High carbon steel coil is widely used in many fields due to its high strength and hardness. The heat treatment process is the key means to regulate the performance of high carbon steel coil, among which quenching and tempering have a significant impact on its hardness. A deep understanding of these effects is of great significance for optimizing the performance of high carbon steel coil and meeting different engineering needs.
Quenching is a heat treatment process in which high carbon steel coil is heated to above the critical temperature, kept warm for an appropriate time, and then rapidly cooled. During the heating process, pearlite in high carbon steel is transformed into austenite. Because austenite has a face-centered cubic structure, the atoms are arranged more closely and can dissolve more carbon. When cooling rapidly, austenite does not have time to transform into equilibrium structures such as pearlite, but transforms into martensite. Martensite is a supersaturated solid solution. The supersaturated state of carbon in it causes the lattice to be distorted, resulting in a strong solid solution strengthening effect, which greatly improves the hardness of high carbon steel coil.
Quenching process parameters include heating temperature, holding time and cooling rate. If the heating temperature is too high, the austenite grains will grow, resulting in a coarse martensite structure after quenching. Although the hardness may increase slightly, the toughness will decrease significantly. At the same time, it may also cause greater quenching stress, increasing the tendency of workpiece deformation and cracking. If the heating temperature is too low, the austenitization is incomplete, and undissolved pearlite or ferrite will remain in the structure after quenching, resulting in insufficient hardness. If the holding time is too short, the carbide cannot be fully dissolved into the austenite, which will reduce the carbon content of the martensite and the hardness will also decrease. The influence of the cooling rate on the hardness is also critical. If the cooling rate is insufficient, the complete martensite structure cannot be formed, and some non-martensite structures such as pearlite or bainite will be generated, thereby reducing the hardness.
Tempering is a heat treatment process in which the quenched high carbon steel coil is heated to a temperature range below the critical temperature, kept warm for a certain period of time, and then cooled. The main function of tempering is to eliminate quenching stress, stabilize the structure and improve toughness. During the tempering process, the supersaturated carbon in the martensite will gradually precipitate in the form of carbides, and the degree of lattice distortion will gradually decrease. With the increase of tempering temperature and the extension of time, carbides will continue to aggregate and grow, and the hardness will gradually decrease. However, proper tempering can significantly improve the toughness and plasticity of the material while reducing the hardness, so that the comprehensive performance of the material can be optimized.
Tempering temperature is a key factor affecting the hardness of high carbon steel coil. During low-temperature tempering (150-250℃), the carbon in the martensite precipitates in the form of fine ε-carbides, the hardness decreases less, and it can maintain high hardness and strength. It is mainly used for parts requiring high hardness and high wear resistance. During medium-temperature tempering (350-500℃), carbides gradually transform into cementite, and the hardness is further reduced, but it has a higher elastic limit and yield strength, and is often used to manufacture parts such as springs. During high-temperature tempering (500-650℃), cementite aggregates and grows, the hardness is significantly reduced, and the material has good comprehensive mechanical properties, which is suitable for structural parts that bear large loads. Tempering time will also have a certain effect on hardness. Within a certain range, as the tempering time increases, the hardness will gradually decrease and tend to stabilize.
Quenching and tempering are complementary heat treatment processes. The martensite structure obtained by quenching provides the basis for high hardness, while tempering improves the toughness and other properties of the material while adjusting the hardness. Reasonable matching of quenching and tempering process parameters can make high carbon steel coil obtain ideal hardness and comprehensive performance. For example, for tools requiring high hardness and high wear resistance, high temperature quenching and low temperature tempering processes can be used; while for mechanical parts requiring high strength and toughness, appropriate temperature quenching and medium or high temperature tempering processes can be used.
Quenching and tempering in the heat treatment process have a complex and important effect on the hardness of high carbon steel coil. Quenching greatly increases the hardness by forming martensite, while tempering adjusts and optimizes the hardness according to different process parameters. In actual production, it is necessary to accurately control the quenching and tempering process parameters according to the specific use and performance requirements of high carbon steel coil, so as to obtain the hardness and comprehensive performance that meet the engineering needs and give full play to the material advantages of high carbon steel coil.
