Difference between Ferrite and Austenite Steel

What is the difference between ferrite and austenite steel?

The classification of stainless steel is made possible on account of the different kinds of microstructure. Out of these four classes, the two most popular classes are ferritic and austenitic stainless steels. The microstructure of these alloys is the internal arrangement of the crystal. The arrangement of the crystal is what gives the alloy its mechanical and it’s chemical characteristics. The microstructure of Ferritic stainless steel consists of ferrite crystals. Ferrite crystals are a kind of iron that contains trace quantities of carbon, which amounts up to 0.025%. The Ferrite crystals tend to absorb a limited amount of carbon. This is because of its body centered cubic crystal structure. The arrangement is such that there is one iron atom at each corner, in addition to one in the center. This central ferrous atom is what gives the ferritic class of stainless steel its magnetic properties. On the other hand, austenitic stainless steel, which is a gamma-phase iron, which is an allotrope of iron. At an elevated temperature range of 1,674 to 2,541 °F, the alpha iron undergoes a phase transition. Therefore, the alpha iron, which was a body-centered cubic or BCC structure gets converted to the face-centered cubic or FCC configuration of gamma iron. This changed configuration of a gamma iron is referred to as austenite.

Ferrite and austenite microstructure

Ferritic steels have a body centered cubic crystal structure. This means there is one ferrous atom present at every one of the eight corners, and one atom at the core. In this setting, each of the eight corners is also the corner of another cube. Hence, the corner ferrous or iron atoms will be shared equally among eight unit cells. On the other hand, austenite, which has a face-centered cubic crystal structure has atoms at the corners. As the name suggests, the atoms are present at the center of the faces of its cellular unit. Atoms in an FCC or face-centered cubic arrangement are packed together in a very snug manner. Hence, the atoms in the microstructure will occupy about 74% of its volume. Since they are packed snugly, this kind of structure is also referred to as cubic closest packing or CCP.

Carbon solubility in ferrite and austenite

In comparison to austenite, the carbon solubility of ferrite is low. Being a solid solution of carbon and iron, a percentage of about 0.025%, which means that the solubility of carbon in ferrous is 0.02%. Since pure iron is already a structure at room temperature, the interatomic spaces are small. Therefore, sphere shaped carbon atoms cannot accommodate the ferrous atoms. This is what makes the solubility of carbon is low in ferrite. Moreover, the carbon atom is small, which makes it impossible for it to act as a substitute, yet, it is too large for an interstitial solid solution.  On the other hand, the carbon solubility in iron in an austenite region is about is 2.11%, which is significantly higher than in ferrite regions. This is because austenite has an fcc structure.  Due to this structure, the interatomic spacing of austenite larger than ferrite. Having a larger spacing makes it easy for austenite to accommodate carbon atoms in their spaces.

The density of ferrite and austenite

BCC is heavier than FCC, which means that Ferrite has a higher density in comparison to austenite. The reason for FCC being lighter is that their symmetry or arrangement is what offers closely packed planes in various directions. This is why a face-centered cubic or FCC crystal structure will manifest more ductility. And so the chances of deformation for austenite are greater under load before breaking, especially if compared to a body-centered cubic structure. The lattice in a  body-centered cubic, although cubic, is not closely packed like the FCC type. Hence, BCC or ferrite tend to be strong metals.

The hardness of ferrite and austenite

Ferrite is known to be harder than austenite. Usually, elements such as chromium, molybdenum, silicon, and niobium foster ferrite. Most ferritic steels contain chromium content at the 13.5% range, which means they are capable of undergoing successive transformations from the alpha to gamma and back to the alpha phase during the formation of ferrite. Along with being magnetic, ferrite crystals are known to be harder and brittle, as compared to the soft and ductile crystals of austenite.

Ferrite stainless steel composition

Ferrite stainless steel grades at a glance

• Type 409 stainless steel
• 430 stainless steel
• 430LI stainless steel
• 434 stainless steel
• 439 stainless steel
• Type 442 stainless steel
• 444 stainless steel
• 446 stainless steel

Mechanical properties of ferrite stainless steel

Ferrite stainless steel Physical properties

Property	Ferritic
Density Value (kg/m3)	7700
Thermal conductivity (20°C, W/m.°C	25
Thermal expansion (0-100°C μm/m/°C)	10.5
Electrical resisivity (nΩ.m)	600
Specific heat range (0-100°C, J/kg.°C	430-460

Mechanical properties of austenite steel

	Tensile Strength	Yield Strength
Austenitic	600	250
Duplex	700	450
Ferritic	500	280
Martensitic	650	350
Precipitation Hardening	1100	1000

Austenitic stainless steel chemical composition chart

Austenitic stainless steel grades list

• 304
• 304L
• 304H
• 304N
• 316
• 347
• 321
• 310
• 309
• 308L (generally filler metal only)

Effect of retained austenite steel

The effect of retained austenite steel depends on the acting stresses on the impact-fatigue strength. Retained austenite help to increase the impact-fatigue resistance at a high-stress level, but under low stress, it decreases.

Characteristics of austenite steel

• Strength at Temperature up to approximately 1900F
• Cold Workability
• Low Thermal Conductivity
• High Formability

Austenite steel equivalent grades

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