TWI Industrial Member Report Summary 859/2006
By J M Nicholas and T Sourmail
There are occasions when it would be beneficial to know whether there has been a problem with hydrogen control when welding a thick section steel before completion of the weld. It may be that a source of hydrogen is introduced during welding (inadequate cleaning or a region of damp flux, for example), and that early detection and immediate action can be taken to avoid hydrogen cracking in the component, thus reducing the repair incidence and the related costs. Such action may be increasing the interpass time to allow additional hydrogen diffusion prior to welding the next pass, or maintaining a postheat on completion of welding to allow hydrogen diffusion before the weld cools. In order to identify such events, a procedural change would be needed, along with the practical tools to assess the risks.
The first aim of this project is to investigate whether a hydrogen flux monitoring device, generally used for corrosion monitoring, can be used during weld manufacture to establish if the hydrogen level exceeds a critical level. As the readings obtained from the monitoring device (flux of hydrogen) are not readily interpretable in terms of hydrogen content, this would require identifying safe conditions in terms of these readings, perhaps during the welding procedure qualification. Readings obtained during welding operation would then have to be compared against those obtained during qualification. As these measurements will possibly be carried out at different temperatures, a mathematical model will be required to establish a correlation between them.
A second aim of the project is to then use these tools to estimate, should the hydrogen level be found to exceed a critical level, the measures required to bring the hydrogen content to a safe level.
- To establish the feasibility of measuring hydrogen flux and temperature in-situ during welding for single layer welds.
- To develop a model to assist in applying the measurements as a comparative tool for weld procedure qualification and production welding, by providing an estimate of post-heat hold time for avoidance of fabrication hydrogen cracking.