Materials Science Chapter 9 Engineering Alloys

Martempering (marquenching)

A quenching process whereby a steel in the austenitic condition is hot-quenched in a liquid (salt) bath at above the Ms temperature, held for a time interval short enough to prevent the austenite from transforming, and then allowed to cool slowly to room temperature. After this treatment, the steel will be in the martensitic condition, but the interrupted quench allows stresses in the steel to be relieved

Mf

The temperature at which the austenite in a steel finishes transforming to martensite

White cast irons

Iron-carbon-silicon alloys , contain large amounts of iron carbide that make them hard and brittle

Gray cast irons

Iron-carbon-silicon alloys, contain large amounts of carbon in the form of graphite flakes. They are easy to machine and have good wear resistance

Ductile cast irons

Iron-carbon-silicon alloys, contain large amounts of carbon in the form of graphite nodules instead of flakes as in the case of gray cast iron. More ductile that gray cast irons

Malleable cast irons

Iron-carbon-silicon alloys. First cast as white cast irons and then are heat-treated.

Intermetallics

Stoichiometric compounds of metallic elements with high hardness and high temperature strength, but brittle

Hardenability

The ease of forming martensite in a steel upon quenching from the austenitic condition. A highly hardenable steel is one that will form martensite throughout in thick sections. Hardenability should not be confused with hardness. Hardness is the resistance of a material to penetration. The hardenability of a steel is mainly a function of its composition and grain size

Shape-memory alloys

Metal alloys that recover a previously defined shape when subjected to an appropriate heat treatment process

Amorphous Metal

Metals with a noncrystalline structure also called glassy metal. These alloys have high elastic strain threshold

Martensite

A supersaturated interstitial solid solution of carbon in body-centered tetragonal iron

Jominy hardenability test

A test in which a 1 in. diameter bar 4 in. long is austenitized and then water-quenched at one end. Hardness is measured along the side of the bar up to about 2.5 in from the quenched end. A plot called the Jominy hardenability curve is made by plotting the hardness of the bar against the distance from the quenched end

Continuous-cooling transformation (CCT) diagram

A time-temperature-transformation diagram that indicates the time for a phase to decompose into other phases continuously at different rates of cooling

Isothermal transformation (IT) diagram

A time-temperature-transformation diagram that indicates the time for a phase to decompose into other phases isothermally at different temperatures

α ferrite

(α phase in the Fe-Fe3C phase diagram): an interstitial solid solution of carbon in BCC iron: maximum solid solubility of carbon in BCC iron is 0.02 percent

Austenite

(γ phase in Fe-Fe3C phase diagram): an interstitial solid solution of carbon in FCC iron; the maximum solubility of carbon in austenite is 2.0 %

Spheroidite

A mixture of particles of cementite (Fe3C) in an α ferrite matrix

Pearlite

A mixture of α ferrite and cememtite (Fe3C) phases in parallel plates (lamellar structure) produced by eutectoid decomposition of austenite

Bainite

A mixture of α ferrite and very small particles of Fe3C particles produced by the decomposition of austenite; a nonlamellar eutectoid decomposition product of austenite

Austempering

A quenching process whereby a steel in the austenitic condition is quenched in a hot liquid (salt) bath at a temperature just above the Ms of the steel, held in the bath until the austenite of the steel is fully transformed, and then cooled to room temperature. With this process, a plain-carbon eutectoid steel can be produced in the fully bainitic condition

Plain-carbon steel

An iron-carbon alloy with 0.02 to 2 % C. All commercial plain-carbon steels contain about 0.3 to 0.9 percent manganese along with sulfur, phosphorous, and silicon impurities

Proeutectoid cementite (Fe3C)

Cementite that forms by the decomposition of austenite at temperatures above the eutectoid temperature

Eutectoid cementite (Fe3C)

Cementite that forms during the eutectoid decomposition of austenite; the cementite in pearlite

Austenizing

Heating a steel into the austenite temperature range so that its structure becomes austenite. The temp for this vary depending on the composition of steel

Cementite

The intermetallic compound Fe3C; a hard and brittle substance

Tempering (of a steel)

The process of reheating a quenched steel to increase its toughness and ductility. In this process martensite is transformed into tempered martensite

Ms

The temperature at which the austenite in a steel starts to transform to martensite

Proeutectoid α ferrite

α ferrite that forms by decomposition of austenite at temperatures above the eutectoid temperature

Eutectoid α ferrite

α ferrite that forms during the eutectoid decomposition of austenite; the α ferrite in pearlite