Microstructure-property relationships of creep-resistant 2.25Cr-1Mo-0.25V submerged-arc weld metal
Research output: Thesis › Doctoral Thesis
2.25Cr 1Mo 0.25V steel is mainly applied for large hydrogen bearing pressure vessels like hydrocracking reactors in the petrochemical industry. Pressure vessels of this kind exhibit high wall thicknesses of several 100 mm, which is why they are usually built by joining multiple components via high performance processes such as submerged-arc welding. Because of the high application temperatures of 400 450 °C in combination with high pressures, a balanced ratio of the base and weld metal’s toughness and creep strength is required for a safe application over several years. In addition, the industry’s demand for higher process efficiency and reduced material and transportation costs requires further improvement of the 2.25Cr 1Mo 0.25V steel’s creep strength while keeping high levels of toughness to increase the process temperature or reduce the reactor’s wall thickness. Although the mechanical properties are severely affected by the stability of the sub microstructure, not much in-depth microstructure investigation has been conducted so far. This doctoral thesis is dedicated to analyzing the impact of numerous factors, such as the welding heat input, post weld heat treatment temperature or time on the 2.25Cr 1Mo 0.25V submerged arc weld metal’s microstructure. This is achieved by combining basic characterization methods, such as light optical microscopy, with more advanced methods such as in situ high temperature laser scanning confocal microscopy and high resolution methods like atom probe tomography and high energy X ray diffraction. The correlation of the microstructural changes on all length scales, ranging from the bead sequence to the dislocation structure with the weld metal's mechanical properties, improves the understanding of fundamental processing structure property relationships. It was found that 2.25Cr-1Mo-0.25V weld metal consists of small amounts of intragranularly nucleated acicular ferrite besides upper, lower and coalesced bainite. The weld metal’s mechanical properties substantially depend on the post weld heat treatment time and temperature, as well as on the weld metal’s bead sequence and the submerged-arc welding heat input. The decrease of the weld metal’s strength and stress rupture time and increase of its impact toughness with longer annealing time or higher annealing temperature is linked to the accelerated recovery of the dislocation structure caused by severe coarsening of fine V and Nb rich MX carbonitrides. The precipitate transformation sequence during annealing at 690 to 720 °C was found to be M3C → M3C+M7C3+MX → M3C+M7C3+MX+M2C+M23C6 → M7C3+MX+M2C+M23C6, whereas prolonged annealing causes the dissolution of M7C3 carbides in favor of MX carbonitrides. The micro hardness distribution over the weld metal’s cross section changes during the post weld heat treatment. Annealing times of more than 12 h at 705 °C evoke an enhanced temper resistance of the heat affected zones between the weld beads of the multi layer weld metal, which is related to a finer prior austenite and ferrite grain size, a reduction of welding related stresses through reaustenitization and a higher amount of larger carbides. The knowledge achieved in this doctoral thesis provides the basis for future adaptions of the 2.25Cr 1Mo 0.25V weld metal’s chemical composition to increase both the creep resistance and toughness.
|Translated title of the contribution||Mikrostruktur-Eigenschaftsbeziehungen von kriechfestem 2,25Cr-1Mo-0,25V Unterpulverschweißgut|
|Publication status||Published - 2022|