Basic analysis of steel fracture

There are thousands of steel varieties used in various industries. Each kind of steel has different trade names due to different properties, chemical compositions or alloy types and contents. Although the fracture toughness value greatly facilitates the choice of each steel, these parameters are difficult to apply to all steels.
The main reasons are: First, because a certain amount of one or more alloying elements needs 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 in the steelmaking and pouring process, especially the 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 billet different parts have different changes, which affect the quality of steel.
Because the toughness of steel mainly depends on the degree of microstructure and defect dispersion (prevent concentrated defects), rather than chemical composition. Therefore, the toughness will change greatly after heat treatment. To deeply explore the properties of steel and the reasons for its fracture, it is also necessary to grasp the relationship between physical metallurgy and microstructure and steel toughness.

Fracture of ferrite-pearlite steel

Ferritic-pearlite steel accounts for the vast majority of total steel production. They are usually iron-carbon alloys with a carbon content between 0.05% and 0.20% and a small amount of other alloying elements added to increase the yield strength and toughness.
The microstructure of ferrite-pearlite is composed of BBC iron (ferrite), 0.01% C, soluble alloy and Fe3C. In carbon steel with very low carbon content, cementite particles (carbides) stay in the ferrite grain boundaries and grains. But when the carbon content is higher than 0.02%, the vast majority of Fe3C forms a flaky structure with some ferrite, which is called pearlite, and tends to act as “grains” and nodules (grain boundary precipitates). Dispersed in the ferrite matrix. In the microstructure of low carbon steel with a carbon content of 0.10% to 0.20%, the content of pearlite accounts for 10% to 25%.
Although the pearlite particles are very hard, they can be widely dispersed on the ferrite matrix and easily deform around the ferrite. Generally, the grain size of ferrite decreases with the increase of pearlite content. Because the formation and transformation of pearlite nodule will hinder the growth of ferrite grains. Therefore, pearlite can indirectly increase the tensile yield stress δ y by increasing D-1 / 2 (D is the average grain diameter).
From the point of view of fracture analysis, there are two kinds of steels with carbon content in low carbon steel, and their properties are of great concern. Firstly, when the carbon content is less than 0.03%, carbon exists in the form of pearlite spheroidization, which has little effect on the toughness of the steel; secondly, when the carbon content is higher, it directly affects the toughness and Charpy curve in the form of spheroidization.

Influence of treatment process

Practice has shown that the impact performance of water-quenched steel is better than that of annealed or normalized steel. The reason is that rapid cooling prevents cementite from forming at grain boundaries and promotes the fineness of ferrite grains.
Many steels are sold in hot rolled state, and rolling conditions have a great influence on impact performance. A lower finishing temperature will reduce the impact transformation temperature, increase the cooling rate and promote the finer ferrite grains, thereby improving the toughness of the steel. Thick plates have a slower cooling rate than thin plates, and ferrite grains are larger than thin plates. Therefore, thick plates are more brittle than thin plates under the same heat treatment conditions. Therefore, normalizing treatment is often used after hot rolling to improve the properties of the steel sheet.
Hot rolling can also produce anisotropic steel and various mixed structures, pearlite bands, inclusions, grain boundaries, and directional ductile steels with the same rolling direction. The pearlite zone and the elongated inclusions are coarsely dispersed into scales, which have a great influence on the notch toughness at low temperatures in the Charpy transition temperature range.

The influence of ferrite-soluble alloying elements

Most of the alloying elements are added to low-carbon steel to produce solid solution hardened steel at certain ambient temperatures and increase the lattice friction stress δi. But at present, it is not possible to predict the lower yield stress only with the formula, unless the grain size is known. Although the determinants of the yield stress are the normalizing temperature and the cooling rate, this research method is still very important, because the range of a single alloy element that can reduce the toughness can be predicted by increasing δi.
The regression analysis of the non-plastic transformation (NDT) temperature and charpy transformation temperature of ferritic steel has not been reported so far. However, these are limited to the qualitative discussion of the influence of a single alloying element on the toughness. The following briefly introduces the influence of several alloying elements on the properties of steel.
1) Manganese. The vast majority of manganese content is about 0.5%. It can be added as a deoxidizer or sulfur fixer to prevent hot cracking of steel. There are the following effects in low carbon steel.

• Steel with a carbon content of 0.05%, air cooling or furnace cooling has a tendency to reduce the formation of cementite film at the grain boundary.
• It can slightly reduce the ferrite grain size.
• Can produce a large number of fine pearlite particles.

The first two effects indicate that the NDT temperature decreases with the increase of manganese content, and the latter two effects will cause the peak of the Charpy curve to be sharper.
When the carbon content of steel is high, manganese can significantly reduce the transformation temperature by about 50%. The reason may be the 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 has a decisive effect on the impact performance of the normalized steel. Because the high hardenability of steel causes austenite to transform into brittle upper bainite instead of ferrite or pearlite.
2) Nickel. Adding to steel acts like manganese and can improve the toughness of iron-carbon alloys. Its effect depends on the carbon content and heat treatment. In steel with a very low carbon content (about 0.02%), the addition of 2% can prevent the formation of hot-rolled and normalized steel grain boundary cementite, and at the same time substantially reduce the starting transformation temperature TS and increase the Charpy impact Peak curve.
If the nickel content is further increased, the effect of improving the impact toughness will decrease. If the carbon content is so low that no carbides appear after normalizing, the effect of nickel on the transformation temperature will become very limited. The biggest benefit of adding nickel to normalizing steel with a carbon content of about 0.10% is to refine the grains and reduce 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 which reduces the temperature at which austenite decomposes.
3) Phosphorus. In pure iron-phosphorus alloys, phosphorus segregation occurs at the ferrite grain boundaries, which reduces the tensile strength Rm and embrittlement between grains. In addition, because phosphorus is also a stabilizer of ferrite. Therefore, adding to steel will greatly increase the δi value and the ferrite grain size. The combination of these effects will make phosphorus an extremely harmful embrittlement, causing transcrystalline fracture.
4) Silicon. The addition of silicon to steel is for deoxidation and at the same time beneficial to improve impact performance. If both manganese and aluminum are present in the steel, most of the silicon will be dissolved in the ferrite, and at the same time, the δi will be increased by solid solution hardening. The combined result of this effect and the addition of silicon to improve the impact performance is that the addition of silicon by weight to the iron-carbon alloy with stable grain size increases the 50% transformation temperature by about 44℃. In addition, similar to phosphorus, silicon is a stabilizer of ferrite and can promote ferrite grain growth. In terms of weight percentage, adding silicon to normalized steel will increase the average energy conversion temperature by about 60°C.
5) Aluminum. There are two reasons for adding alloy and deoxidizer to steel: first, it forms AlN with nitrogen in the solution to remove free nitrogen; second, the formation of AlN refines the ferrite grains. The result of these two effects is that every 0.1% increase in aluminum will reduce the transition temperature by about 40°C. However, when the amount of aluminum added exceeds the requirement, the effect of “solidifying” the free nitrogen will become weaker.
6) Oxygen. The oxygen in the steel will segregate at the grain boundaries and cause intergranular fracture of the iron alloy. The oxygen content in steel is as high as 0.01%, and fracture will occur along the continuous channels produced by the grain boundaries of the embrittlement grains. Even if the oxygen content in the steel is very low, the cracks will concentrate and nucleate at the grain boundary, and then spread through the grain. The way to solve the problem of oxygen embrittlement is to add deoxidizers carbon, manganese, silicon, aluminum and zirconium to combine with oxygen to form oxide particles and remove oxygen from the grain boundaries. Oxide particles are also beneficial substances that delay the growth of ferrite and increase d-/2.

The influence of carbon content between 0.3% and 0.8%

The carbon content of hypoeutectoid steel ranges from 0.3% to 0.8%, and proeutectoid ferrite is a continuous phase and forms first at the austenite grain boundary. Pearlite is formed in the austenite grains, and it accounts for 35%-100% of the microstructure. In addition, there are a variety of aggregated structures formed in each austenite grain, making pearlite become polycrystalline.
Because the strength of pearlite is higher than that of proeutectoid ferrite, the flow of ferrite is restricted, so that the yield strength and strain hardening rate of steel increase with the increase of pearlite carbon content. As the number of hardened blocks increases, the limiting effect of pearlite on the refinement of the proeutectoid grain size increases.
When there is a large amount of pearlite in steel, micro-cleavage cracks will form at low temperature and/or high strain rate during deformation. Although there are some internal aggregated tissue sections, the fracture channel initially runs along the cleavage plane. Therefore, there are some preferred orientations in ferrite grains between ferrite sheets and in adjacent aggregated structures.

Bainitic steel fracture

Adding 0.05% molybdenum and boron to low carbon steel with 0.10% carbon content can optimize the austenite-ferrite transformation that usually occurs at 700～850℃, and does not affect the subsequent austenite at 450℃ and 675℃ Kinetic conditions for body-bainite transformation.
The bainite formed between about 525 and 675°C is usually called “upper bainite”; the bainite formed between about 450 and 525°C is called “lower bainite”. Both structures are composed of acicular ferrite and dispersed carbides. When the transformation temperature drops from 675℃ to 450℃, the tensile strength of untempered bainite will increase from 585MPa to 1170MPa.
Because the transformation temperature is determined by the content of alloying elements, and indirectly affects the yield and tensile strength. The high strength obtained by these steels is the result of the following two effects:

• 1) When the transformation temperature decreases, the size of bainitic ferrite flakes is continuously refined.
• 2) The fine carbides are continuously dispersed in the lower bain. The fracture characteristics of these steels largely depend on the tensile strength and transition temperature.

There are two functions to pay attention to: First, at a certain level of tensile strength, the Charpy impact performance of tempered bainite is far better than that of untempered upper bainite. The reason is that in the upper bainite, the cleavage facet in the spheroid cuts some bainite grains, and the main size of the fracture is the austenite grain size.
In the lower bainite, the cleavage planes in the acicular ferrite are not arranged in a straight line, so the main feature that determines whether the quasi-cleavage fracture plane is fractured 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 is the carbide distribution. In the upper bainite, carbides are located along the grain boundaries, and increase brittleness by reducing the tensile strength Rm. In the tempered lower bainite, the carbides are very evenly distributed in the ferrite, while restricting cleavage cracks to improve the tensile strength and promote the refinement of spheroidized pearlite.
Second, it should be noted that the transformation temperature and tensile strength changes in untempered alloys. In upper bainite, the decrease in transformation temperature will refine the size of acicular ferrite and increase the elongation strength Rp0.2.
In lower bainite, in order to obtain a tensile strength of 830MPa or higher, it can also be achieved by reducing the transformation temperature to increase the strength. However, because the fracture stress of upper bainite depends on the austenite grain size, the carbide grain size at this time is already very large, so the effect of tempering to improve the tensile strength is small.

Martensitic steel fracture

The addition of carbon or other elements to the steel can delay the transformation of austenite into ferrite and pearlite or bainite. At the same time, if the cooling rate is fast enough after austenitization, the austenite will become martensite through the shear process. No need for atomic diffusion.
The ideal martensite fracture should have the following characteristics:

• Because the transformation temperature is very low (200°C or lower), tetrahedral ferrite or acicular martensite is very fine.
• Because the transformation occurs through shearing, the carbon atoms in the austenite will not have time to diffuse out of the crystals, so that the carbon atoms in the ferrite will be saturated, so that the martensite grains will elongate and cause the lattice expansion.
• Martensite transformation takes place beyond a certain temperature range, because the initially formed martensite flakes increase resistance to the subsequent transformation of austenite into martensite. Therefore, the structure after transformation is a mixed structure of martensite and retained austenite.

In order to ensure the stable performance of the steel, it must be tempered. High carbon (above 0.3%) martensite, tempered in the following range for about 1h, undergoes the following three stages.

• 1) When the temperature reaches about 100℃, some supersaturated carbon of martensite precipitates and forms very fine ε-carbide particles, which are dispersed in martensite to reduce the carbon content.
• 2) Any retained austenite may be transformed into bainite and ε-carbide when the temperature is between 100 and 300℃.
• 3) In the third stage of tempering, it depends on the carbon content and alloy composition from about 200°C. When the tempering temperature rises to the eutectoid temperature, the carbide precipitation becomes coarser and Rp0.2 decreases.

Medium-strength steel fracture

In medium-strength steel (620MPa<Rp0.2<1240MPa), in addition to eliminating stress and improving impact toughness, tempering has the following two effects: First, it transforms retained austenite. The retained austenite will transform into ductile acicular lower bainite at a low temperature of about 30°C. At higher temperatures, such as 600°C, retained austenite will transform into brittle pearlite. Therefore, the steel is tempered for the first time at 550-600℃, and for the second time at 300℃ to avoid the formation of brittle pearlite. This tempering system is called “secondary tempering”.
Second, increase the dispersive carbide content (increased tensile strength Rm) and reduce the yield strength. If the tempering temperature is increased, both will cause an impact, and the tempering range will decrease. Because the microstructure becomes finer, the tensile plasticity will be improved at the same strength level.
Tempering brittleness is reversible. If the tempering temperature is higher than the critical range and the transition temperature is lowered, the material can be reheated and processed in the critical range, and the tempering temperature can be increased again. If there are trace elements, it indicates that the brittleness will be improved. The most important trace elements are antimony, phosphorous, tin, arsenic, and manganese and silicon have a debrittle effect. If other alloying elements exist, molybdenum can also reduce temper brittleness, while nickel and chromium also have a certain effect.

High-strength steel fracture

High-strength steel (Rp0.2>1240MPa) can be produced by the following methods: quenching and tempering; austenite deformation before quenching and tempering; annealing and aging to produce precipitation hardening steel. In addition, the strength of steel can be further improved through strain and retempering or strain during tempering.

Fracture of stainless steel

Stainless steel is mainly composed of iron-chromium, iron-chromium-nickel alloy and other elements that improve mechanical properties and corrosion resistance. The corrosion protection of stainless steel is due to the formation of an impermeable layer of chromium oxide that prevents further oxidation on the metal surface.
Therefore, stainless steel can prevent corrosion and strengthen the chromium oxide layer in an oxidizing atmosphere. But in a reducing atmosphere, the chromium oxide layer is damaged. The corrosion resistance increases with the increase of chromium and nickel content. Nickel can comprehensively improve the passivation of iron.
The addition of carbon is to improve the mechanical properties and ensure the stability of the austenitic stainless steel properties. Generally speaking, stainless steel uses microstructure for classification.
Martensitic stainless steel. It belongs to iron-chromium alloy and can be austenitized and post-heat treatment to form martensite. Usually contains 12% chromium and 0.15% carbon.
Ferritic stainless steel. It contains about 14%-18% chromium and 0.12% carbon. Because chromium is a stabilizer of ferrite, the austenite phase is completely suppressed by more than 13% of chromium, so it is a complete ferrite phase.
Austenitic stainless steel. Nickel is a strong stabilizer of austenite. Therefore, at room temperature, below room temperature or high temperature, the nickel content is 8% and the chromium content is 18% (300 type) can make the austenite phase very stable. Austenitic stainless steel is similar to ferritic and cannot be hardened by martensite transformation.
The characteristics of ferritic and martensitic stainless steels, such as grain size, are similar to other ferritic stainless steels and martensitic stainless steels of the same grade.
The austenitic stainless steel FCC structure is impossible to cleavage and fracture at freezing temperatures. After 80% cold rolling of large parts, 310 stainless steel has extremely high yield strength and notch sensitivity, even at temperatures as low as -253℃, it has a notch sensitivity ratio of 1.0. Therefore, it can be used in the liquid hydrogen storage tank of the missile system. Similar type 301 stainless steel can be used in liquid oxygen storage tanks with temperatures as low as 183°C. However, it is unstable below these temperatures. If any plastic deformation occurs, the unstable austenite will become brittle non-tempered martensite. Most austenitic steels are used in anti-corrosion environments. When heated to a temperature range of 500 to 900 ℃, chromium carbides will precipitate on austenite grain boundaries, resulting in complete depletion of the chromium layer near the grain boundaries. This part is very susceptible to corrosion and localized corrosion. If there is stress, it can also cause crystal brittle fracture.
In order to reduce the above-mentioned hazards, a small amount of elements with stronger properties than chromium carbides, such as titanium or niobium, can be added to form alloy carbides with carbon to prevent chromium from being depleted and the consequent stress corrosion cracking. This treatment is often referred to as “stabilization treatment”.
Austenitic stainless steel is also commonly used in high temperature, such as pressure vessels, to prevent and meet corrosion resistance and creep resistance. Certain steel grades are very sensitive to cracks in and near the heat affected zone due to post-weld heat treatment and high temperature environments. Therefore, when the welding is reheated, niobium or titanium carbides will precipitate in the grains and grain boundaries under the action of high temperature, which will cause cracks and affect the service life. This must be paid attention to.

Source: China Pipe Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.epowermetals.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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