The shape of the metal itself has a significant impact on its malleability. A metal with better plasticity is less likely to crack when forged.
The plasticity of a metal is closely related to its organization, with finer grains and a more uniform structure leading to better plasticity. Therefore, refining the grain and uniforming the organization can improve the metal’s malleability.
A metal’s ability to change shape without cracking during pressure processing includes hammer forging, rolling, stretching, extrusion, and other processes in both hot and cold states.
The malleability of a metal is mainly influenced by its chemical composition.
Ⅰ The essence of metal
1.1 Effects of chemical composition
Metals with varying chemical compositions exhibit different degrees of malleability.
As a general rule, pure metals possess superior malleability compared to alloys. The malleability of carbon steel improves with a decrease in the mass fraction of carbon.
However, when steel contains an increased amount of carbide-forming elements such as chromium, tungsten, molybdenum, vanadium, etc., its malleability is noticeably diminished.
1.2 Impact of metal organizations
The malleability of metals varies greatly depending on their tissue structure.
An alloy is malleable when it is a single-phase solid-soluble structure, such as Austenitic.
However, metals with a metallic compound structure, such as carburizing bodies, have poor malleability.
Cast columnar structure and coarse grains are not as malleable as uniform and fine structures after undergoing pressure processing.
Ⅱ Processing conditions
2.1 Deformation temperature
Raising the temperature at which a metal deforms can effectively enhance its malleability.
As the metal heats up, its atoms become more mobile, and the attraction between them weakens, making them more prone to sliding. This leads to improved plasticity, reduced deformation resistance, and significantly enhanced malleability. Consequently, forging is generally carried out at high temperatures.
2.2 Deformation speed
The deformation speed refers to the degree of deformation per unit time. The effect of deformation speed on the malleability of metal is illustrated in Figure 1.
As shown in the figure, the effect of deformation speed on malleability is contradictory. On one hand, as the deformation speed increases, the recovery and recrystallization processes cannot keep up with the processing hardening phenomenon, leading to a decrease in the plasticity of the metal, an increase in deformation resistance, and a deterioration in malleability (point a on the left-hand side of the figure).
On the other hand, during the deformation process, some of the energy consumed in plastic deformation is converted into heat, which heats up the metal and results in an improvement in the plasticity of the metal, a decrease in deformation resistance, and an enhancement of malleability (point A on the right-hand side of the figure).
The thermal effect becomes more pronounced with an increasing rate of deformation.
Figure 1 Effect of deformation speed on plasticity and deformation resistance
2.3 Mode of deformation (stress state)
The stress state of a deformed metal varies depending on the method of deformation used.
For instance, in the case of extrusion deformation, the metal is subjected to three-way compression.
During pulling, the metal experiences compression in two directions and tension in one direction.
In the case of upsetting, the central part of the blank is under three-way compressive stress, while the surrounding part experiences compressive stress in the up-down and radial directions and tensile stress in the tangential direction, as illustrated in Figure 2.
Figure 2 Stress states for several forging methods
