The need for PWHT is mostly due to the residual stresses and micro-structural changes that occur after welding has been completed. During the welding process, a high temperature gradient is experienced between the weld metal and the parent material. As the weld cools, residual stress is formed. SCOPE The procedure covers the general requirements of Post Weld Heat Treatment (PWHT) of weld joints between alloy steel to alloy steel, alloy steel to carbon steel and carbon steel to carbon steel material of piping fabricated in accordance with ASME B31Pressure Piping and ASME BPV Code. REFERENCES ASME Sec. VIII Division I Rules for. Stress relieve is normally a code requirement for pressure equipment above a certain thickness. (Each code has different rules for determining when a stress relieve is required.) For carbon steels, the stress relieve is typically around 580°C – 650°C.
Post Weld Heat Treatment Services ( PWHT ) is defined as one of heat treatments done after welding/machining to improve the Chemical mechanical properties of weldment / machined surfaces. In concept, PWHT covers many different potential treatments. However, in steel fabrication, most common procedure used is Stress Relieving.
Post-Weld Heat Treatment/PWHT purpose
Best recovery tools for mac. The primary reasons that components are required to be subjected to PWHT within the ASME Code rules are that PWHT reduces residual stresses and tempers hardened microstructures. PWHT will achieve both of these results but might not positively benefit the overall properties of the weldment if not properly controlled, particularly in terms of the toughness in the heat-affected zone. When toughness is a requirement, the Codes will impose additional controls on the PWHT, such as time at temperature controls. PWHT done to meet the Code requirements is typically performed at subcritical temperatures.
Residual stresses: can contribute to increases in the susceptibility to corrosion mechanisms and to fatigue. Because residual stresses cannot exceed the yield strength of the materials, an immediate benefit of increasing the temperature of the material during PWHT is a corresponding drop in the yield strength of the material and thus a reduction of the maximum residual stresses in the weldment. In order to further reduce the residual stresses, the weldment will need to be held for Longer periods at the elevated temperatures (this reduction occurs by relaxation-recrystallization or primary creep mechanisms).
Although the reduction of residual stresses is a benefit of PWHT, most of the rules for PWHT specified in the Codes are targeted at the hardened microstructures. This is because the applications that might require a reduction in residual stresses are not addressed specifically in the Codes.
Phase diagram of an iron-carbon alloying system
Post-weld treatment acts as a tempering process by reducing the hardness of the heat-affected zone and the weld metal. Tempering is a heat treatment whereby the material is heated to a temperature below the lower critical temperature (often assumed to be approximately 1340ºF [725ºC] for carbon steels). The PWHT for carbon steels is generally done in the range of 1100-1200ºF (600-650ºC), although some Codes specify only the minimum temperature of 1100ºF (600ºC). A secondary effect of tempering is to allow some additional transformation of the martensitic grain structure into ferrite, but the main objective is tempering the martensite. The result can be increased ductility and toughness in addition to reduced hardness. If the tempering temperature is too high or held too long, some corresponding reduction in the toughness can result.
NITS has a talented crew that uses diesel fuel as source of heating for stress relieving for pressure vessels with the sole objective of reinforcing process and component integrity and quality. The experience heat treaters from NITS have the experience, equipment, and expertise to develop custom configuration for your particular process. Our heating processes include low-Range, Mid-Range & High-Range Temperature Heating.
Furnace equipment:
- Permanent Low Thermal Mass Furnaces for heat treatment by Oil / Gas / Electrical mode of heating. Types of furnaces comes under the category are Bogie Hearth Furnace, Top Hat Furnace, Fixed Hearth Furnace, Pit type Furnace, Box Furnace, Roller Hearth Furnace etc.
- Temporary Low Thermal Mass Furnace for Stress Relieving, Normalizing, Solution Annealing etc.
Heat treatment Equipment:
- High velocity oil & gas fired burner system provides capacity from 5,00,000 kcal/hr up to max. 8,000,000 kcal / hr.
- Heat treatment low voltage power source of 40,50,75,100,125 kva capacity.
- Heat Treatment Distribution Panel.
- Flexible Ceramic Pads operating at 45V, 60V, 80V, 230 V.
- Electric Channel Elements.
- Programmer Controller, 03 Point, 06 Point, 12 Point Inlet.
- Thermocouple attachment unit.
- Twin / Trinary Heat Module for Portable Operation.
- Thermocouple (Simplex / Duplex, Wire type)
Oil/Gas Fired High Velocity Burner Equipment:
The Indotherm Oil / Gas high velocity burner systems provide from 5,00,000 kcal/hr up to 8,000,000 Kcal/hr ( 2000,000 Btu/hr to 32,000,000 Btu/hr ). For On Site Heat Treatment from a flame supervised, automatic / manually controlled, nozzle mix high velocity burner which will operate on all commercially available fuels.
WelderDestiny ›Welding Engineering Tools ›Post Weld Heat Treatment
Weldsare often given a Post Weld Heat Treatment. (PWHT) There are various reasonsfor this, and various methods of applying this post weld heat treatment. There are also certainrisks associated with PWHT. In this web page we will explore the use of PWHT.
Firstlywe will look at different types of heat treatment, and then we will correlatethem with the welding operation.
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Types of Heat Treatment
Twodifferent materials, when subjected to the same thermal cycle, can potentiallyhave significantly different results on the materials. Heat treatments aretherefore generally described in terms of the effect that they will have on thematerial, rather than the thermal cycle. There are however exceptions to thisrule.
Belowis a list of typical heat treatments:
- Annealing:This generally refers to the treatment that is required to get a material to asoft and unstressed condition. For most materials, such as carbon steels, thiswill mean taking the temperature up very high, and then cooling it down veryslowly to room temperature.
- QuenchAnneal: Here the intention is to again achieve a “soft” structure, but a slowcool would be detrimental to the material. Typical of this would be the 300series stainless steels. (Also called austenitic stainless steels.) Thesestainless steels do not undergo any significant phase transformations in thebulk of the material during the heat treatment cycle, but can result in the formationof locally detrimental phases or particles when it is kept in some intermediatetemperature ranges for extended time periods. To prevent this from happening,the material is cooled rapidly (quenched) from the high temperature. (Typically1050°C) This reduces the time that the material will remain in the temperaturerange of 500 – 850°C, where they can experience the formation of grain boundarycarbides which seriously reduces the corrosion resistance of the material. Thisdetrimental process is called sensitization.
- SolutionTreatment: The main intent of this heat treatment is to make sure that all thedifferent alloying elements are evenly dispersed throughout the material, and“dissolved” within the material as far as possible. This is often performed oncastings, because the solidification process during casting tends to result inthe material having relatively large differences in certain alloying elementsin different parts of the structure. There tends to be areas of highconcentration of certain elements and low concentrations of other elements. Byincreasing the temperature to a point where a lot of diffusion takes place,these uneven concentrations of alloying elements will even out. Certain phases(e.g. carbides) will also be “dissolved” (go into solution) by the material atthese high temperatures. To retain as much of the alloying elements insolution, some materials are typically quenched after the solution treatment.This is then very much the same as the quench anneal discussed above.
- QuenchHardening: To achieve high hardness in some material such as carbon steels, andlow alloy steels, the material can be heated to a temperature above which phasetransformations occur in the material. (Typically 950°C for carbon steels.) Thematerial is then rapidly cooled (quenched) to form some meta-stable phases (forinstance Martensite) that results in a high hardness level in the material.When materials are quench hardened, they are also typically brittle.
- Tempering:To soften a material that has already been hardened through a previous heatcycle (e.g. quench hardening) you can again increase the temperature of thematerial to a point below which it starts to experience bulk phasetransformations, (Typically heated to 650 – 700°C) and keeping it there for aperiod of time. During this tempering cycle, the hardened martensite turns to atempered martensite that is not as hard and brittle as the quenched martensite,but is still very strong and tough.
- Quenchand Temper: This is a combination of the two heat treatment cycles describedabove.
- StressRelieve: When performing plastic deformation of ductile metals, they will endup with a lot of residual stresses in the material. Welding also results inthese residual stresses around the weld. By increasing the temperature of themetal, the yield strength of the metal is reduced. (The yield strength is thestress at which the material starts to deform plastically.) When the yieldstrength is reduced, due to the high temperature, below the residual stress level,then the material will “relax”. This relieves the stresses that have beencaptured in the material from the deformation or welding activities. Carbonsteels are typically stress relieved at temperatures around 600°C. At thistemperature, the residual stress is typically reduced to about 30% of the yieldstrength of the material at room temperature. The main reason for stressrelieve treatment is that it improves the fracture toughness of the components.It also reduces the likelihood of certain corrosion mechanisms such as StressCorrosion Cracking. (SCC)
- Normalizing:This is normally performed on carbon steels to provide a stress free, finegrained structure. This is achieved by taking the temperature above thetemperature at which phase transformation occur in the bulk of the material, (Typicallytemperatures of around 950°C) and then allowing it to cool in still air. Byapplying this treatment to ordinary carbon / carbon manganese steels, they willdevelop a structure that is a very good compromise of strength and toughness.This is a very typical heat treatment applied to steel castings, forgings andother steels to achieve a fine grained structure.
- PrecipitationHardening: Some material will experience the formation of very small particleswithin their structure if their temperature is elevated and kept there for atime. These small particles are called precipitates. The presence of thesesmall particles act to strengthen the material. A typical precipitationhardening cycle is as follows: The temperature of the material is taken to apoint where a “solution treatment” will take place. Then the material is“quenched” to ensure that none of the precipitates form. Next, the temperatureis increased again (but to a temperature lower than the solution temperature)which then encourages the precipitates to form. This is called an “aging”treatment. It is important to note that if the temperature is too high, or iskept elevated too long during this part of the cycle, then the material will be“over aged”. An over aged material will have a lower hardness and strength thanone that was properly aged. By the same token, if the aging temperature or timeis too low, then the material will be “under aged” which again results in alower hardness and strength. Under aging is less of a problem, because you canjust increase the temperature again to get further aging, until the optimumresult is achieved. Over aging is a much bigger problem, because it can only beremedied by performing the entire cycle again. (From the solution treatment.)
- Thermo-MechanicallyControlled Process: (TMCP) This is not strictly speaking a heat treatment assuch, but I think this is a good place to discuss it shortly. In the TMCP, asteel is deformed (e.g. undergoes rolling) while simultaneously experiencingtemperatures that are not high enough to cause total phase transformations andrecrystallization, but not so low that it will result purely in cold forming.Every steel mill tends to have their own sequence of temperatures anddeformation to obtain a strong material that is easy to weld. TMCP steels tendto have low carbon equivalents, while still being very strong and tough.
Thereare some other more specialized heat treatments that are sometimes employedunder very special circumstances, but for our purposes, those listed abovewould be the main heat treatments to take note of.
The Effect of Welding on Materials
Fusionwelding is associated with temperatures high enough to melt the materials beingjoined. As such, they introduce a thermal cycle that will result in thematerials close to the weld being heated to temperatures close to the meltingpoint, and the materials far away from the weld seeing hardly any increase intemperature. In other words, there is a thermal gradient experienced by thematerials that span all the way from ambient to melting temperature.
Thepart of the base metal that has had its structure changed by the welding heatis known as the Heat Affected Zone (HAZ) of the weld. The material where thetemperature was not high enough to make any significant difference to thematerial is simply called the base material. The section that was melted due tothe welding operation is termed the weld metal.
Theweld metal structure is going to be a function of the base metal compositions,the filler metal composition and the effects caused by the thermal cycle. As ageneral rule we can choose the filler metal to give the desired results withinthe weld metal. We can however not do anything, in terms of the composition, tothe heat affected zone. (HAZ) The only effect we have in controlling thestructure of the HAZ is to control the thermal cycle.
Itshould also be obvious that the thermal cycle within the HAZ will havesignificant effects on the material’s heat treatment before the weldingoperation. As an example, some part of the HAZ of a carbon steel parent metalwill be raised to a temperature above which phase transformations occur in thesteel. (This is called the critical temperature, and for ordinary carbon steelsthis is around 720°C.)
Whenit is subsequently cooled down, there will again be phase transformations. Ifthe cooling rate is rapid enough, then we may experience some amount of quenchhardening in this region, resulting in a hard brittle structure. If the coolingis slow enough, then we will experience a thermal cycle similar to anormalizing heat treatment. If the cooling rate is very slow, then the thermalcycle will resemble that of the annealing cycle for the steel.
Thismeans that by changing the amount of energy used when welding, (also called theheat input) along with pre-heats and post-heats, different structures can beachieved in both the weld metal and the HAZ of the weld.
Somematerials, such as low alloy steels, almost always result in a quench hardenedstructure within the HAZ when welded. They then need a further Post Weld HeatTreatment (PWHT) to achieve the desired result. In the case of our example of alow alloy steel, they almost always need a tempering treatment to achieve asuitable strong and tough microstructure.
Somematerials achieve a significant amount of their strength from cold working.Cold working just refers to deforming the metal plastically at a temperaturebelow which the deformed grains of the metal will recrystallize. This treatmentresults in an increase in strength of the cold worked material.
Whenwelding a cold worked (also called work hardened) material, a section of theHAZ will experience temperatures high enough to cause recrystallization andphase changes. This will eliminate the cold work, and potentially reduce thestrength of the material in the HAZ significantly. Please note that no post weld heat treatment canreverse this effect.
Thissoftening is often experienced when welding work hardened aluminum alloys. TheHAZ will always be significantly weaker than the cold worked parent metal. Theonly way to effectively deal with this effect is to design the component insuch a way that the weld is placed in a region of lower stress, or the basemetal is made thicker than required around the weld area, to compensate for theloss of strength.
Solitaire collection crack. Theother way that aluminum alloys are typically strengthened, is throughprecipitation hardening. Again, the welding thermal cycle will introduce anarea into the HAZ that is unaged, (Temperature was high enough to returnprecipitates into solution, followed by a quench due to the high cooling ratestypically associated with welding.) and an area that is over aged. (Temperaturewas higher than required for optimum aging, but not high enough for returning precipitatesinto solution.) Thus welding of precipitation hardened (also called aged)materials will result in reduced strength of the HAZ. For small components, itmay be possible to subject the entire component to a precipitation hardeningcycle, but under most circumstances, there is usually nothing that can be doneto remedy this reduced strength in the HAZ of a precipitation hardenedmaterial.
Typical Post Weld Heat Treatments (PWHT)
Whileit is theoretically possible to perform any of the heat treatments describedpreviously as a post weld heat treatment, (PWHT) practically this is not thecase. The typical post weld heat treatments are as follows:
- StressRelieve: Welding introduces high residual stresses into metals in the regionaround the weld. This can result in reduced fracture toughness properties, andsusceptibility to corrosion mechanisms such as Stress Corrosion Cracking. (SCC)The stress relieve is probably the most often performed post weld heat treatment, particularly oncarbon steels and carbon manganese steels. Stress relieve is normally a coderequirement for pressure equipment above a certain thickness. (Each code hasdifferent rules for determining when a stress relieve is required.) For carbonsteels, the stress relieve is typically around 580°C – 650°C. Please note thata stress relieve treatment at the higher end of the range could reduce thestrength of a TMCP steel. When post weld heat treatment is required for TMCP steels, it shouldpreferably be done at the lower end of the range.
- Temper:For low alloy steels, and other materials that naturally harden when welded, atemper is almost always required. Tempering temperatures can vary widely, butfor low alloy steels is typically around 700°C – 750°C. (Some quenched andtempered micro alloyed steels can have significantly lower temperingtemperatures.) Please note that using a post weld heat treatment tempering temperature above thatof the base metal will weaken the base metal. Post weld heat treatment tempering is thereforerecommended to be around 30°C lower than the temper of the base metal.
- Normalizing:Most large and complex shaped structures cannot be subjected to a normalizingPWHT. The reason being that the material becomes so weak at the soaktemperature (Typically 950°C for carbon steels.) that it cannot support thestructure, and results in catastrophic buckling and distortion. A normalizing post weld heat treatmentis therefore only performed on rather simple components that can be easilysupported. A typical example may be a welded dished end for a pressure vessel,before it is welded onto the vessel itself.
Risks Associated With Post Weld Heat Treatment
PostWeld Heat Treatment (PWHT) is not without risk. Below is a list of potentialproblems that can be experienced when performing post weld heat treatment.
- Lossof strength: Excessive times, or too high a temperaturefor a stress relieve post weld heat treatment can result in a reduced strength of the material.Tempering treatments can also result in reduced strengths for quenched andtempered materials. Times and temperatures therefore need to be wellcontrolled.
- Distortion orcollapse: The stress relieve or tempering temperatures result in a materialthat is significantly lower strength while at the elevated temperatures. If astructural component is experiencing some load on it, then during the post weld heat treatmentcycle it could buckle or distort. This could have catastrophic and high costconsequences. It is therefore important to make sure that all structures beingexposed to high temperatures are properly supported. In addition, if onesection of the structure experiences significantly higher temperatures thanother sections, the difference in thermal expansions can also result in severedistortions of the structure. This situation is typically experienced whenshell and tube heat exchangers are subjected to a stress relieve post weld heat treatment where theshell temperature is increased and decreased in temperature much faster thanthe tubes. It is important to make sure that there are no excessive temperaturegradients or differences during a post weld heat treatment operation. To ensure this, there shouldbe enough thermocouples attached to the components to ensure that unacceptabletemperature gradients do not occur. In some furnaces, there could also bepotential flame impingement on some local spots on the vessel. This couldlocally increase the temperature to values above the phase transformationtemperature, (lower critical temperature) resulting in unexpected phase changesand also volume changes. These have the potential to not only reduce themechanical properties, but also to lead to distortion. To prevent this, thereshould be furnace loading sketches showing how flame impingement will beprevented, and there should also be thermocouples placed on the locationsclosest to the burners of gas or oil fired furnaces.
- Embrittlement or Cracking: Some heats ofmaterial may have trace elements in that make it susceptible to “temperembrittlement”. (Typically Chrome) During the post weld heat treatment operation some intermetallicphases can be formed that are very brittle, leading to a significant andpotentially catastrophic embrittlement of the component or structure. To ensurethat this will not happen, the materials should be purchased with testing onsamples that have been exposed to a simulated post weld heat treatment cycle. Some steel componentsmay also have some retained austenite in their structures, due to theirprevious heat treatments. Upon post weld heat treatment, this retained austenite could transform toa martensite like structure that is brittle. Codes will usually require thatfinal inspection and Non Destructive Testing (NDT) be performed after the post weld heat treatmentoperation, to detect any defects that have formed during the PWHT operation. Hardnesstesting after the post weld heat treatment is also good at identifying if any hardening hasoccurred during post weld heat treatment. (Please note that not all embrittling mechanisms are associated with higher hardness of the metal.)
Post Weld Heat Treatment Methods
Thereare 3 typical methods to apply the heat for post weld heat treatment. These methods are as follows:
- Furnace:In furnace post weld heat treatment, the entire component is typicallyplaced within a furnace and the temperature cycle applied to the wholecomponent. Obviously this means that the component or structure must not be solarge that it does not fit into a furnace. Also, the component must be moved towhere the furnace is located. For maintenance work, or for long unwieldy shapedcomponents, this is often impractical. The main advantage of this type of post weld heat treatmentis that variable expansion caused by excessive thermal gradients (differenttemperatures in different parts of the component) can be kept to a minimum. Todo this, the heating and cooling rates are important, as thicker sections willtend to take longer to heat or cool than thinner sections.
- Internal Firing:In this method, the component is insulated on the outside, and heat isintroduced to the inside of the component, until the entire component is heatedto the required temperature. The heat is typically introduced by gas firedburners. Obviously this is only suitable for “hollow” type components such aspressure vessels. It is also quite an expensive option, with risks associatedwith suitable insulation and flame impingement from the burners.
- Locally Applied External Heating: This method of post weld heat treatment issuited to elongated components that only need the heat applied at local areas,rather than the entire component. This is typical on circumferential welds onpiping, or closure welds on long pressure vessels. In this method, some kind ofelement introduces the heat into the outside of the component, (sometimes elementsare placed on the inside and outside to assist with even heating) usually in aband around the full circumference of the component. Suitable insulation keepsthe heat from being lost from the surface through convection and radiation.Heat is however lost through conduction to the unheated sections of thecomponent. It is therefore important to control the thermal gradients from thearea experiencing the PWHT to the areas that are still “cold”. If this is notdone carefully, the variable thermal expansion can actually introduce residualstresses in the areas adjacent to the area being post weld heat treated, whichcan just move the problem from one location to another.
Heat Sources for Post Weld Heat Treatment
Dependingon the method for applying the heat during post weld heat treatment, there are a number of different heatsources. The typical heat sources are:
- Gas or OilBurners: These are normally used in larger furnaces, or when performinginternal firing of a component. The main issue with this is that the burnerscould potentially impinge on the components, resulting in “hot spots” where thetemperature is too high compared with the rest of the component. The furnacelay-out must therefore be carefully planned to make sure that this does nothappen. As a general rule, additional thermocouples will be applied in theareas where flame impingement is possible during the post weld heat treatment.
- ElectricalResistance Heating Elements: These are often used for the local post weld heat treatment ofcomponents. They take the form of mats that have the electrical resistancewires “woven” through ceramic beads. These “heat beads” are then attached tothe surface to be subjected to post weld heat treatment. Often they are simply kept in placeby use of steel wire. When using these heating elements, the elements areplaced against the surface of the metal, and insulation is applied around theoutside, to keep the heat in.
- Induction Heating: In this heat source, anelement (often just a cable wrapped around the pipe) has a high frequencyalternating electrical current passing through it. This alternating currentresults in a magnetic field around the element. The magnetic field in turnresults in eddy currents in the metal. The eddy currents then result inresistive heating in the metal. In essence, the metal being subjected to post weld heattreatment is actually the heating element. (This works in much the same way asinduction heating hot plates for preparing food.) This heat source has a numberof advantages. The first is that it can normally result in much faster heatingthan the use of burners or electrical resistance heaters. The second advantageis that the heating coils can be applied around the outside of the insulationthat is placed against the metal to keep the heat in. This means that theelements are not exposed to the high temperatures, so the system is more robust.The third advantage is that this system is generally more energy efficient thanthe electrical resistance heating method. There are also some disadvantages.The first is that the high magnetic fields can mess around with someinstrumentation, and there is also some concern regarding the long term safetyaspects. If you have a heart pacemaker, you would likely not want to hangaround this type of equipment when it is operating. The other disadvantage isthat if something goes wrong with the control, then it would be much easier forthe material to be severely overheated before the operator realizes that thesystem is out of control. This can be mitigated by ensuring that there areenough “spare” thermocouples, as the problem mostly arises when control thermocouplesget dislodged from the component.
Code Requirements for Post Weld Heat Treatment
Asa general rule, the fabrication codes dictate when post weld heat treatment is required, and theparticular post weld heat treatment cycle. They also generally state the requirements in terms ofthe methods that can be used, and the maximum heating and cooling rates and themaximum thermal gradients that are allowable. Examples of such codes would be ASMEVIII, BS 5500 or AS 4458 for pressure vessels and ASME B31.3 or AS 4458 forpiping.
Thereare also other guideline and recommended practice documents that assist inassuring that the post weld heat treatment is performed correctly. A widely used document in thisregard is AWS D1.10, which is a recommended practice for local post weld heat treatment of piping.
Post Weld Heat Treatment Conclusion
PostWeld Heat Treatment (PWHT) is a specialist area in itself, with manytechnicians specializing in operating post weld heat treatment equipment and furnaces. If you areinterested in getting into this kind of work, then get hold of a company inyour area that does this type of work and see if they can offer you a job. Mostof the time, these companies will also provide you with the necessary trainingto operate their specific equipment.
Anothertechnical issue revolves around comparing different PWHT cycles. As this is abit of a technically detailed subject, I have addressed it in a web page of itsown. Click here to see how differentheat treatment cycles can be compared.
When Pwht Is Required For Carbon Steel Welding
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When Pwht Is Required For Carbon Steel Conductivity
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