Heat Treatment

Heat treatment is defined as one or more controlled heating and cooling processes that are applied one or more, one after the other, according to the location, in order to give the desired properties to the solid metal or alloys. All basic heat treatments applied to steel are related to the transformation of the austenite phase in the iron-cementite equilibrium diagram. In other words, the physical and mechanical properties of a steel depend on the type, quantity and metallographic structure of the transformation products it contains.

Heat treatment of steel starts with austenitization. For the austenitization process, the material is heated to a temperature above the lower critical temperature line (Ac1). In order to prevent distortions, materials subjected to cold deformation, that is, materials with excessive internal stress, should be heated more slowly than materials that do not contain tension. In addition, the differences between the heating or temperature increase rates in thin and thick sections should be taken into account during the heating of the parts with cross section changes. Thin parts of the piece should be heated slower than the thick parts in order to avoid any distortion due to the temperature effect. Steel parts are generally heated slowly to reduce the risk of damage during heat treatment.

Some Annealing Operations;

Soft Annealing ( +A )

Spheroidized Annealing ( +AC )

Normalizing ( +N )

Stress Relieving ( +SR )

Quenching ( +Q )

Tempering ( +T )

Some Hardening Operations;

Quenching and Tempering ( +QT )

Some Surface Hardening Operations;

Case Hardening

Nitration Hardening

Induction Surface Hardening

Flame Surface Hardening

We can collect the steels to be heat treated in two groups according to the carbon content they

contain in their structures.

Sub-eutectoid steels ( %C < 0,8 )

Supereutectoid steels ( %C > 0,8 )

Annealing Operations

In general, annealing is the process of heating metal materials to appropriate temperatures, keeping them at this temperature until the necessary changes are achieved, and then cooling slowly in order to obtain the desired structural, physical and mechanical properties and facilitate chip removal or cold forming.

Soft Annealing

The scientific and technological definition of the softening annealing is very slow cooling by heating sub-eutectoid steels to Ac3 in sub-eutectoid steels and over-eutectoid steels to certain temperatures above the Ac1 lines in order to reduce hardness, facilitate chip removal, and remove the internal stresses in cast and forged parts, after converting their internal structures to austenite and keeping them in the furnace. is the process. The purpose of this process is to reduce the grain size and improve the electrical and magnetic properties of some steels.

Soft Annealing is indicated by the + A icon.

Spheroidized Annealing

Spheroidization annealing is mostly applied to high carbon steels. Low carbon steels are seldom subjected to spheronization annealing. Because; These types of steels become very soft at the end of the spheroidizing annealing and this excessive softening causes some difficulties during machining. Medium carbon steels, on the other hand, are sometimes subjected to spheroidization annealing before plastic forming to gain sufficient ductility. During the spheronization annealing the annealing time should be adjusted well. If the steel is annealed for a longer time than necessary, the cementite particles join together and show elongation and this situation negatively affects the workability of the steel.

Spheroidizing annealing is the process of turning the carbides into spherical shape by slow cooling after keeping the steels around the Ac1 temperature line for a long time and annealing oscillating in this region. This process can also be done with controlled cooling after austenitization. As stated in the soft annealing process, over-eutectoid steels in annealed state are not suitable for processing due to the presence of hard and brittle cementite grains in their internal structures. Spheroidizing annealing is also used in order to facilitate the processing of such steels and to increase the ductility.

Spheroidization Annealing is indicated by the + AC icon.

Normalizing

Normalization annealing is generally defined as the process of heating sub-eutectoid steels to Ac3 and hyper-eutectoid steels to temperatures of 40-50ºC above Acm conversion temperatures, in order to reduce the grain, obtain a homogeneous inner structure and mostly improve mechanical properties, and after annealing, cooling in calm air outside the furnace.

The main purposes of the normalization process are;

Reducing the grain size,

To obtain a homogeneous internal structure,

Distributing the carbide network in grain boundaries in super-eutectoid steels

To facilitate the processing of steels and to improve their mechanical properties,

To increase the hardness and strength of steels subjected to soft annealing.

In the normalization process, since the cooling is done outside the oven and in calm air, the cooling rate is relatively high. Generally, as the cooling rate increases, the transformation temperature of austenite decreases and thinner perlite is obtained. Therefore, the normalized steel is thinner and more perlite than the soft annealed steel.

Normalization Annealing is indicated by the + N icon.

Stress Relieving

Stress relief annealing is the process of heating metallic materials to a suitable temperature below conversion temperatures and then slow cooling, in order to reduce internal stresses caused by casting, welding and cold forming processes. This process is sometimes referred to as conversion temperature or sub-critical temperature annealing. Steel materials are subjected to stress relief annealing at temperatures between 540ºC and 630ºC.

Stress Relieving is indicated by the + SR icon.

Quenching

After the annealing process, when the steels are cooled at a slow or medium speed, the carbon atoms dissolved in the austenite are separated from the austenite structure by diffusion. When the cooling rate is increased, the carbon atoms cannot find enough time to leave the solid solution by diffusion. Even if the iron atoms move a little, a different structure is formed because the carbon atoms are confined in solution. This structure formed as a result of rapid cooling is called martensite.

Martensitic transformation occurs only during cooling. Therefore, the conversion in question is independent of time and depends only on the decrease in temperature, ie cooling. The most important feature of martensite is that it is a very hard phase. In steels, martensite is the hardest phase after cementite. High hardness values can only be obtained in steels containing sufficient carbon.

The main purpose of the hardening process is to obtain a completely martensitic structure. For this, after the annealing process, the material must be cooled at speeds higher than a value called the critical cooling rate. The critical cooling rate is the lowest cooling rate necessary to obtain a completely martensitic structure. The critical cooling rate, which is an important criterion for steel, depends on the chemical composition (ratio of carbon and alloy elemental) and the grain size of austenite.

Quenching Hardening is indicated by the + Q icon.

The quenching media used are listed according to their quenching intensity as follows;

Water containing 10% NaCl (salt water)

Tap water

Melted or liquid salts

Oil and water mixture

Oil

Air

Tempering

In steels, martensitic structure obtained by quenching process is brittle and is not suitable for many applications. In addition, the formation of mertanzite causes internal stresses to occur in the steel. For these reasons, tempering is the annealing process applied to steels that are quenched almost always at temperatures below the Ac1 line. The purpose of tempering is; to remove residual stresses in quenched steel and to increase the ductility and toughness of steel. When quenched steels are tempered, their ductility increases, whereas their hardness and strength decrease.

Steel parts used in applications requiring high hardness and high wear resistance are tempered at temperatures below 205ºC, and parts used in applications requiring high toughness are tempered at temperatures above 425ºC. If there is no notch in the part that causes stress build-up, ductility change can be taken as a good measure of toughness, and in this case, tempering between 205ºC and 425ºC may not be inconvenient. When the tempering temperature reaches 205°C, residual stresses can be largely relieved. At 480ºC, residual stresses are completely eliminated.

Tempering is indicated by the + T symbol.

Core Hardening Operation

By evaluating the produced parts according to the working conditions, it may be requested that all or part of the part gain hardness up to the core. In such cases, different heat treatments should be applied according to the desired feature.

The hardening process is evaluated under different headings, considering the construction properties and final structure properties.

Quenching and Tempering

It is the process of quenching and tempering to obtain the desired hardness and mechanical properties. It is especially used for situations where the entire cross section of the part is desired to be hard.

Quenching process can be defined as the hardening of the material by heating it up to the hardening temperature and cooling it suddenly. Related to the subject, the relationship between the factors such as the selection of the curing temperature, the heating rate, the cooling medium selection and the cooling rate, and the determination of the correct values are subjects that require expertise. The hardening temperature ranges are the values determined by a series of tests to ensure that the maximum hardness is obtained with the smallest grain structure. Heating to be made below or above these values will result in a low hardness value and unsatisfactory final interior structure. In addition, the holding time at the hardening temperature is also important, and it is related to the alloying of the material, the low alloy and the suitability of the grain sizes.

The choice of quenching medium is related to the alloy content of the material. While water and salt baths are preferred for low alloy steels, for high alloy steels, considering the risk of distortion, soft environments such as oil are preferred. Common cooling media can be specified as water, oil, salt bath and air.

Heating after quenching is called tempering. The final structure formed after quenching is very hard and fragile and causes internal stresses that occur during sudden cooling. In order to improve the toughness of the material by tempering, the material should be reheated and kept at the same temperature for a while. The purpose of tempering is; To remove residual stresses of quenched steel, to increase its ductility and toughness. When quenched steels are tempered their ductility increases, whereas their hardness and strength decrease

Quenching Process is indicated by the + QT icon.

Surface Hardening Operations

Steel parts used in some applications are required to have high both wear resistance and impact resistance. For this, the surfaces of the said parts are expected to be hard, and the inner or central regions relatively soft. To ensure this situation, surface hardening processes are applied to the parts. The chemical composition, shape and dimensions of the part should be taken into account in the selection of the most suitable method.

Case Hardening

Case Hardening, also known as carburizing, is applied to steels containing less than 0.20% carbon. The basis of this process is to increase the carbon content of the surface to a sufficient level by giving carbon on the surface of low carbon steels. The cementation of the steels in question is carried out by keeping them in a carbon donor environment and at a temperature above the Ac3 line for a suitable period of time.

The steel parts to be cemented are annealed in the austenite zone at a temperature between 900ºC-950ºC for a sufficient period of time in a carbon donor environment. Carbon donor media can be solid, liquid or gas. A mixture of charcoal and barium carbonate as a solid medium, a molten cyanide bath as a liquid medium, and gases such as carbon monoxide, methane and propane mixed with hydrogen or nitrogen can be used as the gas medium.

The carbon, which becomes atoms during the process, dissolves in the austenite structure of the steel and spreads from the surface in a decreasing rate. The surface layer whose carbon ratio is increased by annealing in a carbon donor medium must be hardened. For this purpose, the easiest way to follow is to give water from the cementation temperature.

Nitration Hardening

Nitration is mostly applied to steels with low carbon and containing alloying elements capable of forming nitrides. Examples of elements with nitride-forming properties are aluminum, chromium and molybdenum. This process is carried out by annealing the steels in question at a temperature of around 500ºC in a nitrogen-donating environment for a long period of 40-90 hours.

Nitrogen donating media are generally used in baths containing sodium cyanide (NaCN) and / or potassium cyanide (KCN). In the nitration process, very thin and dispersed chromium, aluminum or molybdenum nitrides, which are formed by the diffusion of atomic nitrogen to steel, provide a hard layer on the surface of the part. Since nitrogen diffusion rate in steel is low, long-term annealing is required in order to obtain a hard layer of sufficient thickness.

Induction Surface Hardening

In the induction hardening process, it is based on creating an electric current in a piece of metal placed in a rapidly changing magnetic field. The machine used in this process resembles a transformer with a large number of copper coils. The part to be hardened forms the secondary part of the high frequency induction machine. A simple solenoid or helical coil for external heating in high frequency units, a coil for heating the inner part of perforated parts, a plate-shaped coil that provides high current intensity for scanning applications in narrow gaps, a single coil for scanning rotating surfaces and suitable for local or point heating and can be intertwined with each other. five different coils are used.

There is no risk of distortion in parts hardened by induction method. This method can be used on the production line. Since automatic devices are used in the induction hardening process, personal skills are not required much.

Flame Surface Hardening

In this process applied to medium carbon steels, only the surface of the part is heated with flame and austenitized and then hardened by giving water. However, during this process, a significant temperature increase in the interior of the part and therefore a structural change should not be allowed. Intensive heating is provided by the flame of oxygen-flammable gas (acetylene, propane, etc.). In this process, the quenching temperature is higher than the annealing temperature required for normal quenching hardening.

No change occurs in the chemical composition of the steel during the flame surface hardening process. The desired area of the steel part is heated to the appropriate temperature, austenitized, and then hardened by giving water. Water is given by spraying water to the part whose surface is heated to the desired temperature. In some cases, water in oil can be given to the part. After the process, the piece is heated to a temperature between 180ºC-205ºC and subjected to stress relief by cooling in air.

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