Essentials of Heat Treatment of Steel 211

allotropic

The ability of a material to exist in more than one crystal structure. Temperature determines whether iron has a body-centered cubic (BCC) or face-centered cubit (FCC) structure.

annealing

A heating and cooling operation that usually involves slow cooling. Annealing can reduce hardness, improve machinability, facilitate cold working, produce a desired rnicrostructure, and alter mechanical properties.

austenite

The phase at which solid steel recrystallizes and has a face-centered cubic crystal structure. Austenite steel holds a greater amount of dissolved carbon and exhibits increased formability.

bainite

A combination of ferrite and cementite that is harder than pearlite. Bainite contains needlelike grain structures, and it requires an initial rapid cooling followed by gradual cooling.

BCC

Body-centered cubic. The crystal structure that contains an atom in the center and one atom in each corner of a cube. Steel has a BCC structure at certain temperatures.

BCT

Body-centered tetragonal. A crystal structure that has been distorted by the presence of extra atoms of carbon. Martensite has a BCT crystal structure.

body-centered cubic

BCC. The crystal structure that contains an atom in the center and one atom in each corner of a cube. Steel has a BCC structure at certain temperatures.

body-centered tetragonal

BCT. A body-centered cubic crystal structure that has been distorted by the presence of extra atoms of carbon. Martensite has a BCT crystal structure.

brine

A mixture of water and salt. Brine cools metal faster than other quenching mediums.

brittle

Unable to be drawn, stretched, or formed. Brittle materials tend to fracture when subjected to mechanical forces.

carbon

A nonmetallic element that combines with iron to form steel. Carbon content affects a steel's properties and phase changes.

cementite

A compound of iron and carbon that is very hard and brittle. The presence of cementite hardens steel.

chromium

A metallic alloying element that increases hardenability and corrosion resistance. Chromium is often added to steel to improve hardenability.

cold working

Any metal shaping process that takes place when the metal is below its recrystallization temperature. Cold working adds certain properties to the metal, such as increased strength and improved surface finish.

critical temperatures

Temperatures at which a steel changes phases. The upper critical temperature and lower critical temperature are two significant temperatures.

crystal structures

The regular, repeating pattern of atoms in a metal. Crystal structures develop as the metal drops below the recrystallization temperature and solidifies.

ductility

A material's ability to be drawn, stretched, or formed without fracturing. A metal's ductility can be improved through heat treatment.

eutectoid temperature

The lowest temperature at which austenite transforms into ferrite and cementite. Steel with 0.8% carbon transforms at the eutectoid temperature.

face-centered cubic

FCC. The crystal structure that contains one atom in the center of the six sides of a cube and one atom in each corner of the cube. Steel has an FCC structure at certain temperatures.

FCC

Face-centered cubic. The crystal structure that contains one atom in the center of the six sides of a cube and one atom in each corner of the cube. Steel has an FCC structure at certain temperatures.

ferrite

The phase at which solid steel has a body-centered cubic crystal structure. Ferrite steel can hold only a minimal amount of carbon, and it is relatively soft.

forging

Forming a metal by compressing it at elevated temperatures. Forged parts are very strong due to their compressed grain structure.

hardenability

The ability of a metal to be hardened by heat treatment processes. Hardenability can be improved with chromium, manganese, nickel, and other alloying elements.

hardness

The measure of a material's ability to resist penetration, indentation, and scratching. Hardness is a mechanical property.

heat treatment

The controlled heating and cooling processes used to change the structure of a material and alter its properties. Heat treating is often used to adjust a material's hardness.

hypereutectoid steel

Steel that contains more than 0.8% carbon. Hypereutectoid steel consists of pearlite and cementite at room temperature.

hypoeutectoid steel

Steel that contains less than 0.8% carbon. Hypoeutectoid steel consists of ferrite and pearlite at room temperature.

lower critical temperature

The temperature at which austenite begins to form in a steel. The lower critical temperature is 1330°F (721°C).

manganese

A metallic alloying element that increases strength, hardness, and hardenability. Manganese is often added to steel to improve hardenability.

mar

Melted salt. Mar is a quenching medium that cools materials faster than air but slower than oil or water.

martensite

A steel that consists of a distorted, body-centered tetragonal crystal structure. Martensite is very hard and brittle.

microstructure

The shape and alignment of the microscopic components of a metal. Microstructure is key in determining hardness, toughness, and other properties.

microstructures

The shape and alignment of the microscopic components of a metal. A material's microstructure often determines its hardness, toughness, and other properties.

molybdenum

A metallic alloying element that increases hardenability, strength, and wear resistance. Molydenum is often added to steel to improve hardenability.

nickel

A metallic alloying element that increases strength, impact resistance, and corrosion resistance. Nickel is often added to steel to improve hardenability.

normalizing

A heat treatment in which the metal is heated to above the upper critical temperature, held for a period, and then cooled in still air. Normalizing produces a uniform rnicrostructure and relieves stress.

oxidation

A material's chemical reaction with oxygen. Oxidation causes rust and tarnish to form on metal surfaces.

pearlite

A combination of ferrite and cementite. Steel with exactly 0.8% carbon consists of uniform pearlite at room temperature.

phase diagram

A chart that depicts the effects of temperature and pressure changes on a substance. Steel phase diagrams illustrate how carbon content and temperature impact the microstructure of steel.

phases

A portion of material with a uniform crystal structure, consistent properties, and recognizable boundaries. At room temperature, steel consists of two or more phases.

precipitation

The separation of a substance that was previously dissolved in another substance. Carbon appears as a precipitation of cementite in steels with over 0.02 percent carbon.

quenching

The soaking of a metal at a high temperature above the recystallization phase, followed by a rapid cooling process. The quenching of steel creates martensite.

quenching mediums

The substance used to cool a metal during quenching. Water, saltwater, air, synthetic polymers, and oil are common quenching mediums.

silicon

An element that is added to alloys to improve hot-forming properties. Silicon is often added to steel to improve hardenability.

soaking

The heating of a metal at a constant temperature for an extended duration of time. Soaking is part of annealing processes.

synthetic polymer

A polymer that is chemically manufactured from separate materials. Some synthetic polymers can be used as quenching mediums.

tempered martensite

Martensite steel that has been heated and gradually cooled. Tempered martensite has better ductility and toughness than standard martensite.

tempering

The heat treatment of hardened steels to temperatures below the transformation temperature range, followed by a gradual cooling process. Tempering is usually done to improve toughness.

time-temperature-transformation diagram

A diagram that illustrates phase changes that take place in metal due to time and temperature. Time-temperature-transformation diagrams are also called isothermal transformation diagrams.

toughness

A material's ability to absorb mechanical forces before fracturing. A metal's toughness can be improved through heat treatment.

upper critical temperature

The temperature above which steel consists entirely of austenite. The upper critical temperature varies based on the steel's carbon content.