The surface defects of materials have a great influence on the quality of cold extrusion parts. This is because of the difference between cold extrusion and cutting, when cold extrusion, the surface defects of materials will remain or expand to the surface defects of cold extrusion.

In order to prevent the surface cracking of cold extrusion parts and improve the surface quality of extrusion parts, good material surface condition is very important. Generally, the surface of hot-rolled bar is not satisfied with the special requirements of cold extrusion process, and cold drawn high-quality structure and deep drawing steel are generally used in cold extrusion production. In order to improve the dimensional accuracy and reduce the surface roughness of the cold extrusion parts, it is more advantageous to select the surface polished materials. The surface defects of materials, such as impurities, ash inclusions, scratches, cracks, etc., are generally scattered on the surface of materials, even for qualified materials, defects are inevitable during processing. Therefore, it is necessary to strengthen the inspection and acceptance of materials.

In order to find out whether there are defects on the surface of the material and to determine the deformation function of the metal cold extrusion parts, cold upsetting test, cold twisting test or corrosion test are generally used for identification. When cold upsetting the sample, there are deep marks on the surface, and the materials with laminations, cracks and ash inclusions inside will be fully exposed.

The allowable limits of surface defects of materials are determined according to the variety of materials, the shape and use of parts and practical experience. Surface defects have the greatest influence on upsetting extrusion process. Generally speaking, when the surface defect depth of the material is greater than 0.1mm, it is easy to expand into cracks in cold extrusion. Therefore, when the surface defect depth is greater than 0.1mm, it should be eliminated.

Decarburization of the material surface also has a great influence on the quality and function of the cold extrusion parts. For the annealing treatment of steel before cold extrusion, the risk of surface decarburization may occur when the temperature is above 720 ��. The general requirement of the depth of the decarburization layer is that the area of the decarburization ring is less than 5% of the corresponding cross-sectional area.

The basic requirements of the cold extrusion process for the material surface are: the surface shall be clean and smooth, free from cracks, bubbles, scratches, oxide skin and other defects, and the fracture surface of the steel and the structure of the acid pickling test piece shall be free from stratification, shrinkage cavity, non-metallic inclusions and white spots.