, the forging may even break into several pieces, whose section is light gray-blue. For these pieces, when forging, the first upsetting will be broken, there will be coarse grain cracks, sparks splashing; or when being lengthened, a rough curved lateral crack will be formed. When the surface is too shallow, the crack is not visible. Only after pickling, it can be found that cracking occurs at the edge or the flat flange. Under the microscope, the burnt steel does not have coarse grains, and the grain boundaries are cracked, oxidation occurs between the crystal grains, and partial melting may occur at grain boundaries.
The heating temperature for low carbon should not exceed 1300-1350 °C; that for high carbon steel (carbon mass fraction 1.0% -1.2%) should not exceed 1100-1150 °C; alloy steel is more likely to overfire, so heating temperature should be lower. The more oxygen in the furnace gas, the longer the heating time, the easier it is to burn. Over-fired steel is irreparable and can only be re-refined and must be taken care of when heating.
Uneven temperature distribution
On the whole piece of steel, the phenomenons of uneven temperature are as follows:
1. In the same section, the center temperature is lower than the external temperature.
2. In the same section, the external temperature is lower than the center temperature.
3. The temperature of each surface is inconsistent.
4. The temperature at both ends of the same steel material is higher than the middle.
If the billet is not heated uniformly, cracks are prone to occur during forging. In particular, if the alloy steel forgings are not uniformly heated, a center crack will occur when forging. In order to prevent uneven heating, the processes that the forgings need are preheating, heat preservation, and turning over properly.
Cracking
Because the heating speed is too fast, the metal surface is heated and expands rapidly, and the heat is not transmitted to the inside. The internal metal expands relatively little, so it is subjected to the tensile stress of the outer metal. Additionally, considering about the structural transformation or the residual internal force inside the metal during heating, cracks will be easily formed in billets and ingots of high-alloy tool steels with large brittleness, and cold steel ingots with large cross-sections. If such a crack occurs during heating, there is basically no remedy for the forging. Therefore, in order to produce good steel products, steel billets and ingots of large-section cold steel ingots and high-alloy tool steels must be heated according to certain heating specifications.
(Click here to read about oxidation and decarburization.)
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