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

Spheroidite and tempered martensite are two distinct microstructures found in steels. Spheroidite consists of cementite (Fe3C, a hard and brittle compound of iron and carbon) particles dispersed in a ferrite (pure iron, a relatively soft and ductile phase) matrix. Tempered martensite, on the other hand, is formed by heating martensite (a metastable, hard, and brittle phase) at relatively low temperatures (below the lower critical temperature), leading to the formation of a tempered martensite structure. This structure consists of small, dispersed cementite particles within a ferrite matrix.

Spheroidite forms when steel is heated to a temperature within the range of the iron-carbon phase diagram, where both cementite and ferrite are stable. The steel is then held at this temperature for a prolonged period, allowing the cementite and ferrite phases to grow and form a spheroidal-shaped structure. The cementite particles form along the grain boundaries of the ferrite, leading to a continuous network of cementite particles with spherical or globular morphology.

Tempered martensite forms when martensitic steel is heated to a temperature below the lower critical temperature, which is within the tempering range (typically 150-700°C). This process allows carbon atoms, which are trapped in the martensitic lattice during quenching, to diffuse out of the distorted martensite structure, and form small, finely dispersed cementite particles within the ferrite matrix. The resultant microstructure is a combination of ferrite and cementite, but with a much finer and uniform distribution compared to spheroidite.

Spheroidite exhibits lower hardness and strength compared to tempered martensite due to the continuous network of cementite particles along the grain boundaries. The presence of cementite along grains weakens the overall structure by acting as potential points for crack initiation and propagation. However, the spheroidal shape of cementite particles in the ferrite matrix leads to a higher ductility and better impact toughness compared to the more brittle tempered martensite microstructure.

Tempered martensite is much harder and stronger than spheroidite due to its finer, more evenly dispersed cementite particles within the ferrite matrix. This fine distribution creates a more uniform structure with less continuous pathways for crack propagation, resulting in a stronger material. Additionally, the formation of tempered martensite from brittle martensite involves the relief of internal stresses and the dissolution of excess carbon, leading to a microstructure with improved toughness and ductility compared to untempered martensite. In conclusion, the microstructural differences between spheroidite and tempered martensite, which involve the size, shape, and distribution of cementite particles within a ferrite matrix, result in their varying hardness and strength. Tempered martensite is harder and stronger due to its finer, more uniform cementite distribution, while spheroidite exhibits higher ductility due to its continuous network of cementite particles along the grain boundaries.