Hot Working and Cold Working of Metals
Plastic deformation processing is a method of forming and modifying metals by applying pressure. However, when plastic deformation is performed at room temperature, work hardening occurs, which increases the resistance to deformation. Therefore, for metals with large sizes or low plasticity (such as W, Mo, Cr, Mg, Zn, etc.), plastic deformation at room temperature is very difficult. In production, plastic deformation is often carried out under heating conditions. From a metallurgical point of view, the boundary between cold working and hot working is determined by the recrystallization temperature of the metal. Plastic deformation carried out below the recrystallization temperature is called cold working, while plastic deformation carried out above it is called hot working. For example, the recrystallization temperature of lead is below 0℃, so plastic deformation of lead at room temperature already belongs to hot working. The recrystallization temperature of tungsten, however, is about 1200℃; therefore, even deformation at 1000℃ still counts as cold working.
During hot working, two opposite processes occur simultaneously inside the metal: work hardening and softening through recovery and recrystallization. However, recovery and recrystallization at this stage occur concurrently with deformation, so they are called dynamic recovery and dynamic recrystallization. When these processes occur after deformation stops—either during holding at high temperature or in subsequent cooling—they are called static recovery and static recrystallization, which are consistent with the previously discussed recovery and recrystallization processes.
Hot working of metals must be controlled within a specific temperature range. The upper limit of hot working temperature is generally 100–200℃ below the solidus line. If this temperature is exceeded, grain boundary oxidation occurs, leading to a loss of cohesion between grains and a deterioration of plasticity. The lower limit of hot working temperature should be somewhat above the recrystallization temperature. If the working temperature is too high above this point, coarse grains may form; if it is too low, deformation structures may be retained.
Temperature Ranges for Hot Working (Forging) of Common Metallic Materials
| Material | Initial Forging Temperature (℃) | Final Forging Temperature(℃) |
| Carbon structural steel and alloy structural steel | 1200 ~ 1280 | 750 ~ 800 |
| Carbon tool steel and alloy tool steel | 1150 ~ 1180 | 800 ~ 850 |
| High-speed steel | 1090 ~ 1150 | 930 ~ 950 |
| Martensitic stainless steel (1Cr13) | 1120 ~ 1180 | 870 ~ 925 |
| Austenitic stainless steel (1Cr18Ni9Ti) | 1175 ~ 1200 | 870 ~ 925 |
| Pure aluminum | 450 | 350 |
| Pure copper | 860 | 820 |
Effects of Hot Working on Metal Structure and Properties
1. Improvement of Ingot and Billet Structure
Hot working can significantly improve structural defects in steel, such as closing up pores and shrinkage cavities, thus increasing the density of the metal. Coarse columnar and dendritic grains in as-cast structures are broken up, refining the grains. Large primary or eutectic carbides in certain alloy steels are crushed and distributed more evenly, and large inclusions are also broken down and dispersed uniformly. Because the diffusion rate of atoms increases under the combined effects of temperature and pressure, chemical segregation can be partially eliminated, resulting in a more uniform chemical composition. All these changes lead to a marked improvement in material properties.
2. Formation of Fibrous Structure
During hot working, segregation zones, inclusions, secondary phases, and grain boundaries in cast metals gradually elongate in the direction of deformation. Brittle impurities and secondary phases such as silicates, oxides, and carbides are broken into chain-like forms, while plastic inclusions like MnS become banded, linear, or streak-like. On a macroscopic sample, these appear as fine lines along the deformation direction, known as flow lines. The structure delineated by these flow lines is called a fibrous structure.
The formation of a fibrous structure gives rise to anisotropy in mechanical properties—the mechanical strength, plasticity, and impact toughness along the flow direction are higher than those perpendicular to it. Therefore, when designing hot working processes, it is important to control the flow line distribution so that it aligns with the principal stress direction. For components subjected to relatively simple stresses—such as crankshafts, hooks, torsion shafts, gears, and blades—the flow lines should follow the external shape of the part and be closed within it, without breaking the surface, to improve mechanical performance.
3. Formation of Band Structure
In multiphase alloys, different phases may become alternately distributed in bands along the deformation direction during hot working. This is called banded structure, which is often seen in rolled metals, though the causes differ among materials.
One cause is segregation and inclusions in the ingot. During rolling, these segregated regions and inclusions are elongated in the deformation direction, forming banded distributions upon cooling. For example, in hypoeutectoid steels with high phosphorus content, the interdendritic regions in the cast state are rich in phosphorus and poor in carbon. Since phosphorus diffuses much more slowly than iron, this segregation is difficult to eliminate even after hot working. These regions become elongated during deformation, increasing the A₃ transformation temperature locally. When austenite cools to A₃, proeutectoid ferrite preferentially nucleates and grows in these phosphorus-rich regions, forming ferrite bands, while the carbon-rich areas on both sides transform into pearlite bands.
When many inclusions (such as MnS) are present and elongated during deformation, proeutectoid ferrite often precipitates on them during cooling, also forming a banded structure.
In high-carbon, high-alloy steels containing abundant eutectic carbides, the carbide particles may align in bands during hot working, forming a carbide band.
Banded structures cause directional mechanical properties, notably reducing transverse plasticity and toughness, and worsening machinability. For materials that can achieve a single-phase structure at high temperatures, banded structures can sometimes be eliminated through normalizing. However, when severe phosphorus segregation is the cause, a high-temperature diffusion anneal followed by normalizing is required for improvement.
4. Grain Size
Normal hot working generally refines the grains. However, grain refinement depends on the amount of deformation, the hot working temperature—especially the final forging temperature—and the post-forging cooling conditions. In general, increasing the deformation amount favors fine grain formation. When the initial ingot grains are coarse, sufficient deformation is required to achieve refinement. Care must be taken to avoid working within the critical deformation range, which would produce coarse grains instead. If deformation is non-uniform, the resulting grain sizes after hot working will also be uneven. When deformation is very large (greater than 90%) and the temperature is high, secondary recrystallization may occur, leading to abnormally coarse grains. If the final forging temperature is much higher than the recrystallization temperature and cooling is slow, coarse grains also form. Conversely, if the final forging temperature is too low, work hardening and residual stresses remain. Therefore, the hot working process must be carefully controlled to produce fine, uniform grains and achieve improved material properties.
Post time: Nov-07-2025