a) Crankshaft:
- Heat treatment: Nitriding or nitrocarburizing.
- Reasons:
- Improved surface hardness: Enhances resistance to wear and tear from bearing contact and fatigue from cyclic loading, increasing crankshaft lifespan.
- Reduced friction: Smoother surface minimises friction and heat generation, improving engine efficiency and reducing operating temperature.
- Fatigue resistance: Hardened surface resists subsurface cracks from repeated stress, enhancing fatigue strength and preventing shaft failure.
- Corrosion resistance: Nitriding offers additional protection against corrosion, particularly beneficial for marine engines or harsh environments.
(b) Valve Spring:
- Heat treatment: Austempering or oil quenching and tempering.
- Reasons:
- High strength and toughness: Enhanced tensile strength and fatigue resistance allow the spring to withstand high loads and repeated cycles without deformation or failure.
- Heat resistance: The treatment maintains spring properties at elevated engine temperatures, ensuring consistent valve operation.
- Dimensional stability: Minimises spring relaxation and set loss over time, preventing valve clearance issues and maintaining engine performance.
- Ductility: Austempering provides slightly better ductility for absorbing shock loads and preventing brittle fracture.
(c) Used Copper Washer:
- Heat treatment: None.
- Reasons:
- Copper is malleable and soft: Its inherent properties allow it to conform to irregularities and create a tight seal even without heat treatment.
- Heat exposure can weaken copper: Annealing or other heat treatments might reduce its strength and compromise its sealing function.
- Used washers are typically discarded: Repetitive thermal cycling and mechanical stresses often render used washers unsuitable for reuse, regardless of heat treatment.
In summary, different engine components require specific heat treatment based on their function and desired properties. Crankshafts and valve springs benefit from hardening processes for strength, wear resistance, and fatigue resistance. Used copper washers, however, generally don’t undergo any heat treatment due to their inherent properties and potential degradation from past use.