Martensite

China Suppliers




For the transformation, see Diffusionless transformations.

Iron alloy phases electric forklift

v  d  e electric pallet jack

Ferrite (-iron, -iron) narrow aisle truck

Austenite (-iron)

Pearlite (88% ferrite, 12% cementite)

Bainite

Martensite

Ledeburite (ferrite-cementite eutectic, 4.3% carbon)

Cementite (iron carbide, Fe3C)

Steel classes

Carbon steel (2.1% carbon; low alloy)

Crucible steel

Alloy steel (contains non-carbon elements)

Maraging steel (contains nickel)

Stainless steel (contains chromium)

Tool steel (alloy steel for tools)

Other iron-based materials

Cast iron (>2.1% carbon)

Ductile iron

Gray iron

Malleable iron

White iron

Wrought iron (contains slag)

Martensite in AISI 4140 steel

0.35%C Steel, water-quenched from 870C

Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (September 2009)

Martensite, named after the German metallurgist Adolf Martens (18501914), most commonly refers to a very hard form of steel crystalline structure, but it can also refer to any crystal structure that is formed by displacive transformation. It includes a class of hard minerals occurring as lath- or plate-shaped crystal grains. When viewed in cross-section, the lenticular (lens-shaped) crystal grains appear acicular (needle-shaped), which is how they are sometimes incorrectly described.[vague]

In the 1890s, Martens studied samples of different steels under a microscope, and found that the hardest steels had a regular crystalline structure. He was the first to explain the cause of the widely differing mechanical properties of steels. Martensitic structures have since been found in many other practical materials, including shape memory alloys and transformation-toughened ceramics.

The martensite is formed by rapid cooling (quenching) of austenite which traps carbon atoms that do not have time to diffuse out of the crystal structure. This martensitic reaction begins during cooling when the austenite reaches the martensite start temperature (Ms) and the parent austenite becomes mechanically unstable. At a constant temperature below Ms, a fraction of the parent austenite transforms rapidly, then no further transformation will occur. When the temperature is decreased, more of the austenite transforms to martensite. Finally, when the martensite finish temperature (Mf) is reached, the transformation is complete.

One of the differences between the two phases is that martensite has a body centered tetragonal crystal structure, whereas austenite has a face center cubic (FCC) structure. The transition between these two structures requires very little thermal activation energy because it is a martensitic transformation, which results in the subtle but rapid rearrangement of atomic positions, and has been known to occur even at cryogenic temperatures. Martensite has a lower density than austenite, so that the martensitic transformation results in a relative change of volume.

Martensite is not shown in the equilibrium phase diagram of the iron-carbon system because it is a metastable phase, the kinetic product of rapid cooling of steel containing sufficient carbon. Since chemical processes (the attainment of equilibrium) accelerate at higher temperature, martensite is easily destroyed by the application of heat. This process is called tempering. In some alloys, the effect is reduced by adding elements such as tungsten that interfere with cementite nucleation, but, more often than not, the phenomenon is exploited instead. Since quenching can be difficult to control, many steels are quenched to produce an overabundance of martensite, then tempered to gradually reduce its concentration until the right structure for the intended application is achieved. Too much martensite leaves steel brittle, too little leaves it soft.

See also

Eutectoid

Ferrite (iron)

Maraging steel

Spring steel

Tool steel

References

^ Ashby, Michael F.; & David R. H. Jones (1992) . Engineering Materials 2 (with corrections ed.). Oxford: Pergamon Press. ISBN 0-08-032532-7. 

Categories: Metallurgy | Metals | Ceramics | SteelHidden categories: Articles needing additional references from September 2009 | All articles needing additional references | All Wikipedia articles needing clarification | Wikipedia articles needing clarification from February 2009