Heat Treating Terms
Heat treating is your last line of defense against metal part failure
Table of Contents
The following list includes the majority of processes and explanations utilized in today’s heat treating industry. For more information on any of these processes, please contact Phoenix Heat Treating’s metallurgist, Peter Hushek.
Age Hardening, Aging
The change that occurs in the properties of metals and alloys at room temperature or slightly elevated temperatures after heat treating or cooling. This process can be applied to numerous alloys, including aluminum, nickel based super alloys, stainless steels and copper alloyed with beryllium.
A process used for steels that contain sufficient quantities of carbon and other alloys that will harden during air cooling or in controlled cooling conditions using other gasses. The steel is cooled from a specified heat-treated temperature to meet the hardening specs required on the job order. Air hardening is generally used for distortion critical tool steel components which cannot handle the distortion induced from oil quenching.
When maximum ductility and high strength is not needed, ductile iron castings and parts are processed with a full anneal. The metal is heated to a required temperature range and held at that temperature for the specified amount of time, then cooled at a controlled rate. The microstructure is converted to ferrite and spheroidal graphite. Elements that retard annealing, such as manganese and phosphorus, and alloying elements including chromium, nickel, copper and molybdenum are kept low to produce optimum machinability. Annealing will soften metals to a specific microstructure or change the molecular properties of the part. Annealing will also remove stress and align the molecular properties to increase strength. Annealing is often performed on a variety of metals to make magnetic properties more uniform. The Magnetic Anneal requires tight control of descending ramp rates and temperature uniformity.
Austenitizing is the process of forming austenite (the solute is usually carbon), by heating a ferrous alloy above the transformation range. Austenitizing is not a complete process, but is the first step used prior to normalizing, annealing of ferrous alloys or quench-hardening of ferrous alloys.
Brinell Hardness Test
The hardness of a material is determined by forcing a hard steel or carbide ball of a specified diameter into the material. The Brinell Hardness number is the value derived by dividing the applied load in kilograms by the specified area of the resulting impressions in square millimeters. The Brinell test is mostly performed on softer material including cast irons, aluminum alloys and annealed steel alloys.
The absorption and diffusion of carbon into solid ferrous alloys by heating to a temperature range above the transformation temperature of the alloy in a controlled gas atmosphere to cause the absorption of carbon and nitrogen at the surface. Temp ranges required are 1650 to 1900-degrees-F (900 to 1040-C). Heating takes place in a carbon-rich environment of liquid, solid or gas to produce a carbon gradient that extends from the surface into the material. Carbonitrided parts are then cooled at a rate required on the job order to produce the desired properties in the material. Carbonitriding is used for thin, hard, wear-resistant cases on hardware parts manufactured via mass production. Carbonitriding is very similar to the cyaniding process which was performed in a salt bath many years ago. The resulting case structure is nearly identical to cyaniding, though not as economical due to environmental cost process.
This process replaces the carbon lost in the surface layer through previous heat treating or hot working of the metal by carburizing this layer to replenish the original carbon content. The process is also called recarburizing. This process is typically applied to alloy steel forgings or casting which must retain a uniform structure over the entire surface of a part.
A process that causes absorption and diffusion of carbon into solid ferrous alloys by heating the material to temperatures usually above the transformation temp of the alloy, 1650 to 1900-degrees F. (900 to 1040-C). Processing takes place in a carbonaceous environment consisting of gas, liquid or solid. The result is the diffusion of carbon into the material surface to the specified depth, resulting in a hardened surface that is completed by quenching from the carburizing temp or by cooling to room temp; then reaustenitizing and quenching. After quenching, the high-carbon exterior becomes harder, while the low-carbon core remains softer. This process is used for processing gears and requires tight controls on gas composition, temperature uniformity and atmosphere circulation.
This process can result from a chemical composition change of the surface of steel by absorption of carbon and/or nitrogen, and through diffusion, a concentration gradient is created. The processes commonly used are carburizing, carbonitriding, ferretic nitrocarburizing or nitriding. There is another type of case hardening where a localized area is quickly heated to the austenitizing temperature and rapidly cooled. The heating is typically performed by induction or flame in this process.
A loss of carbon from the surface layer of a carbon-containing alloy due to a reaction with chemical substances in a atmosphere that comes in contact with the material surface during processing. This is typically oxygen when present at temperatures above the critical temperature with result in decarburization, such as hot forging or heading.
A process used for hardening the surfaces of hardenable ferrous alloys in which an intense flame is used to heat the surface layers above the upper transformation temperature, after which the part is quenched. This is a form of case hardening.
The process of hardening includes age hardening, case hardening, flame hardening, induction hardening, precipitation hardening and quench hardening. The process increases hardness by heating and cooling. Resulting hardness is measured by indentation via the Rockwell hardness test or other, that measures the depth of penetration by a penetrator into the surface of the material.
Heat Treatment, Heat Treating
The term heat treating is a generic industry term that describes the process of heating and cooling solid metals or alloys to obtain a set of desired properties. Heat treatment can only be applied as a process to material which exhibit the chemical and physical composition capable of accepting the controlled heat and cooling to meet beneficial ends.
A process used to case harden only the surface layer of ferrous metals. The metal is heated by electromagnetic induction to a high critical temperature and then quenched.
Martensite, “Martensitic Transformation”
In an alloy, martensite is a metastable transitional structure between two allotropic modifications whose abilities to dissolve a solute differ, the high temperature phase having the greater solubility. The amount of high temperature phase transformed to martensite depends upon the temperature attained in cooling. Martensite is also a metastable phase of steel, formed by the transformation of austentite below a specified temperature. Martensite is characterized by an interstitial supersaturated solid solution of carbon in iron having a body-centered tetragonal lattice that resembles an acicular, needlelike pattern that can be observed in laboratory testing. Martensitic transformation is a reaction that takes place in some metals during the cooling phase causing the formation of the acircular structures called “martensite.”
Metallurgy involves the science and technology of metals and alloys and changes that occur in the physical properties and compositions caused by varying combinations of heating and cooling in processing. A metallurgist or metallurgical engineer is person who holds a degree metallurgical engineering, and specializes in understanding and prescribing the molecular changes that take place in steels and alloys when they are processed by high temperatures, and then cooled through a controlled process to achieve a desired attribute. An experienced metallurgist is an invaluable asset in heat treating, because trial and error is dramatically reduced, and repeated and documented processes can be duplicated time after time.
The hardness of a material can be known by forcing an indenter tool into the surface of a material under a very light pressure. The indentations are then viewed under a microscope in the laboratory to gauge the hardness of the material. Microhardness of two different constituents within a material can be tested by indenting a sample of the material for measuring in case hardening, for example. Microhardness numbers are correlated for most of the common hardness scales for comparison purposes.
Nitriding is a case-hardening process which consists of the introduction of nitrogen, usually as ammonia gas, to machined and heat-treated ferrous alloys to harden the surface layers without requiring any further quenching treatment. Processing temperatures are commonly 975-degrees-F to 1050F (525 to 565-C). Nitriding produces a hard, wear resistant surface, and results in properties that are resistance to corrosion and surface stresses that cause pre-mature material wear and failure.
Nitriding is a case-hardening process which consists of the introduction of nitrogen, usually as ammonia gas, to machined and heat-treated ferrous alloys to harden the surface layers without requiring any further quenching treatment. Processing temperatures are cThis term is used to describe a large family of processes where nitrogen and carbon are absorbed into the surface layer of a variety of carbon and steel alloys. The process uses salt or gas and temperatures below the transformation temps of the alloy. Nitrocarburizing produces wear-resistant surfaces and increased case strength. When nitrocarburizing is used, cases are very thin, so finishing must be avoided.ommonly 975-degrees-F to 1050F (525 to 565-C). Nitriding produces a hard, wear resistant surface, and results in properties that are resistance to corrosion and surface stresses that cause pre-mature material wear and failure.
Normalizing is the process of heating a ferrous alloy to at least 100-degrees-F above the transformation range and then cooling the material in still air to a temperature lower than the transformation range. The process produces a recrystallization and refinement of the grain in the material that results in uniform hardness and structure. Many high strength components are normalized prior to the harden and temper process for the purpose of optimization of the mechanical properties.
Ovens and Furnaces
The Industrial Heating Equipment Association (IHEA) classifies heating equipment as ovens and furnaces. The difference between the two is determined by temperature. Ovens operate up to about 1000-degrees-F (540-C), and furnaces operate at temperatures exceeding 1000-degrees-F.
A hardening process for corrosion-resistant steels such as stainless steel that is caused by the precipitation of a constituent from a supersaturated solid solution by soaking at a desired temperature for a prescribed period of time. The general practice for nickel based alloys is in heat treating of the material to an elevated temperature, followed by rapid cooling, then age hardening by heating to an intermediate temperature. The metallurgy of precipitation hardening is very complex and should be handled only by a competent heat treating company.
Quenching is the rapid cooling of metal or an alloy from an elevated temperature. This is usually done with water, brine, oil, polymer, or even forced or still air. There are two types of quenching – the first is cooling to obtain an acceptable microstructure and mechanical properties that will meet minimum specs after tempering. The second consists of rapid cooling of iron-base alloys and nonferrous metals to retain uniformity in the material. Quenching is performed to control the transformation of austentite and to form the microstructure. When only selected areas of the material are quenched, the process is called selective quenching.
The formation of a new, stress-free grain structure accomplished by heating and cooling at precise temperature and time cycles results in recrystallization. The creation of a new crystal structure occurs while heating or cooling through predetermined temperatures.
Rockwell Hardness Test
An industry test that determines the hardness of a metal material through indentation of a penetrator into the specimen under certain arbitrarily fixed conditions.
Solution Heat Treatment
This process consists of heating an alloy to a required temperature, then holding at that temperature long enough to cause one or more constituents to diffuse into solid solution, followed by rapid cooling rapidly to hold the absorbed constituents in solution. This process has been called solution annealing in the past.
Applying copper plate to selected areas of a material to prevent carburization, decarburization or nitriding in the plated areas is used to “Stop Off”. Once the material has been processed, the plating is stripped. This process is commonly used for processing gears to produce strength and rigidity in the gear teeth while leaving the core material softer. A commercially available paint can be applied on products which can be utilized when a tolerance for partial diffusion through the coating is acceptable.
For steels and irons, stress relief occurs when a material is heated to a temperature below the lower transformation temperature of the material and holding for a pre-determined period of time. The temperature for nonferrous metals may vary from a few degrees above room temperature to 300-degrees, or so, depending on the alloy and the amount of stress relief that is desired. After heating, the material is cooled slowly to prevent the creation of new stress. The main purpose of stress relieving is to relieve stresses caused from forming, shaping, rolling machining or welding.
Tempering is the process of reheating austenitized and quench-hardened steel or iron (ferrous alloys), to a pre-determined temperature that is lower than the transformational temperature (below 1300-degrees-F, 705C), to obtain different combinations of mechanical properties in the material. Temperatures for hardened steels are usually at, or below, 300-degrees-F (150-C). Tempering is often confused with process annealing or stress relieving because the temperature cycles for the three processes can be the same.
A device that measures temperatures in heat treating environments, that consists of two dissimilar metals or alloys that are electrically joined at one end and connected to a voltage-measuring instrument a the other end. When the alloys are heated, there will be a thermal electromotive force that is roughly proportional to the difference in temperature between hot and cold junctions on the alloys. Thermocouples are used to control heating units and to determine the surface temperature of metal parts during processing.
This process consists of gas carburizing performed at an atmospheric pressure that is less than the normal atmosphere. Carburizing temperatures in vacuum furnaces range from 1650 to 2000-F (900 to 100-C), but are usually at 1800 to 1925-F (980 to 1050-C). First, a vacuum is created in the furnace chamber, after which an enriching atmosphere gas consisting of nitrogen is induced. Graphite heating fixtures are used instead of metals in vacuum furnace construction, thus higher temperatures can be used for carburizing. The advantage of vacuum carburizing is that the process takes less time. Vacuum heat treating is used to prevent reactions at the surface of the material, such as oxidation; to remove surface contaminants such as oxides, traces of lubricants and other dissolved substances from metal; add a carburizing layer to the surface; join metals by brazing or diffusion bonding.