HEAT TREATMENTS ON 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. These forces should otherwise be estimated during the engineering phase of the metal work-piece and subtracted from the admissible load of the joint.
Eliminating internal residual stresses is moreover FURTHERMORE 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 metal items at a temperature at which steel has a very low yield strength (it is red). At soaking temperature, residual stresses reduce themselves until they have no significant effects anymore, through flowing of viscous material.
In case more performing materials are chosen than normal carbon steel for construction, particular consideration should be given to the temperature to be adopted during stress relieving, since mechanical characteristics in quenched and tempered steel, alloy steel and low-alloy steel are conveyed through the classical hardening treatments (hardened and tempered steels), normalizing (alloy steels and low alloying steels) and followed by tempering. The stress relieving temperature must not be higher than the temperature during the tempering process (that should be declared by the supplier); otherwise a decrease in mechanical characteristics is inevitable.
When heterogeneous materials are welded onto the same work piece (e.g. stainless steel), it should be taken into consideration that they have different thermal dilatation coefficients which cause local compression deformation, under the influence of temperature, generating new residual stress conditions, 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 indicated (?), temperatures from 450 °C to 850 °C should be avoided due to the well-known sensitization phenomena, resulting in drastic reduction of the characteristics corrosion resistance; for ferritic steels, treatments between 500°C-and 600°C should not be executed to avoid embrittlement phenomena.
Oven for stress-relieving treatments of welded products
TRATER is provided with 12 ovens for thermal treatments and daily at least two thermal plants are employed for standard distention thermal cycle of metal welded pieces in carbon steel at a 600°C-603°C temperature.
All ovens are qualified according to ASTM A9910/A991M and according to ISO 17663 standard. To fulfill some orders, some ovens are also qualified according to standards API 6A, ISO 10423, GE P28D-AL0001 and NORSOK M-650.
The biggest oven in TRATER (21,5m x 8m x h5,5m) have a uniformity error in the whole loading area of maximum 10°C in comparison to the reference value. In the smaller oven this gap is even below 5% (?).
Loading of ovens and thermal cycles
The ovens, normally used for stress relieving in steel structures, have big dimensions and enable us to treat a large number of items simultaneously, suitably arranged to limit the risks of deformation by pressing 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. The latter reach the same air temperature of the oven in shorter times.
For items with high thickness and mass much longer heating, soaking and cooling cycles are planned. These should be suitable (?) so that also the core of the material reaches the correct temperature.
Particular care is moreover dedicated to the selection and conduction of thermal cycles of geometrically complex structures, with closed chambers and parts with very different thickness welded on the work-piece. In this case, slow heating gradients are adopted to avoid strong temperature differences among the single parts of the item, which would represent a risk of higher heating and dilatation in lighter geometries. These last could be pressed by more massive parts that take longer time to be heated up and could undergo compression deformations during the increase in temperature and traction during the cooling phase, since previously they were (AND BE) permanently deformed.
The items are extracted from the ovens only when their temperature is really low (<150°C). This aims to avoid high temperature differences due to the prevoulsly described phenomena. These differences could be sufficient to generate new stresses and affect the results of relaxation obtained with the applied thermal cycle. Only when temperatures are close to the environment temperature, the items can be delivered to the customer.
The availability of many plants (12 ovens) with proper dimensions, enables us to fulfill the most different requests and deliver back the items in short periods of time with a high quality in work execution. This is not the case with other suppliers on the market that must treat all items at the same time and cannot adapt the heating and cooling rates in function of the thicknesses or of the soaking temperatures (in relation to the presence of parts in hardened and tempered steels), therefore all the load is constrained to low temperature. This causes inadequate results in terms of relieving of residuals stresses even on items on which the maximum level of relieving could be reached.
Temperature control and measurement
As previously stated, the temperature uniformity that should be obtained during all phases of stress relieving thermal cycles represents the factor that most influences the result of the treatment, in terms of relaxation. Therefore, it is very important that the temperature of the pieces is monitored and recorded in a very accurate manner through thermocouples, applied in direct contact with the pieces and opportunely positioned according to:
Geometry of the items;
Temperature uniformity of the ovens and characteristics of the plant;
Criticality of the item’s material (hardened and tempered steel, alloy and low-alloy steel).
Different kinds of thermocouples are available today for companies, handling with thermal treatments; the most practical and accurate for the measurement in contact with items are those with 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 of the thermocouple directly onto the surface of the artifact, 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 used thermometric material is certified.
Heat treatment performance and monitoring in TRATER
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 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 anomalies of the 12 ovens can be immediately be solved.
By interacting with the automatic systems of the ovens, the treatment specialists constantly control thermal cycles during their entire duration; they act on the oven load planning to optimize the treating 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 were not available 24 hours a day, it would not possible to guarantee that all pieces 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 (CONSIDER)soaking temperature when the air in the oven has reached the desired temperature but in that moment the temperature of the items with higher thickness is surely 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 gradients 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.
The quality control staff, in compliance with our Manual, performs standard controls agreed with the Customer on the items that undergo the stress relieving treatment.
On steel structures manufactured with tempered and hardened materials, alloy and/or low alloy metals, hardness measurements are executed before and after treatment with portable and calibrated devices; and it is verified if distortion and damage occurred during the thermal cycle.
Moreover TRATER, the only 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.
Qualification of TRATER
TRATER strictly complies with the requirements of the clients. It cooperates to define the most suitable thermal cycles for each type of work; in general the specifications valid for the heat treatment of pressure vessels or for nuclear industry (for example ASME VIII Div.1 UCS 56), have proved the complete relieving of the residual stresses of welds and then geometric stability. These standards prescribe a maximum heating rate of items as a function of their thickness, to ensure that the heat penetrates up to the core. These rates can also be very slow. In a few moments the maximum possible residual stress inside the joints decreases up to the yield strength of the material at that temperature through invisible viscous flows. For this reason, long soaking times are not needed, but only a careful control of the whole thermal cycle. In general, the higher is the treatment temperature, the lower is the yield strength of the steel at that temperature, therefore the final residual stresses will be lower. The following table indicates the relationship between the thermal cycle temperature and the maximum residual stresses that can be found at the end of a treatment for a commercial carbon steel.
Stress relieving temperature [°C] -> Maximum residual stress [MPa]
30°C -> 360MPa
100°C -> 340MPa
200°C -> 325MPa
300°C -> 270MPa
400°C -> 215MPa
500°C -> 145MPa
600°C -> 80MPa
630°C -> 55MPa
TRATER therefore recommends the following treatment parameters:
Heating rate: 55°C/h maximum (and 222°C/h / thickness [inches] of the plates)
Soaking temperature 600°C ÷ 630°C (if there are not tempered steels)
Soaking time (depending by the thickness of the welds):
for welds thickness < 50mm: 1h/inch (15 minutes minimum)
for weld thickness > 50mm: 2h + 1/4h for each additional inch
Cooling rate: 55°C/h maximum (and 280°C/h / thickness [inches] of the plates)
Opening temperature of the oven <150°C
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”.