Speakers
Description
The integrity of large forged components is strongly influenced by macrosegregation phenomena inherited from ingot solidification. In this work, the failure analysis of a large forged turbine shaft (built by 1.6311 steel) is presented to investigate the relationship between manganese segregation, hydrogen-assisted cracking and MnS inclusion formation.
Ultrasonic inspections revealed an extended internal defect located within the central segregated region of the forging. Chemical characterization showed local enrichment in carbon and manganese, while metallographic investigations highlighted pronounced dendritic structures and severe segregation bands associated with insufficient ingot core consolidation. These segregated regions exhibited significant microstructural heterogeneity, including bainitic and locally martensitic constituents, resulting in hardness values up to 420 HV, compared with approximately 280 HV in the surrounding matrix.
Fractographic and microstructural analyses demonstrated that crack propagation preferentially followed manganese-enriched segregated bands. Secondary microcracks, characteristic of hydrogen-assisted damage, were frequently observed within these areas, indicating that manganese segregation promotes local hydrogen accumulation through microstructural hardening and increased lattice trapping. Furthermore, the same segregation phenomena enhance sulfur partitioning during solidification, favoring the formation of MnS inclusions along interdendritic regions. These inclusions act as additional stress concentrators and hydrogen trapping sites, further reducing local fracture resistance.
The results demonstrate that manganese segregation plays a dual detrimental role: it increases the susceptibility to hydrogen-induced cracking through the formation of hard segregated microstructures and simultaneously promotes MnS precipitation, generating preferential paths for crack initiation and propagation. Both effects originate from the solidification behavior of large steel ingots and are amplified by inadequate homogenization and insufficient forging reduction of the segregated core. The study highlights the critical importance of controlling macrosegregation during ingot production and subsequent forging operations to improve the reliability of large structural steel components.
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