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Railway carriage wheels are subject to heat treatments during manufacture such as hardening of the outer wheel surface and annealing of the centre hub - Leaving Cert Engineering - Question 3 - 2019
Question 3
Railway carriage wheels are subject to heat treatments during manufacture such as hardening of the outer wheel surface and annealing of the centre hub. (i) Describe... show full transcript
Worked Solution & Example Answer:Railway carriage wheels are subject to heat treatments during manufacture such as hardening of the outer wheel surface and annealing of the centre hub - Leaving Cert Engineering - Question 3 - 2019
Step 1
Outline the purpose of annealing the centre hub.
Answer
The purpose of annealing the centre hub is to soften the metal, which enhances its machinability, ductility, and refines the grain size. This heat treatment also aims to reduce internal stresses, resulting in improved performance and longevity of the component.
Step 2
Describe a suitable heat treatment procedure for the outer surface of the carriage wheel.
Answer
The heat treatment process for the outer surface of the carriage wheel typically involves the following steps:
- Heating: The wheel surface is heated uniformly to a temperature range that allows for hardening, generally around 800°C.
- Quenching: The heated wheel is rapidly cooled, usually in a quenching medium like water or oil, to lock in the hardened structure.
- Tempering: After quenching, the wheel is tempered by reheating it to a lower temperature, typically between 220-300°C. This step reduces brittleness and enhances toughness while maintaining the desired hardness.
Step 3
Step 4
Compare the main properties of the metal with 0.2% carbon at X and the metal with 1.5% carbon at Y.
Answer
At X (0.2% Carbon):
- Lower hardness
- Reduced strength
- Excellent formability with good cold working characteristics
- Tough and ductile
- Malleable and weldable
At Y (1.5% Carbon):
- Higher hardness
- Stronger structural integrity
- Suitable for cold forming applications
- Can be hardened further by heat treatment
- More brittle compared to lower carbon content, affecting weld performance.
Step 5
Describe the point C in terms of phase change, temperature and composition.
Answer
Point C represents the eutectoid transformation, where the austenite phase changes into a solid mixture of pearlite and cementite at a temperature of approximately 723°C. This point indicates a significant change in the material's structure, with an optimal composition of around 0.83% carbon.
Step 6
Compare flame hardening and induction hardening.
Answer
Flame Hardening:
- Equipment: Uses an oxy-acetylene flame and a quenchant (water or oil).
- Procedures: Involves direct heating of the steel surface followed by quenching to achieve hardening.
- Quenching: Typically involves rapid cooling, leading to a hardened outer layer while the core remains softer.
Induction Hardening:
- Equipment: Utilizes high frequency induction coils.
- Procedures: Heat is generated directly within the steel surface due to electromagnetic induction, allowing for more precise control.
- Quenching: Similar to flame hardening, but generally offers more uniform hardening due to controlled heating. The depth of hardening can be adjusted based on the frequency of the induction current.