Carburization
Encyclopedia
Carburizing, spelled carburising in the UK, is a heat treatment
process in which iron
or steel
is heated in the presence of another material (in the range of 900 to 950 °C (1,652 to 1,742 F)) which liberates carbon
as it decomposes. Depending on the amount of time and temperature, the affected area can vary in carbon content. Longer carburizing times and higher temperatures lead to greater carbon diffusion into the part as well as increased depth of carbon diffusion. When the iron or steel is cooled rapidly by quenching, the higher carbon content on the outer surface becomes hard via the transformation from austenite
to martensite
, while the core remains soft and tough as a ferritic and/or pearlite
microstructure
.
This manufacturing process can be characterized by the following key points: It is applied to low-carbon workpieces; workpieces are in contact with a high-carbon gas, liquid or solid; it produces a hard workpiece surface; workpiece cores largely retain their toughness
and ductility
; and it produces case hardness depths of up to 0.25 inch (0.635 cm).
packed onto the metal (initially referred to as case hardening
), but modern techniques apply carbon-bearing gases or plasmas (such as carbon dioxide
or methane
). The process depends primarily upon ambient gas composition and furnace
temperature, which must be carefully controlled, as the heat may also impact the microstructure of the rest of the material. For applications where great control over gas composition is desired, carburization may take place under very low pressures in a vacuum
chamber.
Plasma carburization is increasingly used in major industrial regimes to improve the surface characteristics (such as wear and corrosion
resistance, hardness and load-bearing capacity, in addition to quality-based variables) of various metals, notably stainless steel
s. The process is used as it is environmentally friendly (in comparison to gaseous or solid carburizing). It also provides an even treatment of components with complex geometry (the plasma can penetrate into holes and tight gaps), making it very flexible in terms of component treatment.
The process of carburization works via the implantation of carbon atoms in to the surface layers of a metal. As metals are made up of atoms bound tightly into a metallic crystalline lattice, the implanted carbon atoms force their way into the crystal structure of the metal and either remain in solution (dissolved within the metal crystalline matrix — this normally occurs at lower temperatures) or react with the host metal to form ceramic
carbides (normally at higher temperatures, due to the higher mobility of the host metal's atoms). Both of these mechanisms strengthen the surface of the metal, the former by causing lattice strains
by virtue of the atoms being forced between those of the host metal and the latter via the formation of very hard particles that resist abrasion. However, each different hardening mechanism leads to different solutions to the initial problem: the former mechanism — known as solid solution strengthening — improves the host metal's resistance to corrosion whilst imparting its increase in hardness; the latter — known as precipitation strengthening — greatly improves the hardness but normally to the detriment of the host metals corrosion resistance. Engineers using plasma carburization must decide which of the two mechanisms matches their needs.
Gas carburizing is normally carried out at a temperature within the range of 900 to 950 °C.
In oxy-acetylene welding
, a carburizing flame is one with little oxygen, which produces a soot
y, lower-temperature flame. It is often used to anneal
metal, making it more malleable and flexible during the welding process.
A main goal when producing carbonized workpieces is to insure maximum contact between the workpiece surface and the carbon-rich elements. In gas and liquid carburizing, the workpieces are often supported in mesh baskets or suspended by wire. In pack carburizing, the workpiece and carbon are enclosed in a container to ensure that contact is maintained over as much surface area as possible. Pack carburizing containers are usually made of carbon steel coated with aluminum or heat-resisting nickle-chromium alloy and sealed at all openings with fire clay.
gas (CO), sodium cyanide
and barium chloride
, or hardwood charcoal. In gas carburizing, the CO is given off by propane
or natural gas
. In liquid carburizing, the CO is derived from a molten salt composed mainly of sodium cyanide (NaCN) and barium chloride (BaCl). In pack carburizing, carbon monoxide is given off by coke or hardwood charcoal.
Heat treatment
Heat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass...
process in which iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
or steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
is heated in the presence of another material (in the range of 900 to 950 °C (1,652 to 1,742 F)) which liberates carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
as it decomposes. Depending on the amount of time and temperature, the affected area can vary in carbon content. Longer carburizing times and higher temperatures lead to greater carbon diffusion into the part as well as increased depth of carbon diffusion. When the iron or steel is cooled rapidly by quenching, the higher carbon content on the outer surface becomes hard via the transformation from austenite
Austenite
Austenite, also known as gamma phase iron, is a metallic non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of ; other alloys of steel have different eutectoid temperatures...
to martensite
Martensite
Martensite, named after the German metallurgist Adolf Martens , 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...
, while the core remains soft and tough as a ferritic and/or pearlite
Pearlite
Pearlite is often said to be a two-phased, lamellar structure composed of alternating layers of alpha-ferrite and cementite that occurs in some steels and cast irons...
microstructure
Microstructure
Microstructure is defined as the structure of a prepared surface or thin foil of material as revealed by a microscope above 25× magnification...
.
This manufacturing process can be characterized by the following key points: It is applied to low-carbon workpieces; workpieces are in contact with a high-carbon gas, liquid or solid; it produces a hard workpiece surface; workpiece cores largely retain their toughness
Toughness
In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing; Material toughness is defined as the amount of energy per volume that a material can absorb before rupturing...
and ductility
Ductility
In materials science, ductility is a solid material's ability to deform under tensile stress; this is often characterized by the material's ability to be stretched into a wire. Malleability, a similar property, is a material's ability to deform under compressive stress; this is often characterized...
; and it produces case hardness depths of up to 0.25 inch (0.635 cm).
Method
Carburization of steel involves a heat treatment of the metallic surface using a source of carbon. Early carburization used a direct application of charcoalCharcoal
Charcoal is the dark grey residue consisting of carbon, and any remaining ash, obtained by removing water and other volatile constituents from animal and vegetation substances. Charcoal is usually produced by slow pyrolysis, the heating of wood or other substances in the absence of oxygen...
packed onto the metal (initially referred to as case hardening
Case hardening
Case hardening or surface hardening is the process of hardening the surface of a metal, often a low carbon steel, by infusing elements into the material's surface, forming a thin layer of a harder alloy...
), but modern techniques apply carbon-bearing gases or plasmas (such as carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
or methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
). The process depends primarily upon ambient gas composition and furnace
Furnace
A furnace is a device used for heating. The name derives from Latin fornax, oven.In American English and Canadian English, the term furnace on its own is generally used to describe household heating systems based on a central furnace , and sometimes as a synonym for kiln, a device used in the...
temperature, which must be carefully controlled, as the heat may also impact the microstructure of the rest of the material. For applications where great control over gas composition is desired, carburization may take place under very low pressures in a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
chamber.
Plasma carburization is increasingly used in major industrial regimes to improve the surface characteristics (such as wear and corrosion
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
resistance, hardness and load-bearing capacity, in addition to quality-based variables) of various metals, notably stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
s. The process is used as it is environmentally friendly (in comparison to gaseous or solid carburizing). It also provides an even treatment of components with complex geometry (the plasma can penetrate into holes and tight gaps), making it very flexible in terms of component treatment.
The process of carburization works via the implantation of carbon atoms in to the surface layers of a metal. As metals are made up of atoms bound tightly into a metallic crystalline lattice, the implanted carbon atoms force their way into the crystal structure of the metal and either remain in solution (dissolved within the metal crystalline matrix — this normally occurs at lower temperatures) or react with the host metal to form ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
carbides (normally at higher temperatures, due to the higher mobility of the host metal's atoms). Both of these mechanisms strengthen the surface of the metal, the former by causing lattice strains
Strain (chemistry)
In chemistry, a molecule experiences strain when its chemical structure undergoes some stress which raises its internal energy in comparison to a strain-free reference compound. The internal energy of a molecule consists of all the energy stored within it. A strained molecule has an additional...
by virtue of the atoms being forced between those of the host metal and the latter via the formation of very hard particles that resist abrasion. However, each different hardening mechanism leads to different solutions to the initial problem: the former mechanism — known as solid solution strengthening — improves the host metal's resistance to corrosion whilst imparting its increase in hardness; the latter — known as precipitation strengthening — greatly improves the hardness but normally to the detriment of the host metals corrosion resistance. Engineers using plasma carburization must decide which of the two mechanisms matches their needs.
Gas carburizing is normally carried out at a temperature within the range of 900 to 950 °C.
In oxy-acetylene welding
Oxy-fuel welding and cutting
Oxy-fuel welding and oxy-fuel cutting are processes that use fuel gases and oxygen to weld and cut metals, respectively. French engineers Edmond Fouché and Charles Picard became the first to develop oxygen-acetylene welding in 1903...
, a carburizing flame is one with little oxygen, which produces a soot
Soot
Soot is a general term that refers to impure carbon particles resulting from the incomplete combustion of a hydrocarbon. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolyzed fuel particles such as cenospheres,...
y, lower-temperature flame. It is often used to anneal
Annealing (metallurgy)
Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating to above the recrystallization temperature, maintaining a suitable temperature, and...
metal, making it more malleable and flexible during the welding process.
A main goal when producing carbonized workpieces is to insure maximum contact between the workpiece surface and the carbon-rich elements. In gas and liquid carburizing, the workpieces are often supported in mesh baskets or suspended by wire. In pack carburizing, the workpiece and carbon are enclosed in a container to ensure that contact is maintained over as much surface area as possible. Pack carburizing containers are usually made of carbon steel coated with aluminum or heat-resisting nickle-chromium alloy and sealed at all openings with fire clay.
Hardening agents
There are different types of elements or materials that can be used to perform this process, but these mainly consist of high carbon content material. A few typical hardening agents include carbon monoxideCarbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
gas (CO), sodium cyanide
Sodium cyanide
Sodium cyanide is an inorganic compound with the formula NaCN. This highly toxic colorless salt is used mainly in gold mining but has other niche applications...
and barium chloride
Barium chloride
Barium chloride is the inorganic compound with the formula BaCl2. It is one of the most common water-soluble salts of barium. Like other barium salts, it is toxic and imparts a yellow-green coloration to a flame. It is also hygroscopic....
, or hardwood charcoal. In gas carburizing, the CO is given off by propane
Propane
Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...
or natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
. In liquid carburizing, the CO is derived from a molten salt composed mainly of sodium cyanide (NaCN) and barium chloride (BaCl). In pack carburizing, carbon monoxide is given off by coke or hardwood charcoal.
Geometrical possibilities
There are all sorts of workpieces that can be carbonized, which means almost limitless possibilities for the shape of materials that can be carbonized. However careful consideration should be given to materials that contain nonuniform or non-symmetric sections. Different cross sections may have different cooling rates which can cause excessive stresses in the material and result in breakage.Dimensional changes
It is virtually impossible to have a workpiece undergo carbonization without having some dimensional changes. The amount of these changes varies based on the type of material that is used, the carbonized process that the material undergoes and the original size and shape of the work piece. However changes are small compared to heat-treating operations.| Work material properties | Effects of carburizing |
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| Physical |
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| Chemical |
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Workpiece material
Typically the materials that are carbonized are low-carbon and alloy steels with initial carbon content ranging from 0.2 to 0.3%. The workpiece surface must be free from contaminates, such as oil oxides, alkaline solutions, which prevent or impede the diffusion of carbon into the workpiece surface.Comparing different methods
In general, pack carburizing equipment can accommodate larger workpieces than liquid or gas carburizing equipment, but liquid or gas carburizing methods are faster and lend themselves to mechanized material handling. Also the advantages of carburizing over carbonitriding are greater case depth (case depths of greater than 0.3 in are possible), less distortion, and better impact strength. This makes it perfect for high strength and wear applications (e.g. scissors or swordfish). The disadvantages include added expense, higher working temperatures, and increased time.Choice of equipment
In general, gas carburizing is used for parts that are large. Liquid carburizing is used for small and medium parts and pack carburizing can be used for large parts and individual processing of small parts in bulk.See also
- CarbonitridingCarbonitridingCarbonitriding is a metallurgical surface modification technique that is used to increase the surface hardness of a metal, thereby reducing wear. During the process, atoms of carbon and nitrogen diffuse interstitially into the metal, creating barriers to slip, increasing the hardness and modulus...
- Case hardeningCase hardeningCase hardening or surface hardening is the process of hardening the surface of a metal, often a low carbon steel, by infusing elements into the material's surface, forming a thin layer of a harder alloy...
- Cementation processCementation processThe cementation process is an obsolete technique for making steel by carburization of iron. Unlike modern steelmaking, it increased the amount of carbon in the iron. It was apparently developed before the 17th century. Derwentcote Steel Furnace, built in 1720, is the earliest surviving example...
- Crucible steelCrucible steelCrucible steel describes a number of different techniques for making steel in a crucible. Its manufacture is essentially a refining process which is dependent on preexisting furnace products...
- NitridizationNitridizationNitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case hardened surface. It is predominantly used on steel, but also titanium, aluminum and molybdenum....
Further reading
- Geoffrey Parrish, Carburizing: Microstructures and Properties. ASM International. 1999. pg 11