Thread Content
Composition differences: Hypoeutectoid steel: Carbon content ranges from 0.0218% to 0.77%. The carbon content is below the eutectic level (0.77%), and in addition to carbon, it also contains iron along with small amounts of impurity elements such as manganese, silicon, sulfur, and phosphorus. For example, the common 20 steel (with a carbon content of about 0.2%) belongs to the hypoeutectoid steel category. Eutectoid steel: It contains exactly 0.77% carbon. In its equilibrium state, its structure consists entirely of pearlite, which is a layered structure formed by the alternating arrangement of ferrite plates and cementite plates. This structure gives austenitic steel relatively moderate strength and hardness. Hyper-eutectoid steel: It has a carbon content of more than 0.77%, with the carbon level sometimes reaching as high as 2.11%. Apart from carbon, iron, and impurity elements, as the carbon content increases, the amount of cementite also increases. For example, T10 steel (with a carbon content of about 1.0%) belongs to the hypereutectoid steel category. Microstructural characteristics of hypoeutectoid steels: The equilibrium microstructure at room temperature is ferrite and pearlite. As the carbon content increases, the proportion of pearlite gradually increases, while the proportion of ferrite gradually decreases. Ferrite is a interstitial solid solution formed by carbon dissolved in α-Fe; it has relatively low strength and hardness, but good plasticity and toughness ； Pearlite, on the other hand, has relatively high strength and hardness, as well as a certain degree of plasticity and toughness. Eutectoid steel: Its equilibrium structure is mainly pearlite. The interlayer spacing in such materials has an impact on their properties; the smaller the interlayer spacing, the higher the strength and hardness, while plasticity and toughness can also be maintained at a good level. For example, by refining the interlayer spacing of pearlite, it is possible to increase the strength of eutectoid steel while maintaining a certain level of toughness. Hyper-eutectoid steel: The equilibrium microstructure at room temperature is pearlite and secondary cementite. Secondary cementite is distributed in a network around the pearlite; as the carbon content increases, the amount of secondary cementite increases and the network becomes more complete. The presence of this type of reticulated cementite reduces the toughness of the steel, but it increases its hardness and wear resistance. Performance characteristics of hypoeutectoid steel: It has good forgeability, as ferrite possesses good plasticity; thus, the material can be easily deformed during forging, requiring less forging force. Moreover, the pattern of structural transformation upon heating makes it easy to control the forging temperature. Its cutting performance is also moderate; compared to low-carbon steel, it has a slightly higher hardness and less tool wear ； Compared to high-carbon steel, its hardness is not too high, so it does not generate excessive cutting heat during machining. Eutectoid steel: Its forgeability is slightly inferior to that of hypoeutectoid steel, as the pearlite structure is relatively hard and requires greater force during forging. In terms of cutting performance, its moderate hardness allows for a good balance between tool life and cutting quality during machining. Hyper-eutectoid steel: It has poor forgeability, as the presence of reticular carbides makes the material prone to cracking during forging. Before machining, spheroidizing annealing is usually required to improve machinability; otherwise, the high hardness and the presence of networked cementite make machining difficult. Hypoeutectoid steel: It possesses good comprehensive mechanical properties, with moderate strength and hardness, as well as good plasticity and toughness. Its yield strength is generally between 200 and 400 MPa, while its tensile strength ranges from 400 to 600 MPa. The elongation rate can be around 20% to 30%, and it also possesses good impact toughness; the toughness decreases relatively slowly in low-temperature environments. Eutectoid steel: It has higher strength and hardness than hypoeutectoid steel, but its plasticity and toughness are slightly lower. Its tensile strength can reach 600 – 800 MPa, the yield strength is around 400 – 600 MPa, and the elongation rate lies between 10% and 20%. Its properties mainly depend on the interlamellar spacing and morphology of pearlite. Hyper-eutectoid steel: It has high hardness and good wear resistance, but poor toughness. Due to the presence of reticular cementite, its toughness is low, making it prone to brittle fracture. However, its hardness can reach quite high levels; for example, the hardness of certain hypereutectoid steels can be as high as 60 – 65 HRC, which makes it suitable for applications that require high hardness and wear resistance. Process performance Application areas Hypoeutectoid steel: Widely used in industries such as machinery manufacturing, construction, and automotive. It is used to manufacture various shaft components, gears, beams and columns in building structures, as well as automotive chassis parts and frames, etc., primarily due to its excellent comprehensive mechanical properties and processability. Eutectoid steel: It is commonly used to manufacture tools that require certain levels of strength and hardness, as well as a certain degree of toughness, such as taps and dies. In machining, these tools need to have sufficient hardness to cut metal, as well as a certain degree of toughness to prevent breaking. Hyper-eutectoid steel: It is mainly used to manufacture tools and parts with high hardness and wear resistance, such as cutting tools, measuring instruments, and molds. For example, high-speed steel tools belong to hypereutectoid steels; their high hardness and wear resistance meet the requirements for cutting metals. How can sub-eutectoid steel, eutectoid steel, and hyper-eutectoid steel be distinguished through metallographic analysis? The microstructure of hypoeutectoid steel consists mainly of ferrite and pearlite. Under a microscope, ferrite appears as white masses or polygons; due to its very low carbon content, it has a high ability to reflect light, which is why it appears white. Pearlite, on the other hand, is a black layered structure formed by alternating layers of ferrite and cementite. As the carbon content increases from 0.0218% to 0.77%, the proportion of pearlite in the microstructure gradually increases, while the proportion of ferrite gradually decreases. For example, in hypoeutectoid steels with a low carbon content (such as 0.2% carbon), ferrite constitutes the majority of the area in the metallographic images, with only small amounts of pearlite present as black layered structures distributed among the ferrite ； In hypoeutectoid steels with a higher carbon content close to the eutectic composition, the proportion of pearlite increases significantly, while the ferrite region shrinks. The microstructure of eutectoid steel consists mainly of pearlite, which is a completely black, layered structure. The interlayer spacing of pearlite may vary under different heat treatment conditions. In a normal balanced state, the spacing between the layers is relatively uniform, and under a microscope they present a typical black-and-white layered pattern, resembling fine fingerprints. Since the carbon content in eutectoid steel is 0.77%, this ensures that its microstructure consists entirely of pearlite—a specific microstructural form. This is an important characteristic for distinguishing eutectoid steel from other types of steel. The microstructure of hypereutectoid steel is pearlite and secondary cementite. Secondary cementite is distributed in a white network around the pearlite. The carbon content in cementite is as high as 6.69%. It has great hardness and appears as white bright spots or a bright network under a metallographic microscope. As the carbon content increases, the reticular cementite becomes more pronounced and continuous. For example, in hypereutectoid steel with a carbon content of 1.2%, metallographic photographs clearly show white network-like secondary cementite surrounding the black pearlite regions; moreover, the higher the carbon content, the more complete this network structure is, which is a prominent characteristic of the metallographic structure of hypereutectoid steel