Abstract
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.
The latest evolution in these high-strength pipe steels is X-80, carrying a specified minimum yield of 80 thousand pounds per square inch. With even more advanced steels expected soon, unique opportunities are being created for the arc-welding industry to introduce new welding consumables and processes capable of welding these high-strength steels.
The welding industry’s priority is quality. All pipeline fabricators need to provide repeatable, highquality welds – the first time.
Since shielded metal arc welding (SMAW) has been adequate to produce safe, economic pipelines, the industry has been slow to adopt the more recent advances in welding process development. SMAW, i.e. cellulosic electrodes, has remained the most common process for welding pipelines and has a greater ability than other processes to accommodate non-ideal conditions such as poor fit-up to a change in terrain. However, as the strength of the steel increases there has become a greater risk of failure, either due to hydrogen cracking or reduced toughness. It is this risk of hydrogen cracking that is the main issue with the cellulosic SMAW electrodes in higher-strength applications.
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. This moisture originates primarily from the moisture and organic components.
Attempts to adapt welding processes and procedures to X-80 and higher-strength steels must be assessed. To take advantage of the pipe strength, the weld metal must either match the strength of the pipe or exceed it. The welding process and consumables must be controlled to minimise the risk of brittle fracture in the weld during pipeline fabrication and the resulting risk of hydrogen cracking. In addition, the demands of higher productivity and higher quality are always present.
Introduction
Low hydrogen manual processes suitable for 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.
Category:
Transportation
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