Exam Question

Cast iron, particularly grey and ductile iron, is widely used in main engine components due to its castability, durability, and cost-effectiveness. To enhance performance and extend service life, these components are subjected to different heat treatment processes , which modify their microstructure and improve mechanical properties.
(a) Heat treatment processes applied to cast iron engine components
Stress Relief Annealing: This process involves heating the cast iron to a sub-critical temperature (500-650°C for gray iron, 500-550°C for ductile iron) and then slowly cooling it. This treatment's main purpose is to reduce or eliminate residual stresses from casting and machining, preventing warping and distortion.
Annealing: Annealing is used to soften the cast iron, making it more machinable and ductile. The component is heated above its critical temperature range (870-925°C for full annealing) or to a sub-critical temperature (705-720°C for subcritical annealing) and then slowly cooled.