Features of local heat treatment of welds

At the construction site, something is constantly being welded, heated, dug, poured, installed and fixed. Our company, as people associated with metal heating before, during and after welding, is interested we in questions of heating technology of a particular volume of metal.

Of course, a lot depends on the technological conditions, accuracy requirements and features of a particular construction site. In the framework of this article, we will focus on the technology of welded joints heating of pipelines before, during and after welding.

Heating of pipelines welded joints in a furnace is not possible because pipelines usually have a long length and complex configuration. Based on this, for pipelines welded joints, the main method is local heating. In other words, this process is called post weldedheat treatment. There are several main methods for local heat treatment of welded joints: gas heating, heating with high-frequency currents (induction) and heating of welds with resistance electric heaters (heating mats, heating elements, electric heating elements, etc.).

Every method has its advantages and disadvantages.

• Gas heating does not require complicated equipment (you can heat directly with a torch). However, under the influence of a direct flame on the metal surface, undesirable processes occur associated with accelerated oxidation of the metal and local changes in the metal properties due to the large unevenness of heating.
• The induction heating method can provide a very high heating rate. However, the electromagnetic field, which is the source of heating, creates interference in the welding process, which leads to a "withdrawal of the welding arc" and a reduce welding quality, and also creates bad conditions for human work. In addition, the inductor is wound around the entire circumference of the pipe equally, which also leads to uneven heating around the circumference. Based on laws of physics, it is known that heat always rises. Accordingly, if the same heat will be generated in the lower part of the joint as in the upper, the lower part will “warm up” the upper one without taking into account the desire of the operator-thermist. As a result, the upper part of the joint will always be heated more strongly than the lower. This effect will increase with increasing joint diameter. The temperature difference often may exceed the tolerance for heat treatment. Due to the peculiarities of heating due to eddy currents, induction heating is effective, mainly, only before the Curie temperature, at which a sharp change in the ferromagnetic properties of the metal occurs, which makes it difficult to heat above about 720 degrees Celsius.
  
• Method of post welded heat treatment of welded joints made with heating mats is devoided of above disadvantages. In this type of heating, the heating rate is strictly observed by the requirements of standarts, which do not allow uncontrolled fast heating of the weld, but require a equable and controlled process throughout the entire period from the start of heat treatment to the hold temperature. In addition, the larger the diameter of the pipeline welded joint, the more heaters will be needed, which means that the more control and temperature control channels will be involved. Each channel is controlled by its own thermocouple fixed in the corresponding zone, which leads to complete and accurate temperature control around the entire circumference of the weld, excluding underheating and overheating in different zones. Heating mats do not create an electromagnetic field and provide precise control of the thermal field, which makes them indispensable assistants when it is necessary to carry out preheating before and during welding of steel pipelines from different steel grades, including heat-resistant and especially P91. Also, due to the lack of an electromagnetic field, the people working conditions in the immediate vicinity of the heating zone are not worsened. Standard heating mats made of nichrome wire provide heating to temperatures of 800-850 degrees without problems, i.e. they can be used for welds stress relieving made of low-carbon and heat-resistant steels in order to reduce welding stresses, which some people call annealing of welded joints. If it is necessary to heat to higher temperatures, high-temperature heating mats with a fechral core should be used. It provides heating up to 1100 degrees Celsius, allowing austenization of welds, normalization and stabilizing annealing of welded joints subjected to intergranular corrosion (MCC) and corrosion cracking (CR) and working in aggressive environments.

The above arguments show that local heat treatment of welds, performed using heating mats at multi-channel PWHT units, has many advantages, which leads to its widespread use in chemical, oil, petrochemical, energy, shipbuilding and other industries.