Heat treatment of steel pipe

Traditionally, people think that there is no heat treatment process in pipe rolling plant, but in fact, controlled rolling and controlled cooling, on-line normalization and mandrel bright annealing can be considered to belong to the category of heat treatment. In addition, most companies set the heat treatment line in the pipe rolling plant.

Controlled rolling and controlled cooling is a new process since 1980s, which is widely used in plate production. It mainly improves the strength and toughness index by controlling the rolling temperature (in most cases, it needs to combine V, Nb, Ti and other refined grain elements) and the cooling rate after rolling.

On line normalization refers to normalizing (normalizing) heat treatment on the rolling line, that is, heating the raw pipe to austenization and air cooling on the large cooling bed by the sizing machine to improve the strength. The most common steel grades in pipe rolling plant are N80 class I and K55 coupling materials. At present, our company is still developing P110 class casing of on-line quasi quenched quasi bainite.

Definition and significance of heat treatment

Heat treatment of steel pipe is a process method to change the internal structure of steel pipe through the operation of heating, insulation and cooling of steel in solid state, so as to obtain the required properties. Generally, according to the purpose requirements, the heat treatment process of steel can be divided into: annealing, quenching + tempering, quenching and tempering treatment, and surface treatment.
Heat treatment can give full play to the potential of steel pipe, improve the service performance of workpiece (steel pipe), reduce the weight of workpiece (steel pipe), save materials, reduce costs and prolong service life. On the other hand, the heat treatment process can also improve the processing performance, improve the processing quality and reduce tool wear. At the same time, some physical and chemical indexes can be obtained only after heat treatment. Such as: high H2S stress corrosion resistance, strengthening of stainless steel pipe, etc.
For our company, whether it is pipe rolling plant or pipe processing plant, the heat treatment process improves the physical and chemical indexes such as strength and toughness of steel pipe, so as to meet the requirements of users and improve economic benefits. In the pipe rolling plant, the on-line normalization process significantly simplifies the process flow and reduces the production cost; In the pipe processing plant, high value-added steel pipes are mainly produced through quenching and tempering treatment, especially TP series non API petroleum special pipes with corrosion resistance, collapse resistance, both corrosion and collapse resistance and ultra-high strength.

Basic principle of heat treatment

Development process of metal heat treatment process

The internal structure of metals and alloys is changed by heating, heat preservation and cooling to obtain the structure required by the service performance of workpieces. This technology is called heat treatment process. The same material can obtain different properties after different heat treatment. The change of properties is due to the change of microstructure. Therefore, understanding the structural change of steel during heat treatment is the basis for correct heat treatment.
Like other natural sciences, metal heat treatment technology develops with the development of productivity and is closely related to the development of other science and technology. Heat treatment is the result of the development of ancient metallurgical technology. As a part of metallurgical technology, it has gradually developed into a discipline.
In China’s history, heat treatment technology appeared in the iron age, accompanied by the emergence of cast iron. Steel making in ancient times was carried out by decarburization annealing and repeated forging of cast iron, that is, the so-called “hundred refining into steel”. With the development of steelmaking technology, heat treatment technology has also been developed. Since the Han Dynasty, China’s heat treatment technology has been recorded in writing, including general quenching technology, quenching medium and carburizing process, almost involving all aspects of heat treatment technology. For example, “fire and hydration are quenched” is contained in the historical records and heavenly palace book. According to the separate biography of Pu Yuan, Pu Yuan made 3000 swords for Zhuge Liang in the inclined valley around Meixian County, Shaanxi Province. He said: “the water in Hanzhong is dull and weak and does not quench; Shu water is cool and strong “, so he sent someone to Chengdu to get water. Quench it is really sharp. From the textual research of unearthed cultural relics and some written records, it can be clearly seen that China’s heat treatment technology has a long history and superb skills, which was inferior to that of other countries at that time.

Basic theoretical knowledge of heat treatment

Microstructure of steel

Crystal structure of metal
Matter is composed of atoms. According to the different arrangement of atoms, matter can be divided into two categories, crystal and amorphous. The arrangement of atoms in a crystal is regular, that is, “orderly arrangement”. This regular arrangement is called the crystal structure; The arrangement of atoms in non crystal is irregular, that is, “disordered arrangement”. The arrangement of atoms is usually called lattice structure. The arrangement of atoms in all metals is regular. Therefore, metals belong to crystals.
There are two basic lattice structures of iron: body centered cubic lattice( α- FE) and face centered cubic lattice( γ- Fe). The two lattice structures are shown in Fig.1 and Fig.2.
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Figure.1 body centered cubic lattice

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Figure.2 face centered cubic lattice

Metallographic structure of steel

The concept of “phase”: components with the same chemical composition, crystal structure and physical properties in the structure of metal alloy. In the field of metallography, it is called metallography, which includes solid solutions, metal compounds and pure elements.
The concept of “organization”: it generally refers to the whole composed of one or more phases with different shapes, sizes and distribution modes, as well as various material defects and damages seen by metallographic methods.
Generally, we call the alloy composed of iron (FE) and certain carbon (c) as steel, but carbon exists in the form of iron and carbon compound (Fe3C) in steel. Due to the existence of carbon in steel, it will affect the lattice structure of iron and form different structures. Generally, all kinds of structures in steel are collectively referred to as metallographic structure. The microstructure of steel is different, and its properties are very different. Different structures can be obtained by different heat treatment of steel, and finally the properties we need can be obtained. The basic structures of steel are as follows:

  • (1) Austenite: solid solution of face centered cubic structure formed by iron and other elements, generally referring to the combination of carbon and other elements γ Interstitial solid solution in iron.
  • (2) Ferrite: solid solution of body centered cubic structure formed by iron and other elements, generally referring to the combination of carbon and other elements α Interstitial solid solution in iron.
  • (3) Martensite: metastable phase transformed from austenite by non diffusive transformation. In fact, it is a interstitial solid solution in which carbon is supersaturated in iron. The crystal has a body centered square structure.
  • (4) Pearlite: the layered microstructure with alternating ferrite and cementite sheets. It is the direct product of eutectoid reaction of undercooled austenite. It can also be understood as the mechanical mixture of ferrite and cementite.
  • (5) Bainite: the polymerized structure of ferrite and cementite decomposed by undercooled austenite in the range below pearlite transformation temperature and above martensite transformation temperature. The upper bainite decomposed at higher temperature is called upper bainite, which is feathery; The bainite decomposed at lower temperature is called lower bainite, which is similar to the acicular structure of low-temperature tempered martensite.

In addition, in actual production, according to the requirements of product performance and the specific heat treatment process, there will be other structures in the steel, such as sorbite, troostite, granular pearlite, tempered martensite, tempered sorbite, etc., but these structures are not essentially different from the above basic structures.

Transformation of steel during heating

Whether annealing, normalizing, quenching or carburizing, it is necessary to heat the steel parts to the austenitic state first. Austenite is a solid solution of carbon atoms in the face centered cubic lattice gap of iron. The composition, uniformity, grain size and the quantity and distribution of other phases of austenite have a great influence on the decomposition process, decomposition products and properties of austenite during cooling. At the same time, the heating process of steel will also cause changes in surface quality and composition (oxidation and decarburization), which will affect the heat treatment effect of workpiece.
In order to ensure that the heat treatment can achieve the expected purpose, it is necessary to master the laws of Austenite Formation and growth during steel heating, and use these laws to control the heat treatment effect.
(1) Austenite Formation

The temperature range of Austenite Formation in steel during heating can generally be explained according to the iron carbon alloy state diagram (Fig.3). It can be seen from the figure that when the eutectoid steel with pearlite structure is heated from room temperature to below A1 temperature, there is no other structural transformation except that the carbon content of ferrite increases slightly. When the temperature rises slightly above A1, pearlite changes to austenite. Similarly, for hypoeutectoid steel with ferrite and pearlite, when heated slightly above A1, pearlite changes to austenite, while ferrite does not change. With the continuous increase of heating temperature, ferrite continues to change to austenite. When the temperature rises to A3, all ferrite changes to austenite.

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Fig.3 iron carbon alloy state diagram
(2) Formation process of austenite
The transformation from pearlite to austenite can be roughly divided into four stages, namely the formation of austenite crystal nucleus, crystal growth, dissolution of residual carbide and homogenization of austenite (Fig.4).
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Fig.4 formation process of austenite in eutectoid carbon steel
① Austenite nucleation
Austenite nucleation usually occurs preferentially at the phase interface between ferrite and cementite in pearlite, because the atoms are arranged irregularly at the phase interface, it is easy to obtain the conditions of energy and concentration required for the formation of austenite. Under isothermal conditions, with the increase of time, austenite nuclei grow from scratch, from less to more, and grow at the interface between ferrite and cementite.
② Austenite growth
After the formation of austenite nucleation, the distribution of carbon in austenite is uneven. With the progress of carbon diffusion, the carbon concentration at the contact between austenite and ferrite and cementite is constantly changing, that is, there is a repeated cycle process of carbon concentration losing balance and restoring balance, which makes austenite grow to Cementite on the one hand and ferrite on the other, Until the ferrite disappears and completely transforms into austenite.
③ Dissolution of residual carbide (cementite)
At the end of pearlite transformation to austenite, there are still some undissolved carbides (cementite) in the steel. When eutectoid steel is heated to the temperature above A1, austenite begins to form, but carbides remain. With the increase of time, carbides continue to dissolve until they all disappear.
④ Homogenization of austenite
When all the residual carbides are dissolved, the carbon concentration in austenite is still uneven. The carbon content is higher in the original carbide area and lower in the central area of the original ferrite. If we continue to extend the time, the carbon content of austenite can gradually become uniform through carbon diffusion. The austenite formation process of hypoeutectoid steel and hypereutectoid steel is basically the same as that of eutectoid steel, but it also has the characteristics of excess phase dissolution.
The annealing microstructure of hypoeutectoid steel is pearlite and excess ferrite. When heated slowly to AC1, pearlite transforms into austenite, which becomes the mixed structure of austenite and free ferrite; If the temperature and holding time are further increased, the free ferrite will gradually change to austenite. When the temperature exceeds AC1, the free ferrite disappears completely and all the microstructure is fine austenite grains. If the heating temperature and holding time are further increased, the austenite grain will grow.
The annealing microstructure of hypereutectoid steel is pearlite and excess cementite, in which the excess cementite is often distributed in a network. When heated slowly to AC1 point, pearlite transforms into austenite, which becomes the mixed structure of austenite and excess cementite; If the temperature is further increased and the holding time is prolonged, the excess cementite will gradually dissolve in austenite. When the temperature exceeds ACM, the excess cementite is completely dissolved and all the microstructure is austenite. At this time, the austenite grain has been coarsened. For tools or dies made of hypereutectoid carbon steel, the cementite cannot be completely dissolved into austenite during heating, otherwise the brittleness of steel after quenching will increase and even quenching cracks will occur due to the coarse austenite grains. Therefore, the normal quenching heating temperature is controlled within the range of AC1 ~ ACM.

(3) Formation rate of austenite
In order to control the austenitizing state of steel, it is necessary to understand the formation mechanism of austenite. The formation rate of austenite can be reflected in the isothermal formation diagram of austenite. Figure 5 shows the isothermal formation of austenite in eutectoid steel. From the left side of the figure, the first line indicates that 0.5% austenite is formed, which can be used as the starting line of Austenite Formation; The second line indicates that 99.9% austenite is formed, which can be used as the end line of Austenite Formation; The third line indicates that the residual carbide has been dissolved; The four lines indicate that the austenite carbon concentration is basically uniform.
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Fig.5 Austenite Isothermal formation of eutectoid steel
It can be seen from figure 5 that the time required for austenitizing is closely related to the isothermal transformation temperature. When the temperature is slightly higher than A1, the austenitizing time is longer; The austenitizing process accelerates with the increase of temperature.
The formation time of austenite is shorter, the dissolution time of residual carbide is longer, and the homogenization time of austenite is longer. Taking 780 ℃ isothermal as an example, the time to form austenite is less than 10 seconds, but it takes hundreds of seconds to completely dissolve carbides, and it takes 10000 seconds (about 3 hours) to homogenize austenite.
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Fig.6 isothermal formation of austenite in hypoeutectoid steel
In hypoeutectoid steel and hypereutectoid steel, the isothermal formation diagram of austenite is basically the same as that of eutectoid steel. However, hypoeutectoid steel has its own characteristics, as shown in Figure 6, that is, in hypoeutectoid steel with high carbon content, the end line of ferrite transformation and the dissolution line of residual carbide have the characteristics of crossing. When the temperature is high, there are still residual carbides after free ferrite is dissolved; When the temperature is low, the residual carbide dissolves first, and then the free ferrite dissolves again. Below A3 temperature, when the residual carbide dissolves, ferrite still exists and cannot be completely dissolved.
(4) Effects of various factors on Austenite Formation
Austenite formation is a diffusion process. All factors affecting diffusion, such as temperature and composition, will affect the formation of austenite.
1) Effect of temperature
The transformation from pearlite to austenite follows the law of nucleation and growth. The results show that there is a close relationship between austenite nucleation rate (n) and growth rate (g) and isothermal formation temperature. See table 1.

Table.1 effect of isothermal temperature on austenite transformation parameters

Transition temperature (℃) Austenite nucleation rate n (number of cores)/mm(3.s) Austenite growth rate G (mm/s) Time required for transformation to 50% austenite (seconds)
740 2280 0.0005 100
760 11000 0.01 9
780 51500 0.026 3
800 616000 0.041 1

It is pointed out in the table that with the increase of temperature, the nucleation rate and growth rate of austenite increase sharply. When the temperature increases from 740 ℃ to 800 ℃, the nucleation rate (n) of austenite increases by about 270 times and the growth rate (g) increases by about 80 times, which greatly accelerates the transformation speed of austenite.

2) Influence of composition
A. Carbon impact
With the increase of carbon content in steel, the number of cementite increases correspondingly, while the number of ferrite decreases relatively. Because the total amount of phase interface between ferrite and cementite increases, the transformation from pearlite to austenite is accelerated.
B. Influence of alloying elements
The addition of alloying elements to steel does not change the formation process of austenite during heating, but affects the formation rate of austenite.
Because the alloy elements change the position of A1, A3 or ACM points, some elements reduce A1 points, such as NL, Mn, etc; Some elements increase A1 point, such as Cr, Mo, W and Si. When formulating the heat treatment process, the austenitizing temperature should be appropriately increased or reduced according to the influence of alloy elements on the critical point.
Alloying elements affect the diffusion coefficient of carbon atoms in austenite and the dissolution of residual carbides. Nickel (Ni) not only reduces A1 point and increases superheat, but also increases the diffusion coefficient of carbon in austenite, which speeds up the formation of austenite. Silicon (SI) and aluminum (AL) have little effect on the diffusion coefficient of carbon atoms in austenite, but they slow down the formation rate of austenite because they are elements that increase A1 and reduce superheat.
Chromium (CR), molybdenum (MO), tungsten (W), vanadium (V) and titanium (TI) are elements that can form stable carbides. They not only increase A1 point and reduce superheat, but also significantly reduce the diffusion coefficient of carbon in austenite, thus significantly slowing down the formation rate of austenite. The above elements are arranged in the order that the stability of the carbides formed by them is gradually enhanced, and the carbides formed by titanium (TI) are the most stable. The more stable the carbide formed, the more difficult it is to dissolve, and the slower the formation rate of austenite. In actual production, in order to accelerate the dissolution of stable carbides, the measures of greatly increasing the heating temperature are often taken.
C. Influence of original organization
The morphology and dispersion of carbides in pearlite have an effect on the number of ferrite and cementite phase interfaces and the distance between them. At the same temperature, the more phase interfaces, the greater the nucleation rate; The smaller the interlayer distance, the greater the carbon concentration gradient in austenite, and the faster the diffusion speed. In addition, the shorter the diffusion distance, the faster the growth of austenite crystal. Therefore, the finer the original structure, the faster the austenite formation.

Austenite Formation during continuous heating

In actual production, the heat treatment of steel parts mostly adopts the method of continuous heating. In the case of continuous heating, the experiments show that the law of Austenite Formation during continuous heating is basically the same as that of isothermal formation, but it also has its characteristics, mainly in the following aspects:
(1) The heating speed during continuous heating changes the positions of AC1, AC3 and ACM points, which usually increases with the increase of heating speed, especially AC3 point.
(2) During continuous heating, austenite is formed in a temperature range, and with the increase of heating speed, the formation temperature increases and the formation temperature range increases.
(3) The original structure in steel has a great influence on the formation of austenite during continuous heating. The smaller the dispersion in the original structure, especially in the presence of large free ferrite or cementite, the homogenization of austenite will move to high temperature.

Austenite grain growth and its control

The grain size of austenite affects the microstructure and properties of its transformation products. Grain refinement can improve the strength and toughness of steel. Therefore, the study of austenite grain growth is of great practical significance.
(1) Concept of austenite grain size
According to the formation process and grain growth of austenite, austenite grain size can be divided into three types: initial grain size, actual grain size and essential grain size.
The initial grain size refers to the austenite grain size when pearlite has just changed into austenite. Generally, the initial grain size of austenite is relatively small and will grow when heating or holding.
Actual grain size refers to the austenite grain size actually obtained by steel under specific heat treatment or hot working conditions. Its size directly affects the performance of steel parts. The actual grains are generally larger than the initial grains, because there is usually a heating and holding stage in the production of heat treatment, during which the grains grow to varying degrees.
In different grades of steel, the tendency of austenite grain growth is different. The austenite grain of some steels will grow rapidly with the increase of heating temperature, while the austenite grain of some steels is not easy to grow. The austenite grain growth tendency of steel can be divided into two categories, namely, essential fine grain steel and essential coarse grain steel. The difference between these two types of steels is: within a certain temperature range, the austenite grains of essentially coarse-grained steels grow continuously with the increase of temperature, that is, the tendency of grain growth is large; However, the austenite grain growth tendency of intrinsic fine grain steel is small with the increase of temperature“ “Essential grain size” is not an actual measure of grain size, but indicates the growth tendency of austenite grain under specified conditions.
In industrial production, the Steel Deoxidized by aluminum is mostly essentially fine grain steel, and the Steel Deoxidized by silicon is essentially coarse grain steel. Boiling steel is generally essentially coarse grain steel, killed steel is generally essentially fine grain steel, and essentially fine grain steel is generally used for workpieces requiring heat treatment.
(2) Austenite grain growth and its influencing factors
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Fig.7 schematic diagram of isothermal austenite grain growth
Austenite grain growth at high temperature is a spontaneous process. In fact, austenite grain growth is basically a grain boundary migration process. Therefore, all factors affecting austenite grain boundary migration can affect austenite grain growth.
The higher the austenitizing temperature is, the more obvious the grain growth is. When the grain grows to a certain extent, even if the holding time continues to be extended, the austenite grain will not grow significantly, as shown in Fig.7. The austenite grain size is independent of the subsequent cooling rate.
With the increase of carbon content in steel, the austenite grain growth tendency also increases, but when the carbon content exceeds a certain limit, the austenite grain is fine, because when the carbon content exceeds a certain limit, insoluble cementite appears, and cementite can grow.
Prevent the migration of grain boundaries, so the grains grow slowly, and the actual austenite grains are smaller.
The addition of alloying elements to steel also affects the austenite grain growth. All elements that produce stable carbides (such as titanium, vanadium, niobium, tungsten, molybdenum, chromium, etc.) and elements that produce oxides and nitrides insoluble in austenite (such as aluminum) will hinder austenite grain growth. Manganese and phosphorus tend to accelerate austenite grain growth. In the current industrial production, aluminum is widely used to control the austenite grain size. There are ain particles with high melting point in the Steel Deoxidized with aluminum, which hinder the movement of austenite grain boundary and refine the grain. Intrinsic fine grain steel can be obtained when the residual aluminum content in general steel is about 0.02 ~ 0.04%.
In conclusion, the effects of alloying elements on austenite grain growth are as follows:

  • The elements that strongly prevent grain growth are AI, Ti, Zr and V;
  • The elements that can prevent grain growth are w, Mo and Cr;
  • The elements that have weak effect on preventing grain growth are Si, Ni and Cu;
  • The elements that promote grain growth are Mn (referring to high carbon), P and C (referring to dissolution into austenite).

(3) Measures to control austenite grain growth and grain refinement
1) Reasonable selection of heating temperature and heating time
When the heating temperature is higher, the austenite formation rate is faster. The higher the temperature, the greater the tendency of austenite growth, and the coarser the actual grain. During heat preservation, austenite grain growth also occurs with the extension of heat preservation time. However, the effect of heating temperature on grain growth is much more significant than that of heat preservation time, so the reasonable selection of heating temperature is very important.
The time required for Austenite Formation and homogenization of alloy steel is longer than that of carbon steel, so alloy steel generally needs a longer heating time.
2) Reasonable selection of original microstructure of steel
The original structure of steel has an impact on the growth of austenite grains. Generally, flake pearlite is easier to overheat than granular pearlite, because flake carbide dissolves quickly and transforms into austenite quickly. After austenite is formed, it begins to grow earlier. Therefore, in production, the original structure of bearing steel and tool steel is required to be granular pearlite, One reason is that this organization is not easy to overheat.
3) Add a certain amount of alloy elements
Grain growth is realized by the movement of grain boundary atoms. Therefore, the grain growth can be limited and delayed by adding some alloy elements.
One is that the added alloy elements form dispersed compounds in the steel, such as carbides, nitrides, oxides, etc. These dispersed compounds mechanically hinder the migration of grain boundaries and the growth of grains. However, once these compounds are dissolved, the blocking effect will disappear and the grains will grow rapidly. At this time, the grain growth rate is even greater than that of essentially coarse grain steel. After hot processing (rolling, forging, casting, welding, etc.), the grains of the workpiece are easy to be coarse, which will reduce its mechanical properties and adversely affect the final heat treatment, Grains can be refined by recrystallization. For example, for hypoeutectoid steel workpieces with coarse grains, grains can be refined by complete annealing (or normalizing).

Heating defects of steel (including rolling heating) and their preventive measures

(1) Under heating, overheating and overburning
The defects such as under heating, overheating and over burning of steel in the heating process are mainly caused by inaccurate or failure of furnace temperature instrument, improper charging method and uneven furnace temperature.
The so-called under heating means that ferrite appears in the quenched structure of hypoeutectoid steel, resulting in insufficient response of quenched steel; For hypereutectoid steel, there are more insoluble carbides in quenched structure. The so-called overheating refers to the coarsening of austenite grains when the steel is heated, and the coarse martensite is obtained after quenching, which makes the workpiece brittle. Overburning is not only the severe coarsening of austenite grains, but also the oxidation of grain boundaries, and even the melting of grain boundaries, resulting in the scrapping of workpieces.
In order to prevent these defects, temperature measuring instruments should be inspected frequently, and correct heating specifications and charging methods should be adopted. If the steel is under heated or overheated during heating, the steel can be annealed or normalized once, and then re quenched.
(2) Oxidation and decarburization
Oxidation refers to the process in which the surface of steel interacts with oxygen, oxidizing gas and oxidizing impurities in the heating medium to form iron oxide. Due to the formation of iron oxide scale, the size of the workpiece will be reduced, the surface finish will be reduced, and the cooling speed during quenching will be seriously affected, resulting in soft spots or insufficient hardness. Although the oxidation of steel is a chemical reaction, after an oxide film is formed on the surface of steel, the oxidation rate mainly depends on the diffusion rate of oxygen and iron atoms through the oxide film. With the increase of temperature, the atomic diffusion rate increases, and the oxidation rate of steel increases sharply. Especially above 600 ℃, the formed oxide film is mainly FeO, which is not dense. Oxygen and iron atoms are easy to penetrate into the interior through this oxide film, which is thicker and thicker; Below 600 ℃, the oxide film is composed of relatively dense Fe3O4, so the oxidation speed is relatively slow.
Decarburization refers to the oxidation of carbon in the surface layer of steel, which is the reduction of carbon content in the surface layer. The higher the heating temperature, the higher the carbon content of the steel (especially when there are more elements such as silicon, molybdenum and aluminum), the steel is easier to decarburize. Due to the rapid diffusion rate of carbon, the decarburization rate of the steel is always greater than its oxidation rate. Under the oxidation layer of the steel, there is usually a decarburization layer with a certain thickness. Due to decarburization, the carbon content in the surface layer of steel decreases, resulting in insufficient surface hardness and fatigue strength after quenching, and it is easy to form surface cracks on the surface of steel parts.
In order to prevent oxidation and decarburization, protective atmosphere heating, vacuum heating and surface coating packaging heating can be adopted according to the requirements and actual situation of the workpiece; When heating in salt bath, standard deoxidation system shall be established and deoxidizer shall be added regularly.
(3) Widmanstatten structure
In the rolling process, if the rolling temperature is too high, the network widmanstatten structure will be formed at a specific cooling rate in the subsequent cooling process. This will significantly reduce the toughness index. The solution is to eliminate it by normalizing.

Application of heat treatment in pipe rolling plant

Controlled rolling and controlled cooling is a new process since 1980s. The strength and toughness index is mainly improved by controlling the rolling temperature (in most cases, V, Nb, Ti and other refined grain elements) and the cooling rate after rolling. The typical type in our company is X52 ~ X60 steel pipeline pipe. In production, the purpose of strengthening is achieved by adding V, Nb, Ti and other elements or their combination with other elements.
On line normalization refers to normalizing (normalizing) heat treatment on the rolling line, that is, heating the raw pipe to austenization and air cooling on the large cooling bed by the sizing machine to improve the strength. The most common steel grades in pipe rolling plant are N80 class I and K55 coupling materials. At present, our company is still developing P110 class casing of on-line quasi quenched quasi bainite.
Just like the development of on-line normalization, hot charging and direct rolling process, at present, heat treatment processes such as direct quenching + tempering after rolling will be gradually applied, so as to greatly reduce energy consumption.

Source: Network Arrangement – China Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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