What elemental additions serve to increase the hardness of steel____________________?
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Answer: B. martensite
Explanation
Martensite is a supersaturated solid solution of carbon trapped in a distorted body-centered tetragonal crystal lattice. It forms when austenite is cooled rapidly through quenching, preventing carbon atoms from diffusing out of the iron matrix. This severe lattice distortion restricts dislocation movement extensively, making martensite exceptionally hard and strong. Heat treatment processes like hardening and tempering rely on creating martensitic microstructures to achieve wear resistance. The transformation to martensite is diffusionless, meaning it occurs almost instantaneously upon reaching the critical cooling rate. As-quenched martensite is often too brittle for practical engineering use and requires subsequent tempering to restore toughness.
Austenite is the high-temperature face-centered cubic phase of iron that serves as the starting microstructure prior to quenching. Pearlite is a lamellar mixture of ferrite and cementite formed by slower cooling, resulting in much lower hardness than martensite. Cementite is iron carbide, which contributes to hardness in pearlite but does not match the extreme hardness of quenched martensite.
- Subject
- Engineering
- Category
- Mechanical Engineering
- Topic
- —
- Difficulty
- medium
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