The difference between martensitic and pearlite, what are the differences in the structure and prope

Martensite. It is different from pearlite in terms of tissue morphology, crystal structure and formation conditions, and the difference is:

First, the difference in organizational form.

Pearlite: consists of a layer of ferrite.

A duplex formed by alternating parallel stacks with a layer of cementite.

Organization. The spacing between the lamellae of pearlite depends mainly on the degree of supercooling when the pearlite is formed, and the spacing between the layers and the austenite.

Grain size is irrelevant.

Martensite: lath martensite is a typical martensite structure formed in low and medium carbon steel, in which there are several (3 5) martensite lath bundles inside a proto-austenite grain, and the orientation between the lath bundles is arbitrary; Within a lath bundle there are several parallel slats with large-horned grain boundaries between the blocks; Within a slatted block are several martensitic slats parallel to each other, and between the slats are small-angle grain boundaries.

There are a large number of dislocations within martensitic slats.

So the substructure of lath martensitic is a high-density dislocation and dislocation tangling. Lathed martensite is also known as dislocation martensite.

Flake martensitic is a medium and high carbon steel.

A typical martensite structure with many martensite sheets at angles to each other inside a protoaustenite grain. The spatial morphology of the martensite sheet is biconvex lenticular with a needle-like or bamboo leaf cross-section.

The martensite sheet formed first in the original austenite grain runs through the entire grain, dividing the austenite grain, and the martensite sheet formed successively becomes smaller and smaller, so the size of the martensite sheet depends on the size of the original austenite grain. Flake martensite forms at a low temperature, and residual austenite is often present around the martensite sheet.

The internal substructure of flaky martensitic is predominantly twinned.

When the carbon content is high, the midridge can be seen in the martensitic sheet, and the midridge surface is a very dense microtwin area. Due to the mutual impact of the martensite sheets when they are formed, there are a large number of microcracks in the martensite sheets.

2. Differences in crystal structure.

Pearlite: Ferrite: Body-centered cube; Cementite: Complex lattice.

Martensite: The heart is square.

3. Thermodynamics of formation.

The conditions are different.

Pearlite: The dynamic force is the free energy of the system.

The magnitude of the drop depends on the transition temperature. The greater the degree of supercooling, the greater the transformation drive. The pearlite transition temperature is high, the atomic diffusion ability is strong, and the pearlite transition can occur at a small supercooling.

Martensite: The driving force is the difference in free energy between austenite and martensite at the transition temperature, while the transition resistance is the interfacial energy and the interfacial elastic strain energy. The new phase of the martensitic phase transformation is completely coherent with the parent phase, and the volume effect is large, so the interfacial elastic strain energy is large.

In order to overcome this phase change resistance, the driving force must be sufficiently large. Therefore, the martensitic phase transition must have a large degree of supercooling.

Martensite refers to a supersaturated solid solution of carbon in -Fe. When the carbon content is greater than , the section is needle-shaped, called needle martensitic, which is characterized by high hardness and brittleness. When the carbon content is less than , its shape is a bundle of thin strips parallel to each other, called lath martensitus, which has good strength and good plasticity.

When the carbon content is present, it is a mixed structure of needles and slats. The hardness of martensitic mainly depends on the carbon content in martensitic, the higher the carbon content, the higher its hardness, but when the carbon content is greater than , the hardness of quenched steel increases very slowly.

Pearlite is a eutectic of ferrite and cementite formed by the eutectic transformation of austenite (austenite is a gap solid solution in which carbon is dissolved in Fe). Its form is a layered complex with alternating layers of ferrite and cementite layers, also known as flake pearlite. Represented by the symbol p, the carbon content is c.

Its mechanical properties are between ferrite and cementite, and its strength and toughness are good. Its tensile strength is 750 900MPa, 180 280 HBS, elongation is 20 25%, and impact energy is 24 32J

Martensite (M) is a supersaturated solid solution with carbon dissolved in -Fe, which is a metastable phase transformed by austenite through a diffusion-free phase transition. Lathed martensitic is a typical structure of ferrous alloys such as mild steel, martensitic aging steel, and stainless steel. Flake martensitic is commonly found in high and medium carbon steels; High strength and hardness are one of the main characteristics of martensite, and at the same time, flake martensitic is also relatively brittle.

Bainite is a medium-temperature (MS 550) transition product of supercooled austenite in steel, and the complex phase structure of -Fe and Fe3C. The transition product in the area with high temperature is called bainite, which looks like a feather and has poor impact toughness. The transition product in the lower temperature region is called lower bainite (MS 350).

Its impact toughness is good.

Pearlite is a eutectic of ferrite and cementite formed by the eutectic transformation of austenite. It has a pearly luster. Its form is a layered complex with alternating layers of ferrite and cementite layers, also known as flake pearlite. Good toughness.

Martensite Definition 1: (400 series with high carbon). These grades of stainless steel contain chromium as the only major heat treatment added to austenitization by sectional quenching to avoid ferrite, pearlite or shellfish at a temperature fast enough.

According to the isothermal transition kinetic curve (C-curve) of supercooled austenite

Pearlitic transition (high-temperature transition of supercooled austenite).

Temperature range: A1 550 At this time, C and Fe atoms can be diffused).

Pearlite: A eutectic mixture of ferrite and cementite, which are generally distributed in phases. Due to the different transition temperatures of austenite to pearlite, the thickness of ferrite and cementite sheets in pearlite is different, which is generally divided into three names: pearlite, soxenite, and drosphite.

Emphasis: pearlite, soxenite, and drosphite all belong to the pearlitic structure, and there is no essential difference between the three, and there is no strict temperature limit, but the thickness of the sheet is different.

Bainite transition (mesothermal transition of supercooled austenite).

Temperature range: 550 220

At this temperature, the diffusion of C and Fe atoms cannot be carried out sufficiently, and the austenite decomposes into a mixture of mesostable -Fe and carbide bainite (bainite).

Upper bainite: 550 formed at a slightly lower temperature, feathery, performance is not as good as pearlite, no use value.

Lower bainite: formed near the martensitic transition temperature (220 slightly above), also known as acicular bainite, composed of acicular supersaturated -Fe and fine carbides dispersed on it. The plasticity and toughness are better than those of pearlite, and they have use value.

Martensite transition (low-temperature transition of supercooled austenite).

Temperature range: 220 or less.

Supercooled austenite is transformed into martensite in a non-diffusion manner.

Martensite: austenite quenching to below the ms (about 230) line, the supercooling degree is great, the transformation trend is great, the austenite changes from the face-centered cube to the body-centered cube very quickly, and the carbon atoms have no time to diffuse, forming a supersaturated gap solid solution of carbon in Fe, that is, martensite, Martensite (M).

Martensite point (MS): Supercooled austenite must be cooled below a certain temperature for the martensite transition to occur, which is called the martensitic transition start point or martensite point for short.

Martensite transition terminal (mf): The temperature at which the martensite transition stops.

Martensite has a high hardness, but its plasticity and toughness are very low, and its breaking strength is not high, so it cannot be used directly.

Morphology: Determined by the carbon content of the austenite:

When wc, all form needle-like martensite M sheets;

When wc, all form lath-like martensitic m strips;

At that time, mixed martensite was formed.

Due to the supersaturation of carbon in Fe, the crystal lattice is seriously distorted, so the M sheet has high hardness and high strength, but low plastic toughness.

M bar has high hardness and strength, and the plastic toughness is also good.

The main features of the martensitic transition:

The driving force of the transition is great, and there is no atomic diffusion in the transition, and it is formed at high speed.

The transformation is always carried out incompletely, and there is residual austenite a'. However, carbon steel has little residual austenite and can be ignored. Alloy steel cannot be ignored.

It is formed during the cooling process below ms, and the martensitic mass does not increase during the isothermal process.

The transformation process is accompanied by volume expansion, which leads to the deformation of the workpiece.

Normalizing metallographies generally see pearlite and ferrite, and the properties of finer grains after tempering basically depend on the material properties.

Quenched metallographic can generally see a single martensitic (needle), the grain is uniform after tempering, and the comprehensive mechanical properties should be higher than that of raw materials.

The pearlite transition is a near-equilibrium transition, the transition is complete, with a decrease in temperature, there is no obvious transition start and end point, the transformation mode is the diffusion of carbon atoms...

Martensite transition is a kind of non-equilibrium transition with shear transformation as the main transformation mode, with obvious beginning and end points, incomplete transformation, and the transformation product is single-phase.

1. The transformation of bainite requires a certain gestation period, although in some steels its gestation period is extremely short, even to the extent that it is difficult to determine;

2. A process of nucleation and growth during bainite transformation.

3. There is an upper temperature and a lower temperature for bainite transition.

Pearlite: is made of austenite.

Ferrite, which is precipitated at the same time as the eutectic transition occurs.

The structure of the cementite lamellar layers.

Bainite: Steel is supercooled after austenitization to below the pearlitic transition temperature range, martensite.

The metastable structure formed by ferrite and the distribution of diffuse carbides within it is a metastable organization formed by the transition temperature range above the transition temperature range (the so-called "bainite transition temperature range").

Martensite: is a supersaturated solid solution of carbon in -Fe. The ratio of the axial ratio c a is called the square degree of martensite.