Abstract
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.
Highlights
1.1 2.25Cr-1Mo-0.25V weld metalThe heat-resistant V-modified 2.25Cr-1Mo-0.25V-alloy was developed following the conventional low-alloyed 2.25Cr-1Mo steel and approved as ASME Code Case 2098–1 in 1991 [1, 2]. 2.25Cr-1Mo-0.25V-alloy is commonly used in the petrochemical industry where it is applied to hydrocracking reactorsRecommended for Publication by Commission IX - Behaviour of Metals Subjected to Welding2 voestalpine Böhler Welding Germany GmbH, Hafenstraße 21, 59067 Hamm, Germany as well as heavy wall pressure vessels for high-temperature hydrogen service [1, 3]
The microstructure of 2.25Cr-1Mo-0.25 V submerged-arc welding (SAW) weld metal was examined regarding the existence of intragranularly nucleated Acicular ferrite (AF) using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray diffraction (EDX), electron backscatter diffraction (EBSD) and XRD as well as in situ high-temperature laser scanning confocal microscopy (HT-LSCM)
& OM, SEM and EBSD are not sufficient methods to distinguish between the different microstructural constituents formed in the bainite temperature region
Summary
1.1 2.25Cr-1Mo-0.25V weld metalThe heat-resistant V-modified 2.25Cr-1Mo-0.25V-alloy was developed following the conventional low-alloyed 2.25Cr-1Mo steel and approved as ASME Code Case 2098–1 in 1991 [1, 2]. 2.25Cr-1Mo-0.25V-alloy is commonly used in the petrochemical industry where it is applied to hydrocracking reactorsRecommended for Publication by Commission IX - Behaviour of Metals Subjected to Welding2 voestalpine Böhler Welding Germany GmbH, Hafenstraße 21, 59067 Hamm, Germany as well as heavy wall pressure vessels for high-temperature hydrogen service [1, 3]. In 1995, the Italian company Nuovo Pignone was the first to fabricate a reactor from 2.25Cr-1Mo-0.25V-alloy in Europe, leading to the manufacturing of more than 90 further reactors by the end of 2001 [1] This V-modified version of the 2.25Cr-1Mo steel has several advantages such as high strength at elevated temperatures and a good resistance to both hydrogen attack and overlay disbonding. The application of 2.25Cr-1Mo-0.25V-alloy entails some disadvantages too These include low toughness levels in as-welded condition before post weld heat treatment and a higher susceptibility to reheat cracking compared to the conventional Cr-Mo grade [3, 4]. Regarding the application in hydrotreating reactors, 2.25Cr1Mo-0.25V weld metal has to withstand combinations of hydrogen partial pressures up to 13.79 MPa and temperatures up to 482 °C to avoid hydrogen attack [2, 5, 6]
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