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STEEL HEAT TREATMENT

Today, steel is the most important material for many industries and economic sectors On a basic level, all steel consists mainly of iron. What makes the individual steels so special are the other substances that are added, the so-called alloy elements. The most significant alloy element is carbon. Steel with a carbon content of 0.1 % is almost impossible to harden and yet with a carbon content of 0.7 % the maximum possible hardness can be attained. By adding other alloy elements such as chromium, nickel, tungsten, etc., the mechanical characteristics and corrosion resistance of the material can be further improved. But it is not enough to just select the right steel to meet the desired properties of a construction part. The required characteristics for the component can only be achieved by a heat treatment appropriate for the type of steel.

A prerequisite for understanding any heat treatment is knowledge of the iron-carbon diagram. With the aid of the iron-carbon diagram, it becomes clear in which state a low-alloy or non-alloyed steel with a known carbon content is at a certain temperature, and what microstructure can be expected for a given temperature profile. But see for yourself:

Iron - Carbon - Diagram

Hardening is an apparently simple process for the layman, but unfortunately the processes during hardening are not simple. To help understanding: In one single hardening process, more atoms start moving and change their places in the component than during a day in an active nuclear reactor.
In this context it is clear that hardening or heat treatment is a highly complex process that must be optimally controlled and monitored by experienced experts. This is the only way for your components to be optimised and attain the desired properties.

Since the refinement of the components by heat treatment is not immediately visible, and it is only possible to recognise what we have done with your component under the microscope, hardening is a matter of trust. Your components are in safe hands with us.

CASE HARDENING

The process of case hardening is always used when a component is desired with a ductile core and at the same time a wear-resistant and hard surface. The workpiece made of case-hardened steel is carburized in the peripheral area and then quenched (hardened). Case hardening usually takes place at temperatures between 880 and 980 °C in an atmosphere containing carbon, so that the surface of the component is enriched with carbon by diffusion. After carburization, the component is quenched in oil, water or special polymer solutions. This results in a significant increase in hardness in the carburized region. Standard case hardening depths lie between 0.1 to 2.5 mm, but we also have workpieces with extreme case hardening depths of over 6.0 mm in our portfolio.

If nitrogen carriers (ammonia) are also added to the furnace atmosphere besides carbon, this is then called carbonitriding. Nitrogen atoms diffuse in the same way as carbon into the material surface and increase the hardness. The advantage of carbonitriding is that cheaper unalloyed steels and free-cutting and deep-drawing steels can be surface hardened.

After the hardening process, the component is tempered. Tempering is necessary to reduce tension in the component and to set the required operational strength. Case hardening at the Hanomag Heat Treatment Group is done in a gas stream or in a salt bath. In the salt bath there is the option of partial hardening without previously insulating certain regions of the workpiece. A hard and wear-resistant surface layer and a ductile core make this process the preferred heat treatment for all transmission components.

Benefits of case hardening

  • Improvement of the mechanical properties
  • Higher wear-resistance 
  • Ductile core
  • Regions can be covered that are not to be hardened.

GAS NITRIDING / CARBURIZING

During nitriding, the surface layer of a material is enriched with nitrogen. If carbon is additionally diffused in, this is called nitrocarburization.

The process temperatures lie between 480 and 580 °C for nitriding. Depending on the material, the treatment results in a surface hardness of up to 1200 VHN. Due to the relatively low treatment temperatures, no structural conversion takes place, as for example, in hardening. This also explains why only slight dimensional and shape changes occur in nitriding.

The increase in hardness effect is not attained in nitriding by a classical hardening process. The increase in hardness is based on the formation of iron nitrides and special nitrides in the surface layer of the component.

The layer construction is in two parts in nitriding. In the external region it consists of a so-called compound layer with a thickness of 5 -20 µm and comprises almost exclusively iron nitrides. Under the compound layer there is a precipitation zone that supports the compound layer. Here there are special nitrides that cause the increase in hardness. The depth of the precipitation zone correlates with that of the compound layer.
 

The following general conditions apply for the nitriding process:

The longer the duration of the process, the deeper the nitriding hardness depth and the higher the selected temperature, the deeper the penetration of nitrogen. However, the intrinsic hardness of the nitriding layer diminishes with increasing treatment duration. It requires time, experience and tests to optimally balance these process parameters with the material. The result is subsequently a stable nitriding process with reproducible values.

Benefits of gas nitriding

  • Higher wear-resistance
  • Only slight dimensional and shape changes
  • Stable process
  • Regions can be covered that are not to be hardened.

FLAME HARDENING

Flame hardening is a surface layer hardening process to harden material surfaces with subsequent tempering at low temperatures. With flame hardening, high surface hardnesses of up to 800 VHN can be achieved for components with ductile cores. A prerequisite is that the materials in their basic state have a minimum carbon content of 0.4 - 0.6 % (ISO EN 8670). To austenitize the component surface, radiant heat is used with the aid of natural gas burners. After austenitization and depending on the hardening process and material, quenching follows with water, special hardening oil, synthetic quenching media or compressed air.

In the case of flame hardening, usually only partial areas of the component are hardened that are subject to particular wear and tear. Additionally, the fatigue and compression strength are also increased by the compression stresses induced in the region close to the surface. The existing technical options and internal construction of the furnace at Hanomag Heat Treatment’s site at Gevelsberg allow very large workpieces to be treated:

Spin hardening: Diameter 2000 mm x 950 mm high, up to 7,000 kg

Rotation and feeding: Diameter 650 mm x 5.000 mm long, up to 10,000 kg

The Hanomag Heat Treatment Group is one of the few providers in Europe that has mastered this special process.

Benefits of flame hardening

  • Large bulky components or individual parts can be hardened.
  • The transition from the hardening penetration depth to the original state is smoother, so that when the original material has a high strength, no sharp transition develops below the hardened zone.
  • Considerable heating depths can be achieved with spin hardening.
  • Flexible, no inductor manufacturing necessary

ANNEALING

The annealing process refers to the treatment of a workpiece at a specified temperature, taking into account a defined retention time and subsequent cooling down. A distinction is made between the following annealing processes:

Normal annealing is carried out mainly after a preceding hot forming of components. Heating takes place at a temperature somewhat above the hardening temperature with subsequent cooling in a still atmosphere. The adjustment to a finely-grained perlite-ferrite structure should be achieved by normal annealing. As a result, coarse-grained and irregular structures can be transformed into new homogeneous and fine structures. This type of heat treatment is performed at around 20-50° C above the AC3 transition temperature, at which the austenite-ferrite conversion occurs. Stress-free annealing is used to reduce intrinsic tensions in workpieces incurred by cold forming, structure conversion, thermal wear or machine processing. Stress-free annealing is usually carried out between 450-650°C at sufficiently long retention times and followed by very slow cooling without any significant changes in structure or mechanical properties.

Soft-annealing refers to annealing at a temperature just below the conversion point followed by slow cooling, in order to achieve a soft condition. A granular perlite should result, with a soft microstructure, which gives the optimum workability in non-cutting and cutting processes. This takes place over several hours at just below the AC1 temperature. Annealing on spherical cementite is also a soft-annealing process, using pendular annealing followed by slow cooling in order to achieve a high degree of moulding of its carbides. Here, the aim is a microstructure consisting of cementite grains in a ferritic matrix, and which gives the best working properties. This treatment is of crucial importance for a subsequent cold massive forming, for example.

Coarse-grain annealing, also called high annealing, takes place at a temperature above the hardening temperature with an appropriate cooling to obtain a coarser grain size. The aim of coarse-grain annealing is to improve the machinability of components that are subject to major machining. This occurs at temperatures between 950 and 1200 °C. The retention times must be sufficiently long to achieve the desired grain coarsening. Since grain growth is accompanied by a deterioration of the component properties, the microstructure must be restored to its fine-grain condition in the course of the final stages of heat treatment (hardening, quenching and tempering, case hardening etc.) by phase transformation. Diffusion annealing is annealing at very high temperatures in the recrystallisation zone. The aim is, for example, to partially or fully reverse the changes in properties and structure that arose from cold forming. Diffusion annealing is carried out to compensate the local differences in the chemical composition of steels and cast materials caused by segregation, without any conversion in the microstructure occurring. This happens by annealing in the temperature range of 1000 - 1300 °C.

Solution annealing is principally used for austenite steels to dissolve precipitated constituents in solid solutions and to eliminate tensions incurred by a preceding work hardening. Solution annealing is carried out to achieve uniform and homogeneous microstructural and material properties. In the case of ferrous materials, annealing is carried out in the temperature range between 950 and 1200 °C, and for non-ferrous metals in the range of 460 - 540 °C.

Benefits of annealing

  • Improvement of mechanical properties
  • Optimisation of mechanical processing (non-cutting and cutting)
  • Improvement of the microstructure for cold forming
  • Reduction of working and processing tensions
  • Restoration of the initial state

INDUCTIVE HARDENING

Inductive hardening, like flame hardening, is a surface layer hardening process.

However, here the component is heated with electrical alternating voltage, which creates heat by induction in the region near the surface. Here the surface hardening depth is primarily determined by the frequency. There are different CNC - controlled high and medium frequency facilities available in the Hanomag Heat Treatment Group, and their program control enables a high degree of reproducibility - not only for series parts.

Since carbon is primarily the controlling factor for achieving the desired hardness, other higher-alloy and high-alloy materials such as X155CrVMo12 are eligible for inductive hardening besides tempered steels with a carbon content of 0.4 % upwards.

Benefits of inductive hardening

  • Partial hardening according to needs
  • Short process times
  • Relatively slight dimensional and shape changes
  • High reproducibility due to CNC control system

LOW PRESSURE CARBURIZING

Low-pressure carburizing with subsequent quenching is a special form of case hardening Compared to case hardening in gas, the benefits of vacuum technology can be exploited by selecting carburizing temperatures of up to 1070 °C. Thus, the duration cycle can be significantly reduced especially for high case hardening depths.

With low-pressure carburizing, the correct carbon profile in the workpiece is adjusted precisely for the respective case hardening depth with alternating carburization and diffusion steps. The batch is then reduced to the hardening temperature and hardened with the help of high pressure gas quenching. Low-pressure carburizing results in a high surface hardness and a ductile core for the workpieces, which are optimum properties for high-stress use in future operations. By selecting acetylene as the carburizing gas and at a very low treatment pressure, the formation of soot, which was previously a problem with this technology, is completely avoided. At the Hanomag hardening plant at the site in Gommern, the low-pressure carburizing process takes place in a single chamber vacuum furnace. The furnace chamber is 910 mm wide. The maximum batch weight is 1,500 kg

Benefits of low-pressure carburizing

  • Optimum uniformity for complex component geometries and heavy loading.
  • Surface free of oxidation
  • Clean and smooth surface finish (blasting not necessary)
  • Precise adjustment of case hardening depths from 0.05 mm up to several millimetres.

PLASMA NITRIDING / CARBURIZING

Plasma nitriding is a process which is carried out in a low-pressure atmosphere (vacuum). In plasma nitriding, the component is used as a cathode, and the installation wall serves as the anode. An ionised gas atmosphere results. As a result of the given voltage between the batch and housing, the nitrogen molecules become ionised (glow discharge) and are accelerated towards the workpiece and dissociate on the surface. The nitrogen atoms diffuse into the workpiece and form a hard diffusion zone as well as a corrosion-resistant compound layer on the component surface. Pulsed DC current is used and the component heats up on its own through the process.

In plasma nitriding, loading and process knowledge are crucial for the success of the heat treatment and strongly influence the quality of the final product. With plasma nitriding, the structure of the compound layer can be exactly matched to the respective load profile of the component. The hardness, ductility, corrosion resistance as well as adhesion and abrasion properties can be individually varied and adjusted.

Benefits of plasma nitriding

  • Variable compound layer
  • Compound layer can be suppressed 
  • Only slight dimensional and shape changes
  • Minimum tolerances
  • Environmentally friendly, no toxic side-products

SALT BATH NITROCARBURIZING Tenifer® process TF1, Q, QP, QPQ

The TENIFER process (TF1) is a salt bath nitrocarburizing of components in molten salt at temperatures around 580° C.

Besides the addition of nitrogen, carbon also always diffuses into the surface in every TF1 treatment. This is a crucial reason for the positive, specific surface zone properties of salt bath treated components. The TF1 process is currently the most widespread process for nitrocarburization in salt baths, not least because of its environmental compatibility. In many cases the TF1 process is an alternative to other hardening processes, such as case hardening or hard chrome plating, with equal or better quality and ideally suited for individual parts.

An oxidising follow-up treatment in a so-called AB1 bath can improve the corrosion resistance (Q). Further optimisation can be achieved by a follow-up intermediate processing (polishing) (QP) or by an intermediate processing (polishing) with repeated treatment in the AB1 bath (QPQ). QPQ stands for Quench-Polish-Quench and thus includes the Tenifer treatment in combination with a 2-fold oxidative cooling and an intermediate processing (polishing). The components get an aesthetic black surface through oxidation, and in many cases the corrosion resistance of this is superior to that of galvanic surface layers.

Benefits of salt bath nitrocarburizing

  • Short treatment times
  • Good emergency running properties
  • High wear resistance
  • Improved corrosion resistance
  • An aesthetic black surface

SolNit

The Solution-Nitriding process - in short SolNit - is the surface nitriding of rust and acid-resistant steels. The dispersion of active nitrogen produces high nitrogen, austenitic or martensitic surface layers on near-net shape components with a high level of hardness, favourable residual stress and excellent corrosion properties.

The process is carried out in vacuum facilities at temperatures between 1050 and 1150 °C in a pure nitrogen atmosphere at a pressure of a few hundred millibar. The nitriding depths can vary between 0.10 mm and several millimetres. The process is used, for example, in the sector of plastics processing, transmission components, roller bearings for turbines, pumps, valves or surgical instruments.

In the Hanomag Heat Treatment Group, the SolNit process takes place in a single chamber vacuum furnace at the site in Gommern. The furnace chamber is 910 mm wide, 1220 mm long and 910 mm high. The maximum batch weight is 1500 kg

Benefits of the SolNit process

  • Higher corrosion resistance
  • Significantly higher hardnesses
  • Improved wear resistance
  • High residual compressive stress in the surface layer
  • Higher cavitation and erosion resistance
  • Reduced coefficient of friction
  • Low scuffing

QUENCHING AND TEMPERING

Quenching and tempering involves a combined heat treatment procedure of hardening and subsequent tempering.

A particular feature of quenching and tempering is that tempering takes place at high temperatures of up to 700 °C. Tempering of martensite forcibly expels part of the dissolved carbon. With increasing tempering temperature, tensile strength, yield strengths and hardness are reduced, whereas elongation, constriction and notched impact strength increase.

One refers to quenching and tempering similarly to hardening, depending on which quenching medium is used for hardening, i.e. water, oil or air quenching and tempering

Quenching and tempering is particularly useful for workpieces that are subject to dynamic loading and from which high ductility is expected. Quenched and tempered steels have a wide spectrum of applications due to their good strengths with high ductility at the same time, all heat-treatable steels are suitable.

Benefits of quenching and tempering

  • Good notched impact strength
  • High strength with high ductility at the same time
  • Good flexural strength