Machinability
The machinability of metallic materials is a highly complex issue, generally evaluated from aspects such as allowable cutting speed, cutting force, and surface roughness. Factors including the material’s chemical composition, hardness, toughness, thermal conductivity, microstructure, and degree of work hardening all have an impact.
The carbon content in steel has a certain influence on machinability. Low-carbon steels contain more ferrite, which has good plasticity and toughness, but generates higher cutting heat during machining, causing tool adhesion and making chips difficult to break. This leads to poor surface finish and thus poor machinability. High-carbon steels contain more cementite, have higher hardness, and cause severe tool wear, resulting in poor machinability as well. Medium-carbon steels, with an appropriate proportion of ferrite and cementite, have moderate hardness and plasticity, providing better machinability. Generally, steels with a hardness around 250 HB are considered to have good machinability.
The thermal conductivity of steel also plays a significant role in machinability. Steels with an austenitic structure have low thermal conductivity, so little cutting heat is absorbed by the workpiece. Instead, the heat concentrates near the cutting edge, causing the cutting tool to overheat and reducing its service life. Therefore, even though austenitic steels are not very hard, they exhibit poor machinability.
The grain size of steel does not significantly affect its hardness, but coarse-grained steels have lower toughness. Their chips break easily, which improves chip-breaking performance and thus enhances machinability.
The morphology of cementite in pearlite also affects machinability. Hypoeutectoid steels, with a structure of ferrite plus lamellar pearlite, have good machinability. Hypereutectoid steels with lamellar pearlite plus secondary cementite, however, have poor machinability. If the structure is composed of granular pearlite, the machinability can be improved.
Forgeability
Forgeability refers to the ability of a metal to deform plastically under pressure without cracking.
The forgeability of steel is primarily related to its carbon content. Low-carbon steels have good forgeability, but as the carbon content increases, forgeability gradually deteriorates.
Austenite has good plasticity and can easily undergo plastic deformation. When steel is heated to a high temperature to obtain a single-phase austenitic structure, it exhibits good forgeability. Therefore, the initial rolling or forging temperature of steel is generally set about 100–200 °C below the solidus line. The final forging temperature should not be too low; otherwise, the plasticity of the steel decreases and cracks may form. Generally, for hypoeutectoid steels, the final forging temperature should be controlled near and above the GS line; for hypereutectoid steels, near and above the PSK line.
White cast iron, whose structure consists mainly of hard and brittle cementite at both low and high temperatures, has extremely poor forgeability.
Castability
The castability of a metal includes its fluidity, shrinkage, and segregation tendency.
(1) Fluidity
Fluidity refers to the ability of liquid metal to completely fill the mold. It is affected by many factors, the most important being chemical composition and pouring temperature.
Among the alloying elements, carbon has the greatest influence on fluidity. As the carbon content increases, the solidification temperature range of steel widens, which tends to reduce fluidity. However, increasing carbon also lowers the liquidus temperature; therefore, at the same pouring temperature, steels with higher carbon content have a larger temperature difference between the steel temperature and the liquidus temperature—that is, greater superheat—which improves fluidity. Thus, the fluidity of molten steel increases with carbon content. The higher the pouring temperature, the better the fluidity. For a fixed pouring temperature, the greater the superheat, the better the fluidity.
Because cast iron has a lower liquidus temperature than steel, its fluidity is always better than that of steel. For hypoeutectic cast iron, as the carbon content increases, the solidification temperature range narrows, and fluidity improves. Eutectic cast iron has the lowest solidification temperature and solidifies isothermally, thus possessing the best fluidity. For hypereutectic cast iron, fluidity decreases as carbon content continues to increase.
(2) Shrinkage
During cooling from the pouring temperature to room temperature, cast iron experiences a decrease in both volume and linear dimensions, known as shrinkage. Shrinkage is an inherent physical property of casting alloys and the fundamental cause of many casting defects such as shrinkage cavities, porosity, residual stress, deformation, and cracking.
As the metal cools from pouring temperature to room temperature, it undergoes three interconnected stages of shrinkage:
• Liquid shrinkage: Shrinkage occurring from the pouring temperature down to the start of solidification (liquidus temperature).
• Solidification shrinkage: Shrinkage during the solidification process, from the start to the end of solidification (solidus temperature).
• Solid shrinkage: Shrinkage after solidification is complete, from the solidus temperature down to room temperature.
Liquid and solidification shrinkage cause volume reduction, expressed as volumetric shrinkage, and are the primary cause of shrinkage cavities and porosity. Solid shrinkage, although also a volume change, mainly affects the external dimensions of the casting and is expressed as linear shrinkage. It is the main cause of internal stress, deformation, and cracking.
The primary factors influencing the shrinkage of carbon steel are chemical composition and pouring temperature. For steel of constant composition, higher pouring temperatures lead to greater liquid shrinkage. At a constant pouring temperature, as carbon content increases, the temperature difference between the molten steel and the liquidus temperature increases, resulting in greater volumetric shrinkage. Likewise, as carbon content increases, the solidification temperature range widens, causing greater solidification shrinkage. As shown in tables, the volumetric shrinkage of steel increases continuously with carbon content. Conversely, the solid shrinkage of steel decreases as the carbon content increases, especially before the eutectoid transformation, where linear shrinkage decreases significantly.
| w(C)/% | 0.10 | 0.35 | 0.75 | 1.00 |
| Volume shrinkage of steel (%) (from 1600 ℃ to 20℃) | 10.7 | 11.8 | 12.9 | 14.0 |
(3) Dendritic Segregation
The greater the horizontal and vertical distance between the solidus and liquidus lines, the more severe the dendritic segregation. The closer the cast iron composition is to the eutectic point, the less segregation occurs; conversely, the further it is from the eutectic point, the more severe the dendritic segregation becomes.
Post time: Nov-13-2025