Bainite is the phase of steel in which cementite is in particle form.

Explanation:
Bainite is one of the microstructures that can form in steel during the cooling process. It is characterized by a mixture of ferrite and cementite phases. Among these two phases, the cementite is present in particle form.

The cooling process of steel involves various phases, each with different microstructures and properties. Let's briefly discuss the other phases mentioned in the options and why cementite is not in particle form in them:

- Martensite: Martensite is a phase that forms when steel is rapidly cooled from a high temperature. It has a highly distorted crystal structure and is characterized by its hardness. In martensite, the carbon atoms of the steel are dissolved in the iron matrix, and cementite is not present as separate particles.

- Ferrite: Ferrite is the phase that forms when steel is slowly cooled. It has a body-centered cubic (BCC) crystal structure and is relatively soft. In ferrite, the carbon atoms are dissolved in the iron matrix, similar to martensite, and cementite is not present as separate particles.

- Pearlite: Pearlite is a lamellar structure that forms when steel is slowly cooled. It consists of alternating layers of ferrite and cementite. In pearlite, cementite is present in the form of thin lamellae, not as separate particles.

- Bainite: Bainite is a microstructure that forms when steel is cooled at an intermediate rate. It consists of a mixture of ferrite and cementite. In bainite, the cementite is present as fine particles dispersed within the ferrite matrix. These particles give bainite its unique microstructure, which provides a combination of strength and toughness.

In summary, cementite is in particle form in the bainite phase of steel. This is because the cooling rate during the formation of bainite allows for the precipitation of cementite particles within the ferrite matrix.