5 Martensite Morphologies You Should Know

Martensite structure obtained by quenching is an important basis for steel to achieve strength and toughness.

Due to different types and compositions of steel and different heat treatment conditions, the morphology, internal fine structure and tendency to form microcracks of quenched martensite will be greatly changed.

These changes have a great influence on the mechanical properties of martensite.

Therefore, it is very important to master the morphological characteristics of martensite and further understand the various factors that affect the morphology of martensite.

1. Morphology of martensite

Through thin film transmission electron microscopy, the morphology and fine structure of martensite have been studied in detail.

It is found that although the morphology of martensite in steel is diverse, its characteristics can be generally divided into the following categories:

1. Lattice martensite

Lath martensite is a typical martensite structure formed in low and medium carbon steel, maraging steel, stainless steel and other iron series alloys.

The typical structure in mild steel is shown in Fig. 1.

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Fig. 1 Strip Martensite 100X of Low Carbon Alloy Steel (0.03% C, 2% Mn)

Because its microstructure is composed of many groups of laths, it is called lath martensite.

For some steels, the lath is not easy to be etched and exposed, but tends to be blocky, so it is sometimes called blocky martensite.

Because the substructure of this martensite is mainly dislocation, it is usually called dislocation martensite.

This martensite is composed of several lath groups, also known as cluster martensite.

Each strip group is composed of several strips of approximately the same size, which are roughly parallel and arranged in a certain direction.

It can be seen from Fig. 2 that lath martensite is characterized by dislocation with high density in the lath.

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Fig. 2 Thin film transmission microstructure of low carbon alloy steel (0.03% C, 2% Mn) 20000X

In addition, there are sometimes phase transformation twins in the lath, but they are only local, not in large quantity, and are not the main fine structure form.

The crystal orientation relationship between lath martensite and parent austenite is K-S relationship, and the habit plane is (111) γ.

However, the habit plane of lath martensite in 18-8 stainless steel is (225) γ.

According to the research, the crystallographic characteristics of lath martensite microstructure can be shown in Fig. 3.

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Fig. 3 Schematic diagram of crystallographic characteristics of lath martensite microstructure

Where A is a large area composed of lath martensite bundles arranged in parallel, which is called lath group.

A primary austenite grain can contain several lath groups (usually 3-5).

A strip group can be divided into several parallel regions like B in the figure.

When some solutions are used for corrosion, this area sometimes only shows the boundary of lath group, which makes the microstructure appear blocky, hence the name blocky martensite.

When color etching is used (such as 100ccHCl+5g CaCl2+100ccCH3CH solution), black and white tones can be displayed in the lath group.

The same tone region is composed of martensite laths with the same orientation, which is called the homotropic beam.

According to the K-S orientation relationship, the martensite can have 24 different orientations in the parent austenite, including six orientations that can generate lath martensite in parallel (see Fig. 4).

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Fig. 4 Martensite (111) in steel γ Possible orientation when forming on the plane

An isopathic bundle is a bundle of slats transformed from one of them.

Several parallel collinear bundles form a strip group.

Some people think that in a lath group, it is only possible to change the position according to two groups.

Therefore, a lath group is composed of two groups of aligned lath beams alternately, and the two groups of aligned lath beams can be alternated with each other at large angle grain boundaries. However, there is also a case where the lath group is mainly composed of a kind of homotropic bundle, as shown in C in Fig. 3.

An aligned bundle is composed of strips arranged in parallel, as shown in D in Fig. 3.

This situation can be observed with an electron microscope, as shown in Fig. 5.

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Fig. 5 Some microstructures in the isotropic beam of lath martensite in Fe-0.2% C alloy (transmission electron micrograph)

According to the research results in Fe-0.2% C alloy, the strip width distribution is a lognormal distribution as shown in Fig. 6.

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Fig. 6 Strip distribution of film and replica technology

It can be seen from the figure that the width of the lath with the highest occurrence frequency is 0.15~0.20μm, and the distribution curve is very inclined to the side of the small size lath, but a small part of the lath has a width of 1~2μm.

Fig. 7 shows that larger laths are often distributed throughout the lath bundle, which is an important feature of the lath bundle microstructure.

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Fig. 7 Microstructure of lath martensite in Fe-0.2% C alloy (transmission electron micrograph)

The experiment shows that the austenite grain size is changed by changing the austenitizing temperature, but the width distribution of the lath is hardly affected.

However, the size of lath group increases with the increase of austenite grain size, and the ratio of the two is approximately unchanged.

Therefore, the number of lath groups generated in an austenitic grain is generally unchanged.

The area of the lath boundary in unit martensite volume is about 65000cm measured by thin film electron microscopy ²/ cm ³.

The area of small angle crystal in the lath bundle is about 5 times that of large angle crystal boundary.

In the Fe Cr Ni alloy based on 18-8 stainless steel, lath martensite and ε ‘- martensite (closely packed hexagonal lattice) will also be generated.

Its microstructure is quite different from that of the above Fe-C alloy, as shown in Fig. 8.

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Fig. 8 Microstructure of Fe-15% Cr-12&Ni (Ms=- 90 °) alloy lath martensite (aqua regia, glycerin corrosion)

It does not have lath groups and sympositional bundles, but is formed as a thin lath group around a sheet of ε ‘- martensite (strips distributed in parallel in the figure).

However, the electron microscopic structure of this lath martensite is exactly the same as that in Fe-C and Fe Ni alloys.

2. Flake martensite

Another typical martensite structure in iron series alloys is lamellar martensite, which is commonly found in quenched high and medium carbon steels and high Ni Fe Ni alloys.

The typical lamellar martensite structure in high carbon steel is shown in Fig. 9.

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Fig. 9 Superheated quenching structure of T12A steel 400X (heated at 1000 ℃, water quenched)

This martensite is also called lenticular martensite because its spatial form is in the form of a biconvex lens sheet.

It is also called acicular martensite or bamboo leaf martensite because it is intersected with the grinding surface of the sample and appears as needle or bamboo leaf under the microscope.

The substructure of lamellar martensite is mainly twin, so it is also called twin martensite.

The microstructure of lamellar martensite is characterized by the fact that the lamellae are not parallel to each other.

In an austenitic grain with uniform composition, when it is cooled to a point slightly lower than Ms, the first martensite formed first will run through the entire austenitic grain and split the grain into two halves, limiting the size of martensite formed later.

Therefore, the size of lamellar martensite varies, and the smaller the martensite flakes formed later, as shown in Fig. 10.

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Fig. 10 Microstructure of lamellar martensite

The size of the flakes almost entirely depends on the grain size of austenite.

Flaky martensite can often be seen with obvious mid ridge (see Fig. 11).

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Fig. 11 Flake martensite (with obvious mid ridge, T12 steel is carburized at 1200 ℃ for 5 hours and quenched at 180 ℃)

The formation rule of mid ridge is not very clear at present.

The habit plane of lamellar martensite is (225) γ, Or (259) γ; The orientation relationship with parent phase is K-S relationship or Xishan relationship.

It can be seen from Fig. 12 that many fine lines in the martensite are transformation Luan crystals, and the banded thin ribs in the middle joint part are mid ridges.

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Fig. 12 TEM structure of lamellar martensite

The existence of transformation Luan crystal is an important feature of lamellar martensite.

The spacing of Luan crystals is about 50 Å, which generally does not extend to the boundary of martensite.

The edge of the sheet is a complex dislocation array.

It is generally believed that such dislocations are screw dislocations arranged regularly in the [111] α´ direction.

The transformation Luan crystal in lamellar martensite is generally (112)α´ Luan crystal.

However, in Fe-1.82% C (c/a=1.08) alloy, (110) a Luan crystal will be mixed with (112) α´ Luan crystal.

According to the difference of the internal substructure of the lamellar martensite, it can be divided into the transformation twin area (middle part) centered on the middle ridge and the twin free area (in the surrounding part of the lamella, there are dislocations).

The proportion of twin zone varies with alloy composition.

In Fe Ni alloys, the higher the Ni content (the lower the Ms point), the larger the twin zone.

According to the research of Fe-Ni-C alloy, even for the alloy with the same composition, the proportion of twin zone increases with the decrease of Ms point (such as caused by changing the austenitizing temperature).

However, the density of the transformation twins hardly changes. The thickness of twins is always about 50Å.

Lath martensite and lamellar martensite are the two most basic martensite morphology in steel and alloy.

Their morphological characteristics and crystallographic characteristics are listed in Table 1.

Table 1 Types and Characteristics of Martensite in Iron Carbon Alloys

Features

Lath martensite

Lamellar martensite

Habitual surface

(111) γ

(225) γ

(259) γ

orientation relationship

K-S relationship (111) γ lll(110) α ´【110】 γ 【111】 α.’

K-S relationship (111) γ lll(110) α ´【110】 γ 【111】 α.’

Xishan relationship (111) yll (110) α.’ 【211】 γ ll【110】 α.’

Formation temperature

M>350℃

M≈200~100℃

M.<100℃

Alloy composition% C

<0.3

1~1.4

1.4~2

Closed at 0.3~1

Histomorphology

The laths are usually arranged in parallel groups from the austenite grain boundary to the grain interior, and the lath width is usually 0.1~0.2 μ, length less than 10 μ. An austenitic grain contains several lath groups. There are small angle grain boundaries between lath bodies and large angle grain boundaries between lath groups.

The convex lens sheet (or needle, bamboo leaf) is slightly thicker in the middle, the primary one is thicker and longer, and it traverses the austenite grains, while the secondary one is smaller. Between the primary lamellae and the austenite grain boundary, the angle between the lamellae is large, and they collide with each other to form microcracks.

On the same left, there is a middle ridge in the center of the slice, and thin slices with zigzag distribution are common between the two primary slices.

Substructure

Dislocation network (entanglement), dislocation density increases with carbon content, usually (0.3~0.9) × A small amount of fine twins can sometimes be seen at 1012cm/cm3.

The fine twins with a width of about 50 | form the transformation Lie and twin regions with the middle ridge as the center. As the M point decreases, the transformation twin region increases, and the edge of the sheet is a complex dislocation array. The twin plane is (112) α  ※, the twin direction is [11I] α  ´

Formative process

Cooling nucleation, new martensite sheets (laths) are produced only during cooling

The growth speed is low, and a lath is formed in about 10-4s

The growth speed is high, and a sheet is formed in about 10-7s

There is no “explosive” transformation, and the cooling transformation rate is about 1%/℃ within less than 50% of the transformation amount

When M<0 ℃, there is an “explosive” transformation, and the new martensite sheet does not produce uniformly with the temperature drop, but because of the self triggering effect, it forms in groups (in the shape of “Z”) continuously and massively in a very small temperature range, accompanied by a temperature rise of 20~30 ℃

3. Other martensite morphology

3.1 Butterfly martensite

In Fe Ni alloys or Fe Ni C alloys, when martensite is formed within a certain temperature range, martensite with special morphology will appear, as shown in Fig. 13.

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Fig. 13 Microstructure of Dish Martensite

The three-dimensional shape of this martensite is slender rod, and its section is butterfly shaped, so it is called butterfly martensite or butterfly martensite.

It has been found that the butterfly martensite is formed in Fe-31% Ni or Fe-29% Ni-0.26% C alloy in the temperature range of 0~- 60 ℃.

The electron microscope study confirms that its internal substructure is high-density dislocation, and no twins can be seen.

The crystallographic relationship with the parent phase generally conforms to the K-S relationship. Butterfly martensite is mainly formed at 0~- 20 ℃, and coexists with lamellar martensite at – 20~- 60 ℃.

It can be seen that for the above two alloy systems, the formation temperature range of butterfly martensite is between the formation temperature range of lath martensite and lamellar martensite.

The joint of two wings of butterfly martensite is very similar to the mid ridge of lamellar martensite.

It is assumed that the martensite (probably twinning) growing from here to the two sides along different orientations will show butterfly shape.

The joint part of butterfly martensite is similar to the joint part of two pieces of martensite formed by explosion, but there is no twin in it, which is very different from that of sheet martensite.

From the internal structure and microstructure, butterfly martensite is similar to lath martensite, but it does not occur in rows.

Up to now, there are still many problems about butterfly martensite.

However, its morphology and properties are between lath martensite and lamellar martensite, which is an interesting problem.

3.2 Flaky martensite

This martensite was found in Fe-Ni-C alloy with very low Ms point.

It is in a very thin band (the three-dimensional figure is thin), and the bands cross each other, showing twists, branches and other special forms, as shown in Fig. 14c).

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Fig. 14 Fe-Ni-C Alloy Cooled to Ms Point

Microstructure of martensite formed at the same temperature

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The electron microscopic structure of this martensite is shown in Fig. 15.

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 Fig. 15 Electron microscopic structure of lamellar martensite (Fe-31%, Ni0.23% C, Ms=- 190 ℃, cooled to – 196 ℃)

It is a full Luan martensite composed of (112) α´  Luan crystals, without central ridge, which is different from the lamellar martensite.

It is pointed out that the morphology of Fe-Ni-C system martensite will change from lenticular to lamellar with the decrease of formation temperature.

In Fe-Ni-C alloy with carbon content of about 0.25%, when Ms=- 66 ℃, the structure is explosive lamellar martensite, as shown in Figure 14a);

When Ms decreases to – 150 ℃, a small amount of lamellar martensite begins to appear, as shown in Fig. 14 b);

When Ms point drops to – 171 ℃, it is all lamellar martensite (see Fig. 14c).

It is found that the transition temperature from lens sheet to thin sheet increases with the increase of carbon content.

When the carbon content reaches 0.8%, the formation zone of lamellar martensite is below – 100 ℃.

With the decrease of the transformation temperature, when the lamellar martensite transformation is going on, there is not only the continuous formation of new martensite sheets, but also the thickening of old martensite sheets.

Thickening of old martensite sheets is not visible in lamellar martensite.

3.3 ε’ Martensite

All the above martensites are martensites with body centered cubic or body centered square structure (α’ ).

In the alloy with low stacking fault energy of austenite, ε’ martensite with dense hexagonal lattice will also be formed.

This martensite is easy to form in high Mn-Fe-C alloys.

However, the Fe Cr Ni alloy represented by 18-8 stainless steel often coexists with α’-martensite.

ε’ martensite is also thin, as shown in Fig. 16.

Along (111) γ,  the surface is formed in the state of widmanstatten, and its substructure is a large number of stacking faults.

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Fig. 16 Martensite Microstructure of Fe-16.4% Mn Alloy (Corrosion by Nitrate Alcohol)

2. Relationship between chemical composition and martensite morphology and internal substructure of alloys

The content of alloying elements in steel has a significant effect on the morphology of martensite.

The typical example is that the martensite morphology of Fe-C and Fe Ni alloys changes from lath to flake with the increase of alloy content.

For example, in Fe-C alloy, below 0.3% C is lath, above 1% C is flake, and 0.3~1.0% C coexist.

However, in different data, the concentration limits for the transition from lath martensite to lamellar martensite are not consistent.

This is related to the influence of quenching speed. When the quenching speed increases, the minimum carbon concentration for twin martensite formation decreases.

Fig. 17 shows the effect of carbon content on the martensite type, Ms point and retained austenite content of Fe-C alloy.

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Fig. 17 Effect of carbon content on Ms point, lath martensite content and retained austenite content (carbon steel quenched to room temperature)

It can be seen from the figure that there is basically no retained austenite in the steel with carbon content less than 0.4%.

The Ms point decreases with the increase of carbon content, while the amount of Luan crystal martensite and retained austenite increases.

The relationship between martensite morphology and alloy composition of iron binary alloys is summarized in Table 2.

Table 2 Martensite Morphology of Fe Binary Alloys

Alloy system

Lath martensite

Lamellar martensite

Martensite

Alloy composition (%)

Point M (℃)

Alloy composition (%)

Point M (℃)

Alloy composition (5%)

Extended Y-zone

Fe-C

Fe-N

Fe-Ni

Fe-Pt

Fe-Mn

Fe-Ru

Fe-Ir

Fe-Cu

Fe-Co

<1.0

<0.7

<29

<20.5

<14.5

7.5~19

20~48

2~6

0~1

1~24

700~200

700~350

700~25

700~400

700~150

600~200

550~40

700~620

620~800

0.6~1.95

0.7~2.5

29~24

24.6

500~40

350~100

25~195

-30

14.5~27

11~17

35~53

Reduced Y area

Fe-Cr

Fe-Mo

Fe-Sn

Fe-V

Fe-W

<10

<1.94

<1.3

<0.5

<0.3

700~260

700~180

It can be seen from the table that all alloy elements in zone γ are reduced to lath martensite.

As the content of alloy elements in the expanded P zone increases, the general Ms point decreases significantly, and the martensite morphology also changes.

For example, in Fe-C, Fe-N, Fe Ni, Fe Pt and other binary alloys, the martensite morphology changes from lath to flake with the increase of alloy element content.

However, Mn, Ru and Ir can significantly reduce the stacking fault energy of austenite.

Therefore, in the binary iron alloy of these elements, with the increase of alloy element content, the morphology of martensite changes from lath to flake, but to ε´martensite.

Fe Cu and Fe Co alloys are two exceptions to the elements in the expanded γ-zone.

Although Cu belongs to the element of expanding the Y-zone, due to the small amount of solid solution in Fe, the Ms point does not decrease much, so it shows the same tendency as the alloy of shrinking the Y-zone.

Fe Co alloy is different from other alloys. With the increase of Co content, Ms point rises instead, so its situation is special.

Generally, there are many kinds of alloy elements in steel, but if the third element is added to Fe-C or Fe Ni alloy, when the amount is small, it can be considered that the martensite morphology is basically the same as that of binary alloy.

As previously mentioned, lath, butterfly, lens sheet and thin sheet martensite can be formed in Fe-Ni-C alloy.

The relationship between formation temperature of these four forms of martensite and carbon content and Ms point is shown in Fig. 18.

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Fig. 18 Relationship between martensite morphology, carbon content and Ms point of Fe-Ni-C alloy

As shown in the figure, the formation temperature of lenticular and lamellar martensite increases with the increase of carbon content.

The hatched area in the figure shows the formation area of butterfly martensite.

The relationship between the morphology, substructure and crystallographic characteristics of martensite in iron series alloys is summarized in Table 3.

Table 3 Characteristics of Fe System Martensite

Habitual surfaceorientation relationshipMartensite morphologySecond shear typeSubstructure in martensiteM. PointAustenite fault energySteel grade
(111)
(225)(259)
K-S
K-S
Xishan
LathSlip twinDislocationHigh
medium
low
Low
low or medium
high
Low carbon copper, high Mn steel, low Ni steel;
high and medium carbon steel, stainless steel, medium Ni steel;
high Ni steel, extremely high carbon steel

In steel, lath martensite with carbon content less than 0.20% is generally considered as body centered cubic lattice.

The martensite with carbon content greater than this value is a body centered square lattice.

Some people think that body centered cubic martensite is equivalent to dislocation martensite in low carbon steel, while body centered square martensite is equivalent to high carbon twin martensite.

However, in Fe Ni alloys, twin martensite is also body centered cubic.

Therefore, the relationship between crystal structure and substructure is still uncertain.

3. Factors affecting the morphology and substructure of martensite

The law of martensite morphology change caused by alloy composition change is discussed above.

As for what factors affect this change, there is much discussion at present, and there is no consensus.

Many people believe that morphological changes are ultimately changes in substructure, and typical views are as follows:

1. Ms point

Those who hold this view believe that the morphology of martensite depends on Ms point.

They think that in Fe-C alloy, the carbon content increases and Ms point decreases.

When it is lower than a certain temperature (300~320 ℃), it is easy to produce transformation twins, thus forming lamellar martensite.

The relationship between martensite morphology and crystal characteristics of carbon steel and carbon content and Ms temperature is listed in Table 4.

Table 4 Relationship between martensite morphology and crystallographic characteristics of carbon steel and carbon content and Ms point of steel

Carbon content (%)Crystal structureOrientation relationshipHabitual surfaceM. Point (℃)Martensite morphology
<0.3Body centered cubic or squareK-S relationship(111)>350Lath martensite
0.3~1.0Centroid squareK-S relationshipStrip (111), sheet (225)350~200Mixed martensite
1.0~1.4Centroid squareK-S relationship(225)<200Flake martensite with partial twins and dislocations in substructure
1.4~1.8Body · Heart SquareXishan relationship(259)<100Typical lamellar martensite with obvious mid ridge and “Z-shaped” arrangement

The reason for the transformation of martensite morphology from lath to flake with the decrease of Ms point can be explained as follows.

It can be seen from Table 4 that there is a certain relationship between the habit surface and the morphology of martensite.

It is generally believed that the formation temperature of low-carbon martensite is high.

At this time, (111) γ with large shear is taken as the habit plane.

At the same time, slip is easier to occur at a higher temperature than twinning, and there are fewer {111} γ crystal systems in the face centered cubic lattice.

Therefore, the number of initial orientations for the formation of martensite is small, which is conducive to the formation of clustered martensite in the same austenite.

As the temperature of Ms point decreases, twinning becomes easier to occur than slip.

At the same time, {225} γ or {259} γ is used as the habit plane to form martensite.

Because there are many crystal systems, the number of initial orientations of martensite formation increases.

Therefore, it is easy to form a Li crystal lamellar martensite with adjacent martensite sheets not parallel to each other in the same austenite.

It has been proved that if martensite is formed at high temperature, twin lamellar martensite cannot be formed even if austenite is strongly strengthened.

For Fe-Ni-C alloys, the Ms point can be changed by changing the austenitizing temperature.

Using this phenomenon, different Ms points can be obtained in the same alloy.

Observe the change of martensite morphology when the cooling temperature is slightly lower than the corresponding Ms point.

The results show that with the decrease of Ms point, the martensite will change from butterfly shape to sheet shape to sheet shape.

Moreover, as the formation temperature decreases, the transformation twin zone also gradually increases.

Similarly, the morphology of deformation induced martensite formed in the same alloy at various temperatures above Ms point is studied.

The results show that the morphology of martensite will change completely with the change of deformation temperature (that is, the formation temperature of deformation induced martensite).

This test at least determined that the martensite morphology and internal structure of this kind of alloy are only related to Ms point.

In addition, when Ms point decreases under high pressure, transformation twins are easy to occur, and martensite morphology changes from lath to sheet, as shown in Fig. 19.

This is also an experimental fact that supports Ms’s point.

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Fig. 19 Effect of 4000MPa Pressure on Ms Point and Martensite Substructure of Ferromagnetic Alloy

In the actual structure, many martensites are formed successively at various temperatures between Ms and Mf points.

The actual formation temperature of each martensite crystal is different, so the internal structure and morphology of each martensite crystal are certainly different.

Therefore, strictly speaking, it should not be said that Ms point affects the morphology or internal structure of martensite, but the formation temperature of martensite.

2. The stacking fault energy of austenite

This is a hypothesis put forward by Kelly et al. They believe that the lower the stacking fault energy of austenite, the more difficult it is to generate transformation Luan crystals, and the more inclined it is to form lath martensite.

The stacking fault energies of 18-8 stainless steel and Fe-8% Cr-1.1% C alloy are both low. Even at liquid nitrogen temperature, dislocation martensite is formed.

This phenomenon is difficult to explain with Ms point hypothesis, but it can be explained with this hypothesis.

In addition, in the lamellar martensite of Fe-30~33% Ni alloy, the transformation twin zone increases with the increase of Ni content.

Since Ni is considered as an alloying element to increase the stacking fault energy of austenite, this is an experimental phenomenon supporting this hypothesis.

Of course, this experimental phenomenon can also be explained by Ms point theory (because Ni decreases Ms point).

3. Strength of austenite and martensite

This is a hypothesis recently proposed by Davis and Magee.

They used alloying method to change the strength of austenite, and studied the corresponding relationship between the change of martensite morphology and the change of austenite strength.

The results show that the martensite morphology changes with the austenitic yield strength (about 206MPa) at Ms point as the boundary.

Above this boundary, the lamellar martensite with the habit plane of {259} γ is formed.

Below this boundary, the lath martensite with the habit plane of {111} γ or the lamellar martensite with the habit plane of {225} γ is formed.

Therefore, they believe that the strength of austenite is the decisive factor affecting the morphology (habit plane) of martensite.

They also further studied the strength of martensite.

When the strength of austenite is lower than 206MPa, there are two cases. When the strength of martensite formed is higher, it is {225}γ martensite.

When the strength of martensite is low, {111} γ martensite is formed.

This hypothesis can be used to explain the morphological change caused by the change of alloy composition or Ms point, especially the change of {111}γ → {225} γ in Fe Ni alloys and {111} γ → {225}γ → {259}γ in Fe-C alloys.

This hypothesis also has a clear concept of {225} γ martensite, which is not very clear in the past. It is formed when weak austenite transforms into strong martensite.

C has little effect on strengthening austenite, but it is an element that can significantly strengthen martensite, and {225} γ martensite mostly occurs in alloy systems with high carbon content.

This hypothesis is based on the following:

If the relaxation of transformation stress in martensite is only carried out in the form of twinning deformation, the martensite with the habit plane of {259} γ is obtained;

If the relaxation of transformation stress is carried out partly in the austenite in slip mode and partly in the martensite in twinning mode, the martensite with the habit plane of {225} γ is obtained.

If the sliding mode is also used in the martensite, the martensite with the habit plane {111} γ is obtained.

From many experimental results, it can be seen that the above hypothesis is correct to some extent, but it cannot be concluded that further research is still needed in the future.

In addition, the strength of austenite and martensite emphasized by this hypothesis must be closely related to alloy composition, type, Ms point, austenitic stacking fault energy and other factors, so this hypothesis is not isolated.

4. Critical shear stress of martensite slip and twin deformation

This hypothesis emphasizes that the internal structure of martensite depends on whether the deformation mode during transformation is slip or twin, so in the final analysis, it is dominated by the critical shear stress of the two.

Fig. 20 schematically shows the influence of the critical shear stress of martensite slip or twin and the temperature of Ms and Mf on the morphology of martensite formed.

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Fig. 20 Schematic Diagram of the Influence of Critical Shear Stress and Ms Mf Temperature on Martensite Morphology Caused by Martensite Slip or Twin

The arrows in the figure indicate the possible moving direction of the corresponding lines, which is caused by the change of alloy composition.

The movement of the lines will result in the movement of the intersection of the slip twin curves.

It can be seen from the figure that for low carbon steel (Ms point and Mf point are both high), the critical shear stress required to cause slip is lower than that required to cause twinning.

Therefore, lath martensite with high dislocation density is obtained.

On the contrary, if it is a high carbon steel (Ms and Mf are both low), the critical shear stress required to cause twinning is small, thus obtaining lamellar martensite containing a large number of twins.

If the carbon content is medium, Ms and Mf are just as shown in the figure. In the process of martensitic transformation, lath martensite is formed first, and then lamellar martensite can be formed.

That is, the mixed structure of two kinds of martensite is formed.

This view seems to be correct in essence.

However, what factors will cause the change of shear stress, and how the alloy composition or Ms point affect the martensitic slip or twin critical shear stress are still unclear.

There are other viewpoints, such as the transformation to lamellar martensite will be caused by the increase of transformation driving force.

For Fe-C alloy, the driving force limit of martensite morphology change is 1318J/mol, and for Fe Ni alloy, it is 1255~1464J/mol.

Others believe that the ordering caused by the increase of C and N content in martensite is closely related to the morphological transformation, etc.

4. Formation of lamellar martensite microcracks in Fe-C alloy

When high carbon steel is quenched, microcracks are easily formed in martensite.

In the past, it was believed that microcracks were caused by microstress caused by the increase of specific volume during martensitic transformation.

Metallographic observation in recent years shows that microcracks are formed due to the mutual collision of martensite during growth, as shown in Fig. 21.

5 Martensite Morphologies You Should Know 22

Fig. 21 Schematic diagram of microcracks formed by collision of two Fe-C martensite sheets (Section A-A represents the cross section of one martensite sheet, which diffuses to two martensite sheets)

The formation of martensite is very fast. When the martensite collides with each other or with the austenite grain boundary, a considerable stress field will be caused due to impact.

Because the high carbon martensite is very brittle and cannot be relieved by slip or twin deformation, it is easy to form impact cracks.

This congenital defect makes the high carbon martensite steel add brittleness.

Under the action of other stresses (thermal stress and structural stress), the micro cracks will develop into macro cracks.

At the same time, the existence of microcracks will significantly reduce the fatigue life of the parts.

The microcracks in the lamellar martensite of Fe-C alloy often appear at the radial junction of several martensite needles or in the martensite needles, as shown in Fig. 22.

5 Martensite Morphologies You Should Know 23

Fig. 22 Optical microscopic characteristics of microcracks in martensite of Fe-1.39% C alloy

Generally, the area of microcracks appearing in unit martensite volume is taken as the sensitivity of microcracks forming in martensite, expressed in Sv.

The experiment indicates that the sensitivity of martensite to microcrack formation is affected by the following factors:

1. Effect of quenching cooling temperature

With the decrease of quenching cooling temperature, the amount of retained austenite (represented by γR) in the quenching structure of steel decreases, the amount of martensite increases, and the sensitivity to microcrack formation increases, as shown in Fig. 23.

5 Martensite Morphologies You Should Know 24

Fig. 23 Relationship between Fe-C martensite formation microcrack sensitivity and quenching temperature (1.39% C, heated at 1200 ℃ for 1 hour)

2. Effect of martensite transformation amount

Fig. 24 shows the effect of martensite transformation amount on the susceptibility to microcrack formation.

5 Martensite Morphologies You Should Know 25

Fig. 24 The relationship between the micro crack sensitivity (SV) of martensite formation in Fe-1.86% C alloy and the average volume (V) of each piece of martensite, the number of martensite sheets in unit volume (NV) and the transformation of martensite:

It can be seen from the figure that, with the increase of martensite transformation variable, the sensitivity to microcrack formation Sv increases, but when the transformation fraction (f) is greater than 0.27, Sv will not increase.

Although the number of martensite in unit volume Nv increases, because the austenite is continuously divided, the size of the martensite sheet formed (represented by the average volume V of a piece of martensite) will decrease.

Therefore, the size (V) of martensite sheet may have a critical value for the sensitivity Sv to microcrack formation.

When it is greater than this critical value, the sensitivity Sv to microcrack formation will increase with the increase of transformation fraction.

Therefore, the formation of cracks is largely determined by the size of martensite sheets.

Although the total number and area of cracks may increase when the martensite transformation variable increases, the martensite flakes formed at the early stage are large, so the cracks are mainly formed at the early stage of transformation.

3. Effect of martensite sheet length

It has been directly observed in the experiment that the susceptibility of martensite to microcrack formation increases with the increase of martensite sheet length (i.e. the maximum size of the sheet), as shown in Fig. 25.

5 Martensite Morphologies You Should Know 26

Fig. 25 Relationship between the sensitivity of microcrack formation and the length of martensite sheet (the number beside the point is the martensite content%)

This is because long martensite sheets have more chances to be impacted by other martensite sheets.

At the same time, it tends to cross the austenite grain, so there is a great chance to meet the grain boundary.

Many experimental observations have confirmed that microcracks are mainly formed in coarse martensite, while when the martensite is very fine, microcracks rarely occur.

Therefore, there should be a critical martensite size for the formation of microcracks in martensite.

If the austenite composition is relatively uniform, there will also be a critical austenite grain size below which microcracks will not occur.

The view that fine austenite grains can reduce the microcracks of quenched high carbon steel has been applied to production practice.

At present, it is not clear whether the microcrack sensitivity depends on the size of the martensite sheet itself or on the stress field caused by the growth of the critical size martensite sheet.

4. Effect of austenite grain size

In case of the homogeneous austenite, length of the martensite sheet formed at initial stage is related to austenite grain size.

Coarse austenite grains form coarse martensite, which is easy to promote the formation of microcracks.

The experimental results in Figure 26 confirm this. This shows that high carbon steel is easy to form cracks when quenched at a higher temperature.

Therefore, generally, lower quenching temperature should be selected for quenching of high carbon steel.

5 Martensite Morphologies You Should Know 27

Fig. 26 Effect of Austenite Grain Size of Carbon Steel (1.22% C) on Field Microcrack Sensitivity

5. Effect of carbon content in martensite

The influence of carbon content in martensite on the susceptibility to microcrack formation is shown in Fig. 27.

5 Martensite Morphologies You Should Know 28

Fig. 27 Effect of carbon content in martensite on microcrack sensitivity

It can be seen from the figure that the sensitivity of microcrack formation increases with the increase of carbon content in martensite.

When the carbon content in austenite is more than 1.4%, the susceptibility to microcrack formation decreases, which is related to the habit plane of the crystal during martensitic transformation.

When the carbon content in the steel is greater than 1.4%, the martensite morphology changes, the sheets become thick and short, the included angle between the martensite sheets becomes smaller, and the impact opportunity and stress are reduced, so the microcrack sensitivity is reduced instead.

The microcrack sensitivity measured in 1.39% C steel decreases significantly with the decrease of carbon content in martensite, and the results are listed in Table 5.

Table 5 Effect of carbon content in martensite on susceptibility to microcrack formation (grain size 3)

A1~Aw temperature (℃)

Carbon content in martensite (%)

Retained austenite (%)

Carbide quantity (%)

Sensitivity to microcrack formation S. (mm-1)

1010

910

871

857

834

799

768

732

1.39

1.30

1.21

1.18

1.05

1.01

0.92

0.83

33.5

22

15

13

12

8

9

6

3.9

6

6.5

12

15

17.5

20

18

17

13

9

10

4.5

1.5

0.15

The metallographic observation shows that the decrease of microcrack sensitivity is related to the occurrence of more parallel growth lath martensite in the microstructure.

The lath martensite has good plasticity and toughness, and the chance of mutual impact is reduced due to the parallel growth of lath martensite, so the microcrack sensitivity is low.

It can be seen from the above that the high carbon steel is easy to crack due to the coarse austenite grain and the high carbon content of martensite.

Therefore, the production tends to adopt lower heating temperature and shorter holding time to reduce the carbon content in martensite and obtain fine grains.

Generally, hypereutectoid steels obtain cryptocrystalline martensite by incomplete quenching, which is not easy to produce microcracks, which is the reason why they have good comprehensive properties.

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