Heat treatments of pressure vessels and machines for the chemical, energy production and process engineering sectors

Heat treatment of heat exchangers

The heat treatment of heat exchangers is definitely the activity in which Trater has developed the most experience, and is able today to support the Customer in this complex process. The treatment of heat exchangers is complex due to the combination of metallurgical and thermodynamical problems and in general for the geometrical complexity of the items. Furthermore, the use of more and more performing materials, but difficult to be treated and with specific corrosion resistance, mechanical and creep characteristics, makes it necessary to face the heating of exchangers paying the highest attention and commitment, managing the work just after an in-depth engineering and metallurgical study.

A badly performed treatment, applying wrong methods and without an efficient temperature control on the entire item, can bring to macroscopic geometrical deformations (bending of the tubes, deformation of the sheets, screwing and flexion of the tube), breaks of the of the tubes-to-sheet/s welds, and even though not immediately detectable, it can generate a tensile stress state in correspondence of specific geometries of the items (tubesheets, welds between tubes and tubesheets, sections of the tubes) with a sufficient high value to expose the item to phenomena of stress corrosion cracking and hydrogen embrittlement, in particular operative conditions. Insufficient temperatures can bring to an incomplete tempering of the welds and the insufficient relieving of the residual stress state during the welding processes, calandering or mechanical preliminar workmachining, making the joints weak, reducing the resistance to creep of the material and exposing the item to the previously mentioned phenomena of stress corrosion cracking. Furthermore, the consequences of oxidative processes on the surface of the tubes (internal and external) are not to be underestimated due to the influence in operation of the exchanger (pollution of the process products and reduction of the thermal exchange) and due to the generation of exothermal heat, proper of oxidation phenomena.

In many cases, it is necessary to treat the entire tube bundle of an exchanger with “U” tubes or a floating tubesheet. The problems are the same of the ones described for the fixed-tube sheet heat exchangers. In relation to the dimensions of the bundle, the number of tubes, their diameter, thickness, geometry and number of diaphragm present, the most external tubes of the item heat up and cool down with higher speed than the internal ones causing, due to temperature difference and consequent dilatation difference, deformations with remarkable entity, and new residual stress states, due to generated constraints with the baffles and tubesheets. The geometrical distortions can make impossible the insertion of the bundle into the shell and reduce the thermal efficiency of the heat exchanger, while the residual stresses can expose the item to serious damages during operation.
During the treatment, the shell of the exchangers heats up and cools down easily because it is touched by the turbulent gases generated by the burners, while the tube bundle, closed into the shell (in a still environment, without direct heat supply, if not per natural convection which determinates a stratification of the air), heats up and cools down much more slowly, generating big differences in the temperature of the item. In particular, the natural elongation of the shell during the heating phase, since it is not together with the tubes, can cause the deformation of the tube-sheets, the collapse of the tube-to-sheet welds and/or the stretching of the tubes; the local upsettings of the material will surely correspond to tensile stressed zones after the heat treatment. In relation to the mass and the geometry of the tubes, during the cooling instead the tube bundle of the item cools down with lower rates than the shell and compressive efforts beyond the elastic limit of the material (in temperature) can occur with upsettings, deformations and generation of residual tensile stress states, that can expose the item in operation, as already stated, to the known problems of stress-corrosion, fatigue and hydrogen embrittlement. Furhtermore, in the field of the complex geometry of a fixed tube-sheet exchanger, the missing achievement of the correct soaking temperature on the entire item will generate zones, in which there is no stress relieving and tempering of welds and the high hardnesses, acting as points of concentration of the efforts, that can generate cracks in operation. Various studies have demonstrated that hardness values of the weld and the heat-affected zone of the welds under 200HB (for carbon steels) and under 240HB (for low-alloyed or alloyed steels) are optimal for the operation of these items even when subject to corrosive attack, as indicated by the NACE directives.
In the Nineties Trater has developed technologies for the treatment of heat exchangers previously described able to reduce and, in most cases, cancel all of the indicated problems. Thanks to the initial intuition of forcing hot gases with controlled volume and temperature inside the tubes with the aim to uniform the temperature of the items, Trater has developed more sophisticated techniques for heating up and cooling down the items to treat the geometrically more complex exchangers and tube bundles, in carbon steel, low-alloy and alloy steel, alloys with high nickel, titanium, copper content and with particular oxidation problems. Trater has furthermore developed really valid and efficient methods to control temperature in all the most critical parts of the items, using in some cases up to 140 thermocouples, applied into direct contact with the item; through an especially created software, able to calculate in real time the average temperatures and the deviations between the various parts of the exchanger, and a 24-hours a day monitoring through our highly specialized personnel, Trater is able to ensure the best treatment result. Over the years, in Italy and abroad, Trater earned the title of strategic company, supporting the most important industrial realities in Italy and in Europe, specialized in the construction of heat exchangers, boosting the acquisition of important jobs in an industry sector that does not know crisis and has, therefore, always been a driver of the economy, which is evolving at the same level of technologies for the energy recovery.

In Trater the suitable soaking temperatures are adopted for each type of material, in order to obtain its best stress relieving avoiding the reduction of its mechanical characteristics. As already known, indeed, in all materials, when temperature increases, the phenomena of atomical diffusion due to recrystallization, and the carbides precipitation are very accelerated and, in general, involve microstructural variations that can change the behaviour during service of the material.

For example:

  • For manganese carbon steels (C max 0,24% – Mn 1,7% max) a stress relieving and PWHT temperature between 550°C and 630°C is applicable, obtaining a good level of stress relieving and tempering of the instable structures of the heat affected zone (above all for joints with high thickness) due to transformation or carbide’s precipitation, with consequent increase of ductility and tenacity characteristics of the material and a reduction in hardness. Temperatures higher than 650°C are damaging and too long soaking times cause an excessive expansion of the carbides, at the edge of the grain.
  • Microalloyed steels (provided with treatment of controlled rolling, normalizing or thermomechanics) present more complex metallurgical structures and fear the decrease of mechanical characteristics in the HAZ and base material, caused by the interaction with the supply treatment of the material.
  • The quenched and tempered steels instead, can be subject to risks of evident loss of ductility or even of formation of defects, although the treatment is necessary, if at completion of the welding operations structures out of balance are still present.
  • The nickel steels to be used at low temperature have a really variable behaviour in function of the chemical composition, of the manufacturing treatment and of the coupling with the filler metal; the stress relieving temperature must be object of an accurate evaluation.
  • It is therefore evident that the execution of the heat treatment must be object of accurate evaluation and where applied executed with particular attention and only with the certain data for the treatment parameters in relation to soaking times and temperatures, heating and cooling rates.
  • All cycles adopted in Trater are in any case agreed with the Customers and in compliance with manufacturing Codes of the vessels, and with the welding processes qualification.
Execution methods of treatments
Each thermal cycle is composed of three distinct stages: first heating stage; a successive soaking stage for a more or less time; a cooling stage. In each of these stages, situations able to ruin the result of the treatment or the integrity of the processed item being can occur.
During heating, the rates of temperature increase are established in relation to the type of material, the geometry of the item, its thickness and the characteristics of the plant used for the treatment. A valid reference, when applicable, is made by the manufacturing Standards of the pressure vessels, which indicate the heating methods, taking as a reference the maximum thicknesses of the items. In this stage, differences in temperature are easily possible due to a too rapid cooling of the item, to its geometrical complexity or to not suitable thermal exchanges between the environment of the oven and the item under treatment. When the differences are higher than certain values, in relation to the local geometry of the item, plasticization states can be generated in localized areas of the material due to the overcoming of its yield strength value in temperature, with an increase of the stress state of the item and with the risk of cracks and/or permanent deformations of the item. Differences in temperature, even if of limited, amount during treatments with metallurgical transformations, can furthermore cause an increase in the size of the material’s grain or other undesired phenomena, due to the protracting of time of the subsequent soaking phase, necessary to completely uniform the temperature on the item. To avoid the mentioned problems, during this phase, it is necessary to minimize the differences in temperature starting from the beginning of the cycle, by applying appropriate heating rates in relation to the thicknesses, the geometry of the item and the characteristics of the heating plant. Ovens that provide the maximum uniformity guarantees must be used and that cause minimum differences of temperature on the item and temperature measurement thermocouples must be correctly positioned on the items, and hot insertion of the items into the oven must be avoided compatibly with the characteristics of the material.
As previously seen, most part of the physical and metallurgical type of processes happen at soaking temperatures which determinate the final effect of the treatment. During PWHT and stress relieving treatments there is the tempering of the martensitic structures occurred during the welding, the flowing and annihilation of dislocations. As for the heating phase, also during the soaking time, situations can occur that cause the damage of the treatment’s effect or the integrity of the item. In Trater, the soaking stage of the thermal cycle is performed from the start in the correct range of temperature (established in relationship with the type of material and the type of treatment), so that the changes happen in the entire item simultaneously and as per the heating phase, the differences in temperature do not cause local plasticization situations of the material, with the consequent generation of new residual stresses, distortions or breakages.
The cooling parameters are established in relationship with the metallurgical characteristics of the material and they are influenced by the geometry of the item, by its thickness and by the performances of the plant for the treatment, including in this case also the cooling means. Slow coolings and particularly controlled are generally necessary in the stress relieving treatments. As for the previous treatment stages, it is important that, independently from the cooling speed, the maximum uniformity is always researched on the item, also through its thickness, to avoid distortions and the generation of local plasticization states, with the generation of new residual stresses. In case of stress relieving treatments, in Trater, the unloading of the item from the oven at high temperature is avoided; studies conducted by us on geometrically very complex items did not show the emergence of residual effort states if extracted from the oven at a temperature lower than 150°C.
As previously stated several times, the geometry of the item has a decisive influence on the execution method of an heat treatment. The items composed by courses open on the sides, with constant thickness or with a few differences, or cylindrical items, closed by heads with low thickness are to be considered simple. Cylindrical items with high thicknesses (for the temperature differences between the internal and the external walls), closed by heads, welded structures with parts with high thickness differences, cylindrical items with nozzles and flanges, tube bundles of exchangers, heat exchangers with tubes welded on the sheets are instead to be considered as complex geometries. A structure that can be defined “complex” in Trater is always subject of an accurate study by our Technical Office, and the maximum care is applied during the execution of the work.

Even the dimensions and the weight of the items influence in a significant way the execution and the result of an heat treatment:

  • items with high weight are difficult to be heated up and above all to be cooled down.
  • items with big dimensions need big treatment plants, opportunely dimensioned so that the item is correctly positioned inside them.

Trater, provided with big plants and suitable heating and cooling equipment can treat pressure vessels with dimensions up to 30 meters of length and weights up to 300 tons.

The positioning of the item into the oven is also very important to obtain the results from the treatment and avoid damages to the item. When oven with direct flame burners are used, in Trater, the item or the items are position into the oven, far from heat sources to avoid direct contact with the flames that cause very dangerous overheatings, with damage and local plasticization of the material. In case of insertion into the oven of more items, the load is normally performed so that in all cases the best circulation of hot gases on the surface of the items is ensured, and therefore the best temperature uniformity.
The supporting of the item is subject to a deep study by our Technical Office to avoid deformations and simplify its heating. An adequate number of supports are always adopted to avoid distortions due to the supporting and the free expansion of the item under the effect of heat is ensured. The item, in the designing phase, must furthermore be sufficiently stiffened to avoid that at high temperature structural collapses occur due to the own weight of the structure. To this purpose, our technicians are at disposal of our Customers to study the best solutions of stiffening that can be adopted on the items. At the end of the treatment deformations can in any case be detected on the items to be attributed to: in the order of mm or even cm, for items subject to rapid coolings (forced air, water, etc., typical of solution annealing or normalizing treatments); in the order of tenths of a millimetre and of mm, for the relieving of residual stresses in items particularly stressed during the manufacturing phase. Even in this case we can be of help to the Customer to evaluate, if possible, the deformations that can appear on the items.

The most valid method to control and measure temperature during heat treatments performed in the oven is based on the use of thermocouples. Other systems, surely valid for other processes, can easily introduce detection errors of sufficiently high entity to undermine the result of the same treatment. For example, it is scarcely indicated, the measurement through infra-red measurement devices due to problems connected with the reflectivity of the treated materials, depending on the surface conditions of the item, changeable during the thermal cycle.

Various kinds of thermocouples can be used that differ one another for their geometry, temperature of use, and method of fixing to the items. The choice of the correct typology according to the kind of heat treatment that will be executed, the correct application of the sensors, the quality of the material used and the quality of all the elements in the measurement chain are fundamental to avoid false signals that would bring to heat treatment execution errors, undermining the quality and the life time of the components undergoing treatment.

For further details please refer to the specific paragraph in the section “Engineering Company”.

Heat treatment of pressure vessels for the energy and the mining industry.

During the execution of a weld, the constraint conditions and localized heating on the welded joint cause, due to thermal dilatation of the metal and due to the decrease of resistance of the same metal at high temperature, the start of complex stress states inside the welded joint. This particular stress state can reach extremely high levels, even close to the limit of yielding strength of the material and must opportunely be evaluated in relation to the operating conditions of the vessel. In all these conditions or when the presence of shrinkage stresses can ruin the behaviour of the welded metal, a solution can be the execution of post-welding heat-treatment (PWHT) aiming to a decrease, or even better eliminate existing stress state. Through this treatment, following will be obtained;
  • tempering of the zones of welds with major hardness with microstructural transformations, improving the tenacity and ductility of the material;
  • a decrease, through diffusion, of the hydrogen content
  • a reduction of the residual stresses, that as previously stated, should be estimated and subtracted from the admissible load of the joint.
The elimination of internal residual stresses is furthermore necessary to limit the phenomenon of stress corrosion cracking during exercise of the items. Generally, the relieving of stresses happens through the lowering of the mechanical resistance of the joint, as a whole, obtained by a generalized heating of the component in the oven. Heating methods localized to the welds must instead be carefully studied to avoid that, in the impossibility of having a free dilatation of the sections of the item that undergo the treatment, constrained by cold parts, new residual stresses are generated due to local plastic deformation of the material. Under the effect of heat, the stresses are relieved thanks to phenomena of plastic deformation, and their final value is equal to yield strength of the material at the temperature reached during the treatment. Trater has been operating in the sector of heat treatment for forty-five years and is able to execute these treatments ensuring our dear Customers with the expected results.

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Quality control – The inspections after the heat treatment

At the end of the treatment, in some cases, before the execution of the treatment, the Quality Control department executes a series of inspections on the items. Such inspections aim to establish if the treatment has reached the purposes for which it has been performed and, therefore, they are executed with methods that are considered more suitable for this goal.
The following inspections are executed: • inspections of documental type • inspections on the items

The possible inspections on items that are executed through hardness tests, replica micrographic analysis and residual stresses’ measurement. If agreed with the customer, Trater, with its own qualified personnel, can also perform Non Destructive Examinations with the UT, MT e PT methods. Further to the inspections previously listed, the geometrical control of the items is performed to detect possible distortions that can have occurred during the treatment.
At the end of the treatment, the documental inspections regards the diagram. The control is performed by analysing the track of the recording of temperature to verify that the phases of the cycle have been executed according to the dispositions and the reference specifications.

Except when differently agreed or in the case of the application of reference procedures or specifications of the Customer, in Trater, to execute an efficient detection of the hardnesses the following is individuated:

  • for the joints, the different procedures adopted in the welding processes;
  • for the base material, the components of the item with different supply state (laminated, forged, casted or previously heat treated).

Furthermore, as far as the item is concerned the following is identified:

  • the most significant zones related to the geometry of the item and its position into the oven;
  • the surfaces into contact with the liquids or aggressive gases.

The hardness tests are executed on the weld seams, in the heat-altered zone and base material of each process and state of supply. The number is established according to the dimension of the item and is representative of the various zones of the item with at least n°1 detection on its highest thickness, n°1 detection on its minimum thickness and n°1 detection on its superior and inferior part, in relationship with its positioning into the oven, for each material, supply state and procedure. In case of simultaneous heat treatment of more items, the detection points are divided on more items with the same characteristics. The equipment to be used are calibrated and adequate to execute the detections, in relation to the geometry of the item and the welds to be measured. For the alloy and the low-alloy steels, used in the manufacturing of pressure vessels, after heat treatment, the admissible hardness values vary in relationship with the materials, in a field included between 200–250HB, whereas for carbon steels used for the construction of items subject to corrosive attack and Stress Corrosion Cracking (SCC), the maximum hardness value acceptable is 200HB.

Trater is the only heat treatment company that can certify the occurred stress relieving of the welds, ensuring therefore the geometrical stability of the welded structures and the resistance to Stress Corrosion Cracking (SCC) and H2S Service of the components for chemical and petrochemical plants. Trater is provided with a difractometry department, further to the possibility to execute the classical measurements with strain gauges or through the Barkhausen noise. Please, see the chapter “Residual stress measurement” in section “Engineering Company”.
Trater is provided with 12 ovens for heat treatment. They are all qualified according the Standard ASTM A991/A991M and according to ISO 17663. To execute some jobs, some of them are also qualified API 6A, ISO 10423, GE P28D-AL-0001 or NORSOK M-650. The biggest oven in Trater (21.5m x 8m x h5m) have a uniformity error in the entire load volume of maximum 10°C in comparison with the reference values. In the smallest ovens this deviation drops under 5°C.The possibility of Trater to use a high number of plants and their dimensions enable us to perform cycles that last even many days, and shorter cycles, in reasonably rapid times, on the basis of a good planning with the Customer, to grant the quality requested in all the work processes to be we are entrusted with. Trater is furthermore provided with plants for inflating hot gases inside the vessels with complex internal geometries and fixed tubesheet heat exchangers with and, for the protection from oxidation of the internal surfaces of the items, we designed specific plants for the circulation of inert gases inside the items.

More than 50 employees work in Trater. The core of the company is the technical office with 12 persons among specialists, engineers and quality operators. A department of 9 persons, divided into three shifts, which operate 24 hours a day, handles exclusively with the surveillance of the thermal cycles. The operators are instructed during specific courses of metallurgy, welding technologies, modelling thermal systems, combustion systems, electrotechnics and measurements; they attend a continuous training and updating program. Each team is formed by a specialist in electrotechnics/informatics, an electrician and a mechanical, to solve quickly all the possible anomalies of the 12 plants.

Interacting with the automatic systems of the ovens the treatment operators act on the planning of the equipment to optimize the treatment parameters, control constantly that all items reach the soaking temperature and remain in temperature for the time prescribed, in all of their parts. The automotion installed on the plants during the last few years has surely caused an increase in quality of the executed treatment, but if there is not personnel present 24 hours a day, it is not possible to ensure that all the items and the various parts of the same reach, uniformly, the soaking temperature prescribed and cool down with the methods necessary not to cause new residual stresses.

The automatic systems start counting the soaking time when the oven air has reached the desired temperature, but in that moment the temperature of the items with higher thicknesses is surely far from that value and probably, it will not even reach that value before the start of cooling down. Moreover, there are no automatic ovens able to automatically set up the heating and cooling rate, in function of the behaviour of the load, or to discriminate wrong temperature measurements and take immediate action in case of failure.

In a world where the performance and accuracy requests of the manufactured machineries and plants are higher and higher, it is important that the customers can count on a structured and reliable supplier like Trater, which has personnel and means to perform at best thermal cycles.

PWHT – Post Weld Heat treatment on alloy steels and low alloy steels Stress relieving heat treatment of bodies with overlays (valves and parts).

Alloy steels and low alloy steels are used in the construction of items and components subject, in operation, to high mechanical stresses at high temperature. The presence in these materials of alloy percentages such as molybdenum (0.5-1.15%) increases the heat resistance, whereas Chromium (0.5% - 10%) gives greater resistance to oxidation at high temperature. For pressure vessels, for this type of use, the following steels are usually used, defined by ASTM and ASME Standards or equivalent Standards and state of supply (sheets - tubes – forged materials):

  • ASTM / ASME SA387 Grade 12 Class 2 (13CrMo 4-5) – composition 1% of Chromium – 1/2% of Molybdenum – working temperature effective up to 560°C
  • ASTM / ASME SA387 Grade 11 Class 2 – composition 1.25% of Chromium -1/2% of Molybdenum – working temperature effective up to 575°C
  • ASTM / ASME SA387 Grade 22 Class 2 – (10CrMo 9-10) – composition 2.25% of Chromium – 1% of Molybdenum – working temperature effective up to 600°C ASTM / ASME A387 Grade 5 Class 2 – composition 5% of Chromium – 1/2% Molybdenum

Also the following steels are subject to treatment:

  • ASTM / ASME A / SA832-22V with composition 2¼Cr -1Mo ¼ V, used in thick layers, with particular hardness requirements, high toughness and heat resistance;
  • ASTM / ASME A387 Grade 9 -91 Class 2 – composition 7.9-9.6 % of Chromium – 0.85-1 % of Molybdenum, with high resistance characteristics to creep which make these steels optimal for energy production plants and good resistance to the acid attack.

On low alloy steels and alloy steels Trater normally performs normalizing and tempering treatments to obtain the required final mechanical properties and PWHT or stress relieving treatments after welding.

PWHT (Post Weld Heat Treatment) is a treatment to performed on welded structures with the aim to temper the zones of the welds with the highest hardness through microstructural transformations of diffusion and to improve toughness and ductility, obtaining at the same time a reduction in the hydrogen content. At the normally adopted temperatures, included between 630°C and 750°C, a strong decrease of the residual stress state (relieving) occurs, with an improvement of the material’s resistance to phenomena of stress corrosion cracking, when existing in operation.

The soaking time, into the soaking phases of the thermal cycle, must be the minimum one in order to get the maximum benefit in the welds, whereas the temperature must be lower than the one at which the base material has been previously tempered.
For Chromium-Molybdenum-Niobium-Vanadium steels and for Grade 9 and 91 steels, the soaking time and, above all, the PWHT temperature are important factors; a few degrees of difference from the prescribed range can lead to considerable problems to the material. Also in this case, as for all treatments, it is essential that the execution of the work is entrusted to competent companies; Trater can meet all of customer’s requests, with adequate ovens, equipment and knowledge needed to perform these thermal cycles.

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Dry-out of refractory materials

DRYING AFTER HYDRAULIC TEST

Often due to transportation, corrosion, freezing problems, or because the items works with different fluids from water (for example diathermic oil not to be polluted) it is required that, after the hydraulic test, before transporting and/or putting into operation, the items are completely dried out. If all the parts of the items are accessible, the classical aspiration systems or compressed air can be used. When the geometries are complex and closed, it is possible to completely remove the liquids only by evaporation, also at relatively low temperatures, so that possible gaskets normally supplied with the items are not damaged. A complete drying of the item can be ensured only through a careful temperatures’ control, obtainable only by positioning a suitable number of thermocouples in all of the critical points of the items and by applying particular ventilation techniques to the closed chamber, introducing the same item into ovens suitably equipped and dedicated to this kind of operation. Trater has designed procedures and equipment able to detect the presence of water during and after treatment and, at the end of the thermal cycle, an analytical certificate is issued stating the successful result of the drying. The availability in TRATER of a high number of plants and their dimensions enables us to execute these cycles, of the duration also of many hours, in reasonably fast time, in relationship with the dimensions and the geometrical complexity of the items, on the basis of a good planning with the Customer and in order to ensure the requested quality for this kind of work. Trater furthermore makes its spaces available to complete the mounting of items after treatment and tests.

DRY-OUT AND REFRACTORY SINTERING PROCESS OF LININGS

During the Nineties, Trater has studied and designed technology for drying and refractory sintering of items provided with linings by inflating hot air, at controlled temperature and volume; this is an alternative to the method still used by other companies based on the heating through free-flame burners inserted into the items. By using big quantities of gas inflated inside the items, our technology grants the temperature uniformity inside the bodies and the forced removal of water vapour, instead of the classical heat stratification that occurs with burners operating directly inside the items.

Through this technique, a uniform drying of the refractory material is possible and its complete sintering, in the correct period of times and with inferior generation of stress in the material during the treatment. With burners, instead, during the thermal cycle there are, in relation to the single heating and soaking phases, strong differences in temperature due to the stratification of heat, at the direct contact of the flame with the refractory material and to irradiation phenomena, due to which just the upper part of the item dries correctly up, whereas the part directly hit by the flame abundantly exceeds the temperature requested by the process.

Trater is provided with mobile equipment for the execution of this kind of treatments able to operate up to 1200°C, with hot gases capacities over 20 thousand cubic metre/hour, and executes works directly on site and in the plants of our customers.

Also for this kind of work process it is necessary a careful monitoring of temperatures that we perform through special thermocouples, subject to periodical calibration, positioned into contact with the refractory material, above all in the most critical points (nozzles, zones far from the flows, lower zones, etc…), to ensure our dear Customers with the maximum quality of treatment. For the entire duration of the thermal cycles, our specialized personnel monitors the ongoing process 24 hours a day.

For the quality requested also for this kind of treatment, it is necessary that our customers can count on a structured and trustworthy supplier as Trater, which has personnel and means available to perform at best the thermal cycles and deliver items always correctly treated.

Localized heat treatments

Localized heat treatment should be adopted only in cases where, due to the impossibility of transporting the products or the need to carry out the final welds on site, it is not possible to heat the item in the oven. Electrical resistors of appropriate power, power supply and geometry are generally used to locally provide heat to the area to be treated. It is a practical and relatively simple method but each intervention must be the subject to a careful study. Particular attention must be paid to the geometries to be treated if there is a risk of determining, due to differences in expansion, local stresses of a sufficiently high value to cause plastic deformation of the material with the appearance of deformations and the development of new states of residual tension. In the local treatment of cylindrical pressure vessels, as also indicated by the Construction Codes, circumferential bands including the welding to be treated must be heated to allow a uniform radial expansion of the vessel. The width of the heated band must be such as to determine appropriate temperature rates in the longitudinal direction of the vessels. Particularly critical are the nozzles placed in the immediate proximity of the welds to be treated, the heads and the tube sheets which, if not heated consistently in relation to their geometry, will determine the phenomena previously indicated. Generally, it is not advisable to carry out local treatments on welded structures with this technique due to the geometric complexity of the items. Trater, when the difficulties and costs relating to the correct execution of local heating are very high, recommends, if possible, treating the item in an oven. We are, however, available to our esteemed customers to provide all the technical information regarding the performance methods of treatments, especially when they are prescribed on welds of heat exchangers or, in general, for vessels in service in aggressive environments.

Partial treatment in the oven

There is often a need to treat only specific parts of pressure vessels. Frequent cases are:

  • the tubes-to-sheets welds of the tube bundles;
  • the heat exchangers in the channels’ part only,
  • including the weld to the shell;
  • the heads of the cylindrical bodies, including the weld with the shell;
  • the only curved part of the tube bundles.

Partially inserting the vessels into the oven is a valid alternative solution, in some situations, to local treatment with electrical resistances. However, an in-depth study of each case is essential and the execution methods must be decided taking into account the problems typically connected with the inevitable temperature differences that are determined in the partial heating mentioned previously. An effective and accurate control and measurement of the temperature, using thermocouples positioned in direct contact with the item in the most critical points, is necessary to achieve the goals of the treatment (relieving of residual stresses and tempering).

Partial treatment of heat exchangers is not recommended

Trater does not recommend treating heat exchangers with resistive elements. The problems are:

  • Heat treatment of circumferential welds between tube sheet and shell: it is not possible to place resistive elements in the shell side circuit. The weld would reach the PWHT temperature only on the surface the skin (cold tube sheet and shell would continue to remove heat from the core of the weld) and the innermost area of the exchanger would remain untempered (with less resilience) and with higher hardness, creating a “corridor of concentration of the efforts”.
  • Heat treatment of tube-to-sheet welds: to heat these welds, it is necessary that the entire tube sheet gets heated, otherwise carrying out the treatment would be equivalent to making a “hot zone” to the part of the sheet affected by the tubes, therefore tracing the forced material from the cold suburbs. Once the metal has been plasticised, when it becomes cold again, there would be a lack of material, so the area of the tubes would remain under tensile stress, which would be deleterious due to the phenomenon of stress corrosion cracking or the propagation of hydrogen cracks. Each tensile state speeds up crack propagation. In order for the tube sheet to expand, it is necessary that the channels and the shell also expand in the same way, at least in the closest area. To heat such a large portion of material, therefore at least partial treatment in the oven becomes essential. In this way there is also the advantage of being able to also control the expansion of the shell which must be equal to that of the tubes; in localized treatments of tube-to-sheet welds, however, the tubes partially heat up (expand) remaining forced between the length of the shell which remains cold. The upsetting of the tube-to-sheet welds (therefore residual traction at the end of the treatment which is deleterious due to the phenomenon of stress corrosion cracking) is inevitable and certainly shortens the life of the component (tubes frequently need to be closed, each time decreasing the performance of the plant, to the point of having to replace the exchanger). The flexibility during the construction of the vessels offered by localized heat treatments is certainly paid for by the end user.

Outgassing (hydrogen bake-out) heat treatments of operated elements

In case of welds and base materials exposed to the presence of hydrogen or hydrogen containing substances, a periodic execution of outgassing operations is necessary before reparations or replacements through welding, in order to enhance the diffusion of hydrogen and minimize the risks of cracks during the interventions. The treatments can last several days depending on the thickness. Stress relieving heat treatments before exercise also help mitigate the effects; any residual traction states after welding certainly help the spread of cracks of any kind, from those due to stress corrosion cracking to HIC cracks due to hydrogen embrittlement.

PWHT to reduce the levels of hydrogen

Negli acciai bassolegati il PWHT produce l’eliminazione dell’idrogeno introdotto durante la saldatura dei giunti, riducendo così il rischio di cricche a freddo. Quando c’è il rischio che le cricche si aprano prima ancora che inizi il PWHT, è conveniente fare un post riscaldo di qualche ora a circa 300°C, utilizzando le stesse attrezzature del preriscaldo. Per non degradare le caratteristiche del materiale base degli acciai bassolegati e per avere un sufficiente PWHT è fondamentale controllare al meglio le temperature di trattamento termico.

Qualifiche della Trater
Tutte le attività sono svolte in conformità al nostro manuale “Manual of Quality Assurance”, certificato in accordo TUV AD 2000 – Merkblatt W 0 / TRD 2100, HP0. Dispone di personale qualificato fino al massimo livello (comprehensive) nei corsi EWF: “Personnel with the Responsibility for Heat Treatment of Welded Joints Doc.EWF 628-08”. E’ importante che i nostri clienti possano contare su un fornitore strutturato e affidabile come Trater, che ha a disposizione personale e mezzi per eseguire al meglio i cicli termici, e fornire pezzi sempre correttamente trattati.
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