Manufacturer of CNC milling machine manufacturers in Xiamen, China
PART 1 Reasons for the formation of banded tissue 。Manufacturer of CNC milling machine manufacturers in Xiamen, China
Banded structure refers to the phenomenon that pearlite and ferrite are arranged in bands in hypoeutectoid steel, and it is a defect structure formed during the smelting process of steel.
The molten steel is selectively crystallized during the ingot crystallization process, forming a dendrite structure with uneven distribution of chemical components. Coarse dendrites in the steel ingot are elongated along the deformation direction during rolling, and gradually coincide with the deformation direction, thus forming depleted and enriched zones of carbon and alloy elements, which are stacked alternately. Under slow cooling conditions, pro-eutectoid ferrite is precipitated in the depleted zone of carbon and alloy elements (the stability of supercooled austenite is low), and the excess carbon is discharged into the enriched zone on both sides, and finally A band dominated by ferrite is formed; while a band enriched with carbon and alloying elements (higher stability of supercooled austenite) then forms a band dominated by pearlite, and finally forms a band dominated by ferrite and pearlite Ribbons arranged alternately. The more severe the composition segregation, the more serious the banded structure will be.Banded structures are formed when phosphorus segregation exists in steel. When the steel is slowly cooled in the A3-A1 range, the A3 temperature in the high-phosphorus area is high, and ferrite is formed first, and carbon is concentrated to the low-phosphorus area, resulting in a low-phosphorus carbon-rich area, which undergoes eutectoid transformation during subsequent cooling, forming Pearlite, which makes the tissue layered. Manufacturer of CNC milling machine manufacturers in Xiamen, China
Manganese is also an element that promotes the formation of banded segregation. In hot-rolled steel, the manganese content is generally higher at the place where pearlite is formed, and the manganese content is lower at the place where ferrite is precipitated. After the steel is hot-rolled and cooled slowly, the pro-eutectoid ferrite will be preferentially precipitated along the low-manganese part of the deformed fiber distribution direction, and then carbon will advance to the high-manganese part to form pearlite. As a result, the interphase distribution of pearlite and ferrite is striped. Ribbon.
If there are non-metallic inclusions in the steel that are elongated in a strip-like distribution along the rolling direction, during the cooling process, these inclusions may become the core for the preferential precipitation of ferrite, forming a ferrite band, generally It is difficult to eliminate it by normalizing. This banded structure must be improved by high-temperature homogenization annealing and then normalizing.
If the distribution of alloying elements in austenite is not uniform, it will lead to different grain growth tendencies, and undissolved carbides will easily remain in the enriched zone of carbide-forming elements, which will reduce the diffusion rate of carbon atoms, thereby inhibiting the grain growth. grow up; in the depletion area, the grains are easy to grow up, so the mixed crystal structure is easy to appear. During quenching, the hardenability of the alloy element-depleted zone is low, and it is easy to form a non-martensitic structure. During carburizing and quenching, the coarse grains in the mixed crystal form coarse acicular martensite, which will increase the amount of retained austenite. Therefore, the banded structures all have lower mechanical properties after conventional heat treatment. In addition, due to component segregation, the expansion coefficient and the specific volume difference before and after the phase transformation increase, which increases the quenching deformation of the part. Manufacturer of CNC milling machine manufacturers in Xiamen, China
Due to the layered arrangement of the microstructure of the banded structure, its mechanical properties are directional, that is, the tensile strength along the longitudinal direction of the band is high, and the toughness is good, but the transverse performance is relatively poor, not only low in strength, but also low in toughness. Poor, and it will also make the chip performance of the steel worse, and at the same time increase the inhomogeneity of subsequent heat treatment deformation and hardness. If there is a banded structure before quenching, it is impossible to completely eliminate it during the quenching heating process. The remaining banded structure after quenching will cause greater structural stress in the workpiece and even lead to cracking.
Usually, the homogenization temperature of carbon in austenite is higher than 950°C, and the homogenization temperature of alloy elements is higher than 1100°C, and the homogenization time is limited by the bandwidth of the band structure, the concentration difference between bands and the degree of homogenization required . Therefore, it is quite difficult to homogenize the carbon (especially alloying elements) in the banded structure, and it is generally difficult to eliminate the banded structure by conventional heat treatment (such as annealing, normalizing, quenching, carburizing, etc.).
Summarize the reasons for the formation of the banded structure, the outside is due to rolling, and the inside is due to the segregation and inclusions of phosphorus, sulfur and other elements in the steel ingot. The severity of the banded structure can be assessed according to GB/T 13299 "Methods for Evaluation of Microstructure of Steel". Manufacturer of CNC milling machine manufacturers in Xiamen, ChinaPART 2 Method to eliminate band tissue
Conventional heat treatment (such as annealing, normalizing, quenching, carburizing, etc.) cannot eliminate the segregation of alloying elements in the banded structure. Although rapid cooling can suppress the uneven distribution of carbon, the banded structure does not appear or be reduced, but reheating is slow Ribbons form again when cold.
Therefore, the banded structure needs to be eliminated by electroslag remelting, increasing the crystallization speed, increasing the final rolling temperature, increasing the forging ratio, and performing high-temperature homogenization annealing.
PART 3 Conclusion
Banded structure is a phenomenon of uneven distribution of carbon and alloy elements in steel. Depending on the severity of the banded structure, it will have different degrees of influence on the mechanical properties of the workpiece. Manufacturer of CNC milling machine manufacturers in Xiamen, China