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Performance of structural components.






Laboratory class # 5

The “iron- carbon” constitutional diagram, structure,

Properties and application of iron-carbon alloys.

The aim of the work:

1. To consider main phases and structural component of iron-carbon alloys and to consider their properties.

2. To study the ”iron-carbon" constitutional diagram. To consider the crystallization process of a liquid alloy and to study transformations in alloys in a solid condition.

3. To study structures of carbon steels with the various contents of carbon and various marks of cast iron. To study the dependence between materials structure and their properties. To determine application areas of various steels and cast irons.

General ideas.

To receive the necessary properties of steels and cast irons, it is necessary to study the constitutional diagram (equilibrium diagram) “iron-cementite".

Phases and structural component in iron-carbon alloys.

Alloys properties depend on phases presence and phases distribution in solid condition of alloys. In iron-carbon alloys solid phases are the following: ferrite, austenite, cementite and graphite. There are also structural component: perlite and ledeburite.

Ferrite (F) – solid interstitial solution of carbon in a -iron. The ferrite has body centered cubic (BCC) lattice. The maximum carbon solubility of in ferrite is only 0, 02% at the temperature of 727°C. At the cooling up to room temperature the equilibrium contents of carbon in ferrite decrease drops practically up to zero (0, 006%). Ferrite is characterized by small strength, small hardness and high plasticity (sB = 250 MPa, HB=800 MPa, d=50%).

Austenite (A) - solid solution of carbon in iron. Solubility limit of carbon in iron is 2, 14% at the temperature of 1147°C. Austenite has high plasticity.

Cementite (C) - chemical bond of iron with carbon, that is iron carbide Fe3C. The content of carbon in cementite is 6, 67%. A crystalline lattice is complicated (rhombic). The melting temperature of cementite is 1260°C, hardness is very large and equals HV=1000, plasticity is small.

The graphite (G) - has a hexagonal stratified crystalline lattice. Graphite is soft, has electric conductivity and low strength.

Performance of structural components.

Ledeburite (Led) - eutectic, ledeburite is mechanical mixture of austenite and cementite. Ledeburite appears at the temperature of 1147°C. The general contents of carbon in ledeburite equals 4, 3%. Since austenite is stable at cooling only up the temperature 727°C, at lower temperatures ledeburite is small-grain dispersed mixture of perlite and cementite. Ledeburite contains about 50% cementite phase, therefore it is hard and brittle.

Perlite (P) - eutectoid, perlite is mechanical mixture of ferrite and cementite; perlite contains 0, 8% of carbon. Perlite has quite high strength and hardness.

The iron - carbon constitutional diagram is drawn in the figure.

At the crystallization from the liquid alloy the carbon dissolves in liquid iron and changes temperature of crystallization starting of an alloy from 1539°C up to 1147°C.

On the AB line from liquid solution d-ferrite begins to appear, on the BC line austenite begins to appear. On the horizontal line HJB the so-called peritectic reaction takes place. During this reaction d-ferrite crystals with BCC crystalline lattice interact with a liquid phase and rebuild in the face centered cubic (FCC) lattice of austenite.

In BCE area two-phase equilibrium of austenite with liquid solution takes place. At cooling of alloys in this area quantity of austenite crystals increases. The carbon content in austenite changes along the JE line and the carbon content in the liquid phase increases along the BC line.

 

 


Figure 1. The “iron-carbon“ equilibrium diagram: L- liquid alloy; A- austenite; С- cementite; Led- ledeburite; F- ferrite; P- perlite; Сú ú - second-step structural change of solid alloy. Cooling curve.

 


The point D corresponds to the crystallization of pure cementite. The DC line characterizes the solubility limits of carbon in a liquid phase. At cooling of liquid alloys along the DC line the starting of cementite crystals begins. In the DCE area the high-carbon cementite appears. Therefore at cooling of alloys the carbon quantity in liquid phase decreases along the DC line and comes to the eutectic structure.

In the C point (4, 3% of carbon) at the constant temperature 1147°C precipitation from liquid phase of dispersed crystals of cementite and saturated austenite (2, 14% of carbon) takes place. Cementite and austenite create the eutectic structure - ledeburite.

On the ECF line the crystallization of residual liquid phase at the constant temperature (with creation of ledeburite) takes place. It is the line of eutectic transformation.

The line ABCD of crystallization starting is called the liquidus line, the line AHJECF of crystallization finishing is called the solidus line.

All further transformations take place in alloys in solid state by the diffusion. Since cementite remains stable, the structural transformations at further cooling will happen in the austenite phase.






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