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How Does Heat Treatment Of Steel Work?
This substance is everywhere.
It’s in your kitchen, your car, your house, the buildings you work in and the bridges you drive on.
What is it? The same thing we’ve been using to build civilizations for thousands of years:
Starting with blacksmithing, humanity has been working with steel for so long we’ve dedicated entire industries to heating and shaping steel compounds to fit our needs. Today there are countless ways to heat, bend, harden, and chemically alter steel to fit whatever purpose we need it to serve.
Here we briefly discuss how the heat treatment of steel works, and why the heat treatment of metals is so vitally important to our terbaru world.
Read on to learn basics about the steel heat treating process.
Why Steel is Treated
Steel is one of the most common substances in the world, and we would not function as a society without it.
Put simply, steel is an alloy made from a combination of iron and other elements. There are different types of steel depending on what other elements are used alongside iron.
For quality assurance purposes, all the following have to be present in order for an alloy to be called steel:
Aluminum
Carbon
Manganese
Nitrogen
Oxygen
Phosphorus
Silicon
Sulfur
Plenty of other elements can be added to change the properties of the steel as desired, but those listed above must be present. The exact ratio of all these elements will affect the steel’s hardness, durability, flexibility, etc.
There are also specifications for how much of each element there can be in steel alloy before it has to be called something else.
For example, cast iron is an alloy much like steel that contains more than 2.1 percent carbon. Wrought iron is a similar alloy to cast iron but contains very little carbon in comparison, making it easier to twist and bend.
Steel is engineered for its final purpose starting with the elements used to make it. That rough metal alloy then needs to be treated with heat in such a way that it can be shaped and cut into a final product.
So, how does the heat treatment process work?
Methods of Heating and Hardening
Here’s where things start to get interesting.
There is a critical temperature of steel that makes it malleable and begins to change its chemical structure. Steel heated above this temperature enters what is called the austenite phase.
From there, engineers can create the correct shape and quality steel they need. There are many methods used to heat treat steel, including but not limited to:
Annealing – Heating and then slowly cool steel to refine it and make it softer
Carburizing – Adding carbon to the surface with heat and carbon-rich substances
Case hardening – Carburizing and quickly cooling steel to keep the center soft while the rest hardens
Cyanide hardening – Like case hardening, but using molten cyanide salt for the hard case instead of carbon
Decarburization – Removing carbon from the steel alloy either with heat or oxidation
Nitriding – Adding nitrogen to the surface of steel with heat and nitrogen-rich liquid or gas
Drawing or Tempering – Reheating steel that has already been cooled to a specific temperature to remove hardness
Taking unrefined steel alloy through various heat treatment processes is the only way to make all the finished steel parts we use. Not every steel product needs to go through all of the steps above, but all steel needs to be treated.
If you’re starting to get overwhelmed, just remember most rough steel alloy goes through three basic steps:
Annealing
Quenching
Tempering
Let’s break down what each of these means.
Annealing vs Tempering Steel
This is where a lot of people get confused.
Steel is heated beyond its critical temperature for the annealing process. High temperatures used for annealing send steel into its austenite phase. During this phase, the grain quality of the steel changes.
When hot steel is cooled and solid again, it cannot be used as is. Heat treated steel must be tempered before it can be used as a commercial material. Tempering is also heating steel, but this time below the critical temperature.
Tempering steel is a way to make hardened steel less brittle and change its durability to suit your needs. Each steel project may require a different heat level for tempering, but tempering never pushes steel back into the austenite phase.
Quenching (or Setting) Steel
Quenching is the industry term for rapidly cooling hot steel to make it hard again. Depending on the project, hot and malleable steel will be dipped in either water or oil to cool. This brings the steel back to a solid state and often leaves it brittle.
Heat treated steel can also be left to air cool. But as mentioned in the different steps of heat treatment above, the way steel is cooled will greatly affect its finished quality.
As you may have gathered by now, the same steel will be heated and quenched many times. This is to make sure it is ready to support buildings, bridges, or whatever structure for decades to come.
Aren’t you glad all of this is left to the professionals?
Heat Treatment of Steel
There you have it, folks. Looks like you’re ready to build your own skyscraper.
To recap:
All steel is an alloy of iron and a variety of other elements
All steel has to be treated in order to be used in commercial products
The heat treatment of steel generally always involves annealing, quenching, and tempering.
If you found this blog post helpful, check out how we harden and temper our steel right here in our family owned steel mill.
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Riansclub
Do you know that metals in raw form do not yield any such commercial benefit? That is why most industries do heat treatment to alter the mechanical properties of metals. Heat treatment is a very crucial process in the metallurgical industry. So let us learn more about what heat treatment is and the types of heat treatment in this article.What is heat treatment?
Heat treatment is a heating and then cooling process using predefined methods to achieve desired mechanical properties like hardness, ductility, toughness, strength, etc. It is the combination of thermal, industrial, and metalworking processes to alter the mechanical properties and chemical properties of metals.
In simple words, heat treatment is a process of heating the metal, holding it there for some time, and then cooling it back. During the whole process, the mechanical properties get changed due to changes in microstructure.
All metallic metals have grains which are nothing but microstructures of crystals. The nature of those grains determines the behavior of the mechanical properties of a metal. Heat treatment changes that mechanical structure by controlling the rate of diffusion and rate of cooling within that microstructure.
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The properties of heat-treated materials vastly depend on the processes that it has to undergo. Below are those key processes of heat treatment.HeatingHoldingCoolingHeating
The first step in the heat treatment process is heating the metal. The temperature depends on the types of metal and the technique used. Sometimes you need to heat the outer surfaces of the metal, and sometimes you need to heat the whole body. That depends on what kind of alteration you want in the mechanical structure.
Below are different furnaces that are used for heating metals in heat treatment process.Box type furnaceBatch furnaceElevator type furnaceBell-type furnacePit type furnaceSalt bath furnaceFluidized bed furnaceHolding
During the holding process, the metal is kept at the achieved temperature for some period of time. The time required depends on the type of metals and the type of mechanical properties expected.
The holding time also depends on the part size. If the part is large it is kept in a holding state for more time than the same type of metals having a small part size.Cooling
After the holding process, cooling starts. The cooling must be done in a prescribed manner. During cooling, there are some structural changes occur. Different media such as water, oil, or forced air is used to aid in cooling. You can also use furnaces for cooling purposes as the control environments help inefficient cooling.
You may like to read: What is Hooke’s LawHeat Treatment Techniques
Following are few common heat treatment technique used in industries.AnnealingNormalizingHardeningTamperingCurborizationQuenchingAnnealing
Annealing is a heat treatment process that is used to soften the metal. In other words, annealing helps to improve ductility, machinability, and toughness. On the flip side, the hardness of metals gets reduced. Annealing does this by changing the microstructure of metals.
Annealing is done by heating the metals at the above critical temperature, hold them there for some time and then cool it at a very slow rate in the furnace itself. Annealing is usually done on ferrous and non-ferrous metals to reduce hardness after the cold working process. Annealing is also done to enhance the electric conductivity of the metal.Types Of Annealing
There are two types of annealing process which are shown below.Process AnnealingFull AnnealingProcess Annealing
Process annealing is done when metal is heated below the critical temperature, keep it for a suitable time, and then cool it slowly. This process is suitable for low carbon steel like sheet metal and wires. No phase transformation occurs during process annealing, and it’s considerably cheaper than full annealing.Full Annealing
Full annealing is done when metal is heated above the critical temperature. This process is suitable for low and high carbon steel. Phase transformation occurs during the full annealing process, and it is a costly operation than process annealing.
Tampering
Tampering is done on metals that are already hardened. We all know that sometimes our application needs metal to be hardened as well as tough. Tampering helps to achieve the required toughness by sacrificing the hardness. Tampering is a very common process for machine tools, knives, etc.
Tampering is usually done by heating the metal at a relatively low temperature. The temperature depends on the required mechanical properties of metals. If you want high ductility, then you need to heat it at a high temperature. But if you need low ductility, then the low temperature is sufficient.Why Tampering is done?To improve ductilityTo reduce hardnessTo relieve internal tertekanTo reduce brittleness
Hardening
As the name suggests, hardening is used to increase the hardness of a metal. This is usually done by heating the metal above normalization temperature, keeping it at normalization temperature, and then rapidly quenching ( Cooling) it in water, oil, or brine solution.
The heat required depends on the size and the required mechanical properties of the metal. Often after hardening, tampering is done to increase the ductility and toughness of metals. Why hardening is done?To increase the hardness of metalsTo improve the magnetizing propertiesTo reduce ductility and toughnessTo reduce grain size
Types Of HardeningCase Hardening / Surface hardeningDifferential hardeningFlame HardeningCase Hardening / Surface Hardening
Case hardening or surface hardening is a hardening heat-treatment process. In the case of hardening, the complete metal piece is heated. But in the case of case hardening, only the outer surface is heat-treated to make it hardened. The inner metal is still soft and ductile.
Case hardening is widely used for the tool and die industry where the tool surface needed to hardened but the inner metal piece has to remain ductile.Types Of case hardeningNitridingCyaniding
Nitriding is a case hardening process in which nitrogen gas is used to harden the outer surface of the metal. The metal is heated in an ammonia (NH3) atmosphere, and then it is cooled.
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Heat Treatment Processes: Types, Methods, Purposes [pdf]
In this article, you learn what is the heat treatment process? Its methods, types, and the purpose, procedure, application of heat treatment. Heat treatment is an essential operation in the manufacturing process of machine parts and tools.What is Heat Treatment?
Heat treatment is defined as an operation involving the heating and cooling of a metal or an alloy in the solid-state to obtain certain desirable properties without change composition.
The process of heat treatment is carried out to change the grain size, to modify the structure of the material, and to relieve the stresses set up the material after hot or cold working.The heat treatment is done to improve the machinability. To improve magnetic and electrical properties.To increase resistance to wear, heat and corrosion, and much more reason.
Heat treatment consists of heating the metal near or above its critical temperature, held for a particular time at that finally cooling the metal in some medium which may be air, water, brine, or molten salts. The heat treatment process includes annealing, case hardening, tempering, normalizing and quenching, nitriding, cyaniding, etc.Types of Heat Treatment Processes
Following are the different types of heat treatment processes:AnnealingNormalizinghardeningTemperingNitridingCyanidingInduction HardeningFlame Hardening
Read Also:What is Metal Casting Process? A Complete guide List of Mechanical Properties of materials Types of Heat Treatment1. Annealing
Annealing is one of the most important processes of heat treatment. It is one of the most widely used operations in the heat treatment of iron and steel and is defined as the softening process.
Heating from 30 – 50°C above the upper critical temperature and cooling it at a very slow rate by seeking it the furnace. The main aim of annealing is to make steel more ductile and malleable and to remove internal stresses. This process makes the steel soft so that it can be easily machined.1.1 Purpose of AnnealingIt softens steel and to improve its machinability.To refine grain size and remove gases.It removes the internal stresses developed during the previous process.To obtain desired ductility, malleability, and toughness.It modifies the electrical and magnetic properties.1.2 Procedure for Annealing
Depending on the carbon content, the steel is heated to a temperature of about 50° to 55°C above its critical temperature range. It is held at this temperature for a definite period of time depending on the type of furnace and nature of work. The steel is then allowed to cool inside the furnace constantly.1.3 Application of annealing
It is applied to castings and forgings.2. Normalizing
Normalizing: The main aim of normalizing is to remove the internal stresses developed after the cold working process. In this, steel is heated 30 – 50°C above its upper critical temperature and cooling it in the air.
It improves mechanical and electrical properties, machinability & tensile strength. Normalizing is the process of heat treatment carried out to restore the structure of normal condition.2.1 Purpose of NormalizingPromote uniformity of structure.To secure grain refinement.To bring about desirable changes in the properties of steel.2.2 Procedure for Normalizing
The steel is heated to a temperature of about 40° to 50°C above its upper critical temperature. It is held at this temperature for a short duration. The steel is then allowed cool in still air at room temperature, which is known as air quenching.2.3 Application of NormalizingIt is applied castings and forgings to refine grain structure and to relieve stresses.It is applied after cold working such as rolling, stamping and hammering.3. Hardening
Hardening: The main aim of the hardening process is to make steel hard tough. In this process, steel is heated 30° – 40°C above the upper critical temperature and then followed by continues cooling to room temperature by quenching in water or oil. It is the opposite process of annealing.3.1 Purpose of HardeningBy hardening, it increases the hardness of steel.To resist to wearAllows the steel to cut other metals3.2 Procedure for Hardening
The steel is heated above its critical temperature range. It is held at that temperature for a definite period of time. The steel is then rapidly cooled in a medium of quenching.
The quenching medium is selected according to the degree of hardness desired. The air, water, bring, oils and molten salts are used as quenching mediums. A thin section such knife blades are cooled in air. Water is widely used medium but it results in the formation of bubbles on the surface of the metal.
Hence brine solution is used to prevent this. Oil is used when there is a risk of distortion on cracks and is suitable for alloy steels. The molten salts are used to cool thin section to obtain crack-free and impact-resistant products.3.tiga Application of Hardening
It is applied for chisels, sledgehammer, hand hammer, centre punches, taps, dies, milling cutters, knife blades and gears.4. Tempering
Tempering: When the hardening process hardens a steel specimen, it becomes brittle and has high residual stress. It is an operation used to modify the properties of steel hardened by quenching for the purpose of increasing its usefulness.
Tempering or draw results in a reduction of brittleness and removal of internal strains caused during hardening. Steel must be tempered after the hardening process.
The tempering is divided into three categories according to the usefulness of steel required. Low-temperature tempering. Medium temperature tempering. High-temperature tempering.4.1 Purpose of TemperingTo relieve internally stressed caused by hardening.To reduce brittleness.Improve ductility, strength and toughness.To increase wear resistance.To obtain desired mechanical properties.4.2 Procedure for Tempering
The steel after being quenched in the hardening process is reheated to a temperature slightly above the temperature range at which it is to be used, but below the lower critical temperature. The temperature here varies from 100°C to 700°C.
The reheating is done in a bath of oil or molten lead or molten salt. The specimen is held in the bath for a period of time till attains the temperature evenly, the time depends on the composition and desired quality of steel. Now the specimen is removed from the bath and allow to cool slowly in still air.4.tiga Application of Tempering
It is applied to cutting tools, tools, and gears, which are hardened by the hardening process.lima. Nitriding
Nitriding is the process of the case or surface hardening in which nitrogen gas is employed to obtain hard skin of the metal. In this process, steel is heated in the presence of ammonia environment.
Due to this, a nitrogen atom is deposited and makes material hard. Induction hardening and Flame hardening objects are heated by an oxy-acetylene flame.5.1 Purpose of NitridingTo harden the surface of the steel to a certain depth.Increase resistance to wear and fatigue.To increase corrosion resistance.lima.2 Procedure for Nitriding
It is done in the electric furnace where temperature varying between 450° and 510°C is maintained. The part is well machined and finished and placed in an airtight container provided with outlet and inlet tubes through which ammonia gas is circulated.
The container with the part is placed in the furnace and ammonia gas is passed through it while the furnace is heated.
During the process of heating nitrogen gas is released from ammonia in the form of atomic nitrogen, which reacts with the surface of the part, and forms iron nitrate.
The depth of entrance depends upon the length of time spent at the nitriding temperature. The part is taken out and it does not require any quenching or further heat treatment.5.3 Application of NitridingIt is applied for hardening the surface of medium carbon alloy steels.6. Cyaniding
Cyaniding: In this process, steel is heated in the presence of sodium cyanide environment. Due to this, carbon and nitrogen atoms are deposited on the surface of steel and make it hard.6.1 Purpose of CyanidingThis method is effective for increasing the fatigue limit of medium and small-sized parts such as gears, shafts, wrist pins etc.To increase surface hardness.increase wear resistance.To give the clean, bright and pleasing appearance to the hardened surface.6.dua Produce for Cyaniding
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Types Of Aluminum Heat Treatments – L&l Special Furnace Co, Inc.
Aluminum heat treatment is a process by which the strength and hardness of a specific subset of aluminum alloys, namely the wrought and cast alloys that are precipitation hardenable, are increased. Precipitation hardenable aluminum alloys include the 2XXX, 6XXX, 7XXX and 8XXX series. In addition, annealing may be required for parts that have experienced strain hardening during their forming process.
The typical aluminum heat treatments are annealing, homogenizing, solution heat treatment, natural aging, and artificial aging (also known as precipitation hardening). Depending on the exact process being used, furnace temperatures can range from 240 to 1000°F. It is important to keep in mind that the heat treating of aluminum is quite different from steel.Annealing
Aluminum alloys are subject to work hardening, also known as strain hardening. Strain hardening occurs when the aluminum alloy is being shaped by plastic deformation. The plastic deformation causes the grain structures within the aluminum to slide against each other along areas referred to as slip planes. As more and more plastic deformation takes place, there are fewer and fewer slip planes left that are easy to deform. As a result, more force is required to achieve further deformation. When a part has reached this state, it is said to be work hardened. In order to continue plastically deforming the material, the strain hardening has to be removed from the part.
The purpose of annealing is to essentially reset the crystalline grain structure, restoring slip planes and making it possible to continue shaping the part without requiring excess force. To anneal a work hardened aluminum alloy, the metal must be heated to somewhere between 570°F to 770°F for a set amount of time, ranging from just thirty minutes to a full three hours. The time and temperature are depending on two things: the size of the part that is being annealed and the composition of its alloy.
Annealing also relieves internal stresses that can develop in a part during processes such as cold forging or casting, stabilize the dimensions of a part, and resolve issues that result from internal strains (such as warping). Also, annealing can be successfully performed on aluminum alloys that are considered non heat treatable alloys. It is commonly used on aluminum parts that are forged, extruded, or cast.Homogenizing
Homogenizing is used to redistribute the precipitating elements more evenly throughout an aluminum part. This is typically necessary when working with cast aluminum alloy parts. When the part begins to cool, the outside edge that is in direct contact with the mold will cool first. This results in a skin of aluminum grains, or crystals. As the part continues to cool inward, the result is fairly pure aluminum near the skin and some regions near the center. The alloying elements precipitate out, resulting in the aluminum grains being locked into place. The cast part ends up with some regions being soft and others being strong. This segregation between regions can be reduced and the resulting part rendered more workable for forming by going through the homogenizing process.
An aluminum part is homogenized by raising its temperature to just under its melting point, which is usually between 900°F to 1000°F. After the entire part has reached this homogenizing temperature, it is allowed to slowly cool. The result is a cast part with a uniform internal structure.Solution Heat Treatment
While the cooling rate is not a factor in annealing, it is a factor in another similar aluminum heat treatment process called solution heat treatment. During the solution heat treatment process, the elements that are responsible for age hardening (which makes the metal part difficult to work with over time) are dissolved. Those dissolved elements then become spheroids, and the result is a homogenized structure. However, the part must be quenched, or rapidly cooled, to preserve that final distribution of dissolved elements in the alloy that was achieved as a result of heat treatment. The part is then much easier to work with. Over time, though, those trapped elements will precipitate out again and cause age hardening.
The exact temperature for solution heat treatment depends on the alloy composition of the aluminum, but it typically occurs somewhere in the range of 825°F to 980°F — but the temperature used must be within ±10°F of the sasaran temperature. If this temperature is not achieved, the solution heat treatment will not be successful. If the temperature is too low, strength will be lost; if the temperature is too high, then the part could end up discolored, critical elements could melt, or there may be increased strain within the part.
Once the part has reached that narrow window for the target temperature, it needs to soak. This soaking time can be anywhere from 10 minutes for a thin part to 12 hours for larger, thicker parts. However, heat treatment specialists do have a general rule of thumb: one hour for every inch of cross-section in the thickness.
Next comes the quenching step. The objective of quenching here is to “freeze” the trapped elements in place, or to cool the aluminum part rapidly enough that the alloying elements do not have a chance to precipitate out as the part cools. Water is the most commonly used quenchant, and typically the most effective quenchant for aluminum alloys.
Any forming that needs to be done to a solution heat treated part should be done very soon after quenching is complete. Otherwise, natural aging will begin and the part will become more difficult to work with. This is the reverse of what happens with heat treated steels, which are extremely brittle and hard after quenching.Natural Aging
After aluminum has been solution heat treated, the elements that dissolved will begin to precipitate out over time. This causes the grains to lock into position, which in turn increases the natural strength of the aluminum and is called aging.
The natural aging, or age hardening, process takes place at room temperature over a time period of four to five days, with 90% of the hardening occurring within the first day. Because of this effect, aluminum parts often need to be shaped rather quickly after going through a solution heat treatment process.Artificial Aging, aka Precipitation Hardening
For some aluminum alloys to be able to reach maximum hardness, they need to have the dissolved elements fully precipitated out. Not all aluminum alloys can reach sufficient hardness during natural aging at room temperature. Some can only harden to a certain point, but that can be resolved through precipitation hardening, which is sometimes called artificial aging.
In precipitation hardening, the aluminum is heated to an alloy specific temperature between 240°F and 460°F, within ±lima°F of the sasaran temperature. It will then soak for a period of between six to twenty-four hours, followed by cooling to room temperature. The result includes a significant increase in the yield strength of the aluminum, slightly less of an increase in tensile strength, and a decrease in ductility.Issues with Quenching Aluminum Alloys
As discussed earlier, the goal of quenching is to preserve the dissolved elements in the form that is reached at the end of the actual heating process. If quenching is required as part of aluminum heat treatments, then it is critical to quench the part as soon as it comes out of the heat treatment furnace. A delay of more than 15 seconds can be very detrimental. Having a quenchant tank as close as safely possible to the heat treatment furnace is wise.
Water at ambient temperature is typically used as a quenchant for aluminum alloys, but for more complex shapes with varying cross-sections other quenchants or methods may be considered. Other quenchant options for aluminum include:Boiling waterBrine solutionsForced air blastsStill airPolymersGlycolsFast quenching oils
Keep in mind that one of the drawbacks with a fast quench is part distortion (such as warping or twisting) and the development of residual stresses, which makes hot water quenching another common choice.Aluminum Heat Treatment Furnaces
Almost all of the aluminum heat treatments mentioned above require extremely precise control over temperature in order to achieve the desired effects. This involves the use of high-quality furnaces and ovens that can achieve a uniform temperature distribution, extremely accurate control instruments, and skilled technicians that know how to use the equipment properly.
Not only must the correct temperatures be reached, but the temperature-time cycles for the heat treatment process must possess uniformity and continuity. In addition, the furnace must be designed so that the part being treated achieves a uniform temperature throughout.ASM2705E
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Machinemfg
Annealing, normalizing, quenching, tempering, quenching and tempering … totally 12 heat treatment processes. This article will help you sort out.
After heating the steel to a temperature of Ac3 + 30 -50 degrees or Ac1 + 30 – 50 degrees or below Ac1 (you can consult the relevant materials), it is usually slowly cooled with the furnace temperature.
Intentions:To reduce hardness, improve plasticity, cutting and pressure processing functions.To refine grains, improve mechanical functions, and prepare for next steps.To eliminate internal stresses during cold and hot processing.
(1) Suitable for alloy layout steel, carbon east-west steel, alloy east-west steel, high-speed steel forgings, welding parts and raw materials with unsatisfactory supply conditions.
(dua) Usually anneals in the rough condition.02 Normalizing
Heat the steel to 30 – 50 degrees above Ac3 or Accm, after soaking, cool it at a cooling rate slightly larger than that of annealing.
Intentions:To reduce hardness, improve plasticity, cutting and pressure processing functions.To refinesgrains, improve mechanical functions, and prepare for next steps.To eliminate internal stresses during cold and hot processing.
Normalizing is usually used as a pre-treatment process for forgings, weldments and carburized parts. For low- and medium-carbon carbon layout steels and low-alloy steel parts with low functional requirements can be performed with the final heat treatment. For ordinary medium and high alloy steels, air cooling can cause complete or partial hardening, so it cannot be used as the final heat treatment process.03 Quenching
The steel piece is heated to a temperature above the phase transition temperature Ac3 or Ac1, hold for a certain period of time, and then rapidly cooled in water, nitrate, oil, or air.
Quenching is usually done to obtain a martensitic arrangement with high hardness. Sometimes when high-alloy steel (such as stainless steel, wear-resistant steel) is quenched, it is to obtain a single uniform austenite arrangement to improve wear resistance and corrosion resistance.
(1) Usually used for carbon steel and alloy steel with carbon content greater than 0.3%;
(2) Quenching can give full play to the strength and abrasion resistance potential of steel, but together they will constitute a large internal tertekan and reduce the plasticity and impact toughness of the steel. Therefore, tempering is required to obtain better induction mechanical functions.04 Tempering
The quenched steel parts are heated from the beginning to a temperature below Ac1, and after heat preservation, cool them in air, oil or hot water.
Intentions:To reduce or eliminate the internal stress after quenching, reduce the deformation and cracking of the workpiece.To adjust the hardness, improve the plasticity and resistance, and obtain the mechanical functions required by the operation.To stabilize the workpiece size.
(1)Tempering with low temperature when insisting on high hardness and wear resistance of steel after quenching.
(dua)Under the condition of insisting on a certain toughness, tempering at medium temperature is used to improve the elasticity and yield strength of steel.
(tiga)Mainly insist on high impact toughness and plasticity, and use high temperature tempering when there is sufficient strength.
Generally, steel should be prevented from tempering between 230 – 280 degrees and stainless steel between 400 – 450 degrees, because a tempering brittleness occurs at this time.05 Quenching and tempering
After quenching, high temperature tempering is called quenching and tempering. Steel is heated to a temperature that is 10-20 degrees higher than that during quenching. After heat preservation, quenching is performed, and then tempered at a temperature of 400-720 degrees.
Intentions:To improve the cutting function and improve the appearance of processing;To reduce deformation and cracking during quenching;To get outstanding inductive mechanics.
Application key:Suitable for alloy layout steel, alloy east-west steel and high-speed steel with high hardenability.Not only can it be used as the final heat treatment of various more important layouts, but also it can be used as a pre-heat treatment of certain tight parts, such as screws, to reduce deformation.06 Aging
Heat the steel to 80 – 200 degrees, keep it for lima – 20 hours or longer, then take it out of the furnace and cool it in the air.
Intentions:To arrange the steel parts after quenching to reduce the deformation during storage or use time.To reduce internal stress after quenching and grinding, and stabilize shape and size.
(1) Suitable for all steel types after quenching;
(2) It is often used for tight workpieces whose shapes are no longer changed, such as tight screws, measuring things, bed chassis, etc.07 Cold treatment
The quenched steel parts are cooled in a low-temperature medium (such as dry ice, liquid nitrogen) to -60 to -80 degrees or lower, and the temperature is uniformly taken out and then allowed to reach room temperature.
Intentions:
(1)To make all or most of the remaining austenite in the quenched steel part into martensite, and then improve the hardness, strength, wear resistance and fatigue limit of the steel part;
(2) To ensure the steel arrangement to stabilize the shape and size of steel pieces.
(1) Steel parts should be cold treated immediately after quenching, and then tempered at low temperature to eliminate internal stress during low temperature cooling;
(2) Cold treatment is mainly applicable to tight tools, measuring tools and tight parts made of alloy steel.08 Flame-heated surface quenching
The flame incinerated with oxygen-acetylene mixed gas is sprayed onto the surface of the steel part, and the steel is heated rapidly. When it reaches the quenching temperature, to spray with water to cool the steel immediately.
The hardness, wear resistance and fatigue strength of steel parts are improved, and the heart still adheres to the resistance status.
(1) Mostly used for medium carbon steel parts, usually the depth of hardened layer is dua-6mm;
(2) Suitable for single-piece or small-batch production of large workpieces and workpieces requiring partial hardening.09 Induction heating surface quenching
Put the steel parts into the inductor, make the surface of the steel parts induce current, heat to the quenching temperature in a very short time, and then spray water to cool.
To improve the appearance hardness, wear resistance and fatigue strength of steel parts, and adhere to the endurance of the heart.
(1) Mostly used for medium carbon steel and middle hall alloy layout steel parts;
(dua) Because of the skin effect, the high-frequency induction hardened hardened layer is usually 1 to dua mm, the intermediate frequency hardened is usually 3 to 5 mm, and the high frequency hardened is usually greater than 10 mm.10 Carburizing
Put the steel parts in the carburizing medium, heat it to 900-950 degrees and keep it warm, so that the surface of the steel parts can obtain a carburizing layer with a certain concentration and depth.
Intention:
To improve the external hardness, wear resistance and fatigue strength of steel parts, and the heart still adheres to the resistance status.
(1) For low-carbon steel and low-alloy steel parts with a carbon content of 0.15% to 0.25%, the depth of the carburized layer is usually 0.5 to 2.5mm;
(dua) After carburizing, it is necessary to perform quenching to obtain martensite on the surface before carburizing intention is completed.11 Nitriding
By using active nitrogen atoms that are separated out by ammonia gas at 500-600 degrees, the appearance of the steel is saturated with nitrogen to form a nitrided layer.
To improve the hardness, wear resistance, fatigue strength and corrosion resistance of steel parts.
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Here’s What Happens When Metals Undergo Heat Treatment
Before terkini metalworking techniques were invented, blacksmiths used heat to make metal workable. Once the metal was formed into the desired shape, the heated metal quickly was cooled. Quick cooling made the metal harder and less brittle. Modern metalworking has become much more sophisticated and precise, allowing for different techniques to be used for different purposes.Effects of Heat on MetalSubjecting metal to extreme heat causes it to expand in addition to impacting its structure, electrical resistance, and magnetism. Thermal expansion is pretty self-explanatory. Metals expand when subjected to specific temperatures, which vary depending on the metal. The actual structure of metal also changes with heat. Referred to as allotropic phase transformation, heat typically makes metals softer, weaker, and more ductile. Ductility is the ability to stretch metal into a wire or something similar.Heat also can impact the electrical resistance of metal. The hotter the metal gets, the more the electrons scatter, causing the metal to be more resistant to an electrical current. Metals heated to certain temperatures also can lose their magnetism. By raising temperatures to between 626 degrees Fahrenheit and 2,012 degrees Fahrenheit, depending on the metal, magnetism will disappear. The temperature at which this happens in a specific metal is known as its Curie temperature.Heat TreatmentHeat treatment is the process of heating and cooling metals to change their microstructure and to bring out the physical and mechanical characteristics that make metals more desirable. The temperatures metals are heated to, and the rate of cooling after heat treatment can significantly change metal’s properties.The most common reasons that metals undergo heat treatment are to improve their strength, hardness, toughness, ductility, and corrosion resistance. Common techniques for heat treatment include the following:Annealing is a form of heat treatment that brings a metal closer to its equilibrium state. It softens metal, making it more workable and providing for greater ductility. In this process, the metal is heated above its upper critical temperature to change its microstructure. Afterward, the metal is slow-cooled.Less expensive than annealing, quenching is a heat treatment method that quickly returns metal to room temperature after it is heated above its upper critical temperature. The quenching process stops the cooling process from altering the metal’s microstructure. Quenching, which can be done with water, oil, and other media, hardens steel at the same temperature that full annealing does.Precipitation hardening is also known as age hardening. It creates uniformity in a metal’s grain structure, making the material stronger. The process involves heating a solution treatment to high temperatures after a fast cooling process. Precipitation hardening is usually executed in an inert atmosphere at temperatures ranging from 900 degrees Fahrenheit to 1,150 degrees Fahrenheit. It can take anywhere from an hour to four hours to carry out the process. The length of time typically depends on the thickness of the metal and similar factors.Commonly used in steelmaking today, tempering is a heat treatment used to improve hardness and toughness in steel as well as to reduce brittleness. The process creates a more ductile and stable structure. The aim of tempering is to achieve the best combination of mechanical properties in metals.Stress relieving is a heat treatment process that decreases tertekan in metals after they have been quenched, cast, normalized, and so on. Stress is relieved by heating metal to a temperature lower than that required for transformation. After this process, the metal is then slowly cooled.Normalizing is a form of heat treatment that eliminates impurities and improves strength and hardness by altering the grain size to be more uniform throughout the metal. This is achieved by cooling the metal by air after it has been heated to a precise temperature.When a metal part is cryogenically treated, it is slowly cooled with liquid nitrogen. The slow cooling process helps prevent thermal tertekan of the metal. Next, the metal part is maintained at a temperature of roughly minus 190 degrees Celsius for about a day. When it is later heat tempered, the metal part undergoes an increase of temperature up to approximately 149 degrees Celsius. This helps to lower the amount of brittleness that may be caused when martensite forms during cryogenic treatment.
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Stainless Steel Heat Treatment: The Ultimate Guide
Classification and main characteristics of stainless steel
There are many classification methods for stainless steel, such as chemical composition, functional characteristics, metallographic structure and heat treatment characteristics.
From the perspective of heat treatment, it is more practical to divide it according to the metallographic structure and heat treatment characteristics.
1. Ferritic stainless steel
The main alloying element is Cr, or to add a small amount of stable ferrite elements, such as Al, Mo, etc., and the structure is ferrite.
Strength is not high, which can not use heat treatment methods to adjust the performance, there is a certain plasticity and large brittleness.
It has good corrosion resistance in oxidizing media (such as nitric acid) and poor corrosion resistance in reducing media.2. Austenitic stainless steel
It contains a high concentration of Cr, generally greater than 18% and about 8% Ni.
Some using Mn to replace Ni, which is to further improve corrosion resistance, and some have to add Mo, Cu, Si, Ti or Nb and other elements.
Heating and cooling do not occur when the phase change, which can not use heat treatment methods to strengthen, and it has the advantages of low strength, high plasticity and high toughness.
It has strong corrosion resistance to oxidizing media, and has good resistance to intergranular corrosion after adding Ti and Nb.tiga. Martensitic stainless steel
Martensitic stainless steel mainly contains 12~18% Cr, and the amount of C can be adjusted according to needs, generally 0.1~0.4%.
For tools, C can reach 0.8~1.0%, and some are to improve the stability of tempering resistance by adding Mo, V,and Nb etc.
After heating at a high temperature and cooling at a certain speed, the structure is basically martensite.
Depending on the difference of C and alloying elements, some may contain a small amount of ferrite, retained austenite or alloy carbide.
Phase changes occur during heating and cooling.
Therefore, the structure and shape of the structure can be adjusted in a wide range, thereby changing the performance.
The corrosion resistance is not as good as that of austenitic, ferritic and duplex stainless steel.
It has better corrosion resistance in organic acids and poor corrosion resistance in sulfuric acid, hydrochloric acid and other media.4. Austenoferritic steel stainless steel
Generally, the content of Cr is 17~30%, and the content of Ni is tiga~13%.
In addition, alloying elements such as Mo, Cu, Nb, N and W are added, and the C content is controlled very low.
Depending on the proportion of alloying elements, some ferrite, some are mainly austenite, constituting two duplex stainless steels that exist simultaneously.
Because it contains ferrite and strengthening elements, after heat treatment, the strength is slightly higher than that of austenitic stainless steel and the plasticity and toughness are better, which is impossible to adjust the performance by heat treatment.
It has high corrosion resistance, especially in Cl- containing media and seawater, which has good resistance to pitting, crevice corrosion and stress corrosion.lima. Precipitation hardening stainless steel
The composition is characterized by the presence of C, Cr, Ni and other elements, but also contains Cu, Al and Ti etc. that can age precipitate precipitates.
Mechanical properties can be adjusted by means of heat treatment, but its strengthening mechanism is different from martensitic stainless steel.
Because of its reliance on precipitation phase strengthening, so C can be controlled very low, thus its corrosion resistance is better than martensitic stainless steel, and Cr-Ni austenitic stainless steel is equivalent.Heat Treatment of Stainless Steel
Stainless steel is characterized by the composition of a large number of alloy elements mainly composed of Cr, which is the basic condition of stainless steel and corrosion resistance.
In order to fully use alloying elements, to obtain the ideal mechanical and corrosion resistance, it must also be realized through the heat treatment method.1. Heat treatment of ferritic stainless steel
Ferritic stainless steel under normal circumstances is a stable single ferrite tissue heating, cooling does not occur phase change, so it can not use heat treatment to adjust the mechanical properties.
The main purpose is to reduce brittleness and improve resistance to intergranular corrosion.
① σ phase brittleness
Ferritic stainless steel is very easy to generate σ phase, which is a kind of Cr-rich metal compound with hard and brittle characteristics.
It is especially easy to form between crystals to make steel brittle and increase the sensitivity of intergranular corrosion.
The formation of σ phase is related to the composition, except Cr, Si, Mn and Mo, etc. all promote the formation of σ phase;
It is also related to the processing process, especially heating and staying in the range of 540~815℃, which promotes the formation of σ phase.
However, the formation of σ phase is reversible, and reheating above the formation temperature of σ phase will re-dissolve in a solid solution.
② Brittleness at 475℃
Ferritic stainless steel heated for a long time in the range of 400~500℃ will show the characteristics of increased strength, decreased toughness and increased brittleness, especially at 475℃, which is called 475℃ brittleness.
This is because, at this temperature, the Cr atoms in the ferrite will rearrange to form a small Cr-rich region, which is coherent with the parent phase, causing lattice distortion, generating internal tertekan, and increasing the hardness and brittleness of the steel.
When the Cr-rich area is formed, there must be a Cr-poor area, which has an adverse effect on the corrosion resistance.
When the steel is reheated to a temperature higher than 700°C, the distortion and internal tertekan will be eliminated, and the brittleness at 475°C will disappear.
③ High-temperature brittleness
When heating to above 925°C and rapidly cooling down, compounds such as Cr, C, and N will precipitate in the grains and grain boundaries, which causes increased brittleness and intergranular corrosion.
This compound can be eliminated by heating at a temperature of 750~850℃ and then rapidly cooling.
① AnnealingIn order to eliminateσ phase, brittleness at 475°C and brittleness at high temperature, annealing treatment can be used. It needs to heat and hold at 780~830°C, and then to use air cooling or furnace cooling.For ultra-pure ferritic stainless steel (C≤0.01%, strictly control Si, Mn, SandP), the annealing heating temperature can be increased.
② Stress-relieve treatment
After welding and cold-working, parts may have tertekan.
If annealing treatment is not suitable for specific circumstances, heating, heat preservation, and air cooling can be used in the range of 230~370℃, which can eliminate some internal stress and improve plasticity.dua. Heat treatment of austenitic stainless steel
Cr, Ni and other alloying elements in austenitic stainless steel result in the Ms point down to below room temperature (-30 to -70 ℃).
To ensure the stability of the austenitic organization, so that when heating and cooling, the phase change does not occur above room temperature.
Therefore, the main purpose of austenitic stainless steel heat treatment is not to change the mechanical properties, but to improve corrosion resistance.
① Solution treatment of austenitic stainless steel
Effects:
① Precipitation and dissolution of alloy carbides in steel
C is one of the alloying elements contained in the steel.
In addition to having a little strengthening effect, it is detrimental to corrosion resistance, especially when C and Cr form carbides, the effect is even worse, so it should try to reduce its existence.
For this reason, according to the characteristic that C changes with temperature in austenite, that is, the solubility is large at high temperatures, and the solubility is small at low temperatures.
It is reported that the solubility of C in austenite is 0.34% at 1200℃;
It is 0.18% at 1000°C, 0.02% at 600°C, and even less at room temperature.
Therefore, the steel is heated to a high temperature to fully dissolve the C-Cr compound,.
Then, it is quickly cooled to stop precipitation, which is to ensure the corrosion resistance of the steel, especially the intergranular corrosion resistance.
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An Overview Of Heat Treatment Methods & Their Benefits
Heat treatment is the process of heating and cooling metals, using specific predetermined methods to obtain desired properties. Both ferrous as well as non-ferrous metals undergo heat treatment before putting them to use.
Over time, a lot of different methods have been developed. Even today, metallurgists are constantly working to improve the outcomes and cost-efficiency of these processes.
For that they develop new schedules or cycles to produce a variety of grades. Each schedule refers to a different rate of heating, holding and cooling the metal.
These methods, when followed meticulously, can produce metals of different standards with remarkably specific physical and chemical properties.The Benefits
There are various reasons for carrying out heat treating. Some procedures make the metal soft, while others increase hardness. They may also affect the electrical and heat conductivity of these materials.
Some heat treatment methods relieve stresses induced in earlier cold working processes. Others develop desirable chemical properties to metals. Choosing the perfect method really comes down to the type of metal and the required properties.
In some cases, a metal part may go through several heat treatment procedures. For instance, some super alloys used in the aircraft manufacturing industry may undergo up to six different heat treating steps to optimise it for the application.Heat Treatment Process Steps
In simple terms, heat treatment is the process of heating the metal, holding it at that temperature, and then cooling it back. During the process, the metal part will undergo changes in its mechanical properties. This is because the high temperature alters the microstructure of the metal. And microstructure plays an important role in the mechanical properties of a material.
The final outcome depends on many different factors. These include the time of heating, time of keeping the metal part at a certain temperature, rate of cooling, surrounding conditions, etc. The parameters depend on the heat treatment method, type of metal and part size.
Over the course of this process, the metal’s properties will change. Among those properties are electrical resistance, magnetism, hardness, toughness, ductility, brittleness and corrosion resistance.HeatingJet engine parts going into a furnace
As we already discussed, the microstructure of alloys will change during heat treatment. Heating is carried out in line with a prescribed thermal profile.
An alloy may exist in one of three different states when heated. It may either be a mechanical mixture, a solid solution, or a combination of both.
A mechanical mixture is analogous to a concrete mixture where cement binds sand and gravel together. Sand and gravel are still visible as separate particles. With metal alloys, the mechanical mixture is held together by the base metal.
On the other hand, in a solid solution, all the components are mixed homogenously. This means that they cannot be identified individually even under a microscope.
Every state brings along different qualities. It is possible to change the state through heating according to the phase diagram. The cooling, though, determines the final outcome. It is possible for the alloy to end up in one of the three states, depending solely on the method.Holding
During the holding, or soaking stage, the metal is kept at the achieved temperature. The duration of that depends on the requirements.
For example, case hardening only requires structural changes to the surface of the metal in order to increase surface hardness. At the same time, other methods need uniform properties. In this case, the holding period is longer.
The soaking time also depends on the material type and part size. Larger parts need more time when uniform properties are the objective. It just takes longer for the core of a large part to reach the required temperature.Cooling
After the soaking stage is complete, the metal must be cooled in a prescribed manner. At this stage, too, structural changes occur. A solid solution on cooling may stay the same, become a mechanical mixture completely or partially, depending on various factors.
Different media such as brine, water, oil or forced air control the rate of cooling. The sequence of cooling media named above is in decreasing order of effective rate of cooling. Brine absorbs heat fastest, while air is the slowest.
It is also possible to use furnaces in the cooling process. The controlled environment allows for high precision when slow cooling is necessary.Phase Diagrams
Each metal alloy has its own phase diagram. As previously said, heat treatment is done according to these diagrams. They show the structural changes that take place at different temperatures and different chemical compositions.
Let’s use the iron-carbon phase diagram as an example, as this is the most known and widely taught one at universities.
The iron-carbon phase diagram is an important tool when learning about the behaviour of different carbon steels when subjected to heat treatment. The x-axis shows the carbon content in the alloy and the y-axis shows the temperature.
Note that dua.14% of carbon is the limit where steel becomes cast iron,
The diagram displays various regions where the metal exists in different microstates such as austenite, cementite, pearlite. These regions are marked by boundaries A1, A2, A3, and Acm. At these interfaces, phase changes occur when the temperature or carbon content value passes through them.
A1: The upper limit of the cementite/ferrite phase.
A2: The limit where iron loses its magnetism. The temperature at which a metal loses its magnetism is also called Curie temperature.
A3: The interface that separates Austenite + Ferrite phase from the γ (Gamma) austenite phase.
Acm: The interface that separates γ Austenite from the Austenite + Cementite field.
The phase diagram is an important tool to consider whether heat treatment will be beneficial or not. Each structure brings along certain qualities to the final product and the choice of heat treatment is made based on that.Common Heat Treatment Methods
There are quite a few heat treatment techniques to choose from. Every one of them brings along certain qualities.
The most common heat treatment methods include:AnnealingNormalisingHardeningAgeingStress relievingTemperingCarburisationAnnealing
In annealing, the metal is heated beyond the upper critical temperature and then cooled at a slow rate.
Annealing is carried out to soften the metal. It makes the metal more suitable for cold working and forming. It also enhances the metal’s machinability, ductility and toughness.
Annealing is also useful in relieving stresses in the part caused due to prior cold working processes. The plastic deformations present are removed during recrystallisation when the metal temperature crosses the upper critical temperature.
Metals may undergo a plethora of annealing techniques such as recrystallisation annealing, full annealing, partial annealing and final annealing.Normalising
Normalising is a heat treatment process used for relieving internal stresses caused by processes such as welding, casting, or quenching.
In this process, the metal is heated to a temperature that is 40° C above its upper critical temperature.
This temperature is higher than the one used for hardening or annealing. After holding it at this temperature for a designated period of time, it is cooled in air. Normalising creates a uniform grain size and composition throughout the part.
Normalised steels are harder and stronger than annealed steel. In fact, in its normalised form, steel is tougher than in any other condition. This is why parts that require impact strength or need to support massive external loads will almost always be normalised.Hardening
The most common heat treatment process of all, hardening is used to increase the hardness of a metal. In some cases, only the surface may be hardened.
A work piece is hardened by heating it to the specified temperature, then cooling it rapidly by submerging it into a cooling medium. Oil, brine or water may be used. The resulting part will have increased hardness and strength, but the brittleness increases too simultaneously.
Case hardening is a type of hardening process in which only the outer layer of the work piece is hardened. The process used is the same but as a thin outer layer is subjected to the process, the resultant work piece has a hard outer layer but a softer core.
This is common for shafts. A hard outer layer protects it from material wear. When mounting a bearing to a shaft, it may otherwise damage the surface and dislocate some particles that then accelerate the wearing process. A hardened surface provides protection from that and the core still has the necessary properties to handle fatigue stresses.
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Heat Treatment Of Steel
Heat treatment of pipe depends on the way it is manufactured – hot finished or cold finished. Selection of heat treatment methods will depend on types of material and on what material properties you want to restore or further enhancement on existing properties.
In the case of Hot Finished Pipe, no heat treatment is required. As during the manufacturing process, the pipe temperature remains in the range of heat treatment temperature till the final size and thickness are achieved.
Whereas in the case of Cold Finished Pipe, heat treatment is required as per applicable ASTM standard. As the pipe is either cold drawn or temperature is not maintained in the hot finish range.
Heat treatment is the process of heating and cooling metals to change their microstructure and to bring out the physical and mechanical characteristics that make metals more desirable. The temperatures metals are heated to, and the rate of cooling after heat treatment can significantly change metal’s properties. The most common reasons that metals undergo heat treatment are to improve their strength, hardness, toughness, ductility, and corrosion resistance.Maximum Operating Temperatures
[Maximum Operating Temperatures of Stainless Steel Grades – Source: From web]The temperature at which the strength starts to fall sharply is defined as the maximum service temperature.The strength of a material tends to fall quickly when a certain temperature is reached. This temperature limits the maximum operating temperature for which the material is useful.For metals the maximum operating temperature is usually around two thirds of the melting temperature.For prolonged loading the maximum stress will be lower because creep (permanent stretching over time) will occur.When a metal is subjected to a stress for long time at an elevated temperatures, it undergoes plastic deformation; this time dependent accumulation of strain is known as Creep. Creep limits the life time of components.The creep resistant steels for various applications like tubing, boiler drum, main steam pipe, rotors and turbine blades, castings etc. can be put in three categories based on the microstructure: ferritic, bainitic and martensitic.Note that the range of maximum service temperature does not mean the range of temperature in which the material must be used! It may be assumed that any operating temperature below the maximum service temperature down to zero degrees Centigrade is safe in design.Problems can occur when materials are used well below 0oC – for example special steels must be used to contain liquefied gases, since ordinary carbon steels may become brittle at these very low temperatures.The maximum service temperature is important for applications where components become hot. Jet engines, brake discs and extrusion dies are all examples of products which operate at temperatures of 400oC or more – metals and ceramics are then required. Temperatures of only 100oC are enough to cause problems for lower melting point materials such as polymers – for example, plastic cups and kettles.Properties of Steel at Room Temperature
[Properties of Steel at Room Temperature Source: From web]Heat Treatment ProcessHardening:- Hardening is a metallurgical metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield tertekan at the location of the imposed strain. A harder metal will have a higher resistance to plastic deformation than a less hard metal. Precipitation hardening also known as age hardening, is one of the Hardening processes. It creates uniformity in a metal’s grain structure, making the material stronger. The process involves heating a solution treatment to high temperatures after a fast cooling process. Precipitation hardening is usually executed in an inert atmosphere at temperatures ranging from 900 degrees Fahrenheit to 1,150 degrees Fahrenheit. It can take anywhere from an hour to four hours to carry out the process. The length of time typically depends on the thickness of the metal and similar factors.Tempering:- Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air.Annealing:- Annealing is a form of heat treatment that brings a metal closer to its equilibrium state. It softens metal, making it more workable and providing for greater ductility. In this process, the metal is heated above its upper critical temperature to change its microstructure. Afterward, the metal is slow-cooled.Normalizing:-Normalizing involves heating steel, and then keeping it at that temperature for a period of time, and then cooling it in air. The resulting microstructure is a mixture of ferrite and cementite which has a higher strength and hardness, but lower ductility. Normalizing is performed on structures and structural components that will be subjected to machining, because it improves the machinability of carbon steels.Carburization:- Carburization is a heat treatment process in which steel or iron is heated to a temperature, below the melting point, in the presence of a liquid, solid, or gaseous material which decomposes so as to release carbon when heated to the temperature used.Surface Hardening:- In many engineering applications, it is necessary to have the surface of the component hard enough to resist wear and erosion, while maintaining ductility and toughness, to withstand impact and shock loading. This is known as surface hardening. This can be achieved by local austentitizing and quenching, and diffusion of hardening elements like carbon or nitrogen into the surface. Processes involved for this purpose are known as flame hardening, induction hardening, nitriding and carbonitriding.Quenching:- Less expensive than annealing, quenching is a heat treatment method that quickly returns metal to room temperature after it is heated above its upper critical temperature. The quenching process stops the cooling process from altering the metal’s microstructure. Quenching, which can be done with water, oil, and other media, hardens steel at the same temperature that full annealing does.Stress Relieving:- Stress relieving is a heat treatment process that decreases stress in metals after they have been quenched, cast, normalized, and so on. Stress is relieved by heating metal to a temperature lower than that required for transformation. After this process, the metal is then slowly cooled.Cryogenically Treated:- When a metal part is cryogenically treated, it is slowly cooled with liquid nitrogen. The slow cooling process helps prevent thermal tertekan of the metal. Next, the metal part is maintained at a temperature of roughly minus 190 degrees Celsius for about a day. When it is later heat tempered, the metal part undergoes an increase of temperature up to approximately 149 degrees Celsius. This helps to lower the amount of brittleness that may be caused when martensite forms during cryogenic treatment.Or Combination of these.How Does Heat Affect Metal
Metals are elements or compounds with excellent conductivity for both electricity and heat, making them useful for a wide range of practical purposes. The electrical, magnetic and structural properties of metals can change with temperature and thereby provide useful properties for technological devices. Understanding the impacts of temperature on the properties of metals gives you a deeper appreciation for why they’re so widely-used in the modern world.Thermal Expansion – Heating metal can increase its volume, length and surface area, as the heat displaces atoms from their usual position which alters the structure.Magnetism – Iron, cobalt and nickel are all naturally magnetic materials, or ferromagnetic materials. When heat is applied to them it can reduce their natural magnetic properties to a point so low that it is completely gone.Resistance – Some metals are able to effectively reduce, or halt, the flow of an electric current. This is known as resistance and how resistant a metal is depends on how quickly electrons are able to pass through it.The following are our most widely used alloys in manufacturing industrial gaskets, their major characteristics, temperature limits and approximate Brinell hardness (HB).
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Post Weld Heat Treatment (pwht): Pwht Requirements (pdf)
The full form of PWHT is Post-weld heat treatment.PWHT is a controlled process that involves reheating the metal below its lower critical transformation temperature, following a welding process. The material is then held at the elevated temperature for a predetermined period of time to alleviate residual stresses, increase the strength, increase or decrease the hardness, and reduce the risk of cracking by microstructural changes. An array of heating processes can be used to carry out post-weld heat treatment.
This article shall cover PWHT of welded butt joints, branch joints in Carbon Steel and Low Alloy Steel Materials, as well as all types of welded joints in killed carbon steel materials for Low-Temperature Service to guide technicians, supervisors, and other persons involved in the safe execution of PWHT activity so that final result is acceptable.When PWHT is required for Carbon Steel?
Post Weld Heat Treatment or PWHT of Carbon Steel must be performed after every welding in order to ensure the material strength of the part is retained. The exact criteria for PWHT of carbon steel are mentioned in the ASME BPVC code. PWHT ensures the reduction of residual stresses, controlling material hardness, and enhancement of mechanical strength.Advantages of Post Weld Heat Treatment stress due to the redistribution of residual stresses.Tempered metalRemoval of diffusible hydrogen which helps in preventing Hydrogen-Induced Cracking (HIC)PWHT Method and EquipmentThe local post-weld heat treatment of the welded joints on the pipes shall be carried out by the electric-resistance method and after the completion of all welding or repairing operation. The resistance heater is electrically and thermally self-insulated and is built to size for each individual pipes.The applied voltages across the coils are either 220 or 380 volts AC depending on the power requirements.
The power controlling panel of post-weld heat treatment is composed of:A temperature controller indicator and recorder of digital type.A potentiometer device which controls the percentage of power input to the coils.A switch On and Off indicator lights, input, and hasil terminals for power and thermocouple connection.Electrical power contactors of the proper rating.
Each panel will supply a single heating station and therefore for each heating operation, one panel will be needed. Heating and cooling rates are adjusted by manual selection of percentage of power input by means of potentiometers.Requirements for Post Weld Heat Treatment or PWHT
Before applying for the detailed PWHT requirements and exemption in these paragraphs, satisfactory weld procedure qualifications of the welding procedures specification to be used shall be performed in accordance with all the essential variables of ASME SECTION IX including conditions of post-weld heat treatment and including other restrictions listed below.
While carrying out local post-weld heat treatment, the technique of application of heat must ensure uniform temperature attainment at all points of the portion being heat treated. Care shall be taken to ensure that the width of the heated band on either side of the weld edge shall not be less than four (4) times of pipe thickness or 2″ whichever is greater.
Throughout the cycle of post-weld heat treatment, the portion outside the heated band shall be suitably wrapped under insulation so as to avoid any harmful temperature gradient at the exposed surface of the pipe. For this purpose, the temperature at the exposed surface of the pipe should not be allowed to exceed 400°c.
The valves, instruments, and other special items with welding ends, shall be protected, because of the risk of damage during post-weld heat treatment.
No welding shall be performed after PWHT.
Automatic temperature recorders that have been suitably calibrated shall be employed. The calibration chart of each recorder shall be submitted to the owner prior to starting the heat treatment operations and his approval shall be obtained. Recording equipment shall be calibrated at least once every 12 months. Also, the instrument equipment (potentiometer) which is used for calibration of recorders should be supported by a related certificate.Preparation and Attachment of Thermocouple for PWHT
After performing visual inspection and removing surface defects and temporary tack welds (if any) an adequate number of thermocouples (based on the diameter of pipes) shall be attached to the pipe directly and equally spaced location along the periphery of the pipe joint. The minimum number of thermocouples attached per joint shall be 1 for up to 3″ diameter, dua for up to 6″ diameter, and tiga for up to 10″and 4 up to 12″diameter and above. However, the required minimum number of thermocouples to be attached can be increased if it is found necessary.
The thermocouples shall be placed on the joint and in firm contact with the pipe as near as possible to the weld area. Thermocouples should be directly tack welded to the joint or heating band jointly provided that they have a tail of the same material and approved filler wire or electrode not larger than dua.5 mm in diameter is used for tack welding.
In order to avoid incorrect temperature readings due to direct radiation to thermocouples, it shall be protected by ceramic fiber blanked or any other suitable insulation material.
Heating resistance elements shall be laid over the attached thermocouples throughout of heating band and shall be insulated as shown in Fig. 1 below.
Insulating materials shall be mineral wool/glass wool which can overcome the temperature employed. The minimum insulation thickness shall be 50 mm. To hold the insulation material in position wire mesh shall be wrapped around and tied or tied by other suitable means. Fig. 1: PWHT ArrangementPWHT Temperature, Time Record
The post-weld heat treatment temperature and time and its heating and cooling rates shall be recorded automatically and present the actual temperature of the weld area. Each thermocouple shall be connected to the controlling and recording instrument for each treated joint.Heating, Holding and Cooling in PWHT
The heating temperature above 300°c shall be recorded and the heating and cooling rate shall not be more than that specified in related WPS and standards but in no case, more than 200°c/hr and the difference between the temperatures measured by various thermocouples shall be within the range specified.
The heat treatment socking temperature and holding time shall be as specified in related welding procedure specifications. For easy reference, the values for different types of steel are given in the following table.
The cooling down to 300°c shall be controlled cooling. Below that the cooling down to ambient temperature shall take place under insulation coating without controlling.PWHT-TABLE FOR A106-B MATERIALFew Important Notes related to PWHTPWHT operation shall be performed only by trained personnel having a similar experience and approved by the owner.During PWHT joints shall be protected from rain and wind by adequate rain cover and windshield.Hardness tests after PWHT shall be performed to determine if heat treatment has been performed effectively. Normally for Carbon Steel, maximum Brinnel Hardness is 200 HB.Safety Precautions during PWHT
The following safety precautions shall be provided during PWHT:Equipment and panels shall be properly earthed.Electrical technicians shall work with proper safety wears such as rubber gloves, shoes, etc.An only certified electrician will work.Joints under PWHT shall be well cordoned with red-tape/red light and danger display to avoid unknown persons coming in contact with high voltage electrical connections. The adequate platform shall be made for in situ joints to avoid fall of a person.Stainless Steel PWHT
PWHT for stainless steel is usually not required. However, to increase the corrosion resistance or reduce tertekan corrosion cracking susceptibility, stainless steel PWHT may be used depending on the service conditions encountered.PWHT for Piping
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