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Development of Heat-Affected Zone Hardness Limits for In-Service Welding
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Development of Heat-Affected Zone Hardness Limits for In-Service Welding
  • Alternative Title:
    Development of HAZ Hardness Limits for In-Service Welding
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  • Edition:
    Final Report
  • Abstract:
    Welding onto in-service pipelines is frequently required to facilitate a repair or to install a branch connection using the "hot tapping" technique. Welds made in-service cool at an accelerated rate as the result of the ability of the flowing contents to remove heat from the pipe wall. These welds, therefore, are likely to have hard heat-affected zones (HAZs) and an increased susceptibility to hydrogen cracking. To prevent hydrogen cracking, at least one of the three conditions necessary for its occurrence must be eliminated or reduced to below a threshold level. Beyond the use of low-hydrogen electrodes to minimize hydrogen levels, it is prudent to develop and use procedures that minimize the formation of crack-susceptible microstructures. Exactly how crack susceptible microstructures are defined, and what microstructures and hardness levels will lead to cracking for any given set of conditions, is the subject of some debate, however. HAZ hardness is often used as an indicator of the susceptibility of a microstructure to cracking during the evaluation of procedure qualification welds. A widely used value below which it is generally agreed that hydrogen cracking is not expected to occur is 350 HV. Critical hardness level, or the hardness level below which hydrogen cracking is not expected to occur, is not a fixed value, but depends on the hydrogen level of the welding process being used and on the chemical composition (carbon content and carbon equivalent or CE level) of the materials being welded. The ability of welds to tolerate high HAZ hardness is also affected somewhat by joint restraint. The objective of this project was to develop HAZ hardness acceptance criteria that can be used to evaluate welds during the qualification of procedures for welding onto in-service pipelines. The current work focused on the further development and validation of previously-developed criteria, particularly for modern microalloyed materials and material over a wide range of wall thicknesses. The experimental portion of this work was carried out in two parallel phases. One phase involved practical weldability trials to further develop previously established empirical limits for a wider range of conditions. The Tech Report phase involved a more fundamental approach to more confidently prescribe hardness limitations by investigating microstructural susceptibility to hydrogen cracking. A series of graphs was developed that show proposed maximum-allowable hardness as a function of material chemical composition (CE level) for a range of thicknesses. In addition, guidance was developed for several issues related to the application of HAZ hardness limits to welds made onto in-service pipelines. This guidance focuses on the way in which HAZ hardness is measured during the qualification of welding procedures, determination of chemical composition for an in-service pipeline, and control of weld hydrogen levels.
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