Tempering of steel pipes
Tempering of steel pipes
I. tempering definition
Tempering of steel pipe is a heat treatment process in which the steel pipe is heated to a certain temperature below AC1 after hardening, held for a certain time, and then cooled to room temperature. Tempering enables the steel pipe to achieve the required performance
II. Tempering purpose
The quenched steel is seldom used directly, because the structure after quenching is martensite and retained austenite, and there is internal stress. Although martensite has high strength and hardness, it has poor plasticity and high brittleness, and it is easy to deform and crack under the action of internal stress. In addition, the structure after quenching is unstable, and it can decompose slowly at room temperature, resulting in volume change, which leads to workpiece Deformation. Therefore, the quenched parts must be tempered before use.
The purpose of tempering is to:
(1) reduce or eliminate quenching internal stress;
(2) stable tissue, stable size;
(3) reduce brittleness and obtain required mechanical properties
III. change of microstructure and properties during tempering
The microstructure transformation of quenched steel can be divided into four stages: the decomposition of martensite (below 200 ℃), the decomposition of retained austenite (200-300 ℃), the formation of cementite (250-400 ℃), the aggregation and growth of cementite (above 400 ℃). With the increase of tempering temperature, the internal stress in quenching decreases or disappears, the hardness decreases and the plasticity and toughness increase
IV. temper brittleness
The embrittlement of tempered brittleness quenched steel can be divided into the first type and the second type.
Tempering brittleness refers to the phenomenon that the toughness of quenched steel decreases after tempering. During tempering, the hardness and toughness of quenched steel decrease with the increase of tempering temperature, but there are two troughs in the relation curve between tempering temperature and impact toughness of many steels, one is between 200 ~ 400 ℃ and the other is between 450 ~ 650 ℃. With the increase of tempering temperature, the impact toughness decreases. Tempering brittleness can be divided into the first type and the second type.
The first kind of temper embrittlement
(1)The first kind of temper brittleness is also called irreversible temper brittleness. The low temperature temper brittleness mainly occurs when the temper temperature is 250-400 ℃, characteristic is irreversible;
(2) it has nothing to do with the cooling rate after tempering;
(3) the fracture is intergranular brittle fracture.
1. There are three reasons:
(1) residual a transformation theory
(2) carbide precipitation theory
(3) impurity segregation theory
2. Prevention methods
It can't be eliminated that there is no alloy element which can effectively restrain the temper brittleness if it is not tempered in this temperature range
(1) reduce the content of impurity elements in steel;
(2) grain a was refined by Al deoxidization or adding Nb, V, Ti and other alloy elements;
(3) Mo, W, etc. can be added to reduce;
(4) adding CR and Si to adjust the temperature range (pushing to high temperature);
(5) adopt isothermal quenching instead of quenching and tempering process.
The second kind of temper embrittlement
The second kind of temper brittleness is also called reversible temper brittleness and high temperature temper brittleness. The temperature is 400-650 ℃,
Features
(1) it is reversible;
(2) it is related to the cooling rate after tempering;
(3) after tempering and heat preservation, slow cooling appears, but fast cooling does not appear. After embrittlement occurs, it can be reheated and fast cooling eliminated.
(4) tempering in the embrittlement zone, embrittlement after tempering has nothing to do with cooling rate;
(5) the fracture is intergranular brittle fracture.
3. Factors affecting the second type of temper embrittlement
(1) chemical composition
(2) grain size a
(3) hardness after heat treatment
4. Mechanism
(1) when temper embrittlement occurs, Ni, Cr, Sb, Sn, P and so on all tend to converge to the original a grain boundary, and all concentrate on the grain boundary with the thickness of 2-3 atoms. The temper embrittlement increases with the increase of impurity elements. Ni and Cr not only segregate themselves, but also promote the segregation of impurity elements.
(2) no segregation of alloy elements and impurity elements was found in the case of quenching without tempering or tempering without embrittlement treatment.
(3) Mo can inhibit the segregation of impurity elements to a grain boundary, and it does not.
It is shown that the segregation of sb, Sn, P and other impurity elements to the original a grain boundary is the main reason for the second kind of tempering brittleness, while Ni and Cr not only promote the segregation of impurity elements, but also promote the segregation of themselves, so as to reduce the fracture strength of grain boundary and produce tempering brittleness.
5. Prevention methods
(1) improve the purity of steel and minimize impurities;
(2) add some Mo, W and other beneficial alloy elements;
(3) for parts with small size and simple shape, the method of quick cooling after tempering is adopted;
(4) using sub temperature quenching (A1 ~ A3): refine the grain and reduce the segregation. After heating, it is a + F (F is fine strip), impurities will be enriched in F, and f has a greater ability to dissolve impurities, which can inhibit the segregation of impurities to a grain boundary.
(5) the high temperature thermomechanical treatment can make the grain super fine, increase the area of grain boundary, and reduce the concentration of impurity element segregation
V. what tempering defects often appear? How to remedy?
(1) too high hardness and remedial measures
1) it is mainly caused by insufficient tempering. On the one hand, it may be caused by too low tempering temperature, and on the other hand, it may be caused by too short tempering time, resulting in incomplete or incomplete transformation of the structure.
2) remedial measures
Adjust the process, re temper, or extend the tempering holding time.
(2) insufficient hardness and remedial measures
1) the main reason is that the tempering temperature exceeds the normal tempering temperature range, or the holding time is too long, which makes the structure of the workpiece change to the structure with lower hardness, thus causing insufficient hardness.
2) Remedies
The workpiece tempered at low and medium temperature shall be re quenched and tempered after annealing or normalizing; the workpiece tempered at high temperature can be directly quenched and tempered again.
(3) deformation and cracking and remedial measures
For carburized steel parts, tempering must be carried out in time after quenching, otherwise, if the residence time at room temperature is too long, the stress of the parts is large, there will be a risk of self cracking. For the workpiece with complex shape, the slow heating speed should be adopted during tempering heating, and the heating is too fast, which is easy to cause the deformation and cracking of the workpiece.
(4) temper brittleness and remedial measures
For the workpiece tempered at low temperature, the tempering temperature shall be kept away from the first type of tempering brittleness zone; for the steel tempered at high temperature, if it has the second type of tempering brittleness, after tempering, attention shall be paid to rapid cooling, such as water cooling or oil cooling, to prevent the workpiece from power failure from cooling slowly with the furnace.
Vi. tempering type
1. Low temperature tempering
The purpose of low temperature tempering (150-250 ℃) is to reduce the quenching stress and improve the toughness of the workpiece on the basis of ensuring the high hardness and wear resistance after quenching. The martensitic structure is obtained by low temperature tempering, and the hardness can reach 58-64hrc. It is commonly used to process high carbon tool steel, die steel, rolling bearing, carburized steel and other parts.
2. Medium temperature tempering
The microstructure obtained by medium temperature tempering (350-500 ℃) is tempered troostite, which has high elastic limit, yield strength and yield strength ratio, as well as certain plasticity and toughness. The hardness is generally 35-45hrc. It is often used as heat treatment of various springs. It has nothing to do with the formation of medium temperature carbides, so it has nothing to do with tempering and softening. The hardening effect of these elements is mainly achieved by the solid solution hardening mechanism. Another kind of alloy elements, such as chromium, molybdenum, tungsten, vanadium and so on, because they are part of carbide formation, their diffusion rate also affects the tempering softening rate.
3. High temperature tempering
High temperature tempering (500-650 ℃) is usually called tempering after quenching. The tempered sorbite structure obtained by quenching and tempering has good comprehensive properties. In many important mechanical structural parts, such as the connecting rod with complex force, important bolt and gear parts with several weeks, have been widely used. The hardness of medium and high carbon steel after quenching and tempering is generally 200 ~ 350hbw. Compared with normalizing, the mechanical properties after quenching and tempering are not only high in strength, but also good in plasticity and toughness. This is because the cementite in tempered sorbite is granular, while the cementite in normalized sorbite is flaky. The granular cementite is more favorable to prevent the crack growth than flaky cementite in the process of fracture.
I. tempering definition
Tempering of steel pipe is a heat treatment process in which the steel pipe is heated to a certain temperature below AC1 after hardening, held for a certain time, and then cooled to room temperature. Tempering enables the steel pipe to achieve the required performance
II. Tempering purpose
The quenched steel is seldom used directly, because the structure after quenching is martensite and retained austenite, and there is internal stress. Although martensite has high strength and hardness, it has poor plasticity and high brittleness, and it is easy to deform and crack under the action of internal stress. In addition, the structure after quenching is unstable, and it can decompose slowly at room temperature, resulting in volume change, which leads to workpiece Deformation. Therefore, the quenched parts must be tempered before use.
The purpose of tempering is to:
(1) reduce or eliminate quenching internal stress;
(2) stable tissue, stable size;
(3) reduce brittleness and obtain required mechanical properties
III. change of microstructure and properties during tempering
The microstructure transformation of quenched steel can be divided into four stages: the decomposition of martensite (below 200 ℃), the decomposition of retained austenite (200-300 ℃), the formation of cementite (250-400 ℃), the aggregation and growth of cementite (above 400 ℃). With the increase of tempering temperature, the internal stress in quenching decreases or disappears, the hardness decreases and the plasticity and toughness increase
IV. temper brittleness
The embrittlement of tempered brittleness quenched steel can be divided into the first type and the second type.
Tempering brittleness refers to the phenomenon that the toughness of quenched steel decreases after tempering. During tempering, the hardness and toughness of quenched steel decrease with the increase of tempering temperature, but there are two troughs in the relation curve between tempering temperature and impact toughness of many steels, one is between 200 ~ 400 ℃ and the other is between 450 ~ 650 ℃. With the increase of tempering temperature, the impact toughness decreases. Tempering brittleness can be divided into the first type and the second type.
The first kind of temper embrittlement
(1)The first kind of temper brittleness is also called irreversible temper brittleness. The low temperature temper brittleness mainly occurs when the temper temperature is 250-400 ℃, characteristic is irreversible;
(2) it has nothing to do with the cooling rate after tempering;
(3) the fracture is intergranular brittle fracture.
1. There are three reasons:
(1) residual a transformation theory
(2) carbide precipitation theory
(3) impurity segregation theory
2. Prevention methods
It can't be eliminated that there is no alloy element which can effectively restrain the temper brittleness if it is not tempered in this temperature range
(1) reduce the content of impurity elements in steel;
(2) grain a was refined by Al deoxidization or adding Nb, V, Ti and other alloy elements;
(3) Mo, W, etc. can be added to reduce;
(4) adding CR and Si to adjust the temperature range (pushing to high temperature);
(5) adopt isothermal quenching instead of quenching and tempering process.
The second kind of temper embrittlement
The second kind of temper brittleness is also called reversible temper brittleness and high temperature temper brittleness. The temperature is 400-650 ℃,
Features
(1) it is reversible;
(2) it is related to the cooling rate after tempering;
(3) after tempering and heat preservation, slow cooling appears, but fast cooling does not appear. After embrittlement occurs, it can be reheated and fast cooling eliminated.
(4) tempering in the embrittlement zone, embrittlement after tempering has nothing to do with cooling rate;
(5) the fracture is intergranular brittle fracture.
3. Factors affecting the second type of temper embrittlement
(1) chemical composition
(2) grain size a
(3) hardness after heat treatment
4. Mechanism
(1) when temper embrittlement occurs, Ni, Cr, Sb, Sn, P and so on all tend to converge to the original a grain boundary, and all concentrate on the grain boundary with the thickness of 2-3 atoms. The temper embrittlement increases with the increase of impurity elements. Ni and Cr not only segregate themselves, but also promote the segregation of impurity elements.
(2) no segregation of alloy elements and impurity elements was found in the case of quenching without tempering or tempering without embrittlement treatment.
(3) Mo can inhibit the segregation of impurity elements to a grain boundary, and it does not.
It is shown that the segregation of sb, Sn, P and other impurity elements to the original a grain boundary is the main reason for the second kind of tempering brittleness, while Ni and Cr not only promote the segregation of impurity elements, but also promote the segregation of themselves, so as to reduce the fracture strength of grain boundary and produce tempering brittleness.
5. Prevention methods
(1) improve the purity of steel and minimize impurities;
(2) add some Mo, W and other beneficial alloy elements;
(3) for parts with small size and simple shape, the method of quick cooling after tempering is adopted;
(4) using sub temperature quenching (A1 ~ A3): refine the grain and reduce the segregation. After heating, it is a + F (F is fine strip), impurities will be enriched in F, and f has a greater ability to dissolve impurities, which can inhibit the segregation of impurities to a grain boundary.
(5) the high temperature thermomechanical treatment can make the grain super fine, increase the area of grain boundary, and reduce the concentration of impurity element segregation
V. what tempering defects often appear? How to remedy?
(1) too high hardness and remedial measures
1) it is mainly caused by insufficient tempering. On the one hand, it may be caused by too low tempering temperature, and on the other hand, it may be caused by too short tempering time, resulting in incomplete or incomplete transformation of the structure.
2) remedial measures
Adjust the process, re temper, or extend the tempering holding time.
(2) insufficient hardness and remedial measures
1) the main reason is that the tempering temperature exceeds the normal tempering temperature range, or the holding time is too long, which makes the structure of the workpiece change to the structure with lower hardness, thus causing insufficient hardness.
2) Remedies
The workpiece tempered at low and medium temperature shall be re quenched and tempered after annealing or normalizing; the workpiece tempered at high temperature can be directly quenched and tempered again.
(3) deformation and cracking and remedial measures
For carburized steel parts, tempering must be carried out in time after quenching, otherwise, if the residence time at room temperature is too long, the stress of the parts is large, there will be a risk of self cracking. For the workpiece with complex shape, the slow heating speed should be adopted during tempering heating, and the heating is too fast, which is easy to cause the deformation and cracking of the workpiece.
(4) temper brittleness and remedial measures
For the workpiece tempered at low temperature, the tempering temperature shall be kept away from the first type of tempering brittleness zone; for the steel tempered at high temperature, if it has the second type of tempering brittleness, after tempering, attention shall be paid to rapid cooling, such as water cooling or oil cooling, to prevent the workpiece from power failure from cooling slowly with the furnace.
Vi. tempering type
1. Low temperature tempering
The purpose of low temperature tempering (150-250 ℃) is to reduce the quenching stress and improve the toughness of the workpiece on the basis of ensuring the high hardness and wear resistance after quenching. The martensitic structure is obtained by low temperature tempering, and the hardness can reach 58-64hrc. It is commonly used to process high carbon tool steel, die steel, rolling bearing, carburized steel and other parts.
2. Medium temperature tempering
The microstructure obtained by medium temperature tempering (350-500 ℃) is tempered troostite, which has high elastic limit, yield strength and yield strength ratio, as well as certain plasticity and toughness. The hardness is generally 35-45hrc. It is often used as heat treatment of various springs. It has nothing to do with the formation of medium temperature carbides, so it has nothing to do with tempering and softening. The hardening effect of these elements is mainly achieved by the solid solution hardening mechanism. Another kind of alloy elements, such as chromium, molybdenum, tungsten, vanadium and so on, because they are part of carbide formation, their diffusion rate also affects the tempering softening rate.
3. High temperature tempering
High temperature tempering (500-650 ℃) is usually called tempering after quenching. The tempered sorbite structure obtained by quenching and tempering has good comprehensive properties. In many important mechanical structural parts, such as the connecting rod with complex force, important bolt and gear parts with several weeks, have been widely used. The hardness of medium and high carbon steel after quenching and tempering is generally 200 ~ 350hbw. Compared with normalizing, the mechanical properties after quenching and tempering are not only high in strength, but also good in plasticity and toughness. This is because the cementite in tempered sorbite is granular, while the cementite in normalized sorbite is flaky. The granular cementite is more favorable to prevent the crack growth than flaky cementite in the process of fracture.