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## WHAT IS IT?

This model’s goal is to simulate the effects on the microstructure of carbon steel while being exposed to varying temperatures. The idea is to simulate the **normalizing** and **hardening** processes and to observe the changes in the steel’s **grain structure** and its **mechanical properties**.
The main components of carbon steel on which this model is based are iron and carbon. The amount of carbon inside the steel significantly changes its properties. The carbon content for low carbon steel typically ranges from 0.1% - 0.6%. Beyond 0.6% carbon the steel is considered high carbon steel. This model includes carbon content ranges from 0.1% to 1.6%.

#### Grain structure

When looking at the initial grain structure of carbon steel it consists of different types of grains. **Ferrite** grains are made of pure iron determining mechanical properties like ductility inside the steel. Another type of grain, occurring more frequently the higher the carbon content is, is **Pearlite**. Pearlite consists of iron and carbon which is also called iron-carbide, giving the steel the properties of hardness and strength. In steel containing about 0.4% carbon, ferrite and pearlite grains are evenly distributed. The number of pearlite grains increases until 0.8% carbon, from where the steel consists of 100% pearlite grains.

#### Normalizing
The normalizing process is used to adjust the mechanical properties of steel such as **ductility**, **hardness**, and **tensile strength**. The main goal is to recrystallize the steel’s distorted grain structure into an undistorted one.
When applying a certain amount of heat to carbon steel over a certain amount of time, the grain structure changes. While the temperature of the steel is inside the normalizing temperature range, new grains begin to form, originating from the grain boundaries and absorbing the existing grains. The resulting grain structure usually differs from the original in a more uniform grain size and distribution. It is common practice to apply more than one normalizing cycles to carbon steel to acquire an even finer grain structure from cycle to cycle. If the temperature, however, is too high, the new grains forming at the grain boundaries grow at the expense of their neighbors resulting in a coarse-grained structure and giving the steel commonly undesirable mechanical properties.

#### Hardening
The process of hardening is usually applied to carbon steel containing only pearlite grains. The steel is heated above its recrystallization temperature and then quenched in either oil or water. As the temperature drops rapidly, **martensite** begins to form giving the steel a very high hardness but also brittleness. To overcome the brittleness the process of tempering is used, which is not part of this model.

## HOW IT WORKS

The model is setup with an adjustable amount of **initial grains**. Each grain consists of a multitude of **atoms**. The whole world area consists of one atom per patch. Each atom orientates itself to its nearest grain neighbor and inherits its properties (to distinguish between ferrite, pearlite etc.). Depending on the adjustable **carbon content**, ferrite grains exist beside the pearlite grains. Temperature can be applied to the material to simulate the processes of normalizing and hardening. The amount of heat in each atom determines its color by a **color-gradient**, roughly resembling the color real carbon steel would have at this temperature. If the material’s temperature is high enough, the atoms inside the grains will transform to **austenite**.

During normalizing the grains will reorganize themselves and grow in number if the correct amount of heat is applied. They will decrease in number if the average grain temperature exceeds the recommended normalizing temperature range.
At this stage the material can either be **air-cooled** or **quenched** resulting in different grain compositions and mechanical properties. While air-cooling the austenite will transform back into ferrite and pearlite grains. The longer the normalizing has taken place the more grains will have formed as a result.

If the material is quenched after having applied the correct amount of heat, the austenite will transform to martensite if the carbon content is at least 0.8%.

## HOW TO USE IT

### Setup
**1.** Adjust the amount of carbon with the **percent-carbon** slider.
**2.** Adjust the amount of grains with the **initial-grains** slider.
**3.** Click **setup** to form ferrite and pearlite grains depending on **percent-carbon** and **initial-grains**.

### Normalizing
Note the initial yield-strength.

**1.** Set the **state** dropdown to heat and click run.

At first nothing will change except the **Avg Grain Temp** monitor since heat is being applied from the outside and needs to spread through the material.
Observe the color change from grey to red and then to orange and yellow. At a certain temperature, atoms at the grain boundaries will try to re-orientate themselves to the nearest grain.

Observe the formation of austenite at a certain temperature.
If the average grain temperature is inside the normalizing temperature range, observe how the grains will re-orientate and adjust their position to spread more evenly across the area.

**2.** When inside the normalizing temperature range, set the **state** to idle to keep applying the same amount of heat.

Observe how the number of grains grows over time. If the temperature is too high, the number of grains will decrease.

**3.** Set the **state** to cool to “air-cool” the steel.

Observe how the austenite forms back to ferrite and pearlite and color changes back to grey when cooled.

**4.** Compare the resulting **yield strength** with the original one after the setup.

### Hardening
**1.** Select over 0.8% **percent-carbon** to successfully harden the steel and click **setup**.
**2.** Note the **hardness**.
**3.** Set the **state dropdown** to heat and click run.

Observe the same behavior as during normalizing.

**4.** Leave the **state** at heat until the maximum possible temperature is reached (observing the **Avg Grain Temp** monitor)
**5.** Set state to quench.

Observe how the material cools rapidly and the austenite forms back to pearlite and martensite.

**6.** Compare the resulting **hardness** with the original one.

## MONITORS

**Avg Grain Temp**: The average grain temperature.
**Yield Strength**: An indicator specifying at which point the plastic deformation of the material becomes permanent. (Calculated using the Hall-Petch relation)
**Hardness**: An indicator to compare the hardness of the material dependent on its composition.

## PLOTS

**Grains**: Number of grains over time (red). Average grain size over time (blue).
**Number of Atoms**: The number of atoms over time for ferrite (red), pearlite (blue), austenite (green) and martensite (black).

## THINGS TO NOTICE

In reality, new grains begin to form at the grain boundaries while normalizing. This model simulates this by simply hatching grains over time which will re-orientate along with the existing ones. Mechanical properties described in this model are simple indicators to show relations between the grain structure and composition. In reality, the mechanical properties of steel depend on many more factors which are not included in this model.

## THINGS TO TRY

Try to harden the material with less than 0.8% carbon. Is the hardening successful? Is there any transformation into martensite?
Try to normalize after successful hardening. What happens with the martensite after normalizing?

## EXTENDING THE MODEL

Obviously, the missing element in this model is the tempering process where the hardened steel is relieved of its brittleness.

## RELATED MODELS

Crystallization Basic
Crystallization Directed
Crystallization Moving
MaterialSim Grain Growth

## CREADITS AND REFERENCES

Made by Felix Rauchenwald in June 2019.