Analysis of the causes and treatment methods of blackening of castings after shot blasting

　　After the castings are produced, in order to achieve a better use effect, the castings are generally shot blasted. Shot blasting is a cold treatment process, which is divided into shot blasting cleaning and shot blasting strengthening. As the name implies, shot blasting cleaning is to remove impurities such as surface scale and improve appearance quality. Shot blasting strengthening is to use high-speed moving projectiles (60-110m/s) to flow Continuous impact strengthens the surface of the workpiece, forcing the target surface and surface layer (0.10-0.85mm) to undergo the following changes during the cyclic deformation process:

　　1. Modification of microstructure;

　　2. The non-uniform plastic outer layer introduces residual compressive stress, and the inner surface produces residual tensile stress;

　　3. Changes in the outer surface roughness (Ra Rz). Effect: It can improve the fatigue fracture resistance of materials/parts, prevent fatigue failure, plastic deformation and brittle fracture, and improve fatigue life.

　　But sometimes we encounter the situation that the overall surface of the casting is blackened or there are obvious black marks and black spots on the part after the casting is shot blasted. It can be removed on the surface, but it is difficult to handle if it has been immersed in the casting matrix. Reasons for this are:

　　1. Defects that existed before the shot peening brought by the die-casting process:

　　(1) Die casting uses too much black oil;

　　(2) Punch oil splashes when opening the mold;

　　(3) Paint splashing during die casting;

　　2. The product is stored for a long time or the temperature is humid, and the surface is corroded, moldy or dusty.

　　3. The dust removal device of the shot blasting machine fails, and the steel shot contains a lot of dust.

　　4. The operator did not wear gloves as required, and directly touched the surface of the shot-peened casting with his hands, resulting in fingerprints.

　　5. After the shot peening is left for a long time, there will be dust on the surface or water and oil splashed on the surface, and the environment will be humid and oxidized.

　　When the above situation occurs, corresponding measures should be taken:

　　1. Defects do not cover the entire casting surface. Control and management are strengthened by the die casting process.

　　(1) It is black oil with black color;

　　(2) It is punch oil, the color is dark red;

　　(3) It is paint, light color on the surface of castings with different colors;

　　2. After shot blasting, the surface traces are shallow and cannot be removed because they have been immersed in the casting matrix. Strengthen the management of product placement, and it should not be too long. If it needs to be placed for a long time, it must be covered and protected, and placed in a suitable environment;

　　3. The surface of the entire casting is black and dark. Restore dust removal or replace new steel shot;

　　4. The operator must be required to operate according to the operating instructions. Gloves must be worn.

　　5. Shot blasting should be done as timely as possible for final inspection, packing and warehousing. If it needs to be stored for a period of time, strict protection must be carried out.