English name: austinite; the name comes from: William Chandler Roberts-Austen, a British metalworker
Letter code: A, γ.
Definition: solid solution formed by carbon and various chemical elements in γ-Fe.
Characteristics:
- The grain boundary is a relatively straight and regular polygon;
- The residual austenite in the hardened steel is distributed in the space between the martensitic needles.
1. Crystal structure
Austenite (γ-Fe) has a face-centered cubic structure with a maximum void of 0.51 × 10 -8 cm, slightly smaller than the radius of the carbon atom, therefore its carbon dissolving capacity is greater than that of α- Faith.
At 1148 ℃, the maximum dissolved carbon content of γ-Fe is 2.11%.
As the temperature decreases, the dissolved carbon capacity gradually decreases.
At 727 ℃, the dissolved carbon content is 0.77%.
Face-centered cubic structure
2. Properties of austenite
Mechanical properties
(1) Low yield strength and hardness
(2) High plasticity and toughness
(3) High thermal resistance
physical property
(1) Small specific volume, physical performance
(2) Poor thermal conductivity
(3) Large linear expansion coefficient
(4) Paramagnetism
(a) Paramagnetism; (b) Ferromagnetism
Spontaneous arrangement of atomic magnetic moments in a small region.
Application performance
(1) Deformation forming application performance
(2) Corrosion resistance of austenitic stainless steel
(3) Expansion instrument sensitive element
3. Formation of austenite
Thermodynamic conditions for Austenite Formation: there is subcooling or superheating T.
Austenite nucleation
Austenite nucleation is a diffusion-type phase transformation.
Nucleation can be formed at the interface between ferrite and cementite, pearlite and austenite.
These interfaces are easy to satisfy the three conditions of energy fluctuation, structure and nucleation concentration.
Austenite crystal core growth
When heated in the austenite phase region at high temperature, carbon atoms diffuse quickly, iron atoms and substitution atoms can diffuse completely, both interface diffusion and body protection can be realized.
Therefore, the formation of austenite is a diffusion-type phase transformation.
Dissolving peeled carbide
After the ferrite disappears, when the ferrite is maintained or heated at temperature t1, the residual cementite continually dissolves into the austenite as carbon continues to diffuse into the austenite.
Homogenization of austenite composition
When cementite has just been completely separated into austenite, the carbon concentration in the austenite is still uneven.
Only after a long time of heat preservation or continuous heating, and the carbon atoms continue to fully diffuse, can 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 is also the dissolution of the pre-eutectoid phase and the dissolution of the alloy carbide in the austenitization process of hypoeutectoid steel, hypereutectoid steel and alloy steel.
4. Original austenite grain boundary display
The size of the original austenite grain has a great influence on the mechanical and technological properties of metallic 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 place the sample to erode for 10 to 15 minutes.
At this time, the surface of the sample turned black.
Remove and clean the black film on the sample surface with degreasing cotton until it turns gray and dry for observation.
If the corrosion is very superficial, the corrosion may continue; If the corrosion is very deep, polish it gently.
Note: For some samples whose original austenite grain boundaries are difficult to display, erosion polishing, re-erosion, repolishing and repeated multiple times are required.
The erosion and polishing time is less than each time until satisfactory.
Original austenite grain boundary in quenched state 40Cr
5. Factors affecting the rate of austenite formation
Heating temperature
As the heating temperature increases, the diffusion rate of atoms accelerates rapidly, leading to an increase in the austenitization speed and a shortening of the formation time.
Heating speed
The faster the heating speed, the shorter the incubation period. This also results in an increase in the temperature at which austenite begins to transform and the temperature at which the transformation ends. Additionally, it reduces the time required to complete the transformation.
alloy element
Cobalt and nickel have the effect of accelerating the austenitization process, while chromium, molybdenum and vanadium have the effect of slowing it down. On the other hand, silicon, aluminum and manganese have no effect on the bainization process of austenite alloy elements.
It is worth noting that the diffusion speed of alloying elements is much slower compared to that of carbon. As a result, the heating temperature for heat treatment of alloy steel is generally higher and the holding time is longer.
Original fabric
When the cementite in the original structure is in flake form, the rate of austenite formation is faster. Furthermore, the smaller the spacing between the cementite particles, the greater the transformation speed.
The original austenite grain also has a larger carbon concentration gradient, which results in a faster grain growth rate.
Furthermore, spheroidized annealed granular pearlite has fewer phase interfaces, which makes the austenitization process the fastest of all.
6. Factors affecting the growth of austenite grains
Chemical composition
① Within a certain range of carbon content, an increase in the carbon content in austenite leads to an increase in the grain growth tendency. However, if the carbon content exceeds a certain level, the growth of austenite grains will be impaired.
② Adding elements such as titanium, vanadium, niobium, zirconium and aluminum to steel can result in the production of fine-grained steel. This is because carbides, oxides and nitrides are dispersed along grain boundaries, which can inhibit grain growth. On the other hand, manganese and phosphorus have the effect of promoting grain growth.
③ Elements that form strong carbides, when dispersed in austenite, can hinder the growth of austenite grains. On the other hand, non-carbide-forming elements such as silicon and nitrogen have little effect on the growth of austenite grains.
Heating temperature
Austenite grain growth is closely linked to atomic diffusion in the heating temperature system. As a result, the higher the temperature or the longer the time spent at a specific temperature, the coarser the austenite grain becomes.
Heating speed
The faster the heating rate, the greater the superheat and the higher the actual austenite formation temperature. This results in an increase in the nucleation rate, which is greater than the growth rate and makes the austenite grain finer.
In the manufacturing process, rapid heating and short-term heat preservation are often employed to obtain ultrafine grain structures.
Original organization
As a general rule, the finer the original structure of the steel, the greater the dispersion of carbides, which leads to a finer grain structure of the austenite.
