There are different types of stainless steels, among which the precipitation-hardened martensitic stainless steel has the highest strength.

(a) Explore how the phenomenon of peak aging and over aging occurs, and why the size of the precipitates has a great impact on the strength and other mechanical properties of precipitation-hardened martensitic stainless steel.

(b) Use one example to illustrate the application of precipitation-hardened martensitic stainless steel and discuss the critical challenges in the future development of stainless steel as the production of mechanical parts or devices.