(a) Briefly describe the microstructural difference between spheroidite and tempered martensite. (b) Explain why tempered martensite is much harder and stronger.

Spheroidite is a microstructure formed in steels, where cementite particles are arranged in a spherical or globular shape within a matrix of ferrite. This structure is typically obtained by heating a steel with a pearlitic or bainitic microstructure to a temperature within the α + Fe3C phase field for an extended period. Tempered martensite, on the other hand, is formed by heating martensitic steel to a temperature below the lower critical temperature for a specific period of time. This process is known as tempering, and it leads to the formation of a microstructure consisting of fine, dispersed martensite and carbide particles within a matrix of ferrite.

The main microstructural difference between spheroidite and tempered martensite lies in the distribution, size, and shape of the cementite particles within the ferrite matrix. In spheroidite, the cementite particles are larger, arranged in a spherical or globular shape in the ferrite matrix. The Fe3C particles' size and shape reduce the number of interfaces between the ferrite and cementite phases, leading to lower overall hardness and strength. In tempered martensite, the cementite particles are much smaller and are evenly distributed throughout the ferrite matrix. The fine dispersion of carbide particles increases the number of interfaces between ferrite and cementite phases, causing impediments to the movement of dislocations, which results in higher overall hardness and strength.

The enhanced hardness and strength of tempered martensite can be attributed to two main factors: 1. The fine, dispersed distribution of cementite particles in the ferrite matrix: As mentioned earlier, the cementite particles' smaller size and their even distribution throughout the ferrite matrix increases the number of interfaces between the two phases. This increased number of interfaces creates barriers that hinder dislocation movement, making it more difficult for the material to deform under stress. 2. The retained martensite and the presence of carbides: Tempered martensite still contains some martensite, a very hard and strong phase in steel. Additionally, during the tempering process, carbide particles form and contribute to the increase in hardness and strength. The combination of retained martensite and carbides provides tempered martensite with a higher hardness and strength compared to spheroidite. In summary, tempered martensite has a finer distribution of cementite particles and retains some martensite and carbides, resulting in a microstructure that is much harder and stronger than that of spheroidite.