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When it comes to pipe welding in the oil & gas and power generation industries, a lot of emphasis is placed on the new materials in the market and the latest welding technology available. Typically, less thought is given to the joint preparation. In this article, we will discuss the importance of weld preparation and how it can contribute to successful pipe welding.
The Importance of Weld Preparation
In order for metal fabricators to meet stringent project timelines and be within budget, welders need to produce consistently high-quality welds in a timely manner. However, no matter how skilled the welders or sophisticated the welding equipment, poorly machined joint preps can result in increased welder frustration, lower welder efficiency and more rework. Therefore, consistent and repeatable weld joint preparations and its contribution to welding throughput cannot be understated.
Weld Preparation via Cold Cutting
There are two main methods to prepare a weld joint. The first method is thermal cutting (flame or plasma), where heat is applied via a torch to cut the pipe. This is followed by hand grinding to form the bevel. The second method is cold cutting via a machine tool, where a rotating milling tool is used to cut and bevel the pipe simultaneously. Cold cutting is superior to thermal cutting for the following reasons.
Firstly, cold cutting is highly preferred for the repair and maintenance of existing oil & gas pipelines. Due to the presence of hydrocarbons in the pipeline, thermal cutting is obviously not desired due to safety reasons.
Secondly, because cold cutting uses a mechanized process, it is able to deliver a precise and repeatable lathe quality finish for better joint fit-ups. This in turn allows the welders to produce high-quality welds. Weld preparations done by thermal cutting will always result in inconsistencies because the subsequent hand grinding is a manual process that is highly dependent on the grinder’s skill and experience. Hand grinding is also undesirable from a safety point of view, because it can create a lot of airborne contaminants that may be inhaled by the welders.
Thirdly, because heat is used in thermal cutting to cut the pipe, a heat affected zone (HAZ) is created. Because the molecular structure of the metal is altered in the HAZ, thermal cutting could result in undesirable side effects. For example, if the 300 Series Stainless Steel is exposed to a certain temperature range for an extended period of time, sensitization could occur, resulting in reduced corrosion resistance and brittleness of the alloy. With cold cutting, no HAZ is created in the base material.
Lastly, for heavy wall pipes found in the main steam headers of power plants, compound bevels are typically used to reduce the total welding costs. These complex compound bevels cannot be prepared with thermal cutting. In addition, with the increase in adoption of orbital pipe welding systems, “J” bevels are typically recommended and can only be precisely prepared with cold cutting machine tools.
Make an Informed Decision
Improper weld preparation can result in gaps and mismatches during fit-up, which can cause welder frustration and lower efficiencies. If you are looking to improve the throughput of your welding operations, it may be wise not to overlook your existing weld preparation process.