Transformation of Steel During Cooling: Widmanstatten and Martensite

Transformation of Steel During Cooling – Widmanstatten

1. Formation of Widmanstatten structure

In actual production, ωc < 0.6% hypoeutectoid steel and ωc > 1.2% hypereutectoid steel is cooled by air after casting, hot rolling and forging, and the weld seam or heat affected zone is cooled by air, or when the temperature is too high and cooled at a high speed, the pre eutectoid ferrite or pre eutectoid cementite grows from the austenite grain boundary along a certain crystal plane of austenite to the crystal, and precipitates in a needle like manner.

2. Microstructure of Widmanstatten structure

Under the metallographic microscope, the nearly parallel or other regularly arranged acicular ferrite or cementite and the pearlite structure between them can be observed.

This structure is called widmanstatten, and the following figure is the widmanstatten of ferrite and cementite.

Transformation of Steel During Cooling: Widmanstatten and Martensite 1
Transformation of Steel During Cooling: Widmanstatten and Martensite 2

3. Formation mechanism of Widmanstatten structure

The ferrite in widmanstatten is formed according to the shear mechanism, which is similar to that in bainite, and the sample will also be convex.

Because ferrite is formed at a high cooling rate, ferrite can only precipitate along a specific crystal surface of austenite, and there is a crystal orientation relationship with the parent phase austenite.

The acicular ferrite can be precipitated directly from austenite, or the network ferrite can be precipitated along the austenite grain boundary, and then grow into the crystal in parallel from the network ferrite.

When the ferrite in the widmanstatten structure is formed, the carbon in the ferrite diffuses into the parent phase austenite on both sides, so that the austenite carbon content between the ferrite needles increases continuously and finally turns into pearlite.

The ferrite of Widmanstatten structure formed by bainite transformation mechanism is actually carbon free bainite.

4. Influencing factors

The formation of Widmanstatten structure is related to the carbon content, austenite grain size and cooling rate (transformation temperature) in the steel.

The following figure shows the formation temperature and carbon content range of various ferrite and cementite.

It can be seen from the figure that the widmanstatten structure (W zone) can be formed only in a relatively fast cooling rate and a certain carbon content range.

When the mass fraction of carbon in the hypoeutectoid steel exceeds 0.6%, it is difficult to form the widmanstatten structure because of the high carbon content and the low probability of forming the carbon poor zone.

The results show that for hypoeutectoid steel, when the austenite grain is fine, only when the carbon content is in the narrow range of ωc= 0.15% ~ 0.35% and the cooling rate is fast, the widmanstatten structure can be formed.

The finer the austenite grain, the easier it is to form network ferrite, but not to form widmanstatten structure.

The coarser the austenite grain is, the easier it is to form the widmanstatten structure, and the range of carbon content to form the widmanstatten structure becomes wider.

Therefore, widmanstatten structure usually appears with austenite coarse grain structure.

Transformation of Steel During Cooling: Widmanstatten and Martensite 3

5. Properties of Widmanstatten structure

(1) Widmanstatten is a kind of superheated structure of steel.

It can significantly reduce the mechanical properties of steel, especially the impact toughness and plasticity, and increase the brittle transition temperature of steel, so that the steel is prone to brittle fracture.

(2) It is generally pointed out that the strength and impact toughness of the steel will be significantly reduced only when the austenite grain is coarsened, coarse ferrite or cementite widmanstatten structure appears and the matrix is seriously cut.

However, when the austenite grain is relatively fine, even if there is a small amount of acicular ferrite widmanstatten structure, the mechanical properties of the steel will not be significantly affected.

This is due to the finer substructure and higher dislocation density of ferrite in widmanstatten structure.

Therefore, the reduction of the mechanical properties of the steel by the widmanstatten structure is always associated with the coarsening of austenite grains.

(3) When the widmanstatten structure appears in steel or cast steel and reduces its mechanical properties, we should first consider whether it is caused by the coarsening of austenite grain due to the high heating temperature.

(4) For steels prone to widmanstatten structure, the widmanstatten structure can be prevented or eliminated by controlling rolling, reducing the final forging temperature, controlling the cooling rate after forging (binding), or changing the heat treatment process, such as quenching and tempering, normalizing, annealing, isothermal quenching and other processes to refine grain.

6. Appreciation of Widmanstatten structure

Transformation of Steel During Cooling: Widmanstatten and Martensite 4
Transformation of Steel During Cooling: Widmanstatten and Martensite 5
Transformation of Steel During Cooling: Widmanstatten and Martensite 6

Transformation of Steel During Cooling – Martensite

Structure, structure and properties of martensite crystal

1. Definition

(1) Martensitic transformation: the non diffusive phase transformation that occurs when the steel is rapidly cooled from the austenitic state to inhibit its diffusive decomposition (lower than the MS point) is called martensitic transformation.

It is worth noting that the basic characteristics belong to the transformation of martensite, and the transformation products are all called martensite.

(2) Martensite: in essence, martensite in steel is a interstitial solid solution in which carbon is supersaturated in α- Fe.

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Fig. two types of solid solution

2. Crystal structure of martensite

The martensitic crystal structure includes:

  • Body centered cubic: Martensite in low carbon steel or carbon free alloy
  • Body center square: Martensite in steels with high carbon content
  • Hexagonal lattice: Martensite in complex iron base alloys at low temperature
Transformation of Steel During Cooling: Widmanstatten and Martensite 8
Schematic diagram of body centered square lattice of martensite

3. Microstructure of martensite

There are two basic forms of martensite in steel: lath martensite (dislocation martensite) and lamellar martensite (also known as needle martensite).

(1) Lath martensite

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

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Low carbon martensite 500×

a) Structural morphology: martensite lath (D) → martensite bundle (B-2; C-1) → lath group (3-5) → lath martensite.

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Schematic diagram of slat martensite microstructure

b) The dense laths are usually separated by residual austenite with high carbon content.

The presence of this thin layer of residual austenite can significantly improve the mechanical properties of the steel.

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Fig. thin film transmission structure of slat martensite

c) There are a large number of dislocations in lath martensite, and the distribution of these dislocations is not uniform.

It forms a cellular substructure, called dislocation cell, so it is also called dislocation martensite.

(2) Lamellar martensite

High carbon steel( ω C > 0.6%), ωNi = 30% stainless steel and some non-ferrous metals and alloys.

Related reading: Ferrous vs Non-ferrous Metals

a) Structural morphology: the spatial morphology of sheet-like martensite is convex lens.

Because the polishing of the sample is cut off from its cross section, it is needle like or bamboo leaf like under the optical microscope.

Therefore, sheet-like martensite is also called needle like martensite or bamboo leaf like martensite.

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High carbon martensite

b) Microstructure characteristics: the martensite sheets are not parallel to each other.

In an austenite grain, the martensite formed by the first sheet often runs through the whole austenite grain and is divided into two parts, so that the size of the martensite sheets formed later becomes smaller and smaller.

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High carbon lamellar martensite

c) Size: the maximum size depends on the original austenite grain size.

The larger the austenite grain, the coarser the martensite sheet.

d) Cryptocrystalline Martensite: when the largest piece of martensite is too small to be distinguished by an optical microscope, it becomes “cryptocrystalline martensite”.

The martensite obtained by normal quenching in production is generally cryptocrystalline martensite.

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Cryptocrystalline martensite

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

Twins are usually distributed in the middle of the martensite, and do not extend to the edge region of the martensite sheet.

There are high-density dislocations in the edge region.

In the steel with carbon content ωC > 1.4%, the middle ridge line in the martensite sheet can be seen, which is a fine twin region with high density.

f) Microcracks: the formation speed of martensite is very fast, and a considerable stress field will be generated when they collide with each other or austenite grain boundaries.

The lamellar martensite itself is hard and brittle, and the stress cannot be relaxed by sliding or twin deformation, so it is easy to form impact cracks.

Generally, the larger the austenite grain, the larger the martensite sheet, and the more microcracks after quenching.

The existence of microcracks increases the brittleness of high carbon steel parts.

Under the action of internal stress, the microcracks will gradually expand into macro cracks, which can lead to the cracking of the workpiece or the obvious reduction of the fatigue life of the workpiece.

g) The morphology of martensite mainly depends on the carbon content of austenite, which is related to the martensite transformation start temperature MS point of steel.

The higher the carbon content of austenite, the lower the MS and MF points.

Carbon contentShapeFormation temperature (general)
ωC<0.2%lath martensiteAbove 200 ℃
ωC>0.6%plate martensiteBelow 200 ℃
ωC=0.2%~1%Lath and sheet mixed structureThe board horse is formed first, and then the piece horse is formed
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h) Effect of elements on martensite morphology: elements of Cr, Mo, Mn, Ni (decreasing MS point) and Co (increasing MS point) all increase the tendency to form lamellar martensite.

4. Properties of martensite

(1) The mechanical properties of martensite are characterized by high strength and high hardness.

(2) Effect of carbon content on martensite properties: hardness mainly depends on carbon content.

When ωC < 0.5%, the hardness of martensite increases sharply with the increase of carbon content.

When ωC > 0.6%, although the hardness of martensite increases, the hardness of steel decreases due to the increase of residual austenite content.

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(3) Alloying elements have little effect on the hardness of martensite, but can improve the strength.

(4) The hardness of martensite with high strength and high hardness is various, mainly including solution strengthening, phase transformation strengthening and aging strengthening.

The details are as follows:

Solid solution strengthening: the interstitial atoms are in the octahedral gap of the α-phase lattice, causing the square distortion of the lattice and forming a stress field.

The stress field interacts strongly with dislocations to improve the strength of martensite.

Phase transformation strengthening: during the transformation of martensite, lattice defects with high density are caused in the crystal. Both high-density dislocations in lath martensite and twins in lamellar martensite hinder the dislocation movement, thus strengthening martensite.

Aging strengthening: after the formation of martensite, the atoms of carbon and alloy elements diffuse, segregate or precipitate to dislocations or other lattice defects, pin dislocations, and make dislocations difficult to move, thus causing martensite strengthening.

(5) The smaller the size of martensite lath group or martensite sheet, the higher the strength of martensite;

This is because the martensite phase interface hinders the dislocation movement. The smaller the original austenite grain, the higher the martensite strength.

The plasticity and toughness of martensite mainly depend on its substructure.

Twin martensite: high strength, but poor toughness.

Dislocation martensite: high strength and good toughness.

(6) Volume of Martensite: among various structures in steel, the specific volume of austenite is the smallest and that of martensite is the largest;

Therefore, the volume expansion of the steel during quenching is one of the main reasons for the large internal stress, deformation and even cracking of the workpiece during quenching.

Characteristics of martensite transformation

Like other solid phase transformations, the driving force of martensite transformation is the chemical free energy difference between the new phase and the parent phase, that is, the free energy difference between martensite and austenite per unit volume.

Phase change resistance is also the interface energy and strain energy when new phase is formed.

Although there is a coherent interface with austenite during the formation of martensite, and the interface energy is very small, due to the large coherent strain energy, especially the large difference between the specific volume of martensite and austenite and the need to overcome the shear resistance and generate a large number of lattice defects, the large elastic strain energy is increased, which leads to a large resistance to the transformation of martensite, and a sufficient undercooling is required to make the transformation driving force greater than the transformation resistance, so that the transformation from austenite to martensite occurs.

The starting temperature ms of martensite transformation can be defined as the temperature when the free energy difference between martensite and austenite reaches the minimum driving force required for transformation.

Martensite transformation is the transformation of undercooled austenite in the low temperature range.

Compared with pearlite transformation and bainite transformation, it has the following series of characteristics:

1. Non diffusion of martensite transformation

The martensite transformation is carried out by austenite under great undercooling.

At this time, the activity ability of iron atom, carbon atom or alloy element atom is very low. Therefore, the martensite transformation is carried out without diffusion.

Only the reconstruction of lattice rules, the new phase and parent phase have no composition change.

2. Shear coherence of martensite transformation

Shear: the deformation caused by two parallel forces that are close, equal in size and opposite in direction acting on the same object.

During the martensite transformation, the upper surface of the pre polished specimen is inclined and the surface is convex.

This phenomenon shows that the transformation of martensite is directly related to the macroscopical of parent phase, and that martensite is formed by shear.

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Martensite and parent phase austenite remain coherent, and the atoms on the interface belong to both martensite and austenite.

The phase interface is a shear coherent grain boundary, also known as the habitual surface;

Martensite transformation is a phase transformation process in which the new phase is formed on the specific crystal plane and habitual plane of the parent phase, and the coherent relationship is maintained by the shear of the parent phase.

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3. Martensite transformation is carried out in a temperature range

Nucleation position of martensite

It is not uniformly distributed in the alloy, but in some favorable positions in the parent phase (lattice defects, deformation regions, carbon poor regions).

Martensitic transformation process

Like other solid-state phase transitions, they also occur through nucleation and growth;

Martensite transformation is a short-range migration of atoms.

After the formation of crystal nucleus, the growth rate is very fast (102 ~ 106mm / s), and it can still grow at a high speed even at a very low temperature.

Martensite transformation rate

Depending on the nucleation rate of martensite, the transformation is terminated when all the nuclear embryos larger than the critical nucleation radius are exhausted.

The larger the undercooling degree is, the smaller the critical nucleation size is.

Only further cooling can make the smaller nuclear embryo become nucleation and grow into martensite.

For general industrial carbon steel and alloy steel, martensite transformation is carried out during continuous (variable temperature) cooling.

The austenite in the steel is cooled below the MS point at a speed greater than the critical quenching speed, and a certain amount of martensite is formed immediately.

The transformation has no incubation period;

With the decrease of temperature, a certain amount of martensite is formed, and the martensite formed first does not grow.

The martensitic transformation increases with the decrease of temperature.

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Relationship between martensite transformation and temperature

The amount of martensite transformation depends only on the temperature reached by cooling and has nothing to do with the holding time.

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Retained austenite

If the Ms point of high carbon steel and many alloy steels is above room temperature and the Mf point is below room temperature, a considerable amount of untransformed austenite will remain after quenching and cooling to room temperature, which is called retained austenite;

If “cold treatment” is used to completely transform the retained austenite, it can be put into liquid nitrogen for treatment.

Factors affecting the number of Paralympics: the higher the carbon content, the more Paralympics, and the more elements that reduce MS, the more Paralympics.

Mechanical stabilization of retained austenite: the stabilization phenomenon caused by large plastic deformation or compressive stress of austenite during quenching is called mechanical stabilization of austenite.

The retained austenite is related to mechanical stabilization.

The austenite surrounded by martensite is in a compressed state and cannot be transformed and remains.

Deformation induced martensite (deformed martensite)

Plastic deformation of austenite above MS point can cause martensite transformation.

The larger the deformation amount, the more the martensite transformation amount.

This phenomenon is called deformation induced martensite transformation.

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4. Reversibility of martensite transformation

Reversibility: in some iron, gold, nickel and other non-ferrous metals, austenite is transformed into martensite upon cooling, and the martensite formed upon reheating can be transformed into austenite without diffusion.

Generally, the reverse transformation according to the martensite transformation mechanism does not occur in carbon steel, because the martensite has been decomposed into ferrite and carbide during heating;

This process is called tempering.

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