Heat treatment

It is known as thermal treatment when metal alloys are subjected to sudden conditions of high temperature and sudden cooling, residence time, speed, pressure, as long as the alignment of the metals or the alloys are in the solid state, in order to improve their mechanical properties, especially hardness, resistance and elasticity. The materials to which heat treatment is applied are basically steel, glass and cast iron, made up of iron and carbon. Various heat treatments are also applied to ceramics and wood.

Mechanical properties

The mechanical characteristics of a material depend both on its chemical composition and on its crystalline structure. Heat treatments modify this crystalline structure without the chemical composition, through a process of successive heating and cooling until the desired crystalline structure is achieved.

Among these features are:

• Wear resistance: It is the resistance that provides a material to be eroded when in contact with friction with other material.
• Tenacity: It is the capacity that has a material to absorb energy without producing fissures (impact resistance).
• Machinability: It is the ease that has a material to allow the process of machining for chip boot.
• Hardness: It is the resistance that offers a material to let you penetrate or be scratched. It is measured in Brinell units (HB), Rockwell units (HRC), Vickers (HV), etc.

Improvement of properties through heat treatment at all stages of the part

The mechanical properties of alloys of the same metal, and in particular of steels, reside in the chemical composition of the alloy that forms them and the type of heat treatment to which they are subjected. Heat treatments modify the crystalline structure that forms steels, without changing their chemical composition.

This property must have different grain structures with the same chemical composition, it is called allotropy and it is what justifies the heat treatments. Technically, polymorphism is the ability of some materials to present different crystalline structures (see Bravais lattices), with a single chemical composition. Diamond and graphite are polymorphisms of carbon. α-ferrite, austenite, and δ-ferrite are polymorphisms of iron. In a pure chemical element, this property is called allotropy.

Therefore, the different grain structures can be modified, thus obtaining steels with new mechanical properties, but always maintaining the chemical composition. These properties vary according to the treatment given to the steel, depending on the temperature to which it is heated and how it is cooled. The shape that the grain will have and the microconstituents that will make up the steel, knowing its chemical composition (that is, percentage of carbon and iron (Fe₃)) and the temperature at which it is found It can be seen in the iron-carbon diagram.

Attached below as an example is a figure showing how the grain varies as the steel is first heated and then cooled. The micro-constituents referred to above are, in this case, pearlite, austenite and ferrite.

In the figure attached below you can see more clearly how the grain of brass varies according to the temperature variation in a heat treatment.

This will be possible if the control of the heat treatment is carried out, to check if the process meets all the technical requirements that are required in quality control. This control is carried out at all stages of production, taking into account the quality control of the starting materials, the control of the technological processes of heat treatment and the control of the production of the heat treatment workshop.

Mechanical properties of steel

Steel is an alloy of iron and carbon that contains other alloying elements, which give it specific mechanical properties for use in the metal-mechanic industry.

The other main elements of composition are chromium, tungsten, manganese, nickel, vanadium, cobalt, molybdenum, copper, sulfur and phosphorus. These chemical elements that are part of the steel are called components, and the different crystalline structures or a combination of them, constituents.

The constituent elements, according to their percentage, offer specific characteristics for certain applications, such as tools, blades, supports, etc. The difference between the various steels, as has been said, depends both on the chemical composition of their alloy and on the type of heat treatment.

Heat treatments of steel

The heat treatment of the material is one of the fundamental steps so that it can achieve the mechanical properties for which it was created. These types of processes consist of heating and cooling a metal in its solid state to change its physical properties. With proper heat treatment, internal stresses can be reduced, grain size increased, toughness increased, or a hard surface with a ductile interior produced. The key to heat treatments consists of the reactions that occur in the material, both in steel and in non-ferrous alloys, and occur during the heating and cooling process of the parts, with established guidelines or times.

To know what temperature the metal must be raised to receive a heat treatment, it is advisable to have phase change diagrams such as that of iron-carbon. In this type of diagrams, the temperatures at which phase changes occur (changes in crystalline structure) are specified, depending on the diluted materials.

Thermal treatments have acquired great importance in the industry in general, since with the constant innovations metals with greater resistance to both wear and tension are required. The main heat treatments are:

• Temple: Its purpose is to increase the hardness and strength of steel. To do this, the steel is heated at a slightly higher temperature than the higher Ac critique (between 900-950 °C) and then cooled more or less quickly (according to part characteristics) in a medium such as water, oil, etc.
• Revenue: It is only applied to rear tempered steels, to slightly decrease the effects of the temple, keeping part of the hardness and increasing the tenacity. The reseller manages to reduce the hardness and resistance of tempered steels, the tensions created in the temple are eliminated and the tenacity is improved, leaving the steel with the desired hardness or resistance. It is basically distinguished from the temple in terms of maximum temperature and cooling speed.
• Collected: It consists basically of warming up to the austenization temperature (800-925 °C) followed by a slow cooling. With this treatment it is achieved to increase elasticity, while the hardness decreases. It also facilitates the machining of the parts by homogenizing the structure, fineening the grain and softening the material, eliminating the acritude that produces cold work and internal tensions.
• Standardized: Its purpose is to leave a material in a normal state, that is, absence of internal tensions and a uniform distribution of carbon. It is usually used as a pre-warm and revenated treatment.

Thermochemical treatments of steel

Thermochemical treatments are heat treatments in which, in addition to changes in the steel structure, changes are also produced in the chemical composition of the surface layer, adding different chemical products up to a certain depth. These treatments require the use of controlled heating and cooling in special atmospheres.

Among the most common objectives of these treatments are to increase the surface hardness of the parts, leaving the core softer and more tenacious; reduce friction by increasing lubricating power; increase wear resistance; increase fatigue strength or increase corrosion resistance.

• Cementation (C): Increases the surface hardness of a piece of fresh steel, increasing the concentration of carbon on the surface. It is achieved taking into account the medium or atmosphere that surrounds the metal during warming and cooling. The treatment achieves to increase the carbon content of the peripheral zone, obtaining later, by means of tempers and avenues, a great surface hardness, wear resistance and good tenacity in the core.
• Nitrition (N): Like cementation, the surface hardness increases, although it does so to a greater extent by incorporating nitrogen into the composition of the surface of the piece. It is achieved by heating steel at temperatures between 400 and 525 °C within a more nitrogen ammonia current.
• Cyanation (C+N): Surface enduring of small steel parts. Bathrooms with cyanide, carbonate and sodium cianato are used. Temperatures are applied between 760 and 950 °C.
• Carbonitruration (C+N): Like cyanination, it introduces carbon and nitrogen into a surface layer, but with hydrocarbons such as methane, ethanus or propane; ammonia (NH₃) and carbon monoxide (CO). In the process temperatures are required from 650 to 850 °C and it is necessary to perform a temple and a subsequent resell.
• Sulphinization (S+N+C): Increased wear resistance by sulfur action. Sulphur is incorporated into metal by low temperature heating (565 °C) in a salt bath.

Examples of treatments

Steel hardening

The steel hardening process consists of heating the metal uniformly to the correct temperature (see temperature figure for metal hardening) and then cooling it with water, oil, air or in a refrigerated chamber. Hardening produces a fine granular structure that increases tensile (stress) strength and decreases ductility. Carbon tool steel can be hardened by heating to its critical temperature, which is approximately 790-830°C, which is identified when the metal turns a bright cherry red color. When steel is heated, pearlite combines with ferrite, producing a fine-grained structure called austenite. When austenite is quenched with water, oil or air, it transforms into martensite, a material that is very hard and brittle.

Heat treatment of aluminum alloys

The basic heat treatments for improving the properties of aluminum alloys are precipitation treatments. They consist of the stages of solution, tempering and maturation or ageing. Annealing treatments are also carried out.

Designation of metallurgical states of aluminum

‘T’ – Heat treatment (that is, for age hardened alloys) the “T” will always be followed by one or more digits.

F - Raw state for the implementation of advanced manufacturing needs

T1 - Cooled from a high temperature during the shaping process and naturally aged.

T2 - Cooled from a high temperature during the forming process, cold worked and naturally aged.

T3 - Solution heat treated, cold worked and naturally aged.

T4 - Solution heat treated and naturally aged.

T5 - Cooled from a high temperature during the shaping process and artificially aged.

T6 - Solution heat treated and then artificially aged.

T7 - Solution heat treated and then artificially aged.

T8 - Solution heat treated, cold worked and artificially aged.

T9 - Solution heat treated, artificially aged and cold worked.

One or more digits from T1 to T9 may be added to indicate temper variations. T351 - Solution heat treated, controlled drawing to relieve stress. Aluminum does not receive any additional straightening after drawing. It applies to sheets, rods and bars that are cold rolled or finished, drop or press forged of annular products and seamless rolled rings.

T3510 - Solution heat treated, controlled stretch to relieve stresses and naturally aged. Aluminum does not receive any additional straightening after drawing. It is applied to rods, bars, profiles and extruded tubes and drawn tubes.

T3511 - Like T3510, but also refers to products that may receive slight straightening after drawing to meet standard tolerances.

T352 - Applies to compression treated products to relieve stress after solution heat treatment or after being quenched from a hot working process to produce 1 to 5% remaining set.

T651 - Solution heat treated, controlled stretching to relieve stress and then artificially aged. Aluminum does not receive any additional straightening after drawing.

T6510 - Solution heat treated, controlled stretch to relieve stress and then artificially aged. Aluminum does not receive any additional straightening after drawing.

T6511 - As T6510, except that slight straightening is allowed after drawing to meet standard tolerances.

T73 - Solution heat treated then artificially overaged for best stress corrosion resistance.

T732 - Solution heat treated then artificially overaged for the best stress corrosion resistance.

T7651 - Solution heat treated, controlled drawn to relieve stress and then artificially overaged for good exfoliation corrosion resistance. Aluminum does not receive any additional straightening after drawing.

T76510 - Solution heat treated, controlled drawn to relieve stress and then artificially overaged for good exfoliation corrosion resistance. Aluminum does not receive any additional straightening after drawing.

T76511 - As T76510, except that slight straightening is allowed after drawing to meet standard tolerances.

Quenching and tempering: primed and normalized

After the steel has hardened, it is very brittle or fragile, which prevents its handling, since it breaks with the slightest blow due to the internal tension generated by the hardening process. To counteract brittleness, tempering of the steel is recommended (in some texts this process is called tempering and hardening is called tempering). This process makes the steel tougher and less brittle, although it does lose some of its hardness. The process consists of cleaning the piece with an abrasive and then heating it up to the appropriate temperature (see table), and then cooling it outdoors in the same medium that was used to harden it.

Annealed

Annealing is the heat treatment that, in general, has the main purpose of softening steel or other metals, regenerating the structure of overheated steels or simply eliminating the internal stresses that follow cold work. (Cooling in the oven). That is, eliminate residual stresses produced during cold working without affecting the mechanical properties of the finished part, or annealing can be used to completely eliminate strain hardening. In this case, the final part is soft and ductile but still has good surface finish and dimensional accuracy. After annealing, additional cold working can be done as ductility is restored; By combining cold and annealing rework cycles, large total strains can be achieved.

The term "annealed" is also used to describe other heat treatments. For example, glasses can be heat treated or annealed to remove residual stresses present in the glass. Irons and steels can be annealed to maximize their properties, in this case ductility, even when the material has not been cold worked.

There are three stages considered to be the most important in the annealing process:

Recovery

The original microstructure worked at low temperatures is composed of deformed grains that contain a large number of dislocations intertwined with each other. When the metal is first heated, the additional thermal energy allows the dislocations to move and form the boundaries of a polygonized subgranular structure. This means that, as the material heats up, the dislocations disappear and, in turn, the grains become larger. However, the density of the dislocations remains virtually unchanged. This low-temperature treatment removes the residual stresses due to cold working, without causing a change in the density of the dislocations, and is called recovery.

The mechanical properties of the metal remain relatively unchanged, since the total number of dislocations that occur during this stage is not reduced. Since residual stresses are reduced or even eliminated when dislocations are rearranged, recovery is often referred to as stress relief annealing. In addition, the recovery restores the high electrical conductivity of the material, which would make it possible to manufacture wires that could be used to transmit electrical energy, because they would also be highly resistant. Finally, reclamation frequently enhances the corrosion resistance of materials.

Recrystallization

When a previously cold-worked metal is subjected to very high temperatures, rapid recovery eliminates residual stresses and produces the structure of polygonized dislocations. During this instant, the formation of nuclei of small grains occurs at the limits of the cells of the polygonized structure, eliminating most of the dislocations. Because the number of dislocations is reduced on a large scale, the recrystallized metal has low strength but high ductility. The temperature at which a microstructure of new grains that have few dislocations appears is called the recrystallization temperature. Recrystallization is the process during which new grains are formed through heat treatment of a cold worked material. The recrystallization temperature depends on several variables, therefore it is not a fixed temperature.

Growing grains

When the temperatures applied in the annealing are very high, the recovery and recrystallization stages occur more quickly, thus producing a finer grain structure. If the temperature is high enough, the kernels begin to grow, with favored kernels culling smaller kernels. This phenomenon, which can be referred to as grain growth, is accomplished by reducing the area of the grain boundaries. In most materials, grain growth will occur if they are held at a high enough temperature, which is not related to cold working. This means that recrystallization or recovery are not essential for the grains to grow within the structure of the materials.

Ceramics that exhibit almost no hardening show a considerable amount of grain growth. Also, abnormal grain growth can occur in some materials as a result of liquid phase formation.

Types of annealing

Homogenization annealing

In homogenization annealing, typical of hypoeutectoid steels, the heating temperature is that corresponding to A₃+200 °C without ever reaching the solids curve, being carried out in the furnace itself the subsequent slow cooling, its main objective being to eliminate the heterogeneities produced during solidification.

Regeneration annealing

Also called normalized, its function is to regenerate the structure of the material produced by tempering or forging. It is generally applied to steels with more than 0.6% C, while steels with a lower percentage of C are only applied to refine and order their structure.

Example:

After cold rolling, where the grain is elongated and subjected to stress, said treatment returns the microstructure to its initial state.

Globulization annealing.

It is usually desired to obtain globulization in pieces such as thin plates that should have high intoxication and low hardness.

Higher embossment values are generally associated with the globularized microstructure that is only obtained in the range between 650 and 700 degrees Celsius. Temperatures above critical produce formation of austenite which, during cooling, generates pearlite, causing an unwanted increase in hardness. In general, pieces such as the plates for protective boots must be globalized in order to obtain the necessary bends for their use and avoid breaking or cracking. Finally they are tempered to guarantee hardness. It is used for hypereutectoid steels, that is, with a percentage greater than 0.89% of C, to achieve the lowest possible hardness than in any other treatment, improving the machinability of the part. The annealing temperature is between AC3 and AC1.

Example.

The softening of alloy tool steels with more than 0.8% C.

Subcritical Annealing

For a hypoeutectoid carbon steel: The microstructure obtained in this treatment varies according to the annealing temperature. Generally those not exceeding 600 degrees will release stresses in the material and cause some grain growth (if the material was not previously tempered). Generally showing Ferrite-Perlite. Above 600 and below 723 we speak of globulization annealing since it does not exceed the critical temperature. In this case there is no pearlite grain, the carbides are spheroidized and the matrix is completely ferritic. It is used for forging or rolling steels, for which an annealing temperature below AC1 is used, but very close. Through this procedure, the internal stresses produced by its molding and machining are destroyed. It is commonly used for high-strength alloy steels, Cr-Ni, Cr-Mo, etc. This procedure is much faster and easier than the ones mentioned above, its cooling is slow.

Cemented

It consists of hardening the external surface of low carbon steel, leaving the core soft and ductile. As carbon is the one that generates the hardness in the steels in the carburizing method, it is possible to increase the amount of carbon in the low carbon steels before being hardened. Carbon is added by heating steel to its critical temperature while in contact with a carbonaceous material. The three most common cementing methods are: carburizing, liquid bath, and gas packing.

Carburizing by packaging

This procedure involves placing the low carbon steel material in a closed box with carbonized material and heating it to 900 to 927 °C for 4 to 6 hours. During this time, the carbon found in the box penetrates the surface of the part to be hardened. The longer the part is left in the carbon box, the deeper the hard layer will be. Once the piece to be hardened is heated to the appropriate temperature, it is quickly cooled in water or brine. To avoid deformations and reduce surface tension, it is recommended to let the piece cool in the box and then take it out and reheat it between 800 and 845&nababs;°C (cherry red) and proceed to immersion cooling. The most commonly used hardened layer has a thickness of 0.38 mm, however thicknesses of up to 0.4 mm can be obtained.

Carburizing in a liquid bath

The steel to be hardened is submerged in a bath of liquid sodium cyanide. Potassium cyanide can also be used but its vapors are very dangerous. The temperature is maintained at 845 °C for 15 minutes to 1 hour, depending on the depth required. At this temperature the steel will absorb the carbon and nitrogen from the cyanide. The steel must then be rapidly quenched in water or brine. With this procedure, layers with thicknesses of 0.75 mm are achieved.

Gas carburizing

In this procedure, carburizing gases are used for cementation. The piece of steel with low carbon content is placed in a drum to which gas is introduced to carburize as derivatives of hydrocarbons or natural gas. The procedure consists of keeping the oven, the gas and the piece between 900 and 927 °C. after a predetermined time the carburizing gas is cut off and the furnace is allowed to cool. The part is then removed and reheated to 760°C and quenched in water or brine. With this procedure, pieces are obtained whose hard layer has a thickness of up to 0.6 mm, but usually does not exceed 0.7 mm.

Carburizing, cyaniding and nitriding

There are several surface hardening procedures with the use of nitrogen and cyanide, which are usually known as carbonitriding or cyaniding. In all these processes with the help of cyanide and ammonia salts, hard surfaces are achieved as in the previous methods.