This article will take an in-depth look at heat treating.
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This chapter will discuss heat treating and its function.
Heat treating is a process where heat is applied to a material and then cooled to improve its performance, durability, and properties.
Heat treating can be used to soften metal to improve formability. It can be used to harden parts, to improve their strength. Heat treating can be defined as every process that is employed, and changes the physical properties of a material (e.g. metal) by either heating or cooling it.
This section will discuss the theory and function of heat treating.
Every process of heat treatment involves heating and cooling of metals, but there are three major differences in the process, which are the heating temperatures, the rates of cooling, and the types of quenching that are utilized to land on the required properties.
To put a metal under the process of heat treating, proper equipment is needed so that the factors around heating, cooling, and quenching can be fully controlled. The size, type and mixture of the gas for the heating chamber must all be correct for controlling the temperature. The quenching media must also be appropriate to cool the metal correctly.
The three main stages of heat treatment are listed below:
During the stage of heating, the first important aim is to make sure of the uniform heating of the metal. Even heating is obtained by heating slowly. If the metal is unevenly heated, only one part of the metal may expand faster than the other. This results in a section of the metal that is distorted or cracked. The rate of heating must be selected based on the following factors:
This stage serves to maintain the metal at the appropriate temperature until the desired internal structure takes shape. The soaking period is the time duration the metal is kept at the appropriate temperature. For the determination of the right length of time, chemical analysis and mass of the metal are needed. For cross sections that are not even, the soaking period can be determined by using the largest section.
In general, the temperature of the metal must not be brought from the room temperature to the soaking temperature in a single step. Instead, the metal must be heated slowly to a temperature that is just below the temperature where there becomes a change in structure and then hold it until the temperature becomes consistent all over the metal.
After the preheating step, the temperature must be more quickly heated to the final temperature that is needed. Parts having designs that are more complex may require layers of preheating in order to prevent warping.
In this stage, the metal needs to be cooled to room temperature. There are several ways to cool metal depending on its type. The metal may need a cooling medium, a liquid, a gas, or a combination thereof. The cooling rate depends on the metal itself and the cooling medium. It follows that the choices made in cooling are important factors in the metal properties that are desired.
Quenching is when the metal is rapidly cooled in air, water, oil, brine, or other mediums. Most metals that are hardened are cooled rapidly with quenching, this is the reason behind the association of quenching with hardening, but it is not always true that hardening is a result of quenching or otherwise rapid cooling. For instance, water quenching is used for annealing copper, and the hardening of other metals is done by slow cooling.
Not every metal should be quenched because quenching can crack or warp some metals. In general, water or brine can rapidly cool metal, while on the other hand oil mixtures are better for slower cooling. The general guidelines are that water can be used to harden carbon steels, oil for the hardening of alloy steels, and water for the quenching of nonferrous metals. However, as with all treatments, the cooling rate and cooling medium chosen must fit the metal.
Heat treating serves various purposes, which include:
The various types of heat treatment processes include:
This process involves the addition of carbon atoms to the surface and sub-surface of steel to improve its surface hardness. This is for the reinforcement of the metal part’s surface and increases its microstructure and mechanical properties by enabling carbon to diffuse into it.
The level of carbon in the atmosphere, the type of material used, the temperature used, and the length of the metal exposed to that temperature determine the depth at which carbon is able to diffuse. After the quenching of the part is the time when the hardening happens. On top of increasing surface hardness, carburizing enhances fatigue strength and wear resistance.
It is best suitable for steels with low carbon content ranging from 0.05% to 0.3% and can be performed on parts having varying complexity. Carburizing is the treatment of choice for metals requiring improved wear resistance, durability, and fatigue strength for their intended applications at temperatures ranging from 1562 °F to 1832 °F (850 °C up to 1000 °C).
Similar to carburizing, nitriding is a thermochemical case-hardening process that is utilized to improve the hardness, fatigue life of metal parts, and wear resistance. With nitriding, nitrogen is diffused into the surface of the metal to achieve the hardening effect. The process of nitriding includes heat treating a ferrous material then giving it exposure to active nitrogen at subcritical temperatures that are tightly controlled.
Temperatures applied range between 752 °F and 1094 °F (400 °C to 590 °C) during the exposure to active nitrogen, which is below the temperature of the final tempering to ensure that the metal’s mechanical properties are not affected. Nitriding is most effective when carried out on alloy steel materials containing nitride-forming elements. This enables the formation of alloy nitride precipitates with the metal by the nitrogen with ease.
The following are examples of steels that are compatible with the process of Nitriding: 4140, 4130, 4150, 8640, 4340, 15-5, 17-4, 4xx stainless, and nitralloy 135. To add on top of its hardening effect, nitriding can impart anti-galling, anti-seizing, and ant-welding properties to the part of the metal. Metals that have been nitride can maintain their hardness in temperatures of up to 1000 degrees Fahrenheit. These features make nitride metals to highly suit myriad applications that include bearings, dies, gears, shafts, spray nozzles, feed screws, orifice disks, splines, valves, cylinder liners, piston rings.
Hardening is a metal working process that occurs in a vacuum or endothermic atmosphere. In contrast to Nitriding and carburizing, this is a through-hardening process that hardens the part from the surface to the core without changing to the carbon on its surface. The process involves heating the metal above its austenitizing temperature, mostly inside an enclosed furnace. With tempering, metals are heated to temperatures from 698 °F to 1004 °F (370 °C up to 540 °C). For excellent toughness, metals are heated to temperatures between 1004 °F and 1112 °F (540 °C up to 600 °C).
The austenitizing temperature, which varies based on the material, is the temperature that enables the crystal structure of the metal to transform from ferrite to austenite (titanium, aluminum, and high nickel alloys exhibit different structures but make use of the same principle of high temperature and then quenching).
After the transformation to austenite, the metal is quenched rapidly in oil to change the crystal structure to martensite. The metal part is then tempered to reduce its hardness to the level that is desired, thereby lowering its brittleness. Steel is hardened typically through this heating and quenching process. These ways can also be used to harden aluminum which lacks carbon. These hardened metals are utilized in different types of applications, from construction material to components of the automotive field.
| Tempering Temperatures Chart | |||
|---|---|---|---|
| Color | Hardest | Approximate Temperature (°C) | Uses |
| Pale Straw | ↑ | 230 | Lathe tools, Scrapers, Scribers |
| Straw | 240 | Drills, Milling Cutters | |
| Dark Straw | 250 | Taps & Dies, Punches, Reamers | |
| Brown | 260 | Plane Blades, Shears, Lathe Centres | |
| Brown/Purple | 270 | Scissors, Press Tools, Knives | |
| Purple | 280 | Cold Chisels, Axes, Saws | |
| Dark Purple | ↓ | 290 | Screwdrivers, Chuck Keys |
| Blue | Toughest | 300 | Springs, Spanners, Needles |
Annealing is a process whereby a metal part is heated to a predetermined temperature, held at that temperature, and then cooled down slowly. This helps by relieving the residual stresses in the material resulting from processes like cutting, cold working, or machining. By decreasing the hardness, yield strength, tensile strength, metal annealing allows for increased ductility and reduced brittleness.
The major purpose of annealing is to make metals more amenable to processes of manufacture like forming, shaping, stamping, hydroforming, forging, bending, and machining. Stress relief includes heating a metal at a relatively low temperature and then allowing it to cool uniformly. The ideal temperature is below 1000 degrees Fahrenheit for copper or steel and below 400 degrees Fahrenheit for aluminum. Like annealing, stress relief purpose is to reduce internal stresses created during machining, forming, or rolling processes.
However, annealing is performed at substantially higher temperatures at 1600o F (870 °C) or higher for steel and copper and at 600o F (315 °C) or higher for aluminum, which can relieve more stress than a simpler stress relief treatment process. Making use of vacuum, endothermic, and air furnaces, annealing and stress relieving treatments can be performed on all types of metal or plastic.
This is a type of metal hardening process whereby a metal part is cooled at cryogenic temperatures to relieve stress and reduce retained austenite after quenching. Cryogenic services include cryo treatments of -200°F (-128 °C) or colder and sub-zero cooling to -100° F (-73 °C) to improve hardening.
The internal stress relief brought by cryogenic treatment allows for the achievement of tighter tolerances during machining. Cryogenic treatments provide strength to metals for high performance applications in a variety of industries that include automotive, aerospace, medical, and defense. For instance, this process is often utilized to prepare aluminum for exposure to environments that are extremely cold, like those encountered in space. The following are other applications of cryogenically treated metal parts: steel tools, cutting tools, high performance racing parts.
The main objective of this process is to remove the internal stresses that develop after a cold working process. In this, steel is heated beyond its upper critical temperature and then cooled in the air. Normalizing improves the electrical and mechanical properties, machinability, and tensile strength. Normalizing is the heat treatment process that is carried out to restore the structure of normal conditions.
Normalizing treatment procedure includes the application of castings and forgings to refine the structure of the grain and to relieve stresses. Its application is done after cold working, such as stamping, rolling, and hammering.
In this type of process, steel is heated while being in the environment of sodium cyanide. Because of this, there will be a deposition of nitrogen and carbon atoms on the surface of the steel. This will make it hard.
In the cyaniding procedure, the metal part to be treated is sunk deep in a molten cyanide salt bath that is maintained at a temperature of 950 degrees Celsius. The molten salts that are used are sodium carbonate, sodium chloride, soda ash, and sodium cyanide. The article immersed in the molten cyanide is left there at a temperature of 950 degrees Celsius for about 15 to 20 minutes. Sodium cyanide’s decomposition will yield carbon and nitrogen from carbon monoxide, which is then diffused into the surface, causing the surface to harden. The part is then taken out of the bath, and therefore quenched in oil or water.
During the case hardening process, the external layer of the metal part is hardened while the interior part remains soft. For metals that contain a low carbon content such as steel and iron, the infusion of additional carbon must be done to the surface. Case hardening is a process mostly used as a final step after the metal piece has been machined.
Case hardening makes use of high heat in combination with other elements and chemicals to produce an outer layer that is hardened. Because metals can become more brittle due to hardening, case hardening can be useful for applications requiring a flexible metal and durable wear layer. The temperatures for case hardening are between 320 °F to 428 °F (160 °C to 220 °C).
This is a process whereby the hardness and strength of aluminum alloys are increased. These aluminum alloys are a specific subset namely the cast and wrought alloys that are precipitation hardenable. The alloys of aluminum that are precipitation hardenable include the 8xxx, 7xxx, 6xxx, 2xxx series. To add on top, there may be a requirement for annealing for parts that have experienced strain hardening during the process of their forming.
The following heat treatments are the typical aluminum heat treatments: homogenizing, annealing, natural aging, solution heat treatment, and artificial aging. Depending on the process that is exactly used, furnace temperatures range from 203 °F to 401 °F (95 °C up to 205 °C). It must be kept in mind that aluminum heat treating is quite different from steel.
This is a heat treating process involving the addition of a filler metal used for joining two pieces of metal. The filler metal has a melting point that is lower than the melting points of the metals to be joined. It is either fed into the joint during the heating of the parts or pre-placed.
In brazing parts with small clearances, the filler can flow into the joint through capillary action. The molten filler’s temperature exceeds 1544 °F (840 °C). The joints made by the brazing process are usually stronger than soldered joints. Brazing can be carried out on multiple metals. Before initiating the brazing process, it is important to prepare the surfaces by chemical or mechanical cleaning.
This type of heat treating process permits very targeted heating of metals using electromagnetic induction. The process is based on the induced electrical currents within the metal part for the production of heat. It is the most preferred method for bonding, hardening, or softening metals or other conductive materials. In modern manufacturing processes, this type of heat treatment provides a beneficial combination of consistency, speed, and control.
Although the basic principles are known very well, modern advances in solid state technology have changed the process to be a cost effective, simple heating method for applications that involve treating, joining, heating, and materials testing. Induction heat treating allows you to select the best physical characteristics through the highly controllable use of an electrically heated coil.
The characteristics are not only selected for each metal part, but for every section present on the metal part. Induction hardening is able to impart superior durability to shaft sections and bearing journals without the sacrifice of ductility necessary for handling shock loads and vibrations. The valve seats and internal bearing surfaces can be hardened in intricate parts without creating distortion problems. This means that the hardening or annealing of specific areas for durability and ductility in ways that will serve your needs best is possible.
Heat treatment for steel can have various types, which include:
The purpose of the process of annealing is to do the opposite of what hardening does. Metals are annealed to relieve stress, increase ductility, soften the metal, and improve their grain structures. When there is no appropriate preheating stage, welding can lead to a metal part with uneven temperatures, even the areas that are molten and that are next to areas at room temperature.
In such circumstances, welding can weaken the metal; as the weld gets cooler, interna; stresses are developed alongside hard and brittle spots. Annealing is the best solution to fix common problems like these and relieve internal stresses.
Normalizing steel is important for the removal of any internal stresses from machining, heat treatment, forming, forging, casting, or welding. Uncontrolled stress can result in metal failure, so normalizing steel before any hardening will help by giving assurance of the success of projects.
The purpose of hardening is not just to harden the steel but also to make it stronger. While hardening increases strength, it also reduces ductility, making the metal more brittle. After the process of hardening, the metal may need some tempering to remove some of the brittleness.
Most steels are hardened by using the first two stages of heat treatment (slow temperature heat and then comes soaking by a specified time to a temperature that is uniform), the third stage is not the same.
When metals are hardened, they are rapidly cooled by plunging them into oil, water, or brine. Most steels require rapid cooling, known as quenching, to be hardened, but only a few can be air cooled. The cooling rate required to harden steel decreases as alloys are added to steel. If the cooling rate becomes slower, the risk of either warping or cracking is lessened.
After hardening a metal, whether by flame or case, and introducing internal stresses after the inherent rapid cooling of the process, steel becomes both harder than needed and too brittle. The solution may be to temper the steel to decrease that brittleness and relieve or remove the internal stresses. When a steel part is tempered, the hardness that was caused by hardening is reduced and certain physical properties are developed.
Tempering always comes after hardening, and while reducing brittleness, it also softens steel. The softening of steel with tempering is unavoidable, unfortunately. But the amount of hardness lost can be controlled based on the temperature during the process of tempering. Tempering is always done at temperatures below the metal’s upper critical point, which is different in other heat treatment processes, which are done at temperatures above the metal’s upper critical point.
The heat treatment of stainless steel is based on the type of stainless steel and the target requirements of the end item. Heat treatment methods like hardening, annealing, and stress relieving improve the ductility and corrosion resistance properties of the metal part modified during fabrication. These methods can also generate structures that are hard and able to tolerate abrasion and high mechanical stresses. The types of stainless steel include martensite, austenite, and ferrite. These three types of stainless steel have different heat treatment methods.
This is the most used of all stainless steel types. This type of steel is characterized by austenite structure at normal temperature and there are no any changes that take place in the phase. The process of heat treatment can’t harden this type of stainless steel. Rather, it can be hardened by the process of cold working. Solution treatment is the most common heat treatment process for austenitic stainless steel.
This type of stainless steel has a ferrite structure at both normal and high temperatures and has no phase change. But steel forms an austenite structure at a high temperature when it contains a certain amount of austenite forming elements like nitrogen and carbon. This type of steel cannot be strengthened by the process of heat treatment, and can only be put under the process of annealing to drop internal stress and help further processing.
This type of stainless steel has a phase change point that is distinct. It exhibits an austenite structure at high temperatures, and the occurrence of martensite transformation can occur during cooling. After transforming into a martensite structure it hardens. Due to its high chromium content and good hardenability, different heat treating methods like quenching and tempering can be used.
This chapter will discuss the various applications and benefits of heat treating.
The most common application of heat treating is metallurgical. Heat treatment is also utilized in the manufacture of many materials like glass. The following are also areas of application of heat treatment.
Heat treatment can:
Heat treating can change or improve the properties of a metal including:
Heat treatment improves workability and machinability:
By removing internal stresses, heat treating increases manufacturability. For example, if a metal is too hard to bend or machine, it can be stress relieved or annealed to decrease the hardness. If a plastic deforms after machining, it can be annealed or stress relieved to keep it from deforming. Induction flame heat treating can soften just one area of a part, and the rest of the part is left unchanged.
Heat treatment improves wear resistance and durability:
Wear resistance of a part can be improved by heat treating through the process of hardening. Metals can be hardened on the surface or all around to make them stronger, more durable, tougher, and more resistant to wear.
Heat treatment improves strength and toughness:
Toughness and strength are a trade-off, as the strength is increased, the toughness may decrease causing brittleness. Heat treating can specifically affect tensile strength, yield strength, and fracture toughness. Case hardening increases strength, however the parts will need some tempering or drawn back to decrease brittleness. The amount of tempering is determined by the required ultimate strength desired in the material.
Heat treatment improves magnetic properties:
Most materials like 1008 or 316, tend to gain magnetism when they are work hardened. If a special annealing process is used, the magnetic permeability is reduced, which is especially important if the part is going to be used in an environment that is electronic.
Heat treating or heat treatment is a process in which controlled heat is applied to a material to alter its properties like strength, durability, wear resistance, etc. There are many processes of heat treatment available including nitriding, carburizing, induction flame hardening for treating only a single area of a metal part, brazing for joining two metal parts.
All these methods of heat treatment offer different results when applied. Therefore the stages involved in each heat treatment process together with the conditions specified must be taken into consideration when selecting a heat treatment method, in order to come up with a good product with the desired properties.
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