Usually, the heat treatment of large forgings is combined with the cooling of forgings, due to the large section size and complex production process of large forgings. During the heat treatment process, some forgings are prone to white spot defects due to the uneven structure and properties of the forgings. Therefore, in addition to eliminating stress and reducing hardness, the main purpose of heat treatment of large forgings is to prevent white spots from appearing in forgings, improve the uniformity of chemical composition of forgings, and adjust and refine the structure of forgings.
White spots in large forgings White spots are very fine brittle cracks inside the forgings, round or oval silver-white spots with diameters ranging from a few millimeters to tens of millimeters. According to the microstructure observation, no trace of plastic deformation was found in the vicinity of the white point, so the white point belongs to brittle fracture.
The presence of white spots in forgings not only leads to a sharp drop in mechanical properties, but also causes high stress concentration caused by white spots, which may cause parts to crack during heat treatment and quenching, or parts to break suddenly during use, resulting in machine damage.
Therefore, white spots are a defect of forgings, and the technical conditions of large forgings clearly stipulate that once white spots are found, they must be scrapped. There are many theories about the formation of white spots, and the current consensus is that the white spots are the result of the combined action of hydrogen in the steel and internal stress (mainly organizational stress). Without a certain amount of hydrogen and large internal stress, white spots cannot form.
In the cooling process after forging, as the temperature decreases due to austenite transformation, in addition to the internal stress (mainly structural stress) generated inside the forging, the solubility of hydrogen in the steel decreases at the same time. The stress causes dislocations to gather at the subgrain boundaries and form submicroscopic cracks. When hydrogen atoms are desolubilized and precipitated from the solid solution into the submicroscopic cracks, the hydrogen atoms combine into hydrogen molecules in the cracks and generate great pressure. Therefore, in the local brittle place with high hydrogen content in the steel, under the action of structural stress and hydrogen precipitation stress, the submicroscopic cracks continue to expand and rupture, resulting in extremely fine internal cracks, thus forming white spots.