Mechanism of microstructural deterioration preceding type IV failure in weldment of Mod.9Cr-1Mo steel

• Published in: Metals and materials international Vol. 21; no. 4; pp. 639 - 645
• Main Authors: Lee, J. S., Maruyama, K.
• Format: Journal Article
• Language: English
• Published: Seoul The Korean Institute of Metals and Materials 01.07.2015; Springer Nature B.V; 대한금속·재료학회
• Subjects: Characterization and Evaluation of Materials; Chemistry and Materials Science; Chromium molybdenum steels; Creep (materials); Engineering Thermodynamics; Failure; Fracture mechanics; Grains; Heat affected zone; Heat and Mass Transfer; Holes; Machines; Magnetic Materials; Magnetism; Manufacturing; Materials Science; Metallic Materials; Processes; Solid Mechanics; Structural steels; Weldments; 재료공학; embrittlement; creep; alloy; microstructure; fracture
• ISSN: 1598-9623; 2005-4149
• DOI: 10.1007/s12540-015-4569-5

The objective of the present study was to elucidate the cavity formation mechanism of Type IV failure in weldment of advanced high-Cr ferritic steels. A welded joint of Mod.9Cr-1Mo steel was creep tested at 650 °C under 83 MPa. The creep fracture mode was Type IV failure in the heat affect zone (HAZ). Microstructural characterization of the HAZ and the fracture location, were performed before and after the creep test. The Type IV cracking started in the inter-critical HAZ at a location having fine grain size and coarse M 23 C 6 precipitates. Moreover, the grain structure of the inter-critical HAZ, which is a mixture of soft α and hard α’ grains, plays an important role in the stage of cavity evolution into a crack along the grain boundary. This is due to the heterogeneity of local strain between the two kinds of grains. By a synergistic effect of the strain concentration, the coarse precipitates and heterogeneous strain distribution among grains in the inter critical HAZ, facilitates the nucleation and growth of creep cavities, resulting in premature failure of welded joints.