Hardening, bright carburizing, quenching ,tempering, furnaces for the thermal treatment of steel.
Koyo Thermo Systems

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Thermal treatment of steel

Furnaces for annealing, tempering, quenching, hardening

Steel is an alloy of iron and carbon. Its carbon content is usually in the range between 0,02% and 6,5%. The carbon atoms are located in interstitial iron lattice positions, which have different size and the presence of a carbon atom on that position causes unequal lattice deformation and stress. Quite often other metals like Chromium Cr, Cobalt Co, Manganese Mn, etc. are alloyed also, and this modifies as well lattice dimensions and steel properties.
At room temperature and up to 911C pure iron has a body centered cubic configuration (α-iron), called ferrite. At higher temperature between 911C and 1392C exists a face centered cubic configuration (γ-iron), called austenite. Above 1392C steel forms again a small range of body centred cubic lattice called δ-iron or δ-ferrite. Depending on the lattice configuration, carbon is then sitting either in a tetraedic or in an octaedric interstitial position of the iron lattice. Size and lattice deformation is different. With higher deformation, steel gets harder.
Cooling the melt and solid steel slowly results in phase transitions, where austenite and ferrite or mixed phases are generated. During phase transitions, carbon atoms try to migrate into the most favourite lattice positions. However; the capacity of the iron lattice for the absorption of carbon atoms is limited and when the maximum solubility of carbon in iron is reached during cooling, precipitation of cementite or perlite starts. Cementite is an iron carbide Fe3C, while perlite is a mixture of cementite and ferrite. If steel has a higher carbon content, ledeburite is formed, a mixture of austenite and cementite. The various phases formed during slow cooling are described in the iron carbon phase diagram (you can see here a simplified version).

Fe-C-phase diagram, ferrite, austenite, perlite, cementite, martensite

Steel properties like hardness or durability depend on the lattice deformation and the existence of precipitations as well as on the size of various crystallites. These steel properties can be configured or adjusted by various thermal processes.
Koyo Thermo Systems can offer the technology, equipment and furnaces to adjust steel properties accordingly. Most Koyo furnaces use Moldatherm®-heaters.

Crystec Technology Trading GmbH, Germany, www.crystec.com, +49 8671 882173, FAX 882177
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Furnaces for annealing and tempering of steel

While annealing, the workpiece is heated to a certain temperature and then cooled down slowly. This can be done to reach following aims:
With coarse-grain annealing the size of the individual crystallites is wanted to be increased. Consequently; the material's stability and viscosity decreases, which is wanted for several machining processes.
Stress relief heat treatment takes place at relatively low temperatures between 480C and 680C and results in removement of the workpiece's residual stress. This stress is produced by mechanical deformation or processing. Other than that, steel properties should not be changed.
Homogenizing needs up to two days and takes place at relatively high temperatures between 1050C and 1300C and is supposed to accomplish an even distribution of impurities in the metal lattice. Cooling rate determines the development of phases, and consequently; steel characteristics.
Recrystallization annealing is reestablishment of crystallite forms as the ones that existed before strain hardening. To do so, the work piece is heated to temperatures just above the temperature of recrystallization, usually between 550C and 700C. The temperature of recrystallization depends on material and level of deformation.
Normalizing of steel is one of the most important heat treatments. It creates a fine grained structure of crystallites which are distributed evenly over the work piece. Steel with higher carbon proportion needs a temperature of just under 800C for annealing; steel with lower carbon proportion needs a temperature of 950C
With Soft-Annealing of steel, existing precipitations of cementite or perlite is reduced in order to reduce the hardness and strength of steel and in order to make deformation easier. The typical temperature for this process varies between 680C and 780.
Koyo offers continuous and discontinuous furnaces for all annealing processes, for both under atmospheric circumstances and in vacuum.

Normalizing furnace Wire tempering furnace
Continuous normalizing furnace Furnace for tempering steel wires

Crystec Technology Trading GmbH, Germany, www.crystec.com, +49 8671 882173, FAX 882177
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Hardening of steel

Furnaces for steel tempering and quenching

When non-alloy steel is hardened, the work piece is at first heated to a temperature between 800C and 900C until, in case of steel with low carbon proportion, pure austenite is left. For alloyed steel, temperature can vary significantly.
In order to avoid corrosion; exothermic gas can be used inside the furnace. It is also generated from hydrocarbons in a corresponding gas generator and contains CO, H2 and N2 but also CO2 and H2O.

Exothermic gas reactor
Exothermic gas generator

After tempering, steel is cooled down rapidly or quenched in order to keep carbon atoms from migrating to advantageous positions in the lattice during the phase change. This can be avoided because diffusion rate of carbon atoms becomes too low at low temperature for moving between lattice positions.
With sinking temperature the iron lattice still changes its lattice structure, and consequently; so called martensite or martensitic steel is generated. Due to lattice deformation and lattice stress, martensite is very hard but also no longer deformable and brittle.
For thicker work pieces, corresponding high cool-down rates are necessary to harden the whole work piece. In practice, these pieces are put into oil or water baths. The most effective method is quenching with water because of its high thermal conductivity. When dipped into water, a mal conductive steam layer is generated on its surface in the first instance (Leidenfrost phenomenon). It is necessary to pay attention to dipping the work piece in the right manner. Its complete surface has to be touched evenly by the fluid. Also aqueous polymere solutions can be used for quenching.
The work piece can be heated either in a chain conveyor furnace or in a roller hearth furnace, on whose ends the work pieces fall or slide into the quenching bath or in a hood-type furnace which is loaded from below and from which work pieces can be reeled-out quickly.

Bottom-up oven kiln, quenching furnace Continuous mesh belt annealing furnace Vacuum quenching kiln
Bottum-up quenching oven Mesh belt furnace with quenching bath Vaccum quenching kiln

Quite often vacuum furnaces are used for the quenching process. The low oxygen content avoids oxidation and corrosion of the surface of the parts.

Parts Washing

After quenching parts in oil or emulsion, the parts need cleaning before they can be introduced to the next furnace for annealing and tempering. Koyo offers special washing machines for that application. Washing process can also be integrated in a continuous process. Annealing, quenching, washing and tempering can be done in a single piece of equipment.

Washing machine for oily parts after quenching
Parts cleaning after quenching and before tempering.

Washing process technology

Tempering of steel after quenching

After the quenching process, martensitic steel is very hard but also brittle. By tempering the parts again some counter action can be taken.
In the temperature range below 100C first carbon concentration of martensitic steel is increased in the areas of lattice defects. At temperatures between 100C and 200C carbon atoms start migrating from their unfavourable positions in the iron lattice. Precipitation of iron carbide begins. When temperature is increased further this process will be accelerated. Higher than 320C almost all carbon atoms have left their unfavourable interstitial lattice positions. Over 400C no serious micro structure changes happen any more and the steel is getting soft again. However in case of steel alloys with chromium, vanadium, molybdenum and tungsten hardness is again increasing in this temperature range, because special carbides are formed. This secondary hardening is important for parts, which should keep their hardness under conditions of increased temperature.
In general, the hardness of steel decreases with increasing tempering temperature. At the presence of air, the surface is oxidizing which can be seen by the typical colour change of the steel during that process. The colour is referring to the thickness of the grown oxide layer. The necessary anneal time depends on the mass and thickness of the treated parts.

Crystec Technology Trading GmbH, Germany, www.crystec.com, +49 8671 882173, FAX 882177
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Surface Hardening, Case Hardening, Pack Hardening

In contrary to steel hardening by tempering, quenching and annealing, where the bulk material gets hardened, surface hardening technology hardens only the surface of the steel. Hard surface and ductile bulk result in special good steel properties. Several methods can be used for surface hardening.

Carbonisation, Carbonization, Carburization, Carburisation

Carbonisation, Carbonization, Carburization, Carburisation - several words are used for the same process. This surface harding or case hardening or pack hardening method can be used for low carbon steels. The material is treated in a carbon rich, endothermic ghas.
Endothermic gas is generated in a gas generator from methane, ethane or propane and consists mainly from carbonmonoxide CO, hydrogen H2 and nitrogen N2.

Endothermic gas reactor
Endothermic gas generator

The steel is heated at a temperature of 900C to 1000C in a special tempering or annealing furnace or oven or kiln where it absorbs carbon atoms from the endothermic gas atmosphere. Carbon concentration can be increased up to the saturation level of austenite in surface close areas (around 1mm deep). Quenching and annealing follows. Koyo furnaces type KCF can be used for that process. Continous and batch furnaces can be used. In a continous furnace transportation can be realized by a ceramic roller hearth or pusher type system, or by mesh belt transportation.

Continuous pusher-type furnace with ceramic roller hearth transportation system Meshbelt furnace for bright carburizing carburising carbonizing carbonising
Continuous pusher-type furnace with ceramic roller hearth transportation system Meshbelt furnace for carburizing, carburising, carbonizing, carbonising

Rotary drum furnaces can be used for this process also. In those systems tempering, quenching, cleaning and annealing can be intergrated.

Rotary Drum Type continuous carburizing carburising carbonizing carbonising furnace
Rotary Drum Type continuous steel carburizing furnace

Carbonitriding

For carbonitriding not only carbon is diffused into the steel but also some nitrogen and nitride precipitation is caused in surface near areas. Ammonia NH3 is used as nitrogen source usually.
Carbonitriding at low temperature of 650 to 770C results in good nitrogen diffusion and after quenching a thin film of nitride and carbide is formed on the martensite surface. If Carbonitriding is done at high temperature of 770C to 930C, then this nitride and carbide film is not generated because then the carbon diffusion speed is higher. The nitrogen content stabilizes the austenite phase and allows lower quenching rates with increased hardness. However the hard layer is usually thinner compared to carburizing and therefore the material properties change more between surface and bulk material.
Quenching has to follow the tempering process for carbonitridation as well as for carburisation.

carbonitriding furnace flameless bright carburizing furnace and quenching furnace
Carbonitriding furnace Flameless bright carburizing furnace and quenching furnace

Nitriding and Nitrocarburizing

Nitriding is a surface hardening method, where nitrogen is diffusing into the steel surface at a rather low temperature of 500 to 550C. Ammonia serves as a nitrogen source. The nitrogen is diffusing in the steel and occupies interstitial positions in the iron lattice. This causes distortion and stress. The material does not need to be quenched and hardening effect is therefore not a result of the formation of martensite. During cooling, nitride precipitation is the main reason for hardness increase.
For Nitrocarburizing not only nitrogen is diffusing into the material, but also carbon. As a carbon source carbon monoxide or hydrocarbons are usually used. The nitrogen has a higher diffusion depth, while the carbon concentrates only in the areas close to the surface. The solubility of the carbon is low in nitrogen saturated steel and diffusion speed is lower compared to nitrogen. During cooling carbonitrides are formed. Nitrocarburation is a faster process compared to nitriding.
Nitrided and carbonitrided steel have a rather thin, hard and slippery surface. It is not so wear resistant and little bit fragile.

gas nitriding oven two stage nitriding kiln
Gas nitriding oven 2-stage nitriding kiln

Koyo Thermo Systems and Crystec will be pleased to engineer a cost effective system to satisfy your most demanding and exacting requirements.

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