White point: Improper cooling after forging of a wheel forging may produce a white point. The white point is a very fine brittle crack formed inside the wheel forging by the hydrogen and internal stress in the steel. It is round on the longitudinal fracture of the steel Or oval silvery white spots. The white spots in alloy steels are bright, while carbon steels are darker. The size of the white point ranges from a few millimeters to tens of millimeters. Observed from the microstructure, no trace of plastic deformation was found in the area near the white point. Therefore, the white point is purely brittle. The white point not only causes a sharp decrease in mechanical properties, but also causes the part to crack during heat treatment and quenching, or the part undergoes delayed failure and suddenly breaks during use. Therefore, white spot defects are not allowed in wheel forgings. White spots mostly occur in alloy steels with pearlite structure and martensite structure. Carbon steel is lighter. White spot defects are rarely found in austenitic and ferritic steels and bainite alloy steels.
In order to prevent white point defects, ferrite zone isothermal treatment can be used during heat treatment to allow hydrogen to escape, because the solubility of hydrogen in α-Fe is less than the solubility in γ-Fe, and hydrogen diffuses in α-Fe at all temperatures. The coefficients are much larger than the diffusion coefficients in γ-Fe, so isothermal treatment in the ferrite region is conducive to the escape of hydrogen. Carbon steel can be isothermally treated at 620 to 660 ° C, and alloy steel with high white point sensitivity can be subjected to isothermal treatment at 280 to 330 ° C and then heated to 580 to 660 ° C. This can reduce the hydrogen. With the improvement of the current smelting technology and the application of the refining technology outside the furnace, the hydrogen content in the material is reduced, and the harm of hydrogen embrittlement is reduced.
Reticulated carbides: Hypereutectoid steels and alloy steels with higher carbon content have a higher final forging temperature and slow cooling after forging, especially in the slow cooling zone. Austenite will precipitate a large amount of secondary carbides. Carbon atoms have a large mobility and sufficient time to diffuse to the grain boundaries, and then form a network of carbides along the austenite grain boundaries. When the reticulated carbides are severe, it is difficult to eliminate them by ordinary heat treatment, which reduces the impact toughness of the material, and often causes cracks during heat treatment and quenching.
The above-mentioned various defects are related to the cooling rate. Therefore, one of the measures to prevent the defects in the cooling process of the wheel forging is to develop appropriate cooling specifications after forging.