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Analysis of the basic causes of steel pipe fracture

There are thousands of varieties of steel pipes used in various industries. Each steel tube has a different trade name for its different properties, chemical composition or alloy type and content. Although the fracture toughness value greatly facilitates the choice of each steel, these parameters are difficult to apply to all steel pipes. The main reasons are as follows: First, because a certain amount of one or more alloying elements need to be added during the smelting of steel, different microstructures can be obtained by simple heat treatment after the material is formed, thereby changing the original properties of the steel; Second, because the defects generated during steel making and casting, especially concentrated defects (such as pores, inclusions, etc.) are extremely sensitive during rolling, and between different heats of the same chemical composition steel, even in the same steel billet Different parts change differently, which affects the quality of the steel pipe. Because the toughness of steel pipes depends mainly on the microstructure and the dispersion of defects (strictly preventing concentrated defects), rather than chemical composition. Therefore, the toughness changes greatly after heat treatment. To further explore the properties of steel pipes and the causes of their fractures, it is also necessary to understand the relationship between physical metallurgy and microstructure and toughness of steel pipes.

1. Ferrite-pearlite steel fracture

Ferritic-pearlite steel accounts for the vast majority of total steel production. They are typically iron-carbons containing between 0.05% and 0.20% carbon and alloys of other small alloying elements added to improve yield strength and toughness.
The microstructure of the ferrite-pearlite consists of BBC iron (ferrite), 0.01% C, soluble alloy and Fe3C. In carbon steels with very low carbon content, cementite particles (carbides) remain in the ferrite grain boundaries and grains. However, when the carbon content is higher than 0.02%, most of Fe3C forms a sheet-like structure with some ferrite, which is called pearlite, and tends to be a “grain” and a ball node (grain boundary precipitate). Dispersed in the ferrite matrix. In the microstructure of low carbon steel with a carbon content of 0.10% to 0.20%, the pearlite content accounts for 10% to 25%.
Although the pearlite particles are very hard, they are very widely dispersed on the ferrite matrix and are easily deformed around the ferrite. Generally, the grain size of ferrite decreases as the pearlite content increases. Because the formation and transformation of pearlite balls can hinder the growth of ferrite grains. Therefore, the pearlite will indirectly increase the tensile yield stress δy by raising d-1/2 (d is the average grain diameter).
From the point of view of fracture analysis, there are two types of steel in the range of carbon content in low carbon steel, and its performance is of concern.
First, the carbon content is below 0.03%, carbon exists in the form of pearlite ball joints, and has little effect on the toughness of steel. Second, when the carbon content is high, the toughness and Xiabi are directly affected by the spheroidal form. curve.

2. The impact of the process

It has been found that the impact performance of water-hardened steel is better than that of annealed or normalized steel because the rapid cooling prevents the formation of cementite at the grain boundary and promotes the grain size of the ferrite.
Many steel pipes are sold under hot rolling conditions, and rolling conditions have a great influence on impact properties. Lower finish rolling temperatures reduce the impact transition temperature, increase the cooling rate and promote the grain thinning of the ferrite, thereby increasing the toughness of the steel tube. The thick plate is slower than the thin plate because of the cooling rate, and the ferrite grain is thicker than the thin plate. Therefore, the thick plate is more brittle than the thin plate under the same heat treatment conditions. Therefore, normalizing treatment is often used after hot rolling to improve the performance of the steel sheet.
Hot rolling can also produce anisotropic steel and various mixed structures, pearlite strips, and oriented ductile steel with the same grain boundary and rolling direction. The pearlite band and the elongated inclusions are coarsely dispersed into scales, which have a great influence on the notch toughness at the low temperature range of the Charpy transition temperature range.

3. Effect of ferrite-soluble alloying elements

Most alloying elements are added to low carbon steel in order to produce solid solution hardened steel at certain ambient temperatures, increasing the lattice frictional stress δi. However, it is currently not possible to predict lower yield stresses using only formulas unless the grain size is known. Although the determinants of yield stress are normalizing temperature and cooling rate, this method of research is still important because the range of toughness can be reduced by predicting a single alloying element by increasing δi.
The regression analysis of the non-plastic transition (NDT) temperature and the Charpy transition temperature of ferritic steel has not been reported so far, but these are also limited to the qualitative discussion of the effect of adding a single alloying element on toughness. The following is a brief introduction to the effects of several alloying elements on the properties of steel.
1) Manganese. The vast majority of manganese is about 0.5%. The addition of a deoxidizer or a sulfur-fixing agent prevents thermal cracking of the steel. The following effects are also found in low carbon steel.
◆ Carbon-containing 0.05% steel, after air cooling or furnace cooling, has a tendency to reduce the formation of cementite film at grain boundaries.
◆ The ferrite grain size can be slightly reduced.
◆ Produces a large number of fine pearlite particles.
The first two actions indicate that the NDT temperature decreases as the amount of manganese increases, and the latter two effects cause the peak of the Charpy curve to be sharper.
When steel has a high carbon content, manganese can significantly reduce the transition temperature by about 50%. The reason may be due to the large amount of pearlite, rather than the distribution of cementite at the boundary. It must be noted that if the carbon content of the steel is higher than 0.15%, the high manganese content plays a decisive role in the impact properties of normalized steel. Because of the high hardenability of steel, austenite transforms into brittle upper bainite rather than ferrite or pearlite.
2) Nickel. The effect of adding steel to manganese is to improve the toughness of the iron-carbon alloy. The size of the action depends on the carbon content and heat treatment. In steels with a low carbon content (about 0.02%), the addition of 2% can prevent the formation of cemented cementite in the hot-rolled state and normalized steel, while substantially reducing the initial transition temperature TS and increasing the Charpy impact. Curve peak.
Further increase in nickel content and improvement in impact toughness are reduced. If the carbon content is low until no carbide occurs after normalizing, the effect of nickel on the transition temperature will become very limited. The addition of nickel to normal-fired steel containing about 0.10% carbon has the greatest benefit of refining the grains and reducing the free nitrogen content, but the mechanism is still unclear. It may be due to the fact that nickel acts as a stabilizer for austenite, thereby lowering the temperature at which austenite decomposes.
3) Phosphorus. In a pure iron-phosphorus alloy, phosphorus segregation due to ferrite grain boundaries reduces the tensile strength Rm and causes intergranular embrittlement. In addition, since phosphorus is also a stabilizer for ferrite. Therefore, adding steel will greatly increase the δi value and the ferrite grain size. The combination of these effects will make phosphorus an extremely harmful embrittlement agent, undergoing transgranular fracture.
4) Silicon. Silicon is added to the steel for deoxidation and is beneficial for improving impact properties. If both manganese and aluminum are present in the steel, most of the silicon is dissolved in the ferrite, and δi is increased by solid solution hardening. The combined effect of this effect and the addition of silicon to enhance the impact properties is that silicon is added in weight percent in a stable grain size iron-carbon alloy, raising the 50% transition temperature by about 44 °C. In addition, silicon is similar to phosphorus and is a stabilizer for ferrite, which promotes ferrite grain growth. Addition of silicon to normalized steel by weight percent will increase the average energy conversion temperature by about 60 °C.
5) Aluminum. There are two reasons for the addition of alloys and deoxidizers to the steel: first, the formation of AlN with nitrogen in the solution to remove free nitrogen; second, the formation of AlN refines the ferrite grains. The result of both of these effects is that for every 0.1% increase in aluminum, the transition temperature is lowered by about 40 °C. However, when the amount of aluminum added exceeds the need, the effect of “cure” free nitrogen will be weakened.
6) Oxygen. Oxygen in the steel causes segregation at the grain boundaries resulting in intergranular fracture of the iron alloy. The oxygen content of the steel is as high as 0.01%, and the fracture occurs along a continuous channel created by the grain boundaries of the embrittled grains. Even if the oxygen content in the steel is very low, the crack will nucleate at the grain boundary and then diffuse through the crystal. The solution to the problem of oxygen embrittlement is to add deoxidizers carbon, manganese, silicon, aluminum and zirconium to combine with oxygen to form oxide particles, and to remove oxygen from the grain boundaries. Oxide particles are also advantageous materials for retarding ferrite growth and increasing d-/2.

4. The effect of carbon content in the range of 0.3% to 0.8%

The carbon content of the hypoeutectoid steel is between 0.3% and 0.8%, and the pro-eutectoid ferrite is a continuous phase and is first formed at the austenite grain boundary. Pearlite is formed in austenite grains and accounts for 35% to 100% of the microstructure. In addition, a variety of aggregated structures are formed in each austenite grain to make the pearlite polycrystalline.

Since the pearlite strength is higher than that of the pro-eutectoid ferrite, the flow of the ferrite is limited, so that the yield strength and the strain hardening rate of the steel increase as the carbon content of the pearlite increases. The limiting effect increases with the number of hardened blocks, and the pearlite is enhanced by the refinement of the pre-eutectoid grain size.

When there is a large amount of pearlite in the steel, micro-cleavage cracks are formed at low temperatures and/or high strain rates during the deformation process. Although there are some internal aggregated tissue sections, the fracture channels initially travel along the cleavage plane. Therefore, there are some preferred orientations within the ferrite grains between the ferrite sheets and adjacent aggregated structures.

5. Bainitic steel fracture

Adding 0.05% molybdenum and boron to a low carbon steel with a carbon content of 0.10% optimizes the austenite-ferrite transformation, which usually occurs at 700 to 850 °C, without affecting the austenite at 450 ° C and 675 ° C thereafter. Kinetic conditions for bulk-bainite transformation.
Bainite formed between about 525 and 675 ° C is generally referred to as “upper bainite”; and formed between 450 and 525 ° C is called “lower bainite”. Both tissues consist of acicular ferrite and dispersed carbides. When the transition temperature is lowered from 675 ° C to 450 ° C, the tensile strength of untempered bainite will increase from 585 MPa to 1170 MPa.
Because the transition temperature is determined by the alloying element content and indirectly affects the yield and tensile strength. The high strength obtained by these steels is the result of two actions:
1) When the transition temperature is lowered, the bainitic ferrite sheet size is continuously refined.
2) Fine carbides are continuously dispersed in the lower shell. The fracture characteristics of these steels depend to a large extent on tensile strength and transition temperature.
There are two roles to note: First, a certain level of tensile strength, the Charpy impact performance of bainite under tempering is far superior to the untempered upper bainite. The reason is that in the upper bainite, the cleavage facet in the spheroidal light cuts a number of bainite grains, and the main size determining the fracture is the austenite grain size.
In the lower bainite, the cleavage plane in the acicular ferrite is not aligned, so the main feature that determines whether the quasi-cleavage fracture surface is broken is the acicular ferrite grain size. Because the acicular ferrite grain size here is only 1/2 of the austenite grain size in the upper bainite. Therefore, at the same strength level, the lower bainite transformation temperature is much lower than that of the upper bainite.
In addition to the above reasons, the carbide distribution. In the upper bainite, the carbide is located along the grain boundary and increases the brittleness by lowering the tensile strength Rm. In the tempered lower bainite, the carbides are distributed very uniformly in the ferrite, while at the same time increasing the tensile strength and promoting the spheroidized pearlite refinement by limiting the cleavage cracks.
Second, it is important to note the change in transition temperature and tensile strength in untempered alloys. In the upper bainite, a decrease in the transition temperature causes the acicular ferrite to refine the size while increasing the elongational strength Rp0.2.
In the lower bainite, in order to obtain a tensile strength of 830 MPa or more, it can also be achieved by a method of lowering the transformation temperature and increasing the strength. However, since the fracture stress of the upper bainite depends on the austenite grain size, and the carbide particle size at this time is already large, the effect of improving the tensile strength by tempering is small.

6. Martensitic steel fracture

The addition of carbon or other elements to the steel delays the transformation of austenite into ferrite and pearlite or bainite. At the same time, if the cooling rate is fast enough after austenitizing, the austenite will become martensite by the shearing process. Without atomic diffusion.
An ideal martensite fracture should have the following characteristics.
◆ Because the transition temperature is very low (200 ° C or lower), tetrahedral ferrite or acicular martensite is very fine.
◆ Because the transformation occurs by shearing, the carbon atoms in the austenite are too late to diffuse out of the crystal, so that the carbon atoms in the ferrite are saturated and the martensite grains are elongated to cause lattice expansion.
◆ The martensite transformation takes place over a certain temperature range because the initially formed martensite sheet increases the resistance of the subsequent austenite to martensite. Therefore, the transformed structure is a mixed structure of martensite and retained austenite.
In order to ensure the stability of the steel, tempering must be carried out. High carbon (0.3% or more) martensite, tempered for about 1 hour in the following range, and went through the following three stages.
1) When the temperature reaches about 100 ° C, some supersaturated carbon of martensite precipitates and forms very fine ε-carbide particles, which are dispersed in martensite to reduce the carbon content.
2) The temperature is between 100 and 300 ° C, and any retained austenite may be transformed into bainite and ε-carbide.
3) In the third stage of tempering, approximately 200 ° C depends on the carbon content and alloy composition. When the tempering temperature rises to the eutectoid temperature, the carbide precipitate becomes coarse and Rp0.2 decreases.

7. Medium strength steel fracture

In addition to eliminating stress and improving impact toughness, tempering has two effects:
First, transform the retained austenite. The retained austenite will transform into a tough acicular lower bainite at a low temperature of about 30 °C. At higher temperatures, such as 600 ° C, the retained austenite transforms into brittle pearlite. Therefore, the steel is first tempered at 550 to 600 ° C, and the second tempering is performed at 300 ° C to avoid the formation of brittle pearlite, which is called “secondary tempering”.
Second, increase the diffuse carbide content (increased tensile strength Rm) and reduce the yield strength. If the tempering temperature is raised, both will cause an impact and the transition tempering range will decrease. Because the microstructure becomes fine, at the same strength level, the tensile plasticity will be improved.
Temper brittleness is reversible. If the tempering temperature is high enough to exceed the critical range and the transition temperature is lowered, the material can be reheated and treated in the critical range before the tempering temperature can be raised. If trace elements appear, it indicates that the brittleness will be improved. The most important trace elements are bismuth, phosphorus, tin, and arsenic, plus both manganese and silicon have a brittle effect. If other alloying elements are present, molybdenum can also reduce temper brittleness, while nickel and chromium also have a certain effect.

8. High strength steel (Rp0.2>1240MPa) fracture

High-strength steel can be produced by the following methods: quenching and tempering; austenite deformation before quenching and tempering; annealing and aging to produce precipitation hardened steel. In addition, the strength of the steel can be further increased by strain and re-tempering or tempering strain.

9. Stainless steel break

Stainless steel is mainly composed of iron-chromium, iron-chromium-nickel alloys and other elements that improve mechanical properties and corrosion resistance. Stainless steel corrosion protection is due to the formation of a chromium oxide-impermeable layer on the metal surface that prevents further oxidation.
Therefore, the stainless steel can prevent corrosion in the oxidizing atmosphere and strengthen the chromium oxide layer. However, in the reducing atmosphere, the chromium oxide layer is damaged. The corrosion resistance increases as the content of chromium and nickel increases. Nickel can improve the passivation of iron.
Carbon is added to improve mechanical properties and to ensure the stability of austenitic stainless steel properties. In general, stainless steel is classified using microstructure.
◆ Martensitic stainless steel. It belongs to iron-chromium alloy and can be austenitized and post-processed to form martensite. It usually contains 12% chromium and 0.15% carbon.
◆ Ferritic stainless steel. Containing about 14% to 18% chromium and 0.12% carbon. Because chromium is a stabilizer for ferrite, the austenite phase is completely inhibited by more than 13% chromium and is therefore completely
Source: China Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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