Job Knowledge 114 to 116 Heat Treatment

8/2/2019 Job Knowledge 114 to 116 Heat Treatment

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Job Knowledge 114: Heat treatment of weldedjoints

Heat treatment is an operation that is both time consuming and costly. It can affect thestrength and toughness of a welded joint, its corrosion resistance and the level of residualstress but is also a mandatory operation specified in many application codes andstandards. In addition it is an essential variable in welding procedure qualificationspecifications.

Before discussing the range of heat treatments that a metal may be subjected to, there isa need to clearly define what is meant by the various terms used to describe the range ofheat treatments that may be applied to a welded joint. Such terms are often usedincorrectly, particularly by non-specialists; for a metallurgist they have very precisemeanings!

Solution treatment

Carried out at a high temperature and designed to take into solution elements andcompounds which are then retained in solution by cooling rapidly from the solutiontreatment temperature. This may be done to reduce the strength of the joint or to improveits corrosion resistance. With certain alloys it may be followed by a lower temperature heattreatment to reform the precipitates in a controlled manner age or precipitationhardening.

Annealing

This consists of heating a metal to a high temperature, where recrystallisation and/or aphase transformation take place, and then cooling slowly, often in the heat treatmentfurnace. This is often carried out to soften the metal after it has been hardened forexample by cold working; a full anneal giving the very softest of microstructures. It alsoresults in a reduction in both the yield and the tensile strength and, in the case of ferriticsteels, usually a reduction in toughness.

Normalising

This is a heat treatment that is carried out only on ferritic steels. It comprises heating thesteel to some 30-50OC above the upper transformation temperature (for a 0.20% carbonsteel this would be around 910OC) and cooling in still air. This results in a reduction ingrain size and improvements in both strength and toughness.

Quenching


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This comprises a rapid cool from a high temperature. A ferritic steel would be heated toabove the upper transformation temperature and quenched in water, oil or air blast toproduce a very high strength, fine grained martensite. Steels are never used in thequenched condition, they are always tempered following the quenching operation.

Tempering

A heat treatment carried out on ferritic steels at a relatively low temperature, below thelower transformation temperature; in a conventional structural carbon steel this would bein the region of 600-650OC. It reduces hardness, lowers the tensile strength and improvesductility and toughness. Most normalised steels are tempered before welding, all quenchedsteels are used in the quenched and tempered condition.

Ageing or Precipitation hardening

A low temperature heat treatment designed to produce the correct size and distribution ofprecipitates, thereby increasing the yield and tensile strength. It is generally preceded by asolution heat treatment. For steel the temperature may be somewhere between 450-740OC, an aluminium alloy would be aged at between100-200OC. Longer times and/or

higher temperatures result in an increase in size of the precipitate and a reduction in bothhardness and strength.

Stress relief

As the name suggests, this is a heat treatment designed to reduce the residual stressesproduced by weld shrinkage. It relies upon the fact that, as the temperature of the metalis raised, the yield strength decreases, allowing the residual stresses to be redistributed bycreep of the weld and parent metal. Cooling from the stress relief temperature is controlledin order that no harmful thermal gradients can occur.

Post heat

A low temperature heat treatment carried out immediately on completion of welding byincreasing the preheat by some 100OC and maintaining this temperature for 3 or 4 hours.This assists the diffusion of any hydrogen in the weld or heat affected zones out of thejoint and reduces the risk of hydrogen induced cold cracking. It is used only on ferriticsteels where hydrogen cold cracking is a major concern ie very crack sensitive steels, verythick joints etc.

Post Weld Heat Treatment (PWHT)

So what does the term post weld heat treatment mean? To some engineers it is a rathervague term that is used to describe any heat treatment that is carried out when welding iscomplete. To others however, particularly those working in accordance with the pressure

vessel codes such as BS PD 5500, EN 13445 or ASME VIII, it has a very precise meaning.When an engineer talks of post weld heat treatment, annealing, tempering or stress reliefit is therefore advisable to ensure that their heat treatment is the same as the one youhave in mind!

Heat treatment following welding may be carried out for one or more of three fundamentalreasons:

to achieve dimensional stability in order to maintain tolerances during machiningoperations or during shake-down in service

to produce specific metallurgical structures in order to achieve the requiredmechanical properties

to reduce the risk of in-service problems such as stress corrosion or brittle fracture

by reducing the residual stress in the welded component


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The range of heat treatments to achieve one or more of these three objectives in therange of ferrous and non-ferrous metals and alloys that may be welded is obviously far tooextensive to cover in great detail within these brief Job Knowledge articles. The emphasisin the following section will be on the PWHT of carbon and low alloy steels as required by

the application standards although brief mention will be made of other forms of heattreatment that the welding engineer may encounter in the ferrous alloys. There are twobasic mechanisms that are involved, firstly stress relief and secondly microstructuralmodifications or tempering.

Stress Relief

Why is it necessary to perform stress relief? It is an expensive operation requiring part orall of the welded item to be heated to a high temperature and it may cause undesirablemetallurgical changes in some alloys. As mentioned above there may be one or morereasons. The high residual stresses locked into a welded joint may cause deformationoutside acceptable dimensions to occur when the item is machined or when it entersservice. High residual stresses in carbon and low alloy steels can increase the risk of brittle

fracture by providing a driving force for crack propagation. Residual stresses will causestress corrosion cracking to occur in the correct environment eg carbon and low alloysteels in caustic service or stainless steel exposed to chlorides.

What causes these high residual stresses? Welding involves the deposition of molten metalbetween two essentially cold parent metal faces. As the joint cools the weld metalcontracts but is restrained by the cold metal on either side; the residual stress in the jointtherefore increases as the temperature falls. When the stress has reached a sufficientlyhigh value (the yield point or proof strength at that temperature) the metal plasticallydeforms by means of a creep mechanism so that the stress in the joint matches the yieldstrength. As the temperature continues to fall the yield strength increases, impedingdeformation, so that at ambient temperature the residual stress is often equal to the proofstrength( Fig 1.).

To reduce this high level of residual stress, the component is reheated to a sufficiently hightemperature. As the temperature is increased the proof strength falls, allowingdeformation to occur and residual stress to decrease until an acceptable level is reached.The component would be held at this temperature (soaked) for a period of time until astable condition is reached and then cooled back to room temperature. The residual stressremaining in the joint is equal to the proof strength at the soak temperature.

From Fig 1 it can be seen that residual stress in a carbon manganese steel falls reasonablysteadily from ambient to around 600oC but that the high strength creep resistant steelsneed to be above 400oC before the residual stress begins to fall. Stainless steel is hardlyaffected until the temperature exceeds 500oC. There is therefore a range of soaktemperatures for the various alloys to achieve an acceptable reduction in residual stresswithout adversely affecting the mechanical properties of the joint. In carbon manganese

steels this temperature will be between 550-620oC, in creep resistant steels somewherebetween 650-750oC and for stainless steels between 800-850oC.

The next article will cover tempering of ferritic steels and will be followed by furtherinformation on other alloys and methods of applying and controlling heat treatmentactivities.

This article was written by Gene Mathers.


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Job Knowledge 115: Heat treatment of weldedjoints - Part 2

Part 1 of this series of articles gave definitions of some of the heat treatments that may be

applied to a welded joint and dealt with the operation of stress relieving a ferritic steelassembly. The temperature range within which stress relief takes place will also causetempering of those regions in the HAZs where hard structures may have formed.

TemperingTempering is a heat treatment that is only relevant to steels and is carried out to softenany hard micro-structures that may have formed during previous heat treatments,improving ductility and toughness. Tempering also enables precipitates to form and for thesize of these to be controlled to provide the required mechanical properties. This isparticularly important for the creep resistant chromium-molybdenum steels. Temperingcomprises heating the steel to a temperature below the lower critical temperature; thistemperature being affected by any alloying elements that have been added to the steel sothat for a carbon-manganese steel the temperature is around 650OC, for a 2CrMo steel,760OC . Quenched steels are always tempered. Normalised steels are also usually supplied

in the tempered condition although occasionally low carbon carbon-manganese steel maybe welded in the normalised condition only, the tempering being achieved during PWHT.Annealed steels are not supplied in the tempered condition.

Tempering of tool steels may be performed at temperatures as low as 150OC but with theconstructional steels that are the concern of the welding engineer the temperingtemperature is generally somewhere between 550- 760OC, depending on the compositionof the steel.

Post Weld Heat Treatment (PWHT).As mentioned in Part 1, PWHT is a specific term that encompasses both stress relief andtempering and is not to be confused with heat treatments after welding. Such treatmentsmay comprise ageing of aluminium alloys, solution treatment of austenitic stainless steel,

hydrogen release etc. PWHT is a mandatory requirement in many codes and specificationswhen certain criteria are met. It reduces the risk of brittle fracture by reducing the residualstress and improving toughness and reduces the risk of stress corrosion cracking. It has,however, little beneficial effect on fatigue performance unless the stresses are mostlycompressive.

It is an essential variable in all of the welding procedure qualification specifications such asISO 15614 Part 1 and ASME IX. Addition or deletion of PWHT or heat treatment outside thequalified time and/or temperature ranges require a requalification of the weldingprocedures. PWHT temperatures for welds made in accordance with the requirements ofEN 13445, ASME VIII and BS PD 5500 are given below in Table 1.

Table 1: PWHT Temperatures from Pressure

Vessel Specifications

Steel

Grade

BS EN

13445ASME VIII

BS PD

5500

Temp

range

0C

Normal

holding temp

0

C

Temp

range

0

C
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furnace that uses a fixed hearth and a removable cover. Typically, a furnace capable ofheat treating a 150tonne pressure vessel would have dimensions of around 20m long, adoor 5x5m and would consume around 900cu/metres of gas per hour.

Furnaces can be heated using electricity, either resistance or induction heating, natural gasor oil. If using fossil fuels care should be taken to ensure that the fuel does not contain

elements such as sulphur that may cause cracking problems with some alloys, particularlyif these are austenitic steels or are nickel based corrosion resistant cladding for example.Whichever fuel is used the furnace atmosphere should be closely controlled such that thereis not excessive oxidation and scaling or carburisation due to unburnt carbon in thefurnace atmosphere. If the furnace is gas or oil fired the flame must not be allowed totouch the component or the temperature monitoring thermocouples; this will result ineither local overheating or a failure to reach PWHT temperature.

Monitoring the temperature of the component during PWHT is essential. Most modernfurnaces use zone control with thermocouples measuring and controlling the temperatureof regions within the furnace, control being exercised automatically via computer software.Zone control is particularly useful to control the heating rates when PWHTing a componentwith different thicknesses of steel. It is not, however, recommended to use monitoring ofthe furnace temperature as proving the correct temperatures have been achieved in thecomponent. Thermocouples are therefore generally attached to the surface of thecomponent at specified intervals and it is these that are used to control the heating andcooling rates and the soak temperature automatically so that a uniform temperature isreached. There are no hard and fast rules concerning the number and disposition ofthermocouples each item needs to be separately assessed.

As mentioned earlier, the yield strength reduces as the temperature rises and thecomponent may be unable to support its own weight at the PWHT temperature. Excessivedistortion is therefore a real possibility. It is essential that the component is adequatelysupported during heat treatment and trestles shaped to fit the component should beplaced at regular intervals. The spacing of these will depend on the shape, diameter andthickness of the item. Internal supports may be required inside a cylinder such as apressure vessel if so, the supports should be of a similar material so that the coefficients

of thermal expansion are matched.

Whilst heat treating a pressure vessel in one operation in a furnace large enough toaccommodate the entire vessel is the preferred method this is not always possible. In thiscase the pressure vessel application codes permit a completed vessel to be heat treated insections in the furnace. It is necessary to overlap the heated regions the width of theoverlap is generally related to the vessel thickness. BS EN 13445 for instance specifies anoverlap of 5Re where R = inside diameter and e = thickness; ASME VIII specifies anoverlap of 1.5 metres. It should be remembered that if this is done there will be a regionin the vessel (which may contain welds) that will have experienced two cycles of PWHT andthis needs to be taken into account in welding procedure qualification testing. There is alsoan area of concern, this being the region between the heated area within the furnace andthe cold section outside the furnace. The temperature gradient must be controlled by

adequately lagging the vessel with thermally insulating blankets and the requirements aregiven in the application codes.

It is, of course, possible to assemble and PWHT a vessel in sections and then to carry out alocal PWHT on the final closure seam. Local PWHT will be discussed in the next part of thisseries on heat treatment.


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Job Knowledge 116: Heat Treatment Part 3

When it is not possible to place the entire component in a furnace for heat treatment(because of the size of the fabrication, circumferential welds in a pipework system or wheninstalling equipment on site for example) then a local PWHT may be the only option. Local

PWHT needs careful planning to ensure that heating and cooling rates are controlled andthat an even and correct temperature is achieved. Uneven and/or rapid heating can giverise to harmful temperature gradients producing thermally induced stresses that exceedthe yield stress. This may result in the development of new residual stresses when thecomponent is cooled.

Local PWHT may be carried out using high velocity gas burners, infra red burners,induction heating and high or low resistance heating elements. Electrical equipment ismore easily installed and controlled than heating using natural gas or propane, particularlyon site. High voltage resistance heating is rarely used on site due to the need for theradiant heaters to be positioned a set distance from the surface and, more significantlyperhaps, the health and safety risks involved with the use of high voltage current. Lowvoltage electrical resistance heating and induction heating are the two most commonlyused methods.High velocity gas burners are more advantageous when large areas need to be heattreated, particularly if, for example, firing can take place within a pressure vessel whichthen becomes its own furnace. For local PWHT of vessel circumferential seams internalinsulating barriers can be used to localise the heat source. Motorised valves and micro-processor control of the combustion conditions enabled precise management of the heatingcycle to be achieved.

Low voltage electrical resistance heating uses flexible ceramic heating elements,colloquially known as corsets, an appropriate number being assembled to cover the area tobe heat treated. Induction heating uses insulated cables that can be wrapped around thejoint or shaped to fit the area to be heated or specially designed fitting for repetitive PWHToperations as illustrated in Fig 1. To perform the PWHT, temperature controlthermocouples are firstly attached, often by capacitor discharge welding, the elements

placed in position and the area then lagged with thermal insulating blankets to reduce heatloss and to maintain an acceptable temperature gradient.

There are no standard terms used to describe the various regions within the locallyPWHTd area. In this article the terms soak band, heated band, gradient control band,temperature gradient, which may be axial and through thickness, and control zone assuggested by the ASME will be used (see Fig 2).

The soak band is the area that is heated to within the specified PWHT temperature andtime range. It comprises the weld, the two HAZs and part of the surrounding parent metal.The heated band is the area covered by the heating elements, the temperature at the edgeof the heated band generally being required to be at least half that of the soaktemperature. The temperature gradient control band is the region where thermal

insulation, perhaps supplemented by additional heating elements, is applied to ensure thatan acceptable axial temperature gradient is achieved from PWHT temperature to ambient.A control zone is the region where a number of heating elements are grouped together andcontrolled by a single thermocouple, enabling different regions to be heatedindependently; particularly useful with large diameter items or where there are variationsin thickness.

Temperature gradients may be axial (along the length of a pipe or vessel) and throughthickness. The through thickness temperature gradient is caused by heat losses from theinternal surface and is a function of both thickness and internal diameter, the larger thediameter, the greater the effect of radiation and convection losses. Both the width of thesoak band and the temperature achieved can be substantially less than that on the outsideof the pipe or tube. Insulation on the inner surface will reduce the temperature/width

differential but may not be possible on small diameter tubes or pipework systems. Thisthrough thickness gradient is one of the reasons that specifications and codes require thesoak or heated band to be a minimum width, generally related in some way to the


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thickness of the component.As mentioned above, there are rules in the application codes concerning the size of theheated area, normally related to the thickness. In a circular component such as a pipe buttweld or a pressure vessel circumferential seam the width of the band is easy to calculate.ASME VIII for instance requires the soak band width to be twice the thickness of the weldor 50.8mm either side of the weld, whichever is the lesser;

ASME B31.3 requires the soak band width to be the weld width plus 25.4mm either side ofthe weld. BS EN 13445 does not specify a soak band width but instead specifies a heatedband width of 5Rt centred on the weld and where R = component inside radius and t =component thickness. There are no requirements in the ASME codes regarding heatedband width. A very approximate rule of thumb for flat plate is that the heated band shouldbe a minimum of twice the length of the weld although practical considerations mayprevent achieving this ideal.

There are no requirements, in any code or specification, on the width of the thermallyinsulated band although BS EN 13445 recommends 10Re. It is essential that the relevantspecification is referred to for specific guidance on what is required and it is worthremembering that the specification requirements on soak or heated band widths areminima and very little is lost by ensuring the specified dimensions are comfortablyexceeded.

What is an acceptable axial temperature gradient? Again, there is little advice in the codesand specifications. It is generally assumed that if the temperature at the edge of theheated band is above half that of the soak temperature then the temperature gradient willnot be harmful. During heating and cooling BS EN 13445 specifies a maximumtemperature difference of 1500C in 4500mm below 4500C (10C in 3mm) and 1000C in4500mm above 4500C (1OC in 4.5mm).

To ensure that gradients and temperatures are controlled within acceptable limits sufficientthermocouples need to be attached to provide both temperature control and recording. Forsmall diameter tubes, eg less than 100mm diameter, one control zone and one recordingthermocouple are regarded as sufficient; between 100-200mm one control zone and onerecording thermocouple at each of the 12 oclock and 6 oclock positions; above 250mmdiameter one control zone and one recording thermocouple at each 900 quadrant, 12, 3, 6and 9 oclock, are suggested.

These thermocouples should be placed on the centre line of the weld. Thermocouples willalso be needed at the edge of the soak band and the edge of the heated band. Ideally,thermocouples should also be placed on the opposite surface to the heating elements toensure that the correct through thickness temperature has been achieved although this israrely possible on pipe systems. It is advisable to double up on the thermocouples to copewith the possibility of a thermocouple failure.

Thermocouples use a hot and a cold junction to measure the temperature, the hot junctionbeing attached to the component, the cold junction within the temperature recorder. Foraccurate temperature measurement the hot junction must obviously be at the temperatureof the component. Errors can be introduced if the junction is not firmly attached, either bycapacitor discharge (CD)welding, by mechanically fixing the wires to the component or by overheating of thethermocouple junction.

CD welding of the thermocouple wires gives the most accurate results, particularly if thetwo wires are separated by 3-4mm. Mechanically attached wires will probably need to beinsulated by covering the junction with heat resistant putty to prevent overheating of thethermocouple by the overlying heater. If the wire covering is stripped back then the barewires also need to be insulated. It is advisable to specify the positions of thethermocouples on a drawing and to include these within a formal written heat treatmentprocedure document that covers both the specification and best practice requirements.


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Fig 2 Schematic of Temperature bands within a local PWHT.(Reproduced with permission of the American Welding Society (AWS), Miami, Florida,USA.)

Fig 1. Induction PWHT of Pipework


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Fig 2 Schematic of Temperature bands within a local PWHT (Reproduced with permission ofthe American Welding Society (AWS), Miami, Florida, USA)

Job Knowledge 114 to 116 Heat Treatment

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