What is ferrite

What is ferrite?

Ferrite is an interstitial solid solution in which carbon is dissolved in α-Fe, often expressed by the symbol F. With a body-centered cubic lattice, its dissolved carbon capacity is very low, only 0.0008% of carbon can be dissolved at room temperature, and the maximum dissolved carbon capacity is 0.02% at 727 ℃. It is called ferrite or α solid solution, expressed by α or F. α is commonly used in the phase diagram labeling, and F is commonly used in the lines. Austenite of sub-eutectic composition forms ferrite by first eutectic precipitation.

Morphological composition of ferrite

The ferritic grain boundary is smooth, twins or slip lines are rare in the crystal, and the color is light green and bright, and dark after deep corrosion. Ferrite in steel exists in sheet, block, needle and network.
This part of ferrite is called proeutectoid ferrite or structurally free ferrite. With different forming conditions, proeutectoid ferrite has different forms, such as equiaxed, intergranular, spindle, sawtooth and needle. Ferrite is also the matrix of pearlite structure. Ferrite is the main constituent phase in the hot rolling (normalizing) and annealing structures of carbon steel and low alloy steel; The composition and structure of ferrite have an important influence on the technological properties of steel, and also on the service properties of steel in some cases.
Carbon dissolution δ- Interstitial solid solution is formed in Fe, which is in body centered cubic lattice structure. Due to the high temperature, it is called high temperature ferrite or δ Solid solution, with δ Indicates that it exists above 1394 ℃, the amount of dissolved carbon is the largest at 1495 ℃. The mass fraction of carbon is 0.09%.

The main properties of ferritic

Pure ferrite organization has good plasticity and toughness, but the strength and hardness are low; cold working hardening is slow, can withstand a large surface reduction rate of pulling, but the tensile strength of the finished steel wire is hardly more than 1200 MPa. due to the low carbon content of ferrite, its performance is similar to pure iron, plasticity, toughness is very good, elongation δ = 45%-50%. Strength, hardness is low, σb ≈ 250MPa, while HBS = 80.
Pure iron is below 912°C as having a body-centered cubic lattice. The interstitial solid solution in which carbon is dissolved in α-Fe is called ferrite and is denoted by the symbol F. Because α-Fe is a body-centered cubic lattice structure, its lattice gap is very small, and thus the carbon dissolution capacity is very poor, the maximum amount of carbon dissolved at 727 ℃, up to 0.0218%, as the temperature decreases, the amount of carbon dissolved at 600 ℃ is about 0.0057%, at room temperature, the amount of carbon dissolved is about 0.0008%. Therefore, its performance is almost the same as pure iron, its mechanical properties are as follows:

• Tensile strength: 180-280MN/m²
• Yield strength: 100-170MN/m²
• Elongation: 30-50%
• Section shrinkage: 70-80%
• Impact toughness: 160-200J/cm²
• Hardness: HB 50-80

It can be seen that the strength and hardness of ferrite is not high, but has good plasticity and toughness.
The microstructure of ferrite is the same as that of pure iron, with bright polygonal grain organization, sometimes slightly different due to different grain orientation and slightly different degree of corrosion, thus slightly different bright and dark.
Ferrite has ferromagnetism below 770℃, and loses ferromagnetism above 770℃.
(Curie point of ferrite is 770℃)

What is carburite?

Carburite is a metal compound formed by iron and carbon with the chemical formula Fe₃C. Carburite has a carbon content of ωc=6.67% and a melting point of 1227℃. Its lattice is a complex orthogonal lattice, very high hardness HBW = 800, plasticity, toughness is almost zero, brittleness is very large.
There are different forms of carburite in Fe-Carbon alloys, and its number, form and distribution have a direct impact on the performance of Fe-Carbon alloys. They are classified as primary carburite (crystallized from the liquid phase), secondary carburite (precipitated from austenite) and tertiary carburite (precipitated from ferrite).

Physicochemical properties of carburite

The molecular formula of carburite is Fe₃C, which is an interstitial compound with a complex lattice structure. Its carbon content is 6.69%; melting point is about 1227℃; no isotropic heterocrystalline transformation; but there is a magnetic transformation, it has weak ferromagnetism below 230℃, while above 230℃ it loses ferromagnetism; its hardness is very high (equivalent to HB800), while plasticity and impact toughness is almost equal to zero, brittleness is great.
Carburite is not easily corroded by nitric acid alcohol solution and appears white and bright under the microscope, but corroded by alkaline sodium picrate and appears black under the microscope. Carburite has many microstructure forms, and is flaky, granular, reticulated or plate-like when coexisting with other phases in steel and cast iron.
Carburite is the main strengthening phase in carbon steel, and its shape and distribution have a great influence on the properties of steel. At the same time Fe₃C is a meso (sub) stable phase, under certain conditions will decompose: Fe₃C → 3Fe + C, the decomposition of the single carbon for graphite.
Carburite (Fe₃C or Cm): Carburite is a metal compound formed by iron and carbon, containing 6.67% carbon (6.69% in some books), with a complex rhombohedral crystal structure and a melting point of 1227°C. In steel, carburite appears in the organization in different forms and sizes of crystals, which has a great influence on the mechanical properties of steel. By 3% to 5% nitric acid alcohol solution erosion is white bright color, if the hot erosion with sodium picrate solution, it is dyed black-brown, while the ferrite is still white, which can be distinguished from the ferrite and carburized body. The hardness of carburite is very high, reaching more than HB800, brittleness is very large, strength and plasticity is very poor. After different heat treatments, carburite can become flaky, granular or intermittent mesh. Under certain conditions (such as high temperature for a long time or slow cooling), the carburized body can decompose and form graphite-like free carbon: Fe₃C → 3Fe + C (graphite). This process is of great importance for cast iron and graphite steel.

The process of carburite formation

Primary carburite: In the equilibrium crystallization process of Fe-Carbon alloy, the carburite that precipitates when the liquid phase alloy with peri-eutectic composition (peri-eutectic white cast iron) cools below the liquid phase line is called primary carburite.
Eutectic carburite: In the lainite organization, the dotted carburite is uniformly distributed on the austenite matrix, and this carburite is called eutectic carburite.
First eutectic phase and first eutectic carburite: with sub-eutectic and over-eutectic composition of the alloy, before the eutectic transformation, always with the lowering of temperature, first precipitation of a phase constituting the eutectic transformation products, the first phase called first eutectic phase, such as sub-eutectic steel in the first eutectic ferrite and over-eutectic steel in the first eutectic carburite. Due to the formation of different conditions, the form of the first eutectic phase has three major types of block, mesh and Weiss organization.
Eutectic carburite: carburite in pearlite is called eutectic carburite.
Secondary carburite: In the equilibrium crystallization process of iron-carbon alloy, the alloy with eutectic composition (carbon content) above (over-eutectic steel, sub-eutectic white cast iron, eutectic white cast iron, over-eutectic white cast iron) in the slow cooling to a certain extent, the carbon content in austenite reaches saturation, continue to cool down will precipitate carburite along the austenite grain boundary, in the microstructure of the network distribution. This kind of carburite precipitated by the austenite is called secondary carburite.
Tertiary carburite: industrial pure iron in equilibrium cooling to the solid solution line of carbon in iron (Fe-C equilibrium diagram PQ line) below, the solubility of carbon in the ferrite saturated, the temperature drops again, will be precipitated from the ferrite tertiary carburite. Third carburite is precipitated from the ferrite grain boundaries, due to the small amount, generally along the ferrite grain boundaries in intermittent sheet distribution.
Free carburite: It refers to those carburites that are free from the mechanical mixture (tissue) such as pearlite (eutectic tissue) or lysite (eutectic tissue) and exist as an independent phase, such as first eutectic carburite, primary carburite, etc.

The difference between ferrite and cementite

The study of the relationship between the organization of steel materials and metal materials, processes, and properties is a major topic for which people have been working tirelessly for centuries. The identification and analysis of tissue morphology (i.e., metallographic analysis) is a very practical discipline that requires practical experience and reference to relevant materials.
As many materials in different treatment states of the microstructure is very similar, such as mesh ferrite and mesh secondary carburite, needle ferrite and needle carburite, martensite and its tempering products (tempered martensite, tempered taustenite, tempered sothierite), insoluble ferrite and first eutectic ferrite, quenched taustenite and tempered taustenite, ferrite and residual austenite, low carbon lath martensite and feathery upper bainite Although electron microscopy can accurately determine and distinguish these organizations, but the production of practical requirements of rapid inspection often can not do, which brings some difficulties to the inspection of microstructure.
How to use the optical microscope, hardness tester and other conventional instruments, accurate and rapid judgment of microstructure is particularly important. In fact, many microscopic tissue as long as through careful and careful observation of the comparison, will be found under the microscope its independent characteristics. Long-term observation shows that it is feasible and highly realistic and operable to make full use of optical microscope, hardness tester and other common equipment to quickly and accurately identify the microstructure of different metal materials or the same metal material in different treatment states on the basis of theoretical analysis and mutual comparison.

Identification of ferrite and carburite

Steel in the process of hot processing, often appear in the network of ferrite and network of secondary carburite and needle ferrite and needle carburite.
If sub-eutectoid steel austenite grains larger, higher carbon content, the cooling rate is slow, the first eutectoid ferrite will be along the austenite grain boundaries in a mesh precipitation; if the austenite grain coarse, moderate cooling rate, carbon content in 0.15% -0.5% (mass fraction), the first eutectoid ferrite will be along some crystal surface precipitation, lamellar (or needle), this organization is called Weiss organization.
In the over-eutectoid steel first eutectoid carburite morphology is generally reticulate or lamellar (needle), no massive carburite. Reticulated carburite is generally formed when the cooling rate is slow, while needle-like carburite (i.e., Weiss organization) is produced at a carbon content greater than 1.1% and appropriate cooling rate conditions. First eutectic ferrite and first eutectic carburite by 4% (v/v) nitric acid alcohol solution erosion, optical microscope observation are white, not easy to distinguish.
The following four methods can be used to identify the two types of white reticulated or needle-like ferrite and carburite with similar morphology.

01. Microscopic observation method

If the white mesh organization is relatively coarse, uneven thickness, but also clear to see the presence of fine and uniform black lines in the mesh organization (i.e. grain boundaries), the organization can be judged as ferrite, the metal material is sub-eutectoid steel.

Figure 1.1 White reticulated ferrite in sub-eutectoid steel
As shown in Figure 1.1, the microstructure of φ13.2mm 45# steel supply state is composed of white reticulated ferrite and sothite.

Figure 1.2 White reticular secondary carburite in hypereutectoid steel
If the white mesh is very fine and uniform, it can be judged as a mesh secondary carburite, and the metal material is a peri-eutectoidal steel, as shown in Figure 1.2 for the microstructure of T12 steel after complete annealing, which consists of white mesh secondary carburite and lamellar pearlite precipitated along the grain boundaries.

02. Microhardness method

This method is suitable for the coarser white reticulation, and is generally used for the microhardness test with a smaller load.
The measured value above 600HV can be determined as carburized body.
The measured value of 200HV or less can be determined as ferrite, and the steel can be determined as sub-eutectoid.

03. Chemical reagent erosion method

For fine reticulation, use alkaline sodium picric acid aqueous solution for hot etching, that is, the test specimen is immersed in which boiled for 5min, remove the specimen to rinse clean with running water, and then blow dry and observe under the optical microscope.
If the white reticulated tissue becomes black-brown or darker black, it can be identified as carburized body.
If its color remains unchanged and still white (not subject to erosion), it can be determined to be ferrite.

04. Hard needle scribing method

This is the most simple and easy to use a method, as long as there is a light microscope can be carried out.
First, the sample is made into a metallographic specimen, and ordinary erosion, and then a trace is etched on it and put under the optical microscope to observe.
If the mark becomes thicker at the white tissue, the white tissue is considered to be ferrite; conversely, if the mark is finer at the white tissue or the direction changes, it is considered to be carburized.

Figure 1.3 Needle ferrite
As shown in Figure 1.3, for T8 steel in 1200 ℃ holding 320s, first 700 ℃ isothermal 1.5min, and then 600 ℃ isothermal 2min, and then water quenching microstructure, due to quenching heating temperature is particularly high, so that T8 steel produced a serious decarburization, in the subsequent cooling process to get a white mesh and needle ferrite, dark brittleness and needle ferrite between the gray plate The martensite was obtained during subsequent cooling.

Figure 1.4 Needle carburite
The microstructure of T12 steel after carburizing and normalizing at 1150°C is shown in Figure 1.4 and consists of coarse white acicular and reticulated carburite and dark brittlenite.

Identification of ferrite and residual austenite

Since ferrite and residual austenite are not subject to corrosion and are observed to be white under the microscope, it is often easy to get confused if you do not observe carefully.
However, it is easy to distinguish these two differences by mastering the method. The 2 methods commonly used are shown below.

1. Distinguish from the tissue morphology

Ferrite and residual austenite together in the microstructure, generally sub-eutectoid quenched steel parts. Sub-eutectoid steel quenched ferrite in the presence of roughly three forms: polygonal insoluble ferrite, massive pre-eutectoid ferrite and mesh or semi-mesh pre-eutectoid ferrite, the color is brighter. The polygonal and massive ferrite have distinct boundaries and are often found in the blank area of the martensitic pinning angle, and fine-tuning the focus will reveal that the white phase is in the same plane as the martensitic phase. Reticulated or semi-reticulated ferrite is distributed along the original austenite grain boundaries and is relatively fine.
The residual austenite has no obvious boundary line and its shape varies with the shape of the martensite pinning distribution. Sub-eutectoid steel quenching organization of residual austenite is generally not alone, but with the organic combination of quenched needle martensite, so the color is slightly darker than ferrite, often faintly visible needle martensite floating convex phenomenon.

2. Inferred from the heat treatment process

If the sub-eutectoid steel quenching heating and holding time is insufficient or low temperature, will make the quenching organization appears white polygonal insoluble ferrite.

Figure.2 White polygonal insoluble ferrite
As shown in Figure 2 is the microstructure of 45# steel heating and holding at 760 ℃ after 25min water quenching, for white polygonal insoluble ferrite + black carbon quenching martensite + light gray martensite + residual austenite matrix.
If the furnace workpiece more, out of the furnace time is too long, the cooling rate of the workpiece in the furnace is greater than the annealing furnace cooling rate and less than the normalizing air cooling rate, or out of the furnace after the workpiece in the air for too long, will make the quenching organization appears block first eutectic ferrite.

Figure.3 White block first eutectic ferrite
As shown in Figure 3 is 45# steel 840 ℃ heating and holding 25min water quenching, and then 600 ℃ after holding 60min tempering microstructure morphology, where the white block organization that first eutectic ferrite, the rest is tempered sothite.
This is the test, due to the heating furnace contained more workpieces, quenching is not required to operate one by one and close the door at any time; but in the opening of the door of the first specimen quenching, until the last specimen quenching is over, the door has been open. Therefore, about half of the quenched specimens showed massive pre-eutectoid ferrite in the late quenching period. The amount of massive pre-eutectic ferrite increased with the quenching time, and the last quenched specimen contained up to 40% (volume fraction) of massive pre-eutectic ferrite. As the furnace door is not closed, when the furnace workpiece temperature is reduced to below Ac3, because the cooling rate of the furnace workpiece is greater than with cooling (equivalent to annealing) and less than air cooling (equivalent to normalizing), so the precipitation of lumpy first eutectic ferrite.
If the quenching and cooling rate is not enough, the first eutectic ferrite in the steel is generally networked or semi-networked along the original austenite grain boundary distribution.

Figure.4 White mesh first eutectic ferrite
As shown in Figure 4 is 45# steel 900 ℃ heating and holding 25min after oil quenching microstructure morphology, for white fine mesh first eutectic ferrite + black quenching bainite + feather on bainite + light gray martensite + residual austenite matrix.
Only when the quenching heating is severely overheated, in the quenching organization can be observed and martensite is not in the same plane of the residual austenite, the normal quenching organization in the residual austenite is not obvious.

Figure.5 White residual austenite
As shown in Figure 5 is the microstructure of 45# steel 900 ℃ after heating and holding 25min water quenching morphology, black quenching carbon martensite + white residual austenite, where the residual austenite morphology varies with the martensite cross-angle.

Source: China Metal Flanges Manufacturer – Yaang Pipe Industry (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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