Microstructural evolution of 2.25Cr-1Mo-0.25V submerged-arc weld metal

Heat-resistant V-modified 2.25Cr-1Mo-0.25V-weld metal is commonly used in petrochemical industry for heavy wall pressure vessels in high-temperature hydrogen service. In order to improve the reactor efficiency, the weldments have to endure even higher temperatures and pressures. Acicular ferrite (AF) is often regarded as the optimum microstructure due to its good combination of strength and toughness. As few literature about the evolution of microstructure and the final microstructure constituents of 2.25Cr-1Mo-0.25V weld metal is available, the current paper intends to provide comprehensive information by means of microscopy, crystallographic examination via electron backscatter diffraction and in situ observation of the austenite to ferrite phase transformation via high-temperature laser scanning confocal microscopy (HT-LSCM). The investigated weld metal exhibits a high density of complex aluminium-silicon-manganese oxides with a spherical shape and large prior austenite grains, which in combination is beneficial for intragranular nucleation of AF. Nonetheless, the examination of the transformed final microstructure was not sufficient to make an unambiguous statement about the presence of AF within the 2.25Cr-1Mo-0.25V weld metal. Via in-situ HT-LSCM of the phase transformation, intragranular nucleation of AF at non-metallic inclusions within the austenite grains was detected, which confirms that even though the microstructure of 2.25Cr-1Mo-0.25V weld metal is mainly bainitic, small amounts of AF are present.