Common Defects and Countermeasures of Steel Pipe Heat Treatment(Chapter Three)

 Third, the corrosion resistance of steel pipe is poor

Steel pipes that work in corrosive environments are mostly made of stainless steel. Stainless steel is the general term for iron-based alloys containing more than 5% chromium. But from the point of view of real corrosion resistance, the chromium content (w) in the steel must exceed 10%.

In order to improve the machinability of ferritic stainless steel and adjust its grain size, it is necessary to adopt annealing treatment. The annealing temperature is too low, the recrystallization is not complete; If the annealing temperature is too high, the grain will be coarsened significantly, and the grain boundary precipitation will occur during the cooling process, which will reduce the corrosion resistance of the steel.

In order to improve grain boundary corrosion resistance, a small amount of niobium, titanium and other alloy elements that can form stable carbides are often added to steel. The common heat treatment processes of such steel pipes are solution treatment, stabilization treatment and stress relief treatment. The purpose of solution treatment is to fully dissolve chromium-containing carbides into austenite by heating, and then quickly cool to inhibit the precipitation of carbides to obtain single-phase austenite structure, so that the steel has excellent corrosion resistance.

Heat treatment defects of austenitic stainless steel are caused by insufficient heating and cooling speed. Chromium carbides are easy to precipitate along grain boundaries, which leads to the decrease of intergrain corrosion resistance.

In order to improve the intergranular corrosion resistance of austenitic stainless steel containing Ti and Nb, the solution temperature is usually higher than the solution temperature of Cr23C6, (Cr, Fe) 23C6, and lower than the solution temperature of TiC and NbC

3.1 Influence of heat treatment on intergranular corrosion and spot corrosion of steel pipe

Intergranular corrosion of steel pipe is a selective corrosion occurring along grain boundaries, and it is also a type of corrosion most susceptible to thermal treatment. The main reason of intergranular corrosion is chromium deficiency at grain boundaries. When the steel pipe is heat treated at a certain temperature and time parameter or used for a long time at a certain temperature range, the corrosion resistance of the steel pipe will decrease sharply due to the precipitation of chromium carbides along the grain boundary.

Spot corrosion of steel pipe is a typical local corrosion. Under the condition that most of the metal surface remains passive, the worm's eye corrosion caused by the local destruction of passivation film is called spot corrosion. The decline of point corrosion performance of stainless steel is also related to local chromium deficiency.

3.2 Influence of heat treatment on stress corrosion cracking of steel pipe

Stress corrosion cracking of steel pipe is one of the most common corrosion states. The factors affecting the stress corrosion cracking of steel pipe include metallurgy, stress state and environment. It is generally believed that the existence of tensile stress is a necessary condition for stress corrosion cracking. Therefore, if the residual tensile stress on the surface of the steel pipe is not completely eliminated or the residual tensile stress is generated on the surface of the steel pipe due to improper heat treatment, the stress corrosion resistance of the steel pipe will be reduced. Uneven microstructure is easy to produce stress corrosion. The sensitized stainless steel is prone to intergranular stress corrosion cracking.

 

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Common Defects and Countermeasures of Steel Pipe Heat Treatment (Chapter Two)

 Second, the tensile and fatigue properties of the steel pipe are not qualified

Annealing, normalizing and quenching of steel pipes are the most widely used integral heat treatment processes. Annealing and normalizing of steel pipe are mainly used as preparatory heat treatment. Its purpose is to eliminate the defects of casting and forging, improve the cutting performance of the workpiece, and prepare the organization for the final heat treatment. The defects caused by annealing and normalizing of steel pipes are mainly caused by heating, such as oxidation, decarbonization, overheating and overburning.

2.1 The tensile property of the steel pipe is unqualified

The heat treatment quality of quenched and tempered steel pipe is usually controlled by hardness, and it is worth noting that the final hardness of the steel pipe does not mean that other mechanical properties are the same. Steels with different degrees of quenching can obtain the same hardness by changing the tempering temperature, but their mechanical properties are very different.

The tensile strength of the steel pipe depends on the final tempering hardness and is almost unaffected by the quenching hardness. The yield strength, elongation and section shrinkage of the steel pipe not only depend on the tempering hardness, but also have a great relationship with the quenching hardness, under the condition that the tempering gets the same hardness, these performance indicators increase with the quenching hardness. Therefore, the tempered steel pipe can not only control the hardness after tempering, but also check the hardness after quenching

2.2 Steel pipe fatigue performance is not qualified

When quenching is insufficient or unquenched due to low heating temperature, large workpiece size or insufficient quenching cooling speed, even if the hardness after tempering meets the technical requirements, the fatigue strength often cannot meet the requirements of use and may lead to early failure of the workpiece. This situation should be regarded as a heat treatment defect, and the workpiece should be re-heat treated or other measures taken to remedy it.

When gas carburizing and carbonitriding are carried out in endothermic atmosphere, black network non-martensitic structure appears on the surface after carburizing and quenching, which is called internal oxidation. The inner oxide layer depth is generally not more than 0.05mm. The internal oxidation reduces the surface hardness of the carburized workpiece and forms residual tensile stress on the surface, thus greatly reducing the fatigue strength of the steel pipe. When the inner oxide layer depth is less than 0.013mm, the fatigue strength is not affected. When it exceeds 0.016mm, the fatigue strength can be reduced by 25%.

When the activity of carburizing agent is too large, the carburizing time is too long and the cooling rate after carburizing is too slow, the network or large block carbide is easy to form in the carburizing layer, the formation of carbide leads to the local alloying element dilution and hardenability decline around it, and the non-martensitic structure is easy to form after quenching. The formation of network and large massive carbide and non-martensitic structure reduces the favorable residual compressive stress in the permeated layer, which can reduce the fatigue strength of carburized steel pipe parts by 25%~30%.

The serious decarburization of the steel pipe will make the surface non-martensitic structure, reduce the surface hardness, make the surface show residual tensile stress state, and reduce the fatigue strength of the steel pipe.

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Common Defects and Countermeasures of Steel Heat Treatment (chapter one)

 The purpose of heat treatment to steel is to obtain the desired microstructure of metals and alloys by heating and cooling, in order to change the processing performance of the material or improve the service performance of the workpiece, thereby extending its service life. The mechanical properties of heat treated workpiece fail to meet the design technical requirements, which is a common quality defect of heat treatment. The reasons are improper selection of materials, inherent defects of materials, improper heat treatment process, improper heating or cooling methods, and lax execution of heat treatment process.

1.the hardness of steel is not qualified

The hardness of steel metal materials has a certain empirical relationship with its static tensile strength and fatigue strength, and has a certain degree of relationship with the cold formability, machinability and weldability of metal processing properties.

Hardness failure is one of the most common defects in heat treatment. It is mainly caused by insufficient hardness, insufficient quenching cooling speed, surface decarburization, insufficient hardenability of steel, excessive residual austenite after quenching, insufficient tempering and other factors. The phenomenon of low hardness in the local area of the quenched workpiece is called soft point. Most of the onlookers in the soft spot area are martensite and tostenite mixed structures distributed along the grain boundary of protoaustenite. Uneven soft point or hardness is usually caused by uneven quenching heating or uneven quenching cooling. The main cause of uneven heating is uneven furnace temperature and insufficient heating temperature or holding time. The uneven cooling is mainly caused by the bubbles attached to the quenching medium on the surface of the workpiece during quenching cold, the quenching medium is polluted (such as oil suspended beads in the water) or the quenching medium is not stirred sufficiently. In addition, the steel structure is too thick, there is serious segregation, large carbides or large free ferrite will also cause uneven hardening to form soft points.

1.1 steel Soft Points

The purpose of quenching heating is to make the workpiece complete the microstructure transformation during quenching. For this purpose, it must be heated to the appropriate temperature and have sufficient holding time. Due to the low heating temperature and insufficient holding time, the original pearlite structure can not be completely transformed into austenite and the austenite composition is uneven, and the martensitic structure can not be completely obtained after quenching, resulting in the formation of soft points after quenching.

When the quenching medium is not stirred enough, the workpiece does not move enough in the quenching medium or the workpiece enters the medium in the wrong direction, it often delays the steam film rupture on some parts of the workpiece surface, resulting in a reduction in the cooling rate, resulting in high temperature decomposition products, forming soft points or local hardness reduction.

1.2 Insufficient hardness of steel 

Insufficient heating often leads to insufficient hardness of quenched parts. However, improper cooling is a common cause of insufficient hardness of the workpiece. Too long pre-cooling time from the workpiece out of the oven to before quenching, improper selection of cooling medium or high temperature control of cooling medium, resulting in insufficient cooling capacity, oxide skin on the surface of the workpiece or attached salt solution, and too high temperature when the workpiece is removed from the quenching medium after quenching may lead to the decomposition of supercooled austenite in the pearlite transition area of the C-curve. The formation of non-martensitic structures such as sotensite and totensite makes the hardness of the workpiece insufficient.

The existence of a large amount of residual austenite in the quenched structure is an important reason for the insufficient hardness of the quenched workpiece. The residual austenite volume is related to the chemical composition of austenite, when the carbon content is greater than 0.5%~0.6%, the existence of residual austenite can be obviously observed in the quenched structure, and the residual austenite volume rises sharply, when the mass fraction of carbon is 1.4%, the residual austenite volume (volume fraction) reaches 30%. All alloying elements dissolved in austenite by replacement will cause the increase of residual austenite volume. When the volume of residual austenite is small, it has no obvious effect on the hardness. When the volume of residual austenite is large, the hardness will decrease. The quenching hardness of residual austenite with 20% volume fraction will decrease by about 6.5HRC.

1.3 The soft point and hardness of high-frequency quenching and carburizing steel pipes are insufficient

The soft points of high-frequency hardening steel workpiece include two kinds of residual soft points which are not partially hardened on the surface surface and the depth of the hardening layer is not uniform. These hardness defects are caused by improper material selection, poor original structure, improper electrical parameters of high-frequency quenching heating, sensors and cooling devices.

The lack of hardness and soft points of carburized steel pipes are mostly caused by insufficient carburizing, decarburization during quenching, too low quenching temperature, insufficient quenching cooling speed, excessive residual austenite volume on the surface, excessive tempering, unclean surface of the workpiece, uneven carburizing or uneven cooling.

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Annealing and normalizing of 45# steel pipe

The same steel material can have different mechanical properties (strength, hardness, plasticity and toughness), and different steel materials can also have similar mechanical properties, which are closely related to the heat treatment of steel.

Steel pipes through heat treatment to obtain a certain organization, in order to achieve the required performance. Heat treatment is the means, the use of performance is the purpose, and the structure of the steel is the basis and guarantee of performance.

45 steel pipe annealing is to heat the steel to 30-50 ° C above Ac3, and cool it with the furnace after heat preservation. Due to the relatively slow cooling rate, the microstructure is close to the equilibrium state, and the pearlite accounts for about 55% of the entire field of view.

45 steel normalizing is the steel heated to Ac3 above 30-50℃, after heat preservation in the air natural cooling, it is the main difference with complete annealing is that the cooling speed is faster, the degree of supercooling is large, so the pearlite layer is obviously finer than the annealing, the pearlite volume is significantly increased, the grain is relatively small, so the normalizing hardness is higher than the annealing.

45 steel pipe after normalizing can improve the casting or forging of the organization, fine austenite grains, the formation of fine and uniform ferrite and pearlite, thereby improving the strength, hardness and toughness of steel.

45 steel has a high strength and good plasticity, can be used to manufacture a variety of important parts, such as compressors, various chemical pumps and moving parts (crankshaft, connecting rod, piston rod), can also manufacture turbine impeller. Usually large size parts are used in the normalizing state, and small size parts can be adjusted into tempered sorbite.

45 steel is also the most commonly used tempered steel, which must be normalized before quenching and high temperature tempering to obtain a uniform and fine texture.

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Several Common Steel Annealing Processes Purpose and application (CHAPTER THREE)

 5.

Diffusion annealing

Diffusion annealing, also known as homogenization annealing, is mainly used for alloy steel ingot or castings, which always produce dendrite segregation of alloying elements during the solidification process after pouring, that is, chemical composition non-uniformity. Diffusion annealing is to eliminate or weaken dendrite segregation by heating austenitizing at high temperature for a long time, so that the uneven distribution of elements through diffusion.

The commonly used diffusion annealing temperature is 1100℃-1200℃, and the holding time is 10-15 hours. The higher the alloying element content in the steel pipe, the higher the heating temperature used. After a long time of high temperature heating diffusion annealing, austenite grains have been overgrown, such as no more hot processing, must be a complete annealing or normalizing to refine the grains.

6.

Low temperature annealing

Low temperature annealing is to heat steel to a temperature lower than Ac1 annealing, also known as stress relief annealing, mainly used to eliminate castings, forgings, welding parts, cold stamping parts and machining parts in the residual stress, improve stability, to prevent hardening deformation cracking. It includes softening annealing and recrystallization annealing.

The commonly used softening annealing temperature is 650-720℃, and it is air-cooled after heat preservation. After softening and annealing the ingot, the internal stress is eliminated, the ingot cracking is avoided, and the hardness is reduced to facilitate the surface cleaning of the ingot. After softening and annealing, the internal stress and hardness of forged rolled alloy structural steel can be eliminated, and the hardness reduction effect is more significant for the alloy steel with high stability of supercooled austenite. Recrystallization annealing is the process by which cold-work hardened steel is heated to a temperature between T and -Ac1, usually 650-700 ° C. The purpose is to restore the deformed grains to equiaxed grains by recrystallization, thus eliminating work hardening.

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Several Common Annealing Process Purposes and Applications (CHAPTER TWO)

 

3.

Isothermal annealing

Isothermal annealing is to heat the steel to a critical temperature (hypereutectoid Ac1 or hypoeutectoid Ac3) above austenitizing, and then move the steel into another temperature slightly lower than Ar1 in the furnace isothermal stay, not too high and not too low. Too high isothermal time is too long, and the hardness is low; Too low and high hardness. The principle is to choose a lower isothermal temperature as far as possible under the condition of ensuring qualified hardness, in order to shorten the isothermal time and improve labor productivity. When the transformation is complete, the oven is air-cooled to room temperature.

The transformation is easy to control during isothermal annealing, and it is more suitable for alloy steels with high stability of supercooled austenite, which can save the time of steel parts in the furnace and improve the turnover rate of annealing furnace.

4.

Spheroidizing annealing

Spheroidizing annealing is a heat treatment process to spheroidize carbides in steel and obtain granular pearlite, which is mainly used in hypereutectoid steels, such as carbon tool steel, low alloy tool steel and ball bearing steel.

The purpose of spheroidizing annealing is to reduce hardness, improve cutting performance, and obtain a uniform structure, and prepare the structure for the final quenching treatment. Its heating temperature range is generally 20-30℃ above Ac1 after spheroidizing annealing of the advantages of the structure:

(1) From sheet to granular pearlite, reduce hardness, improve cutting performance.

(2) When granular pearlite is heated, austenite grains are not easy to grow, allowing a wide quenching temperature range, and the deformation and cracking tendency are small during quenching, that is, the quenching process performance is good.

(3) The best quenched structure can be obtained, that is, the martensite sheet is small, the residual austenite volume is small, and a certain amount of uniformly distributed granular carbide is retained.

In addition, the steel with obvious network carbide structure must be normalized to eliminate the carbide network and then spheroidized annealing.

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Several common annealing process characteristics and application scenarios for steel ( Chapter One)

1. Complete annealing

The ingot and cast steel after casting and mold cooling, or the hot forging rolled parts with high final rolling temperature, have coarse grain, easy to get Weischell structure, and there are internal stresses. It can be completely annealed to fine grain, uniform structure, eliminate internal stress, reduce hardness, facilitate cutting, and prepare the structure for quenching of processed parts.

Full annealing is only suitable for hypoeutectoid steel and should not be used for hypereutectoid steel. If the hypereutectoid steel is heated to the single phase austenite region above Acm, the network secondary cementation will be precipitated after slow cooling, which greatly reduces the strength, characteristic and toughness of the steel.

2. Incomplete annealing

Hypoeutectoid steel is heated in the two-phase zone between Ac1-Ac3 or hypereutectoid steel is heated in the two-phase zone between Ac1-Accm, and the heat treatment process of slow cooling is called incomplete annealing.

If the final rolling termination temperature of hypoeutectoid steel is appropriate, it does not cause grain coarsening, and the distribution of ferrite and pearlite is not abnormal, partial recrystallization can be carried out by incomplete annealing to refine the grain, improve the structure, reduce hardness and eliminate internal stress. The incomplete annealing temperature of hypoeutectoid steel is generally 740-780 ° C, which has the advantages of low heating temperature, good operating conditions, saving fuel and time.

The annealing of hypereutectoid steel is to refine and even the structure, reduce the hardness and eliminate the internal stress.

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The role of nitrogen in steel pipes

 

First, the effect of nitrogen on the microstructure and heat treatment of steel pipes

①Nitrogen and carbon can be solidly dissolved in iron, forming interstitial solid solutions.The effect of nitrogen on the microstructure and heat treatment of steel pipes

② Nitrogen expands the austenitic phase region of steel, which is a strong forming and stabilizing austenitic element, and its effectiveness is about 20 times that of nickel, which can replace some nickel in steel within a certain limit.

(3) Nitrogen and chromium, aluminum, vanadium, titanium and other elements that penetrate the surface of the steel can generate extremely stable nitrides, which become surface hardening and strengthening elements.

④ Nitrogen makes the structure of high chromium and high chromium nickel steel dense and solid.

(5) Excessive residual nitrogen content in steel can lead to loose macro organization or porosity.

Second, the influence of nitrogen on the mechanical properties of steel pipes

①  Nitrogen has a solid solution strengthening effect, which can improve the hardenability of steel.

② In nitrogen-containing ferritic steel, when tempering after fast cooling or staying at room temperature for a long time, precipitation hardening can occur due to the precipitation of ultra-microscopic nitrides, and nitrogen also causes strain aging phenomenon of low carbon steel. While the strength and hardness increase, the toughness of the steel decreases and the notch sensitivity increases. The brittleness of steel caused by nitrogen is similar to that of phosphorus, and its effect is much greater than that of phosphorus. Nitrogen is also the main cause of blue brittleness in steel.

③ Improve the strength and impact toughness of high chromium steel and high chromium nickel steel, while the plasticity is not reduced.

④ Improve the creep strength and high temperature endurance strength of steel.

Third, the impact of nitrogen on the physical, chemical and technological properties of steel pipes

(1) Nitrogen has no significant effect on corrosion resistance of stainless steel pipes.

②When the mass fraction of nitrogen is greater than 0.16%, the oxidation resistance will deteriorate.

③ The cold working deformation hardening rate of nitrogen-containing steel is higher.

(4) Nitrogen can reduce the grain growth tendency of high chromium ferrite steel, thereby improving its weldability.

The application of nitrogen in steel

(1) Nitrogen as an alloying element, the content in steel is generally less than 0.3%(mass fraction), and can be as high as 0.6% in special cases.

② Mainly used in nitriding tempered steel, ordinary low alloy steel, stainless acid-resistant steel and heat-resistant non-peeling steel, of which heat-resistant non-peeling steel can be manufactured turbine components.

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The Role of Silicon Element In Steel and The Influence of Heat Treatment

Silicon can be dissolved in ferrite and austenite to improve the hardness and strength of steel pipes, its role is second only to phosphorus, and stronger than manganese, nickel, chromium, tungsten, molybdenum and vanadium. However, when the silicon content exceeds 3%, the plasticity and toughness of the steel pipes will be significantly reduced. Silicon can improve the elastic limit, yield strength and yield ratio (σs/σb), and fatigue strength and fatigue ratio (σ-1/σb) of steel pipe, which is the reason that silicon or silicon-manganese steel can be used as spring steel.

Silicon can reduce the density, thermal conductivity and electrical conductivity of steel pipe. It can promote the coarsening of ferrite grains. Reduce coercive force. It has the tendency to reduce the anisotropy of the crystal, so that the magnetization is easy, the magnetic resistance is reduced, and it can be used to produce electrical steel, so the hysteresis loss of the silicon steel sheet is low, and the silicon can improve the permeability of the ferrite, so that the silicon steel sheet has a higher magnetic induction strength under the weak magnetic field. However, under strong magnetic field, silicon reduces the magnetic induction strength of steel pipe. Silicon has a strong deoxidation, thus reducing the magnetic aging effect of iron.

When the steel containing silicon is heated in an oxidizing atmosphere, a layer of SiO2 film will be formed on the surface, thereby improving the oxidation resistance of the steel at high temperatures.

Silicon can promote the growth of columnar crystals in cast steel and reduce plasticity. If the silicon steel is heated or cooled faster, due to the low thermal conductivity, the temperature difference between the inside and outside of the steel is large, and it is easy to crack.

The role of silicon element in steel and the influence of heat treatment.

Silicon can reduce the weldability of steel pipe. Because the affinity of silicon with oxygen is stronger than iron, it is easy to generate low melting point silicate during welding, which increases the fluidity of molten slag and molten metal, causing splashing phenomenon and affecting the quality of weld. Silicon is a good deoxidizer. The deoxidation capacity of aluminum can be significantly improved by adding a certain amount of silicon when deoxidizing aluminum. Silicon has a certain residual in steel, which is brought in as a raw material by iron and steel making. In boiling steel, silicon is limited to < 0.07%, and when it is intended to be added, ferrosilicon alloy is added in steelmaking.

(1) The effect of microstructure and heat treatment of steel

A, as an alloying element in steel, its content is generally not less than 0.4%. It exists as a solid solution in ferrite or austenite, reducing the austenite phase region.

B, increase the annealing, normalizing and quenching temperature, improve the hardenability in hypoeutectoid steel.

C, silicon does not form carbide, has a strong role in promoting the graphitization of carbon, in the silicon content of high carbon and high carbon steel, such as does not contain strong carbide forming elements, easy to occur at a certain temperature graphitization.

D, in carburized steel, silicon reduces the thickness of the carburized layer and the concentration of carbon.

E, silicon has a good deoxidation effect on molten steel.

(2) Effect on mechanical properties of steel

A, improve the hardness and strength of ferrite and austenite, its effect is stronger than Mn, Ni, Cr.W, Mo, V, etc.; The elastic limit, yield strength and yield to strength ratio (σs/σb) of steel are significantly improved. The fatigue strength and fatigue ratio (σ-1/σb) are also improved.

B, when the silicon content exceeds 3%, the plasticity and toughness of steel are significantly reduced; Silicon increases the plastic/brittle transition temperature.

C, silicon is easy to form a ribbon structure in the steel, so that the transverse performance is lower than the longitudinal performance.

D, improve the wear resistance of steel.

(3) The effect on the physical, chemical and technological properties of steel

A, reduce the density of steel, thermal conductivity, electrical conductivity and resistance temperature coefficient.

B. The eddy current loss of silicon steel sheet is significantly lower than that of pure iron, and the coercive force, magnetoresistance and hysteresis loss are lower. High permeability and magnetic induction. But in strong magnetic fields, silicon reduces the magnetic sensitivity.

C, improve the oxidation resistance of steel at high temperature, but when the silicon content is high, the surface decarbonization is intensified.

D, silicon content of more than 2.5% of the steel, its deformation processing is more difficult.

E, silicon reduces the weldability of steel.

(4) Application in steel

A, in the ordinary low alloy steel pipe to improve the strength, improve the local corrosion resistance, in the tempered steel to improve the hardenability and tempering resistance, is one of the main alloy components in the multi-alloy structural steel.

B, SiMn or SiMnB steel with silicon content of 0.5%-2.8% (carbon content of 0.5%-0.7%) is widely used in high-load elastic yellow materials, while adding W, V, Mo, Nb, Cr and other strong carbide forming elements.

C, silicon steel sheet is low carbon and ultra-low carbon steel containing silicon 1.O % -4.5%, used in motors and transformers.

D, in stainless steel and corrosion resistant steel, with Mo, W, Cr, Al, Ti, N, etc., improve the corrosion resistance and high temperature oxidation resistance.

E, graphite steel with high silicon content is used for cold working mold materials.

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The role of molybdenum element in steel pipe

Molybdenum in the steel pipe can improve the hardenability and thermal strength. Prevent tempering brittleness, increase remanence and coercivity, and resist corrosion in certain media.

In tempered steel, molybdenum can make the larger section of the parts quenched deep, quenched through, improve the tempering resistance or tempering stability of the steel, so that the parts can be tempered at higher temperatures, so as to more effectively eliminate (or reduce) residual stress, improve plasticity.

In addition to the above functions in carburized steel, molybdenum can also reduce the tendency of carbide to form a continuous network on the grain boundary in the carburizing layer, reduce the residual austenite in the carburizing layer, and relatively increase the wear resistance of the surface layer.

In the forging die steel, molybdenum can also maintain the steel has a relatively stable hardness, increase the resistance to deformation, cracking and wear.

 

In stainless acid-resistant steel pipes, molybdenum can further improve the corrosion resistance of organic acids (such as formic acid, acetic acid, oxalic acid, etc.) and hydrogen peroxide, sulfuric acid, sulfite, sulfate, acid dyes, bleaching powder liquid. In particular, the addition of molybdenum prevents the point corrosion tendency caused by the presence of chloride ions.

(1) The effect of microstructure and heat treatment of steel pipes

A. Molybdenum can be solidly dissolved in ferrite, austenite and carbide in steel, and it is an element that reduces the austenitic phase region.

B, when the steel content is low, the composite cementite is formed with iron and carbon; When the content is higher, the special carbide of steel can be formed.

C, molybdenum to improve the hardenability of steel, its role is stronger than chromium. And a little less than manganese.

D, molybdenum improve the tempering stability of steel, as a single alloying element, increase the tempering brittleness of steel; When co-existing with chromium, manganese, etc., molybdenum reduces or inhibits the temper brittleness caused by other elements.

(2) The effect on the mechanical properties of the steel pipe

A. Molybdenum has A solid solution strengthening effect on ferrite. At the same time, it also improves the stability of the carbide, thus improving the strength of the steel.

B, molybdenum to improve the ductility and toughness of steel and wear resistance play a favorable role.

C, due to molybdenum, the softening and recovery temperature and recrystallization temperature after deformation strengthening are increased, and the creep resistance of ferrite is strongly increased, and the accumulation of cementite is effectively inhibited at 450-600℃. Promote the precipitation of special carbides, and thus become the most effective alloying element to improve the thermal strength of steel.

(3) The effect on the physical, chemical and technological properties of the steel pipe

A, in the magnetic steel containing 1.5% carbon, 2%-3% molybdenum improves the residual magnetic induction and coercivity.

B, in reducing acid and strong oxidizing salt solution can passivate the steel surface. Therefore, molybdenum can generally improve the corrosion resistance of steel and prevent pitting corrosion of steel in chloride solution.

C, molybdenum content is higher (>3%) to deteriorate the oxidation resistance of steel.

D, the steel containing no more than 8% molybdenum can still be forged and rolled, but when the content is higher, the deformation resistance of the steel to hot processing is increased.

(4) Application in steel

A. It has been widely used in tempered and carburized structural steel, spring steel, bearing steel, tool steel, stainless acid-resistant steel, heat-resistant steel and magnetic steel.

B, chromium molybdenum steel in many cases can replace chromium nickel steel to manufacture important parts.

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