Question

Determine which of the following statements are true.
1. Tempered martensite is as hard as martensite.
2. Microstructure of bainite consists of fine elongated particle of cementite.
3.Sphericity microstructure consists of large cementite particle in alpha ferrite matrix.
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Answer 1

Answer:

1. The statement is False.

2. The statement is False.

3. The statement is False.

Explanation:

Tempered martensite is not as hard as martensite. Tempering reduces the hardness of martensite by causing some of the carbon atoms to precipitate out of the martensite as fine carbide particles. This reduces the amount of carbon in solid solution, which lowers the strength and hardness of the steel. However, the tempered martensite is tougher and more ductile than martensite.

Therefore, the statement is False.

The microstructure of bainite consists of fine elongated particles of ferrite and/or pearlite, rather than cementite. Bainite forms when austenite is cooled rapidly to a temperature above the Ms (martensite-start) temperature, and then held at that temperature until the transformation to bainite is complete. During this time, the carbon atoms diffuse to form ferrite and/or pearlite, depending on the temperature and composition of the steel. Therefore, the statement is False.

The spheroidite microstructure consists of a matrix of ferrite with spheroidized cementite particles dispersed throughout. Spheroidizing heat treatment is used to soften and improve the machinability of high-carbon steels, which have been hardened and tempered. During spheroidizing, cementite particles are heated to a temperature below the eutectoid temperature, causing them to transform into rounded or spheroidal shapes. Therefore, the statement is False.

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