STRESS RELIEVING HEAT TREATMENT AND PWHT OF PRESSURE VESSELS
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 the decrease of resistance of the same metal to 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 elimination of the 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 plastic, local 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.
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 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 thicknesses) 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 controlled thermomechanics treatment) present more complex metallurgical structures and fear the decrease of mechanical characteristics in the HAZ and base material, caused by the interaction with the treatment of supply of the material.
The hardened 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, even though 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 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 an only with the certain data for the treatment parameters in relation to soaking times and temperatures, the heating and cooling rates.
All cycles adopted in Trater are in any case agreed with the Customers and in compliance with manufacturing standards of the vessels, and with the welding processes qualification.
Execution method 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 item being worked 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 pressure vessels.
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 an the item under treatment. When the differences are higher than certain values, in relation to the local geometry of the item, plasticization can outcome in localized areas of the material due to the overcoming of its yield strength value at 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 in limited amount, can furthermore cause increase in the size of the grain or other undesired phenomena during treatments with metallurgical transformations, for the passing time of the subsequent soaking phase, necessary to completely uniform the temperature on the item.
To avoid the cited problems, during this phase it is necessary to minimize the differences in temperature starting from the beginning of the cycle, by applying the appropriate heating rate 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 in relation 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 ruining of the effect of the treatment 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 are generally necessary and particularly controlled 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 it, 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, in Trater, the unloading of the item form the oven at high temperature is avoided; studies conducted by us on geometrically very complex item did not show the outcoming of residual effort states if extracted from the oven at a temperature lower than 150°C.
The geometry of the item
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 shells 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.
As complex geometries must be considered the 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.
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.
The dimensions and the weight
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 and above all to be cooled.
items with big dimensions need great treatment plants, opportunely dimensioned so that the item is correctly positioned into them.
Trater, provided with big plants and suitable heating and cooling equipment can treat pressure items with dimensions up to 30 meters of length and weights up to 300 tons.
The positioning of the item into the oven
The positioning of the item into the oven is also very important to reach 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 i the best circulation of hot gases on the surface of the items is ensured, and therefore the best temperature uniformity.
The supporting of the items into the oven
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 supports 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 happen due to the own weight of the structure.
To this purpose, our technician 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 connected with:
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 occur on the items.
Temperature control and measurement
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 ruin the result of the same treatment.
For example, it is not 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 execution errors of the heat treatment, 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".
Quality control - The testings 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 testings on the items.
Such testings 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 testings are executed:
testings of documental type
testings on the items
At the end of the treatment, the documental testings 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.
Testing 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 testings previously listed, the geometrical control of the items is performed to detect possible distortions that can have occurred during the treatment.
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 thermally 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 welding chords, 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 the 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 alloyed and the low-alloyed steels, used in the manufacturing of pressure, 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.
The measurement of residual stresses
Trater is the only heat treatment company that can certify the stress relieving of the welds, ensuring therefore the geometrical stability of the welded structures and the resistance to Stress Corrosion Cracking (SCC) and or the 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 in section “Other activities”.
Treatment plants in Trater
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 personnel is 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 is lower than 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 performed.
Trater is furthermore provided with plants for inflating hot gases inside the tanks with complex internal geometries and heat exchangers with fixed sheets 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 40 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 metallurgical courses, welding technologies, models of 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 fastly 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 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 with the same method necessary not to cause new residual stresses.
The automatic systems start to count the soaking time when the 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. Moreover, there are no automatic ovens able to set up automatically, in function of the behaviour of the load, the heating and cooling rate, and they do not discriminate wrong temperature measurements and take immediate action in case of failure.
In a world where the performance and accuracy requests of the machineries and plants manufactured 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.