Austenite: Do You Know Its Metallographic Structure?

English Name: austinite; the name comes from: William Chandler Roberts-Austen, a British metallurgist

Letter code: A, γ.

Definition: solid solution formed by carbon and various chemical elements in γ-Fe.

Features:

  • The grain boundary is relatively straight and regular polygon;
  • The residual austenite in quenched steel is distributed in the space between martensitic needles.
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1. Crystal structure

Austenite (γ-Fe) has a face centered cubic structure with a maximum void of 0.51 × 10-8cm, slightly smaller than the carbon atom radius, so its carbon dissolving capacity is greater than that of α-Fe.

At 1148 ℃, the maximum dissolved carbon content of γ-Fe is 2.11%.

With the decrease of temperature, the dissolved carbon capacity gradually decreases.

At 727 ℃, the dissolved carbon content is 0.77%.

Austenite: Do You Know Its Metallographic Structure? 2

Face centered cubic structure

2. Properties of austenite

Mechanical properties

(1) Low yield strength and hardness

(2) High plasticity and toughness

(3) High thermal strength

Physical property

(1) Small specific volume, physical performance

(2) Poor thermal conductivity

(3) Large linear expansion coefficient

(4) Paramagnetism

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(a) Paramagnetism; (b) Ferromagnetism

Spontaneous arrangement of atomic magnetic moments in a small region.

Application performance

(1) Application performance of deformation forming

(2) Corrosion resistance of austenitic stainless steel

(3) Sensitive element of expansion instrument

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3. Formation of austenite

Thermodynamic conditions for Austenite Formation: there is undercooling or superheating T.

Nucleation of austenite

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(a) A-nucleation

The nucleation of austenite is diffusion type phase transformation.

Nucleation can be formed on the interface between ferrite and cementite, pearlite and austenite.

These interfaces are easy to satisfy the three fluctuation conditions of nucleation energy, structure and concentration.

Growth of austenite crystal nucleus

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(b) A grow up

When heated to the austenite phase region, at high temperature, carbon atoms diffuse rapidly, iron atoms and replacement atoms can fully diffuse, both interface diffusion and body protection can be carried out.

Therefore, the formation of austenite is a diffusion type phase transformation.

Dissolution of stripped carbide

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(c) Residual FeC dissolution

After the ferrite disappears, when the ferrite is kept or heated at t1 temperature, the residual cementite continuously dissolves into the austenite as the carbon continues to diffuse in the austenite.

Homogenization of austenite composition

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(d) A homogenization

When the cementite has just been completely separated into austenite, the carbon concentration in austenite is still uneven.

Only after a long time of heat preservation or continuous heating, and the carbon atoms continue to fully diffuse, can the austenite with uniform composition be obtained.

Note: there are some differences in the austenite nucleation process of various steels.

In addition to the basic process of Austenite Formation, there are also the dissolution of pre eutectoid phase and the dissolution of alloy carbide in the austenitizing process of hypoeutectoid steel, hypereutectoid steel and alloy steel.

4. Display of original austenite grain boundary

The size of original austenite grain has a great influence on the mechanical properties and technological properties of metal materials.

Reagent formulation

50 ml of distilled water, 2-3 g of picric acid and 1-2 drops of detergent.

Matters needing attention

Heat the prepared reagent to about 60 ° C, and then put the sample into erosion for 10-15 minutes.

At this time, the surface of the sample has become black.

Take out and wipe the black film on the surface of the sample with degreasing cotton until it is gray, and dry it for observation.

If the corrosion is too shallow, the corrosion can be continued; If the corrosion is too deep, gently polish it.

Note: for some samples whose original austenite grain boundaries are difficult to be displayed, erosion polishing, re erosion, re polishing and repeated several times are required.

The time of erosion and polishing is shorter than that of each time until satisfactory.

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Grain boundary of original austenite in 40Cr quenched state

5. Factors affecting austenite formation rate

Heating temperature

With the increase of heating temperature, the diffusion rate of atoms is rapidly accelerated, and the austenitizing speed is greatly increased and the forming time is shortened.

Heating speed

The faster the heating speed is, the shorter the incubation period is, the higher the temperature at which austenite begins to transform and the temperature at which transformation ends, and the shorter the time required for the end of transformation.

Alloying element

Cobalt and nickel accelerate the austenitizing process;

Chromium, molybdenum and vanadium slow down the austenitizing process;

Silicon, aluminum and manganese do not affect the bainization process of austenite alloy elements.

Since the diffusion speed of alloy elements is much slower than that of carbon, the heat treatment heating temperature of alloy steel is generally higher and the holding time is longer.

Original tissue

When the cementite in the original structure is flake, the austenite formation speed is faster, and the smaller the cementite spacing, the faster the transformation speed.

At the same time, the carbon concentration gradient in the original austenite grain is larger, so the growth group speed is faster.

The spheroidized annealed granular pearlite has fewer phase interfaces, so the austenitization is the fastest.

6. Factors affecting austenite grain growth

Chemical composition

① In a certain range of carbon content, the carbon content in austenite increases and the grain growth tendency increases.

However, when C% exceeds a certain amount, the austenite grain growth is hindered.

② The addition of titanium, vanadium, niobium, zirconium, aluminum and other elements to the steel is conducive to obtaining essentially fine grain steel, because carbides, oxides and nitrides are dispersed on the grain boundary, which can hinder grain growth.

Manganese and phosphorus promote grain growth.

③ Strong carbide forming elements, which disperse in austenite and hinder austenite grain growth;

Non carbide forming elements Si and n have little effect on austenite grain growth.

Heating temperature

Because the growth of austenite grain is closely related to the atomic diffusion in the heating temperature system, the higher the temperature, or the longer the holding time at a certain temperature, the coarser the austenite grain.

Heating speed

The faster the heating speed, the greater the superheat, the higher the actual formation temperature of austenite, and the increase of nucleation rate is greater than the growth rate, which makes the austenite grain finer.

In production, rapid heating and short-time heat preservation are often used to obtain ultra-fine grains.

Original organization

Generally speaking, the finer the original structure of steel, the larger the carbide dispersion, the finer the austenite grain.

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