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Intercritical austenite formation in the coarse grain heat affected zone of X80 line pipe steels Zhang, Tianbi

Abstract

The intercritically reheated coarse grain heat affected zone (ICCGHAZ) is a region of low impact toughness in multi-pass welded steel structures such as pipelines. Its microstructure consists of a bainite matrix (the coarse grain heat affected zone, CGHAZ) and martensite-austenite (M/A) constituents. The former is a result of full austenitization in the first weld pass, and the latter of intercritical annealing in the subsequent weld pass. Size and fraction of M/A constituents are the major factors affecting the impact toughness. Therefore, analyzing the formation of intercritical austenite in the CGHAZ during welding is crucial to select steel chemistries and welding parameters for safe pipeline construction and operation. In this work, a two-pass heat treatment cycles adapted from gas-metal arc welding (GMAW) processes was employed to produce representative bulk ICCGHAZ samples in two X80-grade line pipe steels in a Gleeble thermo-mechanical simulator. Various heating rates (50-500 °C/s) were studied for austenite formation kinetics and used to produce ICCGHAZ samples. ICCGHAZ microstructures were analyzed by a new protocol for processing and segmentation of optical micrographs. It was found that fraction of intercritical austenite formed as a function of temperature is insensitive to heating rate, while the resulting microstructure depends strongly on heating rate. To further characterize the formation process of intercritical austenite, a 2D phase-field model (PFM) was created in the commercial software MICRESS® using the multi-phase field formulation coupled with carbon diffusion, interface properties and Thermo-Calc. The initial microstructure was simplified from the bainitic CGHAZ microstructure and nucleation of austenite was simulated using empirical parameters to replicate experimental particle densities and ensure quasi-random particle distribution. An effective mobility for ferrite-austenite interfaces was introduced to implicitly incorporate the effect of diffusion and solute drag by substitutional atoms such as Mn and Nb. Simulated transformation kinetics and microstructures agreed well with experimental observations by tuning the effective mobility and nucleation parameters.

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