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Medium carbon steel
Carbon steel with a carbon content of 0.25% to 0.65%. It includes most high-quality carbon structural steel and a portion of ordinary carbon structural steel. This type of steel is mostly used to make various mechanical parts, and some are used to make engineering structural components.
Medium carbon steel belongs to hypoeutectoid steel, and its annealed structure consists of pearlite and ferrite. As the carbon content in steel increases, the number of pearlite in the structure increases, while the number of ferrite decreases.
The quenched structure of steel with a carbon content greater than 0.40% is martensite; When the carbon content is greater than 0.40%, there is a small amount of residual austenite in addition to martensite, and the amount of residual austenite increases with the increase of carbon content in the steel.
The final heat treatment methods for medium carbon steel include quenching and tempering, low-temperature tempering after quenching, low-temperature tempering after high-frequency quenching, isothermal quenching, and medium temperature tempering after quenching.
(1) Tempering and tempering. The organization is tempered martensite. This type of organization has good comprehensive mechanical properties, high strength, good plasticity and toughness. Steel used for quenching and tempering should have good hardenability to ensure uniform microstructure and properties across the entire cross-section of the quenched and tempered part. Compared with alloy steel, carbon steel has poorer hardenability, so it is only suitable for quenching and tempering of medium carbon steel parts with small cross-sectional sizes.
(2) Low temperature tempering after quenching. The organization is tempered martensite, which has high strength and appropriate plasticity and toughness.
(3) High frequency quenching followed by low-temperature tempering. The microstructure of the high-frequency quenching layer is extremely fine hidden needle martensite. After low-temperature tempering, tempered martensite is obtained, which can achieve a similar effect to carburizing treatment. Before high-frequency quenching, it is generally subjected to tempering or normalizing. So after high-frequency quenching and tempering, the core of the part has higher strength, good plasticity and toughness, while its surface hardness is high and wear resistance is good. In addition, the surface of high-frequency quenched parts generates compressive stress, resulting in high fatigue limit and long service life.
(4) Isothermal quenching. The organization is composed of bainite, which has high strength, good plasticity, and toughness.
(5) Quenching followed by medium temperature tempering. The organization is tempered martensite.
Before the final heat treatment, some medium carbon steels undergo normalizing as a pre heat treatment, with the aim of refining grain size, reducing structural inhomogeneity in the steel, obtaining uniform pearlite and ferrite structures, and preparing for the final heat treatment.
After quenching and tempering treatment, medium carbon steel is mainly used to make various transmission shafts, connecting rods, clutches, shaft pins, bolts, etc. After high-frequency quenching and low-temperature tempering, medium carbon steel is used for gears, lathe spindles, spline shafts, camshafts, and half shafts that are subjected to impact loads and require wear resistance.
Medium carbon steel in normalized state or without heat treatment, used for making low load rods, sleeves, fasteners, washers, handles, etc. Medium carbon steel with high carbon content is used to make springs and steel wires.
Introduction to Heat Treatment Process of Medium Carbon Steel
Heat treatment of steel: It is a process of heating, insulating, and cooling solid steel in an appropriate manner to obtain the desired microstructure and properties. Heat treatment can not only be used to strengthen steel and improve the performance of mechanical parts, but also to improve the processability of steel.
Their commonality is that they only change the internal organizational structure without altering the surface shape and size.
The heat treatment process can significantly improve the mechanical properties of steel, increase the strength, toughness, and service life of parts, and improve hardness and wear resistance. So important machine parts and tools need to undergo heat treatment. Heat treatment can also improve the processing performance of workpieces, thereby increasing productivity and processing quality.
Therefore, heat treatment plays a very important role in the mechanical manufacturing industry. Taking 45 steel and 40Cr steel as examples.
In production, quenching followed by high-temperature tempering is called "tempering treatment". After tempering treatment, the parts have good comprehensive mechanical properties and are widely used in various important structural parts, especially those working under alternating loads such as connecting rods, bolts, gears, and shafts. But the surface hardness is low and not wear-resistant. Surface hardening and tempering can be used to improve the surface hardness of parts.
1. 45 Steel - High Quality Medium Carbon Structural Steel
45 steel is a term used in GB, S45C in JIS, 1045080M46 in ASTM, and DIN C45； 45 steel is a high-quality carbon structural steel with chemical composition: carbon (C) content of 0.42~0.50%, Si content of 0.17~0.37%, Mn content of 0.50~0.80%, Cr content<=0.25%. The cold and hot processing performance is good, the mechanical performance is good, and the price is low and the source is wide, so it is widely used. Its biggest weakness is low hardenability, and it is not suitable for workpieces with large cross-sectional dimensions and high requirements.
Recommended temperature for heat treatment of 45 steel: normalizing 850, quenching 840, tempering 600
① Before tempering, if the hardness of. 45 steel is greater than HRC55 (up to HRC62) after quenching, it is qualified. The highest hardness for practical application is HRC55 (high-frequency quenching HRC58).
② The heat treatment process of carburizing and quenching is not used for. 45 steel.
Quenching and tempering of 45 steel: The quenching temperature of 45 steel is A3+(30-50) ℃, and in practical operation, it is generally set at the upper limit. A higher quenching temperature can accelerate the heating speed of the workpiece, reduce surface oxidation, and improve work efficiency. To homogenize the austenite of the workpiece, sufficient insulation time is required. If the actual furnace load is large, it is necessary to extend the insulation time appropriately. Otherwise, there may be insufficient hardness due to uneven heating. However, if the insulation time is too long, it can also lead to coarse grains and severe oxidation decarburization, which affects the quality of quenching. We believe that if the furnace loading exceeds the requirements of the process documents, the heating and insulation time needs to be extended by 1/5.
Due to the low hardenability of 45 steel, a 10% saline solution with a high cooling rate should be used. After the workpiece is immersed in water, it should be quenched, but not cooled through. If the workpiece is cooled through in salt water, it may cause cracking. This is because when the workpiece cools to around 180 ℃, austenite quickly transforms into martensite, causing excessive structural stress. Therefore, when the quenched workpiece is rapidly cooled to this temperature range, a slow cooling method should be adopted.
Due to the difficulty in controlling the outlet temperature, it is necessary to operate based on experience. When the shaking of the workpiece in the water stops, the outlet can be air-cooled (preferably with oil cooling). In addition, when the workpiece enters the water, it should move rather than remain still, and should move according to the geometric shape of the workpiece in a regular manner. The combination of a stationary cooling medium and a stationary workpiece results in uneven hardness and stress, leading to significant deformation and even cracking of the workpiece.
The hardness of quenched and tempered parts made of 45 steel should reach HRC56~59 after quenching. The possibility of a larger cross-section is lower, but it cannot be lower than HRC48. Otherwise, it indicates that the workpiece has not been completely quenched, and there may be martensite or even ferrite structure in the structure. This structure remains in the matrix after tempering and cannot achieve the purpose of quenching and tempering. The high-temperature tempering of 45 steel after quenching is usually carried out at a heating temperature of 560~600 ℃, with a hardness requirement of HRC22~34. Because the purpose of quenching and tempering is to obtain comprehensive mechanical properties, the hardness range is relatively wide.
But if the drawing has hardness requirements, the tempering temperature should be adjusted according to the drawing requirements to ensure hardness. If some shaft components require high strength, the hardness requirements will be high; However, some gears and shaft parts with keyways require lower hardness due to the need for milling and insertion processing after quenching and tempering.
Regarding the tempering insulation time, it depends on the hardness requirements and the size of the workpiece. We believe that the hardness after tempering depends on the tempering temperature and is not closely related to the tempering time, but it must be fully penetrated. Generally, the tempering insulation time for workpieces is always more than one hour.
If 45 steel is carburized, hard and brittle martensite will appear in the core after quenching, losing the advantages of carburization treatment. The materials currently using carburizing technology have low carbon content, and the core strength can already reach a very high level at 0.30%, which is not commonly used in applications. 0.35% have never seen any examples, only introduced in textbooks. The process of quenching and high-frequency surface quenching can be used, which has slightly lower wear resistance than carburizing.
2. 40Cr Steel - Alloy Structural Steel
40Cr belongs to GB3077 "Alloy Structural Steel". The carbon content of 40Cr steel is 0.37% to 0.44%, slightly lower than that of 45 steel. The content of Si and Mn is comparable, with a Cr content of 0.80% to 1.10%. In the case of hot rolling supply, this 1% Cr is basically ineffective, and their mechanical properties are roughly equivalent. Due to the fact that the price of 40Cr is about half that of 45 steel, for economic reasons, those who can use 45 steel do not need 40Cr.
Quenching and tempering treatment of 40Cr steel: The main role of Cr in heat treatment is to improve the hardenability of the steel. Due to the improved hardenability, the mechanical properties such as strength, hardness, and impact toughness of 40Cr after quenching (or tempering) treatment are significantly higher than those of 45 steel. However, due to the strong hardenability, the internal stress of 40Cr during quenching is also greater than that of 45 steel. Under the same conditions, the cracking direction of 40Cr material workpieces is also greater than that of 45 steel material workpieces.
Therefore, in order to avoid cracking of the workpiece, oil with lower thermal conductivity is mostly used as the quenching medium for 40Cr quenching (sometimes double liquid quenching method is also used, commonly known as water quenching oil cooling), while water with higher thermal conductivity is used as the quenching medium for 45 steel. Of course, the choice of water and oil is not absolute and is closely related to the shape of the workpiece. Simple shaped 40Cr parts can also be quenched with water, while complex shaped 45 steel parts may require oil quenching or even salt bath.
The quenching and tempering of 40Cr workpieces are specified in various parameter process cards. Our experience in practical operation is as follows:
(1) After quenching, 40Cr workpieces should be cooled with oil. 40Cr steel has good hardenability and can be hardened by cooling in oil. Moreover, the deformation and cracking tendency of the workpiece are small. However, in situations where oil supply is tight, small businesses can quench workpieces with uncomplicated shapes in water without discovering any cracks, but operators need to strictly control the temperature of the water entering and exiting based on their experience.
(2) After quenching and tempering, the hardness of 40Cr workpiece is still relatively high. The second tempering temperature needs to be increased by 20-50 ℃, otherwise, it will be difficult to reduce the hardness.
(3) After high-temperature tempering of 40Cr workpieces, those with complex shapes are cooled in oil and those with simple shapes are cooled in water to avoid the influence of the second type of tempering brittleness. After tempering and rapid cooling, the workpiece should be subjected to stress relief treatment if necessary.
The highest hardness that can be achieved after heat treatment of medium carbon steel is about HRC55 (HB538), with a σ b of 600-1100MPa. So among various applications with moderate strength, medium carbon steel is the most widely used, not only as a building material, but also extensively used in the manufacture of various mechanical parts.
As long as the temperature and insulation time of medium carbon steel are sufficient, it is generally possible to reach this hardness value. It is impossible without deformation. One suggestion is to have machining allowance and then use a grinder for processing, and the other is surface quenching.
Carbon steel with a carbon content of 0.25% to 0.65%. It includes most high-quality carbon structural steel and a portion of ordinary carbon structural steel. This type of steel is mostly used to make various mechanical parts, and some are used to make engineering structural components.
Medium carbon steel belongs to hypoeutectoid steel, and its annealed structure consists of pearlite and ferrite. As the carbon content in steel increases, the number of pearlite in the structure increases, while the number of ferrite decreases.
The quenched structure of steel with a carbon content greater than 0.40% is martensite; When the carbon content is greater than 0.40%, there is a small amount of residual austenite in addition to martensite, and the amount of residual austenite increases with the increase of carbon content in the steel.
The final heat treatment methods for medium carbon steel include quenching and tempering, low-temperature tempering after quenching, low-temperature tempering after high-frequency quenching, isothermal quenching, and medium temperature tempering after quenching.
(1) Tempering and tempering. The organization is tempered martensite. This type of organization has good comprehensive mechanical properties, high strength, good plasticity and toughness. Steel used for quenching and tempering should have good hardenability to ensure uniform microstructure and properties across the entire cross-section of the quenched and tempered part. Compared with alloy steel, carbon steel has poorer hardenability, so it is only suitable for quenching and tempering of medium carbon steel parts with small cross-sectional sizes.
(2) Low temperature tempering after quenching. The organization is tempered martensite, which has high strength and appropriate plasticity and toughness.
(3) High frequency quenching followed by low-temperature tempering. The microstructure of the high-frequency quenching layer is extremely fine hidden needle martensite. After low-temperature tempering, tempered martensite is obtained, which can achieve a similar effect to carburizing treatment. Before high-frequency quenching, it is generally subjected to tempering or normalizing. So after high-frequency quenching and tempering, the core of the part has higher strength, good plasticity and toughness, while its surface hardness is high and wear resistance is good. In addition, the surface of high-frequency quenched parts generates compressive stress, resulting in high fatigue limit and long service life.
(4) Isothermal quenching. The organization is composed of bainite, which has high strength, good plasticity, and toughness.
(5) Quenching followed by medium temperature tempering. The organization is tempered martensite.
Before the final heat treatment, some medium carbon steels undergo normalizing as a pre heat treatment, with the aim of refining grain size, reducing structural inhomogeneity in the steel, obtaining uniform pearlite and ferrite structures, and preparing for the final heat treatment.
After quenching and tempering treatment, medium carbon steel is mainly used to make various transmission shafts, connecting rods, clutches, shaft pins, bolts, etc. After high-frequency quenching and low-temperature tempering, medium carbon steel is used for gears, lathe spindles, spline shafts, camshafts, and half shafts that are subjected to impact loads and require wear resistance.
Medium carbon steel in normalized state or without heat treatment, used for making low load rods, sleeves, fasteners, washers, handles, etc. Medium carbon steel with high carbon content is used to make springs and steel wires.
Introduction to Heat Treatment Process of Medium Carbon Steel
Heat treatment of steel: It is a process of heating, insulating, and cooling solid steel in an appropriate manner to obtain the desired microstructure and properties. Heat treatment can not only be used to strengthen steel and improve the performance of mechanical parts, but also to improve the processability of steel.
Their commonality is that they only change the internal organizational structure without altering the surface shape and size.
The heat treatment process can significantly improve the mechanical properties of steel, increase the strength, toughness, and service life of parts, and improve hardness and wear resistance. So important machine parts and tools need to undergo heat treatment. Heat treatment can also improve the processing performance of workpieces, thereby increasing productivity and processing quality.
Therefore, heat treatment plays a very important role in the mechanical manufacturing industry. Taking 45 steel and 40Cr steel as examples.
In production, quenching followed by high-temperature tempering is called "tempering treatment". After tempering treatment, the parts have good comprehensive mechanical properties and are widely used in various important structural parts, especially those working under alternating loads such as connecting rods, bolts, gears, and shafts. But the surface hardness is low and not wear-resistant. Surface hardening and tempering can be used to improve the surface hardness of parts.
1. 45 Steel - High Quality Medium Carbon Structural Steel
45 steel is a term used in GB, S45C in JIS, 1045080M46 in ASTM, and DIN C45； 45 steel is a high-quality carbon structural steel with chemical composition: carbon (C) content of 0.42~0.50%, Si content of 0.17~0.37%, Mn content of 0.50~0.80%, Cr content<=0.25%. The cold and hot processing performance is good, the mechanical performance is good, and the price is low and the source is wide, so it is widely used. Its biggest weakness is low hardenability, and it is not suitable for workpieces with large cross-sectional dimensions and high requirements.
Recommended temperature for heat treatment of 45 steel: normalizing 850, quenching 840, tempering 600
① Before tempering, if the hardness of. 45 steel is greater than HRC55 (up to HRC62) after quenching, it is qualified. The highest hardness for practical application is HRC55 (high-frequency quenching HRC58).
② The heat treatment process of carburizing and quenching is not used for. 45 steel.
Quenching and tempering of 45 steel: The quenching temperature of 45 steel is A3+(30-50) ℃, and in practical operation, it is generally set at the upper limit. A higher quenching temperature can accelerate the heating speed of the workpiece, reduce surface oxidation, and improve work efficiency. To homogenize the austenite of the workpiece, sufficient insulation time is required. If the actual furnace load is large, it is necessary to extend the insulation time appropriately. Otherwise, there may be insufficient hardness due to uneven heating. However, if the insulation time is too long, it can also lead to coarse grains and severe oxidation decarburization, which affects the quality of quenching. We believe that if the furnace loading exceeds the requirements of the process documents, the heating and insulation time needs to be extended by 1/5.
Due to the low hardenability of 45 steel, a 10% saline solution with a high cooling rate should be used. After the workpiece is immersed in water, it should be quenched, but not cooled through. If the workpiece is cooled through in salt water, it may cause cracking. This is because when the workpiece cools to around 180 ℃, austenite quickly transforms into martensite, causing excessive structural stress. Therefore, when the quenched workpiece is rapidly cooled to this temperature range, a slow cooling method should be adopted.
Due to the difficulty in controlling the outlet temperature, it is necessary to operate based on experience. When the shaking of the workpiece in the water stops, the outlet can be air-cooled (preferably with oil cooling). In addition, when the workpiece enters the water, it should move rather than remain still, and should move according to the geometric shape of the workpiece in a regular manner. The combination of a stationary cooling medium and a stationary workpiece results in uneven hardness and stress, leading to significant deformation and even cracking of the workpiece.
The hardness of quenched and tempered parts made of 45 steel should reach HRC56~59 after quenching. The possibility of a larger cross-section is lower, but it cannot be lower than HRC48. Otherwise, it indicates that the workpiece has not been completely quenched, and there may be martensite or even ferrite structure in the structure. This structure remains in the matrix after tempering and cannot achieve the purpose of quenching and tempering. The high-temperature tempering of 45 steel after quenching is usually carried out at a heating temperature of 560~600 ℃, with a hardness requirement of HRC22~34. Because the purpose of quenching and tempering is to obtain comprehensive mechanical properties, the hardness range is relatively wide.
But if the drawing has hardness requirements, the tempering temperature should be adjusted according to the drawing requirements to ensure hardness. If some shaft components require high strength, the hardness requirements will be high; However, some gears and shaft parts with keyways require lower hardness due to the need for milling and insertion processing after quenching and tempering.
Regarding the tempering insulation time, it depends on the hardness requirements and the size of the workpiece. We believe that the hardness after tempering depends on the tempering temperature and is not closely related to the tempering time, but it must be fully penetrated. Generally, the tempering insulation time for workpieces is always more than one hour.
If 45 steel is carburized, hard and brittle martensite will appear in the core after quenching, losing the advantages of carburization treatment. The materials currently using carburizing technology have low carbon content, and the core strength can already reach a very high level at 0.30%, which is not commonly used in applications. 0.35% have never seen any examples, only introduced in textbooks. The process of quenching and high-frequency surface quenching can be used, which has slightly lower wear resistance than carburizing.
2. 40Cr Steel - Alloy Structural Steel
40Cr belongs to GB3077 "Alloy Structural Steel". The carbon content of 40Cr steel is 0.37% to 0.44%, slightly lower than that of 45 steel. The content of Si and Mn is comparable, with a Cr content of 0.80% to 1.10%. In the case of hot rolling supply, this 1% Cr is basically ineffective, and their mechanical properties are roughly equivalent. Due to the fact that the price of 40Cr is about half that of 45 steel, for economic reasons, those who can use 45 steel do not need 40Cr.
Quenching and tempering treatment of 40Cr steel: The main role of Cr in heat treatment is to improve the hardenability of the steel. Due to the improved hardenability, the mechanical properties such as strength, hardness, and impact toughness of 40Cr after quenching (or tempering) treatment are significantly higher than those of 45 steel. However, due to the strong hardenability, the internal stress of 40Cr during quenching is also greater than that of 45 steel. Under the same conditions, the cracking direction of 40Cr material workpieces is also greater than that of 45 steel material workpieces.
Therefore, in order to avoid cracking of the workpiece, oil with lower thermal conductivity is mostly used as the quenching medium for 40Cr quenching (sometimes double liquid quenching method is also used, commonly known as water quenching oil cooling), while water with higher thermal conductivity is used as the quenching medium for 45 steel. Of course, the choice of water and oil is not absolute and is closely related to the shape of the workpiece. Simple shaped 40Cr parts can also be quenched with water, while complex shaped 45 steel parts may require oil quenching or even salt bath.
The quenching and tempering of 40Cr workpieces are specified in various parameter process cards. Our experience in practical operation is as follows:
(1) After quenching, 40Cr workpieces should be cooled with oil. 40Cr steel has good hardenability and can be hardened by cooling in oil. Moreover, the deformation and cracking tendency of the workpiece are small. However, in situations where oil supply is tight, small businesses can quench workpieces with uncomplicated shapes in water without discovering any cracks, but operators need to strictly control the temperature of the water entering and exiting based on their experience.
(2) After quenching and tempering, the hardness of 40Cr workpiece is still relatively high. The second tempering temperature needs to be increased by 20-50 ℃, otherwise, it will be difficult to reduce the hardness.
(3) After high-temperature tempering of 40Cr workpieces, those with complex shapes are cooled in oil and those with simple shapes are cooled in water to avoid the influence of the second type of tempering brittleness. After tempering and rapid cooling, the workpiece should be subjected to stress relief treatment if necessary.
The highest hardness that can be achieved after heat treatment of medium carbon steel is about HRC55 (HB538), with a σ b of 600-1100MPa. So among various applications with moderate strength, medium carbon steel is the most widely used, not only as a building material, but also extensively used in the manufacture of various mechanical parts.
As long as the temperature and insulation time of medium carbon steel are sufficient, it is generally possible to reach this hardness value. It is impossible without deformation. One suggestion is to have machining allowance and then use a grinder for processing, and the other is surface quenching.
