Defects in Metal Material Forming Processing, Part 1

Defects in metal forming processes can arise due to various factors, including material properties, process parameters, and environmental conditions. Here are some common defects associated with metal forming:

Overheating and overburning

In the process of heating or processing metal, due to high temperature and long time, the phenomenon of coarse organization and grain is called overheating; when the metal is heated near the melting temperature or severely overheated, the local low melting point component between grains melts or the grain boundary weakens, which is called overburning.

After overheating, rough pitting, orange peel, coarse grains and other phenomena appear on the surface. Although the strength of the overheated alloy does not decrease much, the impact toughness and plasticity at room temperature are greatly reduced.

The material becomes brittle, and coarse crystalline particles appear on the fracture. Under high magnification, in addition to coarse equiaxed crystals, there may be coarse second phases or coarse Widmanstatten structure. Overheating is not absolutely waste, and sometimes it can be corrected or downgraded by re-deformation, reprocessing and other methods.

After overburning, the surface is rough, the grain boundaries become coarse, straight, hairy, and even cracked, and there will be easy-to-melt and oxidized film layers; overburning greatly reduces the metal bonding strength; the microstructure shows grain boundary coarsening, melting voids or eutectic balls, melted liquid phase network, and even irregular voids at the junction of several grains; grain boundary cracks, plate side cracks, bar head blooming, mouth cracks or cracks into pieces appear during rolling or after extrusion, and coarse dendrites and traces of melting can be seen at the cracked parts.

The main reasons for overheating and overburning are: high heating temperature, long time or long time in the high temperature source; the end temperature of hot working is too high or the stay time in the high temperature zone is too long; there are low melting point components in the alloy, or there are many low melting point inclusions.