High carbon steel
High carbon steel
What is high carbon steel
Carbon steel with carbon content greater than 0.65%. It includes carbon tool steel and part of carbon structural steel. It is mainly used for making tools and springs with small size.
Characteristics of high carbon steel
The annealed structure of hypoeutectoid carbon steel with carbon content less than 0.77% is pearlite and a small amount of ferrite, the annealed structure of eutectoid carbon steel with carbon content of 0.77% is pearlite, and the annealed structure of hypereutectoid carbon steel with carbon content more than 0.77% is pearlite and a small amount of network cementite. When hypereutectoid carbon steel is forged, air-cooled or heap cooled after rolling, the net cementite precipitates along the austenite grain boundary, which reduces the plasticity and toughness of the steel, increases the brittleness and makes the tool easy to crack. Therefore, hypereutectoid carbon steel used for tool manufacturing should prevent and eliminate net cementite. For this reason, forging must be repeated forging at the temperature zone of the eutectoid carburizing, breaking the precipitated network cementite, then cooling rapidly, or stopping the forging temperature to control the critical temperature Arcm above the hypereutectoid carbon steel. After stopping forging, it can be quickly cooled by blowing and spraying, and it can also inhibit the precipitation of the network cementite. In the process of forging and cooling after rolling of high carbon steel, lamellar pearlite structure is formed, which has high hardness, poor machinability and easy to produce quenching deformation and cracking. Hypereutectoid carbon steel should be spheroidized and annealed to spheroidize the cementite in pearlite, and the structure of spheroidal Cementite on ferrite matrix can be obtained. The key of spheroidizing annealing is the selection of heating temperature and the control of cooling rate. Generally, the heating temperature of spheroidizing annealing is 20-30 ℃ above the critical temperature AC1, and the cooling rate is% 26le; 20 ℃ / h. Spheroidizing annealing methods include general spheroidizing annealing, isothermal spheroidizing annealing and periodic spheroidizing annealing. The quenching structure of eutectoid carbon steel and hypoeutectoid carbon steel is martensite and a small amount of retained austenite. The quenching structure of hypereutectoid carbon steel is incomplete quenching, which includes martensite, spheroidal cementite and retained austenite. The hardenability of carbon steel is poor, and non quenched structure will appear in the center of larger size workpiece.
The steel after spheroidizing annealing should have lower hardness and better plasticity. However, it is sometimes found that although the hardness is very low, the brittleness is very large, and the fracture is black once broken. Metallographic examination found that part of the cementite in the steel decomposes and graphite appears, which is called graphitization. The main reason is that the annealing temperature is too high, the holding time is too long, and the cooling is too slow. High silicon content, low manganese content and high residual aluminum content in steel also promote graphitization. Once graphite appears in steel, it can't be eliminated by heat treatment, so we should pay attention to prevention.
During the heating process, the surface carbon of steel is oxidized with the surrounding medium, which reduces the carbon content of the steel surface and causes the surface decarburization. The decarburization tendency of high carbon steel is larger. Due to the low carbon content of decarburized layer, the surface hardness after quenching is insufficient and the wear resistance is low. Therefore, decarburized cutting tools have lower cutting performance and shorter tool life. Surface decarburization also makes the steel easy to form surface microcracks during quenching and reduces the fatigue life of the workpiece, which is very harmful to the life of the spring. Therefore, during the heat treatment of high carbon steel parts, the methods of deoxidization in salt bath, atmosphere control and coating protection should be used to prevent decarburization.
Application of high carbon steel
High carbon steel has high strength, good elasticity, high hardness and good wear resistance, but its plasticity and toughness are low, and its hot workability and machinability are poor. High carbon steel is mainly used for making all kinds of woodworking tools, files, saw blades, taps, planers, small feed turning tools, drills and other metal cutting tools as well as measuring tools and simple molds such as calipers and calipers. High carbon steel can also be used to make various types of spring, steel wire and roller with small load.
What is high carbon steel
Carbon steel with carbon content greater than 0.65%. It includes carbon tool steel and part of carbon structural steel. It is mainly used for making tools and springs with small size.
Characteristics of high carbon steel
The annealed structure of hypoeutectoid carbon steel with carbon content less than 0.77% is pearlite and a small amount of ferrite, the annealed structure of eutectoid carbon steel with carbon content of 0.77% is pearlite, and the annealed structure of hypereutectoid carbon steel with carbon content more than 0.77% is pearlite and a small amount of network cementite. When hypereutectoid carbon steel is forged, air-cooled or heap cooled after rolling, the net cementite precipitates along the austenite grain boundary, which reduces the plasticity and toughness of the steel, increases the brittleness and makes the tool easy to crack. Therefore, hypereutectoid carbon steel used for tool manufacturing should prevent and eliminate net cementite. For this reason, forging must be repeated forging at the temperature zone of the eutectoid carburizing, breaking the precipitated network cementite, then cooling rapidly, or stopping the forging temperature to control the critical temperature Arcm above the hypereutectoid carbon steel. After stopping forging, it can be quickly cooled by blowing and spraying, and it can also inhibit the precipitation of the network cementite. In the process of forging and cooling after rolling of high carbon steel, lamellar pearlite structure is formed, which has high hardness, poor machinability and easy to produce quenching deformation and cracking. Hypereutectoid carbon steel should be spheroidized and annealed to spheroidize the cementite in pearlite, and the structure of spheroidal Cementite on ferrite matrix can be obtained. The key of spheroidizing annealing is the selection of heating temperature and the control of cooling rate. Generally, the heating temperature of spheroidizing annealing is 20-30 ℃ above the critical temperature AC1, and the cooling rate is% 26le; 20 ℃ / h. Spheroidizing annealing methods include general spheroidizing annealing, isothermal spheroidizing annealing and periodic spheroidizing annealing. The quenching structure of eutectoid carbon steel and hypoeutectoid carbon steel is martensite and a small amount of retained austenite. The quenching structure of hypereutectoid carbon steel is incomplete quenching, which includes martensite, spheroidal cementite and retained austenite. The hardenability of carbon steel is poor, and non quenched structure will appear in the center of larger size workpiece.
The steel after spheroidizing annealing should have lower hardness and better plasticity. However, it is sometimes found that although the hardness is very low, the brittleness is very large, and the fracture is black once broken. Metallographic examination found that part of the cementite in the steel decomposes and graphite appears, which is called graphitization. The main reason is that the annealing temperature is too high, the holding time is too long, and the cooling is too slow. High silicon content, low manganese content and high residual aluminum content in steel also promote graphitization. Once graphite appears in steel, it can't be eliminated by heat treatment, so we should pay attention to prevention.
During the heating process, the surface carbon of steel is oxidized with the surrounding medium, which reduces the carbon content of the steel surface and causes the surface decarburization. The decarburization tendency of high carbon steel is larger. Due to the low carbon content of decarburized layer, the surface hardness after quenching is insufficient and the wear resistance is low. Therefore, decarburized cutting tools have lower cutting performance and shorter tool life. Surface decarburization also makes the steel easy to form surface microcracks during quenching and reduces the fatigue life of the workpiece, which is very harmful to the life of the spring. Therefore, during the heat treatment of high carbon steel parts, the methods of deoxidization in salt bath, atmosphere control and coating protection should be used to prevent decarburization.
Application of high carbon steel
High carbon steel has high strength, good elasticity, high hardness and good wear resistance, but its plasticity and toughness are low, and its hot workability and machinability are poor. High carbon steel is mainly used for making all kinds of woodworking tools, files, saw blades, taps, planers, small feed turning tools, drills and other metal cutting tools as well as measuring tools and simple molds such as calipers and calipers. High carbon steel can also be used to make various types of spring, steel wire and roller with small load.
