Heat Treating Terms and Definitions

Engineering Metals and Materials Table of Contents
Heat Treating Terms and Definitions #2

AIR HARDENING STEEL -An alloy steel which does not require quenching from a high temperature to harden but which is hardened by simply cooling in air from above its critical temperature range.

ANNEALING - Applies normally to softening by changing the microstructure and is a term used to describe the heating and cooling cycle of metals in the solid state. The term annealing usually implies relatively slow cooling in carbon and alloy steels. The more important purposes for which steel is annealed are as follows: To remove stresses; to induce softness; to alter ductility, toughness, or electric, magnetic or other physical and mechanical properties; to change the crystalline structure; and to produce a definite microstructure.

AUSTEMPERING - This is a method of hardening steel by quenching from the austenitizing temperature into a heat extracting medium (usually salt) which is maintained at some constant temperature level between 400 and 800 and holding the steel in this medium until austenite is transformed to bainite. The austempering process is limited to sections less than `/2 diameter. The advantages of this method of interrupted quenching are increased ductility and toughness at the resulting hardness of RC 45-55.

AUSTENITE - The solid solution of iron and carbon which is attained by heating to high temperatures above the upper critical temperature. This temperature or temperature range is called the austenitizing temperature and must be attained to obtain the proper microstructure and full hardness of steel in heat treating. The austenitizing temperature varies for the different grades of carbon, alloy and tool steels.

BAINITE - A decomposition or transformation product of austenite which is a type of microconstituent or structure in steel. This term is used by metallurgists to describe a particular structure of steel when the steel is polished, etched and examined with a microscope.

BRINELL HARDNESS - A hardness number determined by applying a 3000 kilogram load to the surface of the material to be tested through a hardened steel ball of 10mm. The diameter of the depression is measured and the hardness is the ratio of load to spherical area of the impression. Tables of numbers have been prepared, and the hardness is read from the table from the diameter of the depression.

CARBURIZING - Adding carbon to the surface of steel by heating the metal below its melting point in contact with carbonaceous solids, liquids, or gases.

CASE HARDENING - A heat treatment or a combination of heat treatments of surface hardening involving a change in the composition of the outer layer of an iron-base alloy in which the surface is made substantially harder by inward diffusion of a gas or liquid followed by appropriate thermal treatment. Typical hardening processes are carburizing, cyaniding, carbo-nitriding and nitriding.

CYANIDING - Surface hardening by carbon and nitrogen absorption of a steel article or a portion of it by heating at a suitable temperature in contact with cyanide salt, followed by quenching.

DECARBURIZATION - When steel is subjected to high temperatures, such as are used in hot rolling, forging, and heat treating in a media containing air, oxygen, or hydrogen there is a loss of carbon at the surface which is known as decarburization. This resultant loss of carbon or chemistry change at the surface of the steel part reduces the strength of the part by reducing the size of the section and produces a softer surface hardness than the core of the part.

FLAME HARDENING - A heat treat method used to harden the surface of some parts where only a small portion of the surface is hardened and where the part might distort in a regular carburizing or heat treating operation. The operation consists of heating the surface to be hardened by an acetylene torch to the proper quenching temperature followed immediately by a water quench and proper tempering. Generally wrought or cast steels with carbon contents of .30 to .40%, low alloy steels, and ductile and malleable cast irons are suitable for flame hardening.

HARDENABILITY- This relates to the ability of steel to harden deeply upon quenching, and takes into consideration the size of the part and the method of quenching. The test used to determine the hardenability of any grade of steel is the Jominy Test.

HARDENING - The heating and quenching of certain iron-base alloys from a temperature above the critical temperature range for the purpose of producing a hardness superior to that obtained when the alloy is not quenched. This term is usually restricted to the formation of martensite.


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