Advantages of high-strength steel pipes

Low hydrogen manual processes suitable for the pipeline girth welding have been available for some time, but despite their availability and advantages in terms of reduced crack susceptibility, they have rarely been used in production up to the present time. With the advent of the new high strength steels such as X-80, any large pipeline project will, consider if not use, these grades of pipe steel, and therefore contractors and engineers must accept the advancements in the welding processes. New, high-strength steels are offering many advantages to the pipeline industry. These steels allow for the use of thinner pipe at higher operating pressures.

These steels can also drive down total construction costs. However, as pipeline owners and designers look to these new steels, they are presenting a challenge for the welding and fabricating industry that must respond with cost-effective ways of joining them. To complicate the matter, these high performance steels have surpassed conventionally fabricated weld metal in terms of hydrogen crack resistance and fracture toughness. It is manufactured using combinations of heat treatment and mechanical treatment that produce higher strength without significant higher alloy. Such advancements in thermomechanical processing in steels (TMCP) make it possible to achieve the balance in strength and toughness, but with a greater resistance to heat affected zone cracking. With all the positive factors regarding the higher yield materials the biggest challenge is how to weld these steels. The pipe steel is no longer the limiting metallurgical factor.
For example, steels are no longer as sensitive to hydrogen cracking as are conventional weld materials. This is why it is important to examine hydrogen cracking and potential solutions in relation to the weld metal.

Hydrogen cracking
It is the main issue with cellulosic SMAW electrodes in higher strength applications. This hydrogen originates primarily from the burning of the electrode coating, which contains moisture and organic components Hydrogen dissolves in the molten puddle during welding.  Upon cooling, diffusable, as opposed to chemically bound, hydrogen can cause porosity during solidification and cracking in the finished weldment. That is why it is critical that hydrogen levels be minimised for the weld to be considered sound.

With respect to the hydrogen issue, the challenge is to minimise the risk of weld metal cracking by controlling the factors know to be influential.
1. Minimise the amount of hydrogen that is available through judicious selection of consumable and/or welding process controls.
2. Minimise stresses, both residual and applied.
3. Minimise weld metal strength, thus controlling the susceptibility of the microstructure. Some industries have successfully employed undermatching strength weld metals, contingent upon design requirements.

It should be noted that with respect to hydrogen, the risk of cracking might be minimised, but it is impossible to eliminate. Since all steel microstructures are susceptible somewhat to cracking, it simply becomes a question of controlling hydrogen and stress levels simultaneously. A report published in 1996 by The Welding Institute titled "Evaluation of Low Hydrogen Processes For Pipeline Construction in High Strength Steel", (PR - 164-9330) which investigated suitable processes for the welding of X80 grade pipe reported, "The most successful root welding performance was obtained using the Lincoln Electric STT? power source and the LA90 electrode wire. The STT power source provided very precise control of short circuiting metal transfer, which resulted in good handling characteristics, well-fused beads, minimal spatter and lower fume emissions. The TWI welder involved in the trials was depositing satisfactory root beads within two hours of being introduced to the welding machine. The root welding speeds were comparable with celZapatillas Trail Running