Defects and preventive measures for heat treatment and quenching of forgings
1) Oxidation and decarburization
When the steel of the forged round manufacturer is heated, the surface of the surface has a layer of crunchy iron oxide scale called oxidation; decarburization refers to the sign of low carbon content on the surface. Oxidation and decarburization will lower the hardness and euphemism of the upper layer of the forging and also affect the size of the forging. In order to prevent oxidation and decarburization, it is usually heated in a salt bath furnace. When higher requirements are required, a protective agent may be applied to the surface of the forging or heated in a protective air and a vacuum.
2) Overheating and overburning
When the forging is heated by quenching, the sign of significant coarsening of austenite grains is called overheating. If the heating temperature is too high, the sign of grain boundary oxidation and first melt on one side is called over-burning. Overheating of the forgings not only reduces the mechanical properties of the steel (especially toughness), but also causes quenching deformation and cracking. The overheated structure can be corrected by normalizing the fire, and the overheated forgings can only be scrapped. In order to prevent overheating and overheating of forgings, the heating temperature and the holding time must be strictly controlled.
3) Deformation and cracking
When the forging is quenched and cooled, the stress caused by the difference in temperature and the difference in structural transformation between the different parts is called the internal stress of quenching. When the quenching stress exceeds the falling strength of the steel, the forging will be deformed; when the quenching stress exceeds the tensile strength of the steel, the forging will generate cracks and become garbage. In order to prevent deformation and cracking of forgings, different quenching techniques (such as step quenching or isothermal quenching) and process planning measures (such as layout symmetry, symmetry of the bevel, and sharp corners) can be used, although the quenching stress is reduced. And after the quenching, timely tempering is carried out.
4) less than hardness
Because the heating temperature is too low, the heat preservation time is not enough, the cooling rate is not enough, or the surface decarburization becomes the hardness, it can be eliminated by the new quenching technique (but an annealing or normalizing treatment is performed before quenching), and Be aware of the following two differences.
(1) The difference between the hardenability and the effective hardening depth of the actual forging. The same steel and differential section forgings are quenched under the same austenitizing conditions, and the hardenability is similar, but the effective hardening depth is different due to the difference in the style, size and cooling medium of the forging. Hardenability is an inherent feature of steel itself. For a steel, it is deterministic and can be used for comparison between different steel grades. The effective hardening depth of the actual workpiece, in addition to the hardenability of the steel, is related to the external components such as the style and size of the forging and the cooling medium used.
(2) The hardenability and hardenability of steel are two different viewpoints. Hardenability refers to the very high hardness that steel can reach after quenching, and it depends on the carbon content of martensite. The hardenability of steel with good hardenability is not necessarily high. For example, low carbon alloy steel has a good hardenability, but its hardenability is not high; high carbon steel has high hardenability, but its hardenability is poor.