Heat treatments on welded structures

STRESS RELIEVING HEAT TREATMENT OF METAL STRUCTURES

The standard heat treatment on metal structures is stress relieving. It aims to eliminate traction and compression forces that are inside welds and in adjacent areas after cooling, which should otherwise be estimated during the engineering phase of the item and subtracted from the admissible load of the joint.

Eliminating internal residual stresses is moreover necessary:

to obtain geometrical stability of the items during their work-machining;

to avoid misalignments of the surfaces during exercise, misalignments in the shafts’ axis, missing balance in rotating elements etc.;

to increase the resistance to fatigue of the material, reducing the eventuality of cracks’ propagation.

The treatment consists in heating up the items at a temperature at which steel has a very low yield strength (it is red). At soaking temperature, residual stresses reduce themselves, through flowing of viscous material, until they have no significant effects anymore.

When a metal structure to be subjected to heat treatment is designed, it is necessary to require reinforcements, even temporary ones, so that the items with the lighter geometries do not collapse under their own weight during stress relieving.

If the metal structures are made up of tubes and box sections, it will be necessary to provide appropriate air exhaust holes from the closed chambers (diameter 3 mm, for example, which can also be obtained by not completing the welds) to avoid their swelling due to the increase in internal pressure of the air, which at 600°C triples its volume, exerting, if constrained, a pressure (3 atmospheres) sufficient to deform the material which has no mechanical characteristics due to the high temperature.

In case more performing materials are chosen than normal carbon steel for construction, particular consideration should be given to the temperature to be adopted for stress relieving, since mechanical characteristics for quenched and tempered steel, for martensitic steels and for alloy steel and low-alloy steels are conveyed through the classical treatments of quenching (quenched and tempered steels), normalizing (alloy steels and low alloying steels) and subsequent tempering. The stress relieving temperature must not be higher than the temperature at which the tempering process occurred (that should be declared by the supplier); otherwise a decrease in mechanical characteristics is inevitable. When heterogeneous materials are welded onto the same item (e.g. stainless steel and carbon steel), it should be considered that they have different thermal expansion coefficients and, under the influence of temperature, the different expansion causes local upsettings, generating new residual stress states, geometrical deformations, and worst of all, cracks.

Furthermore, it is necessary to study the influence of stress relieving temperature on the corrosion resistance characteristics of austenitic stainless steels (with a C>0,03% content) and ferritic steels. For the first class of material indicated, temperatures included between 450°C to 850°C must be avoided due to the well-known sensitization phenomena, resulting in drastic reduction of the characteristics corrosion resistance; for ferritic steels, treatments around 500°C- 600°C must not be executed to avoid embrittlement phenomena.
TRATER is provided with 12 ovens for heat treatments and daily at least two plants are employed for the standard stress relieving cycle of metal welded items in carbon steel at a temperature of 600°C-630°C for one ore two hours. All ovens are qualified according to ASTM A9910/A991M standard and according to ISO 17663. To perform some jobs, some ovens are also qualified according to standards API 6A, ISO 10423, GE P28D-AL0001 and NORSOK M-650. The biggest ovens in TRATER (21,5m x 8m x h5,5m) have a uniformity error in the whole loading volume of maximum 10°C in comparison to the reference. In the smaller ovens this deviation drops even below 5°C.
The ovens, normally used for stress relieving of metal structures, have big dimensions and enable us to treat a large number of items simultaneously, suitably arranged to limit the risks of deformation by crushing and to optimize temperature uniformity. Apart from temperature uniformity of plants, in TRATER oven loads are planned according to the thickness of items we receive for treatment and to the cycles agreed with the customers. Items with low thickness are never treated together with items with high thickness because the more massive items, even with the same surface, take longer time to heat up in comparison to lighter geometries, which reach the same air temperature of the oven in much shorter times. For items with high thickness and mass much longer heating, soaking and cooling cycles are set, suitable so that also the core of the material reaches the correct temperature. Particular care is moreover dedicated to the study and conduction of thermal cycles of geometrically complex structures, with closed chambers and with very different thicknesses one another. In this case, slow heating gradients are adopted to avoid the creation of strong temperature differences among the single parts of the item, for the risk that light geometries heat up and dilate earlier and, forced among the more massive slower parts, undergo upsetting during the increase in temperature, with subsequent residual traction during the cooling phase. The items are extracted from the ovens just when their temperature is really low (<150°C) to avoid that, due to the previously described phenomena, high temperature differences are caused, sufficient to generate new stresses and affect the results of relieving obtained with the applied thermal cycle. Only when temperatures are close to the ambient temperature, the items can be given back to the customer. The availability of many plants (12 ovens) with proper dimensions, enables us to fulfil the most different requests and deliver back the items in short periods of time with a high quality in work execution, opposed to other suppliers today on the market, who must treat all items at the same time, and cannot adapt the heating and cooling rates of the items in function of the thicknesses, or the soaking temperatures in relation to the presence of parts in quenched and tempered steel, therefore all the load is constrained to low temperature, with inadequate results in terms of relieving of residuals stresses even on items on which the maximum level of relieving could be reached.
As previously stated, the temperature uniformity to be obtained during all phases of stress relieving thermal cycles represents the factor that most influences the result of the treatment, in terms of relieving. Therefore, it is very important that the temperature of the items is monitored and recorded in a very accurate manner through thermocouples, applied in direct contact with the items and opportunely positioned according to:
  • the geometry of the items;
  • the thicknesses;
  • the masses;
  • the temperature uniformity of the oven and characteristics of the plant;
  • the criticality of the item’s material (quenched and tempered steel, alloy and low-alloy steel).
Different kinds of thermocouples are available today to the heat treatment companies; the most practical and accurate for the measurement in contact with items are those with single-use disposable wire for the following reasons: they have unlimited length and can be placed in every point of the item; the hot joint of measure is obtained by welding, through capacitive discharge, the two wires that make up the thermocouple directly onto the surface of the item, reducing completely the measurement error due to the interference with the environment of the oven. Each month, Trater uses thousands of meters of thermocouple wires of this type, to ensure the customer a product without “hidden defects” (residual stresses). All the thermometric material used is calibrated and certified.
More than 40 employees work for TRATER. The core of the Company is the Technical Department with 12 employees among specialized workers, engineers and quality control staff. A branch with 9 persons, divided in 3 work-shifts, operating 24 hours a day, handles exclusively with the monitoring of thermal cycles. This personnel has been instructed during specific courses in metallurgy, welding technology, modelling of thermal systems, combustion systems, electrotechnics and measurements and follows a continuous instruction and updating programme. Each team is made up of a specialist in electronics/informatics, an electrician and a mechanic, so every possible anomaly of the 12 ovens can be immediately be solved. By interacting with the automatic systems of the ovens, the treatment specialists constantly control the progress, taking action on the programming of equipment to optimize the treatment parameters. The automation system installed on the plants in recent years surely has caused an increase in quality of the executed treatments, but if personnel are not available 24 hours a day, it would not possible to guarantee that all items and their various components uniformly reach the planned soaking temperature and that they cool in the correct way in order not to generate new residual stresses. Automatic systems start to count the soaking temperature when the air of the oven has reached the desired temperature, but in that moment the temperature of the items with high thicknesses is far from that soaking temperature value and probably it will not reach that value even before the cooling calculated by informatics systems starts. Moreover, there are not automatic ovens that can automatically set up heating and cooling rates in function of the load condition, they cannot even discriminate wrong heating and cooling temperatures and immediately take action in case of malfunctioning. It is important for customers to count on a structured and trustworthy supplier like Trater, in a world where performance and equipment precision requests are getting higher and higher. Trater has personnel and equipment to execute thermal cycles in the best way and deliver items correctly stress-relieved, guaranteeing our customers that they can machine-work them without the risk of distortions or misaligning due to residual stresses.
The quality control staff, in compliance with our manual, performs standard controls and/or agreed with the Customer on the items that undergo the stress relieving treatment. On steel structures manufactured with quenched and tempered materials, alloy and/or low alloy metals, hardness tests are executed before and after treatment with portable and calibrated devices; and it is verified if distortions and damages occurred during the thermal cycle. Moreover Trater, unique heat treatment Company in Italy and Europe, is provided with two devices to measure residual stresses through X-Ray diffractometry and a device for measurements with strain gauge (Hole drilling), that we use to measure and certify the residual stress values on particular constructions.
All activities are performed in compliance with our manual “Manual of Quality Assurance”, certificated in agreement TUV AD 2000 – Merkblatt W 0 / TRD 2100, HP0. The staff in TRATER is qualified to the highest level (comprehensive) in EWF courses: “Personnel with the Responsibility for Heat Treatment of Welded Joints Doc.EWF 628-08”.

Would you like to receive further information?

Please, contact us

ANNEALING OR STRESS RELIEVING HEAT TREATMENTS OF OF CAST IRON

Cast iron is an alloy containing 2% or more carbon and in which iron is the predominant element by weight.

Other alloying elements such as Al, Cu, Mo, Ni, S, Ti and Va are contained, overall, in percentages lower than 10%.

Gray cast irons

Gray foundry cast irons or mechanical cast irons, due to their good castability and modest price compared to steel, are widely used in mechanics, in all those cases where particular toughness of the item is not required.

They have excellent wear resistance due to the anti-seize power of the free carbon which is always present in the casting. Gray cast iron is particularly suitable for the production of casings for pumps, turbines and gearboxes, piston rings, cylinder liners, crankshafts, brake drums, gears and casings of internal combustion engines, parts of ovens, boilers, burners and crucibles. Furthermore, this material is widely used in the construction of bases for machine tools, subject to vibrations, due to its high damping properties.

Stress relieving heat treatment

Trater mainly carries out stress-relieving heat treatments on even particularly complex gray cast iron castings with the aim of eliminating residual stresses generated during the casting process. By applying particular care in the execution of this process, a significant reduction in the residual stress state can be achieved, guaranteeing optimal geometric stability of the product during mechanical processing and reducing the probability of initiation and propagation of cracks in service.

The treatment consists in heating the castings to a temperature such that the cast iron has a very low yield strength and through viscous flows of the material the relieving of the tensile and compressive stresses present in the piece occurs.

As with steel structures, also in this case, during the stress-relieving treatments it is very important that the products are heated and cooled uniformly, therefore avoiding high temperature differences between the single parts of the items and their thickness to which different dilations would correspond with inevitable local plasticization of the material with the generation of stresses of a sufficiently high magnitude to cause residual stresses (at the end of the treatment) and, in the worst cases, breaks of the material (in all phases of the treatment).

All high thickness cast iron castings can be subject to these phenomena given the poor heat transmission by conduction, typical of this material, and castings with complex geometry in which there are strong variations in thickness, partially closed chambers and complex ribs.

In Trater, in these cases, particularly slow heating and cooling cycles are adopted and very particular attention is paid to the control and measurement of the temperature in the most critical parts of the items, by placing an adequate number of thermocouples in contact with the material. It is also very important to avoid extracting castings from the oven at a temperature higher than 150°C to prevent an uneven cooling of the item from causing the previously indicated phenomena.

Stress relieving thermal cycles

They are carried out at a temperature lower than the transformation temperature of pearlite into austenite in a range generally between 540 and 565°C, up to a maximum of 595°C. The soaking time is generally calculated based on the maximum thickness of the casting even if a few minutes are sufficient to obtain relieving of the item. As with all stress relieving treatments, the higher the temperature reached during the soaking phase, the greater the effectiveness of the relieving.

Workability heat treatment by annealing

When a cast iron, after casting, is particularly hard, impossible to work mechanically or has carbides immersed in the ferritic-pearlitic matrix, the so-called “workability” heat treatment can be performed, which lowers the hardness of the material by a few points, homogenizing the casting structure and partially solubilising the carbides. It is a treatment that is carried out at a fairly high temperature, in the range between 700°-760°C, decomposing the iron carbide into ferrite + graphite.

Normalizing heat treatment

It is carried out to improve the mechanical properties (hardness and tensile strength) of the cast iron. It eliminates the effects of treatments such as annealing or preheating or postheating associated with repair welding processes. It is carried out at a temperature included between 885°C – 925°C with a soaking calculated as one hour for each inch of maximum thickness. Subsequent cooling occurs in still air.

Performance of treatments

In Trater a department of 9 people divided into three shifts that operate 24 hours a day, deals exclusively with the preparation of treatments and the monitoring of thermal cycles. The workers are trained with specific courses in metallurgy, welding technologies, modeling of thermal systems, combustion systems, electrical engineering and measurements; they follow a continuous training and upadating programme. By interacting with the automatic systems of the ovens, the operators constantly check that the items uniformly reach the soaking temperature and remain there for the prescribed time. It is important that our customers can count on a structured and reliable supplier like Trater, with personnel and means available to best perform thermal cycles and supply items that always comply with specifications.

HEAT TREATMENTS ON QUALITY STEEL

The treatment consists in heating the items to such a temperature that the steel has a very low yield strength. At the soaking temperature, the residual stresses, through viscous flow of material, are reduced until they no longer produce significant effects.

When designing an welded structure item to be subjected to heat treatment, it is necessary to prescribe reinforcements, even temporary ones, so that the products with the lighter geometries do not collapse under their own weight during stress relieving. If the steel structure is made up of tubes and box sections, it will be necessary to provide appropriate air exhaust holes from the closed chambers (diameter 3 mm, for example, which can also be obtained by not completing the welds) to avoid their swelling due to the increase in internal pressure of the air, which at 600°C triples its volume, exerting, if constrained, a pressure (3 atmospheres) sufficient to deform the material which has no mechanical characteristics due to the high temperature.

Loading of ovens and thermal cycles

The ovens, normally dedicated to the stress relieving of metal structures, are large in size and allow us to simultaneously treat a large number of items, appropriately arranged to limit the risks of deformation due to crushing and to optimize temperature uniformity.

Quality control

Trater's staff has more than 40 employees. The heart of the company is the technical office with 12 people including specialists, engineers and quality workers. The quality control staff carries out, in accordance with our manual, standard checks and/or checks agreed with the Customer on the items subject to stress relieving.
This site is registered on wpml.org as a development site.