How to Read Iron Iron Carbide Phase Diagram

Alloy metals can exist in different phases. Phases are physically homogeneous states of an alloy. A stage has a precise chemical limerick – a certain arrangement and bonding betwixt the atoms.

This structure of atoms imparts different backdrop to different phases. We tin can choose the stage nosotros want and use it in our applications.

Only some special alloys can be in multiple phases. Heating the metal to specific temperatures using heat treatment procedures results in dissimilar phases. Some special alloys tin can be in more than one stage at the aforementioned temperature.

What Are Stage Diagrams?

Phase diagrams are graphical representations of the phases nowadays in an alloy at different weather condition of temperature, pressure, or chemical composition.

The diagram describes the suitable conditions for 2 or more phases to exist in equilibrium. For example, the water phase diagram describes a point (triple point) where water can coexist in three different phases at the same time. This happens at but above the freezing temperature (0.01°C) and 0.006 atm.

Using the Diagrams

There are four major uses of alloy phase diagrams:

• Development of new alloys based on application requirements.
• Production of these alloys.
• Development and control of appropriate heat treatment procedures to improve the chemical, physical, and mechanical backdrop of these new alloys.
• Troubleshooting problems that ascend in application of these new alloys, ultimately improving production predictability.

When it comes to alloy development, phase diagrams accept helped forbid overdesign for applications. This keeps toll and process fourth dimension downward. They besides assist develop culling alloys or same alloys with alternative alloying elements. It can help to reduce the need for using deficient, chancy, or expensive alloying elements.

Performance-wise, phase diagrams help metallurgists sympathize which phases are thermodynamically stable, metastable, or unstable in the long run. Appropriate elements tin can then be chosen for alloying to prevent machinery breakdown. Cloth for frazzle piping, for instance, if not chosen properly, may lead to breakdown at higher temperatures.

The service life likewise improves as phase diagrams testify us how to solve problems such as intergranular corrosion, hot corrosion, and hydrogen damage.

Fe-Carbon Phase Diagram

The iron-carbon stage diagram is widely used to empathize the dissimilar phases of steel and cast iron. Both steel and cast iron are a mix of iron and carbon. Also, both alloys contain a pocket-sized amount of trace elements.

The graph is quite complex merely since nosotros are limiting our exploration to Fe3C, we will only be focusing up to 6.67 weight percent of carbon.

This iron carbon phase diagram is plotted with the carbon concentrations by weight on the X-axis and the temperature scale on the Y-axis.

Atomic number 26 crystal structures explained

The carbon in iron is an interstitial impurity. The alloy may class a face centred cubic (FCC) lattice or a body centred cubic (BCC) lattice. It will form a solid solution with α, γ, and δ phases of atomic number 26.

Types of Ferrous Alloys on the Phase Diagram

The weight percentage scale on the X-axis of the iron carbon stage diagram goes from 0% up to vi.67% Carbon. Up to a maximum carbon content of 0.008% weight of Carbon, the metal is only called iron or pure fe. It exists in the α-ferrite form at room temperature.

From 0.008% upward to two.fourteen% carbon content, the iron carbon blend is called steel. Inside this range, there are dissimilar grades of steel known equally low carbon steel (or mild steel), medium carbon steel, and high carbon steel.

When the carbon content increases beyond 2.14%, we reach the phase of cast iron. Cast iron is very difficult but its brittleness severely limits its applications and methods for forming.

Boundaries

Multiple lines tin be seen in the diagram titled A1, A2, A3, A4, and ACM. The A in their name stands for the discussion 'arrest'. Every bit the temperature of the metallic increases or decreases, phase change occurs at these boundaries when the temperature reaches the value on the boundary.

Usually, when heating an alloy, its temperature increases. Only along these lines (A1, A2, A3, A4, and ACM) the heating results in a realignment of the construction into a different stage and thus, the temperature stops increasing until the stage has changed completely. This is known as thermal abort every bit the temperature stays constant.

Blend steel elements such as nickel, manganese, chromium, and molybdenum affect the position of these boundaries on the phase diagram. The boundaries may shift in either management depending on the chemical element used. For example, in the iron carbon phase diagram, addition of nickel lowers the A3 purlieus while the improver of chromium raises information technology.

Eutectic Point

Eutectic point is a point where multiple phases meet. For the atomic number 26-carbon alloy diagram, the eutectic betoken is where the lines A1, A3 and ACM meet. The formation of these points is coincidental.

At these points, eutectic reactions accept place where a liquid phase freezes into a mixture of two solid phases. This happens when cooling a liquid alloy of eutectic limerick all the manner to its eutectic temperature.

The alloys formed at this point are known as eutectic alloys. On the left and correct side of this point, alloys are known equally hypoeutectic and hypereutectic alloys respectively ('hypo' in Greek means less than, 'hyper' means greater than).

Phase Fields

The boundaries, intersecting each other, marker sure regions on the Fe3C diagram.

Inside each region, a different phase or two phases may exist together. At the purlieus, the stage change occurs. These regions are the phase fields.

They indicate the phases present for a certain composition and temperature of the alloy. Let'due south learn a footling almost the different phases of the fe-carbon alloy.

Different Phases

α-ferrite

Existing at low temperatures and depression carbon content, α-ferrite is a solid solution of carbon in BCC Iron. This phase is stable at room temperature. In the graph, it can be seen every bit a sliver on the left edge with Y-axis on the left side and A2 on the right. This phase is magnetic below 768°C.

It has a maximum carbon content of 0.022 % and it will transform to γ-austenite at 912°C as shown in the graph.

γ-austenite

This phase is a solid solution of carbon in FCC Fe with a maximum solubility of 2.14% C. On further heating, it converts into BCC δ-ferrite at 1395°C. γ-austenite is unstable at temperatures below eutectic temperature (727°C) unless cooled quickly. This phase is non-magnetic.

δ-ferrite

This phase has a similar construction equally that of α-ferrite simply exists only at high temperatures. The stage can be spotted at the top left corner in the graph. It has a melting signal of 1538°C.

Fe3C or cementite

Cementite is a metastable phase of this alloy with a fixed composition of Fe3C. It decomposes extremely slowly at room temperature into Atomic number 26 and carbon (graphite).

This decomposition time is long and information technology will take much longer than the service life of the application at room temperature. Some other factors (high temperatures and addition of certain alloying elements for instance) can affect this decomposition as they promote graphite formation.

Cementite is hard and brittle which makes information technology suitable for strengthening steels. Its mechanical properties are a function of its microstructure, which depends upon how it is mixed with ferrite.

Fe-C liquid solution

Marked on the diagram as 'L', it tin can exist seen in the upper region in the diagram. As the name suggests, it is a liquid solution of carbon in iron. As we know that δ-ferrite melts at 1538°C, it is evident that melting temperature of iron decreases with increasing carbon content.

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Source: https://fractory.com/iron-carbon-phase-diagram/

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