What is Austenite?
The iron allotrope austenite, also known as gamma-phase iron, is a type of austenite. As a result, it can be classified as both metallic and nonmagnetic at the same time. At different temperatures, this allotrope can be found in various iron alloys. This allotrope, for example, exists at 727°C in plain carbon steel while it exists at room temperature in stainless steel. This allotrope has a face-centred cubic structure. This austenite allotrope forms from ferrite when the temperature is raised from 912 °C to 1,394 °C. It’s a procedure we call austenitization. Austenite is a malleable metal with a low melting point. Consequently, it is able to incorporate more carbon into its solid solution.
What is Ferrite?
Ferrite is an alpha-phase-iron allotrope of iron. Paramagnetic in nature, it has a ceramic appearance. Because of its body-centered cubic design, it’s lightweight and portable. The dissolution of carbon in this allotrope, on the other hand, is extremely low. It’s also ceramic-like. Many electronic devices rely on it. We can find this iron in cast iron and steel because it is hard and brittle.
What is the Difference Between Austenite and Ferrite?
The iron allotrope austenite, also known as gamma-phase iron, is a type of austenite. In spite of its metal-like appearance, this material is surprisingly soft for its weight. Additional advantages include the fact that it is both ductile and non-magnetic. We refer to ferrite as alpha-phase iron because it is an iron allotrope. It looks like ceramic and is extremely hard. In addition, it is brittle and paramagnetic in structure. As you can see, austenite and ferrite are very different materials. The different microstructures of stainless steel allow for the classification of the material. There are four types of stainless steel: austenitic, ferritic, martensitic, and bainitic. The crystal arrangement within these alloys determines their microstructure. A metal’s mechanical and chemical properties are determined by the arrangement of its crystals. Ferritic stainless steel has a microstructure that is composed of ferrite crystals. Iron crystals called ferrite contain up to 0.025 percent carbon, making them a special type of iron. Carbon can only be absorbed by the Ferrite crystals to a limited extent. A body-centred cubic crystal structure is to blame. Each of the four corners has an iron atom, as well as one in the centre. The magnetic properties of ferritic stainless steel are due to the presence of a central ferrous atom. Allotropes of iron can be found in austenitic stainless steel, which has gamma-phase iron. The alpha iron undergoes a phase transition at a temperature of 1,674 to 2,541 °F. Thus, the alpha iron, which had a BCC structure, is transformed into gamma iron, which has an FCC structure. A gamma iron that has undergone this transformation is called austenite.
Microstructure of ferrite and austenite
The crystal structure of ferritic steels is body-centred cubic. A single ferrous atom can be found on all eight corners and in the centre. There are eight corners of each cube in this arrangement. There will therefore be an equal distribution of corner ferrous or iron atoms among the 8 unit cells. Austenite, on the other hand, has atoms at the corners of its face-centred cubic crystal structure. Cellular units have atoms at the centre of their faces, as the name suggests. In an FCC or face-centred cubic arrangement, the atoms are packed together tightly. Thus, the atoms in the microstructure will account for approximately 74% of its volume. This structure is also known as cubic closest packing (CCP) because of how tightly it is packed.
Solubility of carbon in austenite and ferrite
Ferrite’s carbon solubility is lower than austenite’s. Solubility is 0.02 percent for carbon in ferrous because it is a solid solution with about 0.025 percent carbon. The interatomic spaces in pure iron are small because it already has a structure at room temperature. As a result, carbon atoms with sphere shapes cannot accommodate ferrous atoms. Because of this, carbon has low solubility in ferrite. Aside from its size and inability to serve as a replacement, carbon is incompatible with interstitial solid solutions. Austenite regions have a significantly higher carbon solubility in iron than ferrite regions (2.11 percent). This is due to the fcc structure of austenite. Austenite’s interatomic spacing is greater than that of ferrite because of this structure. Austenite is able to hold more carbon atoms because of its wider spacing.
Ferrite and austenite both have a high density.
Ferrite has a higher density than austenite because BCC is heavier than FCC. It is because of their symmetry or arrangement that FCC planes are lighter because they are able to pack planes together in multiple directions. A face-centred cubic or FCC crystal structure has a higher ductility because it has more face-centres. As a result, austenite has a greater chance of deformation before breaking than a body-centred cubic structure. Although cubic, the lattice in a body-centred cubic is not tightly packed like the FCC type. As a result, metals like BCC and ferrite tend to be strong.
Ferrite and austenite are both extremely hard.
Ferrite has been found to be more durable than austenite, according to previous research. There are a number of elements that help to promote the formation of ferrite. Most ferritic steels have a chromium content of 13.5 percent or higher, making them capable of going through the alpha, gamma, and back to alpha phases multiple times during the ferrite formation process. In comparison to austenite’s soft and ductile crystals, ferrite’s harder and brittler crystals are well-known for their magnetic properties as well.
Properties of selected austenitic steel-grades
| AISI 201 Annealed | AISI 205 Annealed | AISI 301L Annealed | AISI 303 Annealed | AISI 304L Annealed | AISI 316 Annealed | |
| Composition | Fe 67.5 – 75 % | Fe 62.6 – 68.1 % | Fe 70.7 – 78 % | Fe 66.4 – 74.9 % | Fe 64.8 – 74.5 % | Fe 62 – 72 % |
| Cr 16 – 18 % | Cr 16.5 – 18.5 % | Cr 16 – 18 % | Cr 17 – 19 % | Cr 17.5 – 20 % | Cr 16 – 18.5 % | |
| Mn 5.5 – 7.5 % | Mn 14 – 15.5 % | Ni 6 – 8 % | Ni 8 – 10 % | Ni 8 – 12 % | Ni 10 – 14 % | |
| Ni 3.5 – 5.5 % | Ni 1 – 1.7 % | S 0 – 0.03 % | S 0.15 – 0.35 % | S 0 – 0.03 % | Mo 2 – 3 % | |
| S 0 – 0.03 % | N 0.32 – 0.4 % | Si 0 – 1 % | Si 0 – 1 % | Si 0 – 1 % | C 0 – 0.08 % | |
| Elastic modulus | 200 GPa at 20 °C | 220 GPa at 20 °C | 220 GPa at 20 °C | 200 GPa at 20 °C | 200 GPa at 20 °C | 200 GPa at 20 °C |
| Yield strength | 310 MPa at 20 °C | 460 MPa at 20 °C | 250 MPa at 20 °C | 240 MPa at 20 °C | 200 MPa at 20 °C | 240 MPa at 20 °C |
| Elongation | 46 % at 20 °C | 46 % at 20 °C | 52 % at 20 °C | 52 % at 20 °C | 42 % at 20 °C | 42 % at 20 °C |
| Tensile strength | 660 MPa at 20 °C | 810 MPa at 20 °C | 630 MPa at 20 °C | 600 MPa at 20 °C | 550 MPa at 20 °C | 580 MPa at 20 °C |
| Electrical conductivity | 1.45E+7 S/m at 20 °C | – | – | 1.39E+7 S/m at 20 °C | 7.54E+7 S/m at 20 °C | 7.54E+7 S/m at 20 °C |
| Coefficient of thermal expansion | 1.3E-5 1/K at 20 °C | 1.4E-5 1/K at 20 °C | 1.2E-5 1/K at 20 °C | 1.7E-5 1/K at 20 °C | 1.7E-5 1/K at 20 °C | 1.6E-5 1/K at 20 °C |
| Thermal conductivity | 15 W/(m·K) at 20 °C | 11 – 21 W/(m·K) at 20 °C | 11 – 21 W/(m·K) at 20 °C | 16 W/(m·K) at 20 °C | 16 W/(m·K) at 20 °C | 15 W/(m·K) at 20 °C |
| Melting point | 1375 – 1450 °C | 1375 – 1450 °C | 1375 – 1450 °C | 1400 °C | 1400 °C | 1380 °C |
| Specific heat capacity | 500 J/(kg·K) at 20 °C | 460 J/(kg·K) at 20 °C | 450 J/(kg·K) at 20 °C | 500 J/(kg·K) at 20 °C | 500 J/(kg·K) at 20 °C | 490 J/(kg·K) at 20 °C |
Properties of selected ferritic stainless steels
| ASTM A493 Grade 409Cb Lightly Drafted | ASTM A493 Grade 430 Lightly Drafted | ASTM A240 Grade 405 Heat-Treated | ASTM A240 Grade 434 Heat-Treated | ASTM A276 Grade 444 Annealed; Cold-Finished | |
| Composition | Cr 10.5 – 11.7 % | Cr 16 – 18 % | Cr 11.5 – 14.5 % | Cr 16 – 18 % | Cr 17.5 – 19.5 % |
| Si 1 % | Si 1 % | Si 1 % | Si 1 % | Mo 1.75 – 2.5 % | |
| Mn 1 % | Mn 1 % | Mn 1 % | Mn 1 % | Mn 1 % | |
| Ni 0.5 % | P 0.04 % | Ni 0.6 % | Mo 0.75 – 1.25 % | Ni 1% | |
| C 0.06 % | C 0.04 % | C 0.08 % | C 0.12 % | C 0.03 % | |
| Elastic modulus | 220 GPa at 20 °C | 220 GPa at 20 °C | 220 GPa at 20 °C | 220 GPa at 20 °C | 220 GPa at 20 °C |
| Tensile strength | 550 MPa at 20 °C | 595 MPa at 20 °C | 415 MPa at 20 °C | 450 MPa at 20 °C | 415 MPa at 20 °C |
| Electrical resistivity | 6E-7 – 7.5E-7 Ω·m at 20 °C | 6E-7 – 7.5E-7 Ω·m at 20 °C | 6E-7 – 7.5E-7 Ω·m at 20 °C | 6E-7 – 7.5E-7 Ω·m at 20 °C | 6E-7 – 7.5E-7 Ω·m at 20 °C |
| Coefficient of thermal expansion | 9.3E-6 – 1.2E-5 1/K at 20 °C | 9.3E-6 – 1.2E-5 1/K at 20 °C | 9.3E-6 – 1.2E-5 1/K at 20 °C | 9.3E-6 – 1.2E-5 1/K at 20 °C | 9.3E-6 – 1.2E-5 1/K at 20 °C |
| Thermal conductivity | 11 – 22 W/(m·K) at 20 °C | 11 – 22 W/(m·K) at 20 °C | 11 – 22 W/(m·K) at 20 °C | 11 – 22 W/(m·K) at 20 °C | 11 – 22 W/(m·K) at 20 °C |
| Melting point | 1375 – 1450 °C | 1375 – 1450 °C | 1375 – 1450 °C | 1375 – 1450 °C | 1375 – 1450 °C |
| Specific heat capacity | 420 – 586 J/(kg·K) at 20 °C | 420 – 586 J/(kg·K) at 20 °C | 420 – 586 J/(kg·K) at 20 °C | 420 – 586 J/(kg·K) at 20 °C | 420 – 586 J/(kg·K) at 20 °C |
Conclusion
Iron has two allotropes: austenite and ferrite. It is the face-centred cubic gamma iron configuration of austenite and the body-centred cubic alpha iron configuration of ferrite that distinguish the two.
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