The Proper Way of Joining Pipe
The proper way of joining pipe depends on many variables, including fit-up, accessibility, terrain, whether it is a tie-in or crossing, whether it is a repair weld, the welder skill and a number of other factors.
Each possible welding process has its own merits, and there is no single clear-cut answer for every application. Attempts to adapt welding processes for higher-strength steels must be considered. In addition, the demands of higher productivity and quality are always present.
Root Pass Welding
Traditionally, the welding of the root and hot pass have been completed using a high hydrogen cellulosic electrode. However, on X-80 type material, restrictions do apply. Deposition of small weld passes at high travel speeds are possible with sufficient preheat temperature control. Preheat is required to slow the rate at which the weld cools, thus allowing hydrogen to diffuse, in addition, the use of preheat can also help control residual stresses in the weld zone.
While this approach sacrifices nothing in terms of deposition rate compared with current practices on lower-yield pipe, the need for potentially high levels of preheat can limit overall productivity.
It is currently not advisable to weld with cellulosic electrodes on X-80 that is more than 10mm thick.
The use of basic coated ‘low hydrogen’ electrodesfor the root pass weld are productivity issues. Albeit, they can produce one-tenth of the level of hydrogen compared with a cellulosic electrode, the major disadvantage is that the process is slow when compared to any other process than gas tungsten arc welding.
Whilst SMAW as been used, and can be used to create acceptable quality welded joints in the highyield steels, productivity issues make SMAW less attractive than welding with a continuous wire process. Since there is a strength ceiling for the cellulosic electrodes because of the high hydrogen levels, and the poor productivity levels with the basic low hydrogen electrodes, alternatives are required.
Welding Productivity
For field welding of pipelines, it is the elapsed time to remove the pipe clamp that is the critical indicator of welding productivity. The elapsed time will depend on a number of factors including the strength of the weld deposit, the dimension of the welded ligament and the level of hydrogen present.
The root pass welding speed determines the advancement of the construction and is, therefore, the focal point that lends itself best to technological advances. Mechanised systems can weld faster than manual or semi-automatic systems, provided the requisite care is taken in joint preparation and fit-up to ensure proper process control.
Preparation and Presentation
In manual and semi-automatic welding, the journeyman or pipe fitter has a tolerance in which the welder can compensate for variation in the joint fit-up. In automatic welding that tolerance is more critical, therefore the bevel geometry, line-up and spacing need to be addressed mechanically.
Preparation - Pipe Facing Machines
The roundness and consistency of the joint are more important when making welds using an automated system. Therefore, the use of a pipe facing machine in the field is critical to the success and quality of weld, regardless of whether an internal or external root pass machine is used.
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