Pipelines are the safest and most cost-effective method of oil and gas transportation to storage and processing facilities. Large diameter API 5L welded pipes fabricated by submerged arc welding (SAW) are the preferred product in many cases for pipeline construction. Furthermore, pipelines are constructed by welding segments of pipe, typically by single or dual torch Gas Metal Arc Welding (GMAW). During welding, both during pipe fabrication and girth welding, the Coarse Grain Heat Affected Zone (CGHAZ) experiences rapid thermal cycles with peak temperatures up to the melting temperature of the base metal. Controlling the microstructure evolution in the heat affected zone (HAZ) during welding of line pipe steels is critical to ensure that these products meet the Charpy impact testing and CTOD requirements by clients and specifications.

In particular, the CGHAZ is of concern where austenite grain growth occurs readily due to the combination of high temperature and precipitate dissolution. Controlling the CGHAZ austenite grain size is critical to obtain final microstructures with acceptable impact properties. In this study, austenite grain growth has been measured and modeled for thermal conditions relevant for the CGHAZ in 27 steels including industrial as well as laboratory steels with systematic variations of alloying element content

Austenite grain size was measured using a Laser Ultrasonics for Metallurgy (LUMet) sensor attached to a Gleeble, which enables high-throughput in-situ monitoring of austenite grain growth. A classical grain growth model has been developed based on a standard test. The grain growth kinetics are described by combining curvature driven grain growth with pinning due to TiN precipitates. A phenomenological relationship has been developed for the grain boundary mobility that decreases with C, Nb and Mo alloying which is consistent with their expected grain boundary segregation. The pinning parameter is rationalized in terms of volume fraction and size of TiN particles. The proposed model has been validated for CGHAZ heat treatment cycles.

The results of this study provide a model that can predict the austenite grain size in the CGHAZ as a function of steel chemistries and heat treatment paths, i.e. welding parameters. Processing maps for austenite grain size control have been constructed as a function of welding parameters and line pipe steel chemistry. Thus, the model can be used both for steel chemistry design and for optimizing welding of steels with known chemical composition to minimize the CGHAZ austenite grain size both during pipe fabrication and girth welding.


Austenite Grain Size Control During Welding of Line Pipe Steels

Division / Sponsors: 
Pipeline Systems Division

Paper Proceedings: https://asmedigitalcollection.asme.org/IPC/IPC2020/volume/84447

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