company news about Comprehensive Explanation of Common Metallographic Structures Used in Industrial Steel Production

一、Basic solid solution structure

1. Austenite（A [Feγ (C)] ）

Austenite is a solid solution formed by dissolving carbon and alloying elements in γ -Fe. In the alloy steel system, it is a stable structure in which carbon and various alloying elements are dissolved together in γ -Fe. Its notable feature is excellent plasticity, yet its hardness and yield point are relatively low, with Brinell hardness values typically ranging from 170 to 220HB. It is the microstructure with the smallest specific volume among steels. Under high-temperature conditions, austenite has a strong ability to dissolve carbon. At 1147°C, the amount of carbon dissolved can reach 2.11%, and as the temperature drops to 727°C, the amount of carbon dissolved decreases to 0.77%. Under a metallographic microscope, austenite exhibits a regular polygonal shape due to maintaining the face-centered cubic lattice structure of γ -Fe. This microstructure endows steel with excellent cold working properties. During hot working processes such as forging and rolling, the presence of austenite helps with the plastic deformation of the steel.

2. Ferrite（F [Feα (C)] ）

Ferrite is a solid solution formed by the dissolution of carbon and alloying elements in α -Fe. Its performance is similar to that of pure iron, with a relatively low hardness, approximately ranging from 80 to 100HB, but it has excellent plasticity. When alloying elements are dissolved in ferrite, they can effectively enhance the strength and hardness of steel. At 727°C, the solubility of carbon in ferrite is only 0.022%, and at room temperature, it is as low as 0.008%. Ferrite maintains the body-centered cubic lattice structure of α -Fe and exhibits typical polyhedral metallographic characteristics of pure metals in metallographic structures. The presence of ferrite endows steel with good toughness and cold formability, and it is often used in structural components with high plasticity requirements.

二、Compounds and mixed structures

1. Cementite（Fe₃C ）

Cementite, a compound composed of iron and carbon, is also known as iron carbide. At room temperature, the majority of carbon in iron-carbon alloys exists in the form of cementite. According to the iron-carbon balance diagram, cementite can be classified into three types based on its precipitation path and morphology: Primary cementite crystallizes and precipitates from the liquid along the CD line, mostly presenting a columnar form; Secondary cementite precipitates along the ES line from γ -solid solutions and often appears in a white reticular form. Tertiary cementite precipitates along the PQ line from the α -solid solution, and is also mostly a white network. Cementite has weak magnetism in low-temperature environments. Its magnetism disappears when the temperature exceeds 217° C. Its melting point is approximately 1600°C, and its carbon content is 6.67%. The hardness of cementite is extremely high, far exceeding 700HB, but it is extremely brittle and has almost no plasticity. In steel, the morphology and distribution of cementite have a significant impact on the steel's strength, hardness and wear resistance. For instance, granular cementite can enhance the toughness of steel while maintaining a certain strength.

2. Pearlite（P ）

Pearlite is a mechanical mixture of ferrite and cementite, and it is the product of eutectoid transformation of carbon steel with a carbon content of 0.77%. Its microstructure is a lamellar structure with ferrite and cementite alternately arranged. The size of the pearlite sheet spacing depends on the degree of undercooling during the decomposition of austenite. The greater the degree of undercooling, the smaller the pearlite sheet spacing formed. Based on the difference in lamellar spacing, it can be further classified into pearlite, sorbite and troostite, but essentially they are all pearlite type structures. Coarse lamellar pearlite is the product of austenite decomposition in the high-temperature range of 650-700 °C, with a hardness of approximately 190-230 HB. Fe₃C sheets can be distinguished using a general metallographic microscope (below 500 times magnification). Sorbitite is the product of the decomposition of austenite within the temperature range of 600-650 °C, with a hardness of approximately 240-320HB. It requires a high-power microscope (magnified 1000 times) to distinguish Fe₃C sheets. Troostenite is the product of austenite decomposition at high temperatures of 550-600 °C, with a hardness of approximately 330-400 HB. Fe₃C sheets can only be distinguished through an electron microscope (magnified 10,000 times). Under specific heat treatment conditions, such as spheroidizing annealing or high-temperature tempering, cementite can be uniformly distributed in granular form on the ferrite matrix, forming spheroidal pearlite, also known as granular pearlite. This microstructure can effectively improve the machinability and toughness of steel.

3.Martensite（M ）

Martensite is a supersaturated solid solution of carbon in α -Fe. When steel undergoes high-temperature austenitizing treatment and is cooled at an extremely fast rate below the martensite point, due to the unstable structure of γ -Fe in low-temperature environments, it will rapidly transform into α -Fe. However, due to the extremely rapid cooling rate, the carbon atoms in the steel have no time to diffuse, thus retaining the austenite composition of the parent phase at high temperatures. Therefore, martensite is the product of a non-diffusive phase transformation that occurs when steel is rapidly cooled below the martensite point after austenitizing. Martensite is in a metastable state. Due to the supersaturation of carbon in α -Fe, the body-centered cubic lattice of α -Fe is distorted, forming a body-centered square lattice. This endows martensite with extremely high hardness, approximately between 640 and 760HB, but it also makes it highly brittle, with low impact toughness, and the reduction of area and elongation are almost close to zero. Due to the lattice distortion caused by supersaturated carbon, the specific volume of martensite is larger than that of austenite. When martensite forms in steel, it will generate a relatively large phase transformation stress. Under normal quenching process conditions, martensite presents white needle-like structures at certain angles to each other in the metallographic structure. However, not all martensitic structures are hard and brittle. For instance, low alloy high strength steels containing alloying elements such as manganese, chromium, nickel, and molybdenum, after quenching and tempering treatment, have a tempered low-carbon martensite microstructure. This structure combines high strength with good toughness and is widely used in construction, mechanical manufacturing, and other fields.

• Special metallographic structure
  
  1.Bainite（B ）

Bainite is a mixture of supersaturated ferrite and cementite formed by the phase transformation of undercooled austenite in the medium-temperature range (approximately 250-450 °C). Bainite can be further classified into upper bainite and lower bainite based on the difference in its formation temperature. Upper bainite is a microstructure formed near the pearlite formation temperature. Its characteristic is that α -Fe sheets are arranged in parallel in the same direction within the grains starting from the grain boundaries, with cementite particles interspersed between the sheets. In the metallographic structure, it appears feather-like and can be symmetrical or asymmetrical. The strength of upper bainite is lower than that of fine lamellar pearlite formed at the same temperature, and it is more brittle. Lower bainite is a structure formed around 300°C and appears as black needle-like structures in metallographic structures. Both upper and lower bainite are essentially combinations of ferrite and cementite, but they differ in morphology and carbide distribution. The strength of lower bainite is similar to that of tempered martensite at the same temperature, and its comprehensive performance is superior to that of upper bainite. In some cases, it is even better than tempered martensite. For some parts that require a good fit of strength and toughness, such as shaft parts made of medium carbon steel, obtaining a lower bainite structure through appropriate heat treatment can increase the service life of the parts.

2. Wei's Organization

Widmanstatten structure usually occurs in hypoeutectoid steel. It is formed due to overheating of the steel and the formation of coarse-grained austenite. Under specific undercooling conditions, in addition to the precipitation of massive α -Fe at the boundaries of the original austenite grains, there will also be plate-like α -Fe growing from the grain boundaries into the interior of the grains. These flaky α -Fees have a certain crystalline orientation relationship with the original austenite, presenting in the grains as flaky forms that are at a certain Angle to each other or parallel to each other, which is commonly referred to as the Widmanstatten structure of hypoeutectoid steel. Overheated hypoeutectoid steel is prone to develop Widmanstatten structure at a relatively fast cooling rate. When the Widmanstatten structure is severe, it will lead to a significant decrease in the impact toughness and reduction of area of the steel, making the steel brittle. However, through complete annealing treatment, the Welmanstatten structure can be eliminated and the properties of the steel can be restored. In the process of steel production, controlling the heating temperature and cooling rate is the key to avoiding the formation of Widmanstatten structure.

3. Banded tissue

Banded structure is a microstructure feature of low-carbon structural steel after hot working, specifically manifested as a banded structure in which ferrite and pearlite are distributed in layers parallel to the processing direction. This microstructure will cause the mechanical properties of steel to show anisotropy. There are differences in the performance of steel in the direction parallel to and perpendicular to the strip direction, and it will also reduce the impact toughness and reduction of area of steel. During the steel rolling process, by controlling the final rolling temperature, cooling rate and reasonable rolling ratio and other process parameters, the formation of banded structure can be reduced or avoided.

4. δ phase

The δ phase is a small amount of ferrite present in chromium-nickel stainless steel, especially those containing elements such as niobium and titanium. In austenitic stainless steel, the δ phase plays a significant role. It can effectively prevent the formation of crystalline cracks in stainless steel welds, reduce the tendency of intergranular corrosion and stress corrosion, and at the same time enhance the strength of stainless steel. However, when the amount of δ ferrite exceeds a certain limit (for example, more than 8%), it will increase the pitting tendency of stainless steel. Moreover, under high-temperature conditions, the δ phase is prone to transform into the σ phase, and this transformation can cause metal embrittlement. When designing the composition of stainless steel and formulating the heat treatment process, it is necessary to precisely control the content of the δ phase to balance its beneficial and harmful effects.

5. σ phase

The - σ phase was discovered as an alloy phase when studying the brittleness phenomenon of Fe-Cr alloys. At room temperature, the σ phase is non-magnetic and has the characteristics of being hard and brittle. When σ phase exists in the alloy, especially when it is distributed along the grain boundaries, it will significantly reduce the plasticity and toughness of the steel. The σ phase generally requires a high-temperature environment of 550-900 °C for a relatively long time to gradually form, and its formation process will lead to the deterioration of the material's performance in use. The formation of the σ phase is related to many factors of steel, such as its composition (including the content of elements like chromium and nickel), microstructure, heating temperature, holding time, and pre-deformation. In high-chromium and nickel-chromium stainless steels, the higher the chromium content, the easier it is to form the σ phase. In addition, the δ ferrite in austenitic steel is prone to transform into the σ phase, and the cold deformation process also promotes the formation of the σ phase, causing the temperature range at which the σ phase forms to shift downward. During the production and application of stainless steel, it is necessary to closely monitor the formation of the σ phase and avoid its adverse effects on material properties through reasonable process control.