4.1 State TWO properties of the cementite structure of carbon steel - NSC Mechanical Technology Welding and Metalwork - Question 4 - 2016 - Paper 1

The purpose of case hardening on a camshaft is twofold:

1. To ensure hardness at the surface, providing resistance to wear under operational stress.

2. To maintain a softer, tougher core that can absorb shock loads, preventing catastrophic failures.

The purpose of tempering hardened steel is:

1. To reduce brittleness, which can occur after the hardening process, making the steel less likely to fracture under impact.

2. To increase toughness, enabling the steel to withstand greater deformation without failure.

On the iron-carbon equilibrium diagram, the horizontal axis represents the carbon content from 0% to 1.4%, and the vertical axis indicates temperature in °C.

This section will cover various phase transformations occurring at different carbon contents.

The temperature range is plotted on the vertical axis between 0 °C and 1000 °C. Key phase boundaries such as the formation of ferrite, austenite, and cementite will be marked accordingly based on carbon content.

The AC1 line represents the temperature at which austenite begins to form from ferrite and cementite. It will be depicted as a line that slopes upwards on the diagram.

The AC3 line indicates the temperature above which all iron is in the austenite phase. This line will also slope upwards and intersect with the carbon content axis.

The austenite structure can be represented in the diagram as the area that exists between the AC1 and AC3 lines, highlighting its stability at given temperature and carbon levels.

The ferrite structure is illustrated in the diagram in the areas below the AC1 line, indicating the regions where the iron retains a body-centered cubic (BCC) structure.

The pearlitic structure can be found in the diagram in the regions that intersect with the phase transformation lines, particularly along the eutectoid composition at approximately 0.76% carbon.