Define an Fe-C martensite. (b) Describe the following types of Fe-C martensites

Question

Define an Fe-C martensite. (b) Describe the following types of Fe-C martensites that occure in plain-carbon steels: (i) lath martensite, (ii) plate martensite. (c) Describe some of the characteristics of the Fe-C martensite transformation that occurs in plain carbon steels. (d) What causes the tetragonality to develop in the BCC iron lattic when the carbon content of Fe-C martenistes exceeds about 0.2 percent? (e What causes the high hardness and strength to be developed in Fe-C martensites of plain carbon steels when their carbon conent is high?

Explanation / Answer

A metastable transitional structure formed by a shear process during a phase transformation, characterized by an acicular or needlelike pattern; in carbon steel it is a hard, supersaturated solid solution of carbon in a body-centered tetragonal lattice of iron.

Martensite

the structure of crystalline solids arising as the result of a slip, diffusionless polymorphic transformation on cooling. It is named for the German specialist in physical metallurgy A. Martens (1850-1914).

As a result of a lattice deformation, known as cooperative displacement, during the transformation, a relief appears on the surface of the metal. Internal strain arises in the bulk and plastic deformation occurs, which limit crystal growth. The rate of growth reaches 103 m/sec and is independent of temperature. Hence, the rate of martensite formation is usually limited by crystal nucleation. The reaction of the internal strain shifts crystal nucleation greatly below the thermodynamic phase equilibrium point and may stop the transformation at constant temperatures. Thus, the amount of martensite formed usually increases with an increase in supercooling.

Since the elastic energy must be minimal, martensite crystals take the shape of plates (needles in cross section) regularly oriented relative to the initial lattice. The internal strain is also reduced by plastic deformation and thus the crystal contains many dislocations (up to 1012 cm-2) or is split into twins with a thickness of 10-100 nanometers (100-1000 angstroms). Intragranular boundaries and dislocations strengthen martensite.

Martensite is a typical product of low-temperature polymorphic transformations in pure metals such as iron, cobalt, titanium, zirconium, and lithium; in solid solutions based on these metals; and in intermetallic compounds such as CuZn, Cu3Al, NiTi, V3Si, and AuCd.

Martensite in steel is a supersaturated Fe-C solution obtained by hardening from austenite. The ordered position of carbon atoms that results from martensite displacement transforms the body-centered lattice of a-iron from cubic to tetragonal. The distortion of the lattice near the interstitial atoms results in hardness. The tetragonality and the hardness increase with the carbon content. Hardness increases up to 1,000 HV (Vickers hardness).

Carbon martensite is the major structural component of most high-strength steels. The carbon content in the solid solution and the martensite subgranular structure change upon tempering, which is used for increasing the ductility of steel.

Carbon is the most important factor of martensite strength in steel. The strength of noncarbon maraging steel results from the separation of intermetallic compounds upon aging.

Related Questions

Navigate

• Browse (All)
• Browse D
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.