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Metallurgical and Micostructural Effect on CVN Impact Toughness in 2,25Cr1Mo Weld Metal

Product Number: MPWT19-15280
Author: Bertoni Dott, Markus Gustafson, Eva-Lena Bergquist
Publication Date: 2019

2,25 Cr 1 Mo steel is required to comply with very stringent technical requirements when such a steel is applied in the Refinery Industry.Most of the attention is focused on the impact toughness performance of the weld metal which provides information on the structural stability of the pressure vessel taking into account the defect’s growth resistance in the welded joint.The assessment of welded joint toughness by performing CVN tests as a measure of this resistance remains the simplest and most widespread. Bainitic microstructure with very low ferrite content and small grain size of the primary austenite are effective in improving the CVN impact toughness at low temperature after min and max PWHT. The aforementioned microstructure is promoted by lowering the temperature of the Bs in addition to the heat input. Carbide precipitation at grain boundaries doesn’t significantly affect the impact properties of the weld metal after the industrial min and max PWHTs. The micro slag inclusions are investigated. Their chemical composition, volume and size is limited in the weld metal and they do not affect the CVN weld metal property. Min PWHT and max PWHT transform the Martensite/Austenite microstructure developed in the weld metal in as welded condition to Bainitic microstructure and the size and distribution of the carbide precipitated into the matrix. The PWHTs do not modify significantly the grain size of the microstructure which is mainly managed by chemistry and heat input. The type of current DC+ and AC significantly affect the chemistry of the weld metal providing different content of C, Mn, Si, Oxygen. Their contributions enhance the CVN impact properties especially when AC is used