In case of welds and base materials in 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 thicknesses.
PWHT to reduce the levels of hydrogen
In low alloyed steels, the PWHT generates the elimination (eliminates?) of the hydrogen introduced during the welding of the joints, reducing the risk of cold cracks.
When risk occurs that cracks open up even before the PWHT begins, it is convenient to perform a post-heating of some hours at about 300°C, using the same equipment of the pre-heating.
In order not to degrade the characteristics of the base material of low-alloyed steels and to have a sufficient PWHT, it is essential to control at best the temperatures of the heat treatment.
Ovens for the stress-relieving treatment of items
TRATER is provided with 12 ovens for heat treatments, therefore for these so critical components it is always possible to organize ad hoc thermal cycles, that can also last for different days.
All ovens are qualified according to the standard ASTM A991/A991M and according to ISO 17663. Some of them are qualified according to the Standards API 6A, ISO 10423, GE P28D-AL-0001 e NORSOK M-650 for the execution of some jobs.
The biggest ovens in TRATER (21.5m x 8m x h5m) have a uniformity error of maximum 10°C in the entire load volume in comparison with the reference value. In the smallest ovens this deviation is even lower than 5°C.
Load of the ovens and thermal cycles
For items with high thickness and mass, much longer thermal, soaking and cooling cycles are set up, in order for the core of the material to reach the correct temperature.
Particular care is furthermore focused to the choice and execution of thermal cycles of geometrically complex structures, with closed chambers and very different thickesses one to the other (?) welded onto the item. In this case, slow heating rates are adopted to avoid the generation of strong temperature differences among the single parts of the items, with the risk that the light geometries heat up and expand faster and can undergo upsettings during the stage of temperature increase and tensile stresses during cooling because they had previously deformed, constrained by the parts with higher mass that heat up and expand more slowly,.
For stress-relieving treatments, in TRATER items are taken from the ovens only when their temperature is very low (< 150°C), to avoid the generation of high temperature differences, sufficient to generate new stresses and ruin the results obtained through the applied thermal cycle, for the same phenomena previously described. Only when the temperatures are next to ambient temperature, the items are delivered to customers.
Since there are many plants of suitable dimensions, TRATER is therefore enabled to satisfy the most various requests of thermal cycles’ application and to deliver the materials back in a short period of time with high quality in the execution of the work, in comparison with other realities on the market today that with a few plants and personnel few qualified cannot obtain the same results.
Temperature control and measurement
As previously seen, the temperature uniformity to be obtained during all phases of thermal cycles of the stress-relieving heat treatment represents the factor that mainly influences the result of the treatment, in terms of stress relieving. It is very important, therefore, that the real temperature of the items is measured and recorded in a really accurate way, through thermocouples applied into direct contact with the items and opportunely positioned in relation to:
the geometry of the items;
the temperature uniformity of the oven and the characteristics of the plant;
the criticality of the item's material.
Different types of thermocouples are available to companies nowadays; the most practical and accurate ones for the measurement into contact with the items are those with single-use wires, because:
have an unlimited length and can be positioned in whichever point of the item;
the measurement hot joint has been obtained by welding, through capacitive discharge, two wires that make the same thermocouple directly on the surface of the item, completely reducing the measurement error due to the interference with the oven environment.
Every month, TRATER uses thousands of metres of thermocouple wires of the described type, to ensure the customer with a product free of "hidden defects" (residual stresses). Please, see the specific chapter "temperature measurement" in the section dedicated to the engineering company.
Execution and monitoring of heat treatments in Trater
TRATER employs more than 45 employees. The core of the company is represented by the technical office with 12 persons among specialists, engineers and quality personnel.
A department of 9 persons, divided into three shifts, operating 24 hours a day handles only with the monitoring of the thermal cycles. The operators have been instructed during specific metallurgical courses, welding technologies, modelling of thermal systems, combustion systems, electrotechnology and measurements, and attend a continuous training and updating program.
Each team is formed by a specialist in electronics/computer science, an electrician a mechanic, to quickly solve all possible anomalies in the 12 ovens.
Interacting with the automatic systems of the ovens, the treatment operators constantly control the execution of the thermal cycles for their entire duration; they take action on the programming of the equipment to optimize the treatment parameters.
The automation installed on the plants, during the last few years, has surely caused an increase in the quality of the performed treatments, but if personnel were not available 24 hours a day, it would not possible to grant that all items and their various parts uniformly reach the prescribed soaking temperature and cool up with the necessary methods, so that no residual stresses are generated.
The automatic systems start counting the soaking time when the air in the oven has reached the needed temperature, but at that stage the temperature of the item with high thicknesses is far from that value and probably will not reach it before the start of cooling calculated by computer systems. Moreover, there are no automatic ovens able to set automatically up, in function of the load's behaviour, the heating and cooling rates, and not even to discriminate wrong temperature measurements and take immediate action in case of failure.
All the activities are performed in compliance with our manual "Manual of Quality Assurance", certified according to TUV AD 2000 - Merkblatt W 0 / TRD 2100, HP0.
TRATER employs personnel qualified to the maximum level (comprehensive) in the EWF courses: "Personnel with the Responsibility for Heat Treatment of Welded Joints Doc.EWF 628-08".
It is important that our customers can count on a structured and trustworthy supplier such as TRATER, provided with personnel and means to execute thermal cycles at best and deliver always correctly treated items.