Date of Award
31-8-2024
Document Type
Thesis
School
School of Mechanical Engineering
Programme
Ph.D.-Doctoral of Philosophy
First Advisor
S.Raghuraman
Keywords
Modified 9Cr-1Mo, Fiber Laser Welding, Pure CO2 Shielding Gas, Underfill, Reduced HAZ width
Abstract
Creep Strength Enhanced Ferritic (CSEF) martensitic modified 9Cr-1Mo (P91) steel is extensively employed in fast breeder nuclear reactors and thermal power plants owing to its superior resistance to thermal fatigue and oxidation corrosion compared with other low-alloy steels. Despite its excellent high-temperature performance, the weldability of this steel remains a critical concern.
Conventional arc welding processes generate excessive heat input and complex thermal cycles, resulting in heterogeneous microstructures, retention of δ-ferrite, and subsequent degradation in service life. In contrast, high energy density beam welding techniques such as laser and electron beam welding offer precise thermal control and minimal heat-affected zones (HAZ). However, research on the laser welding of P91 steel remains limited. The present investigation aims to comprehensively evaluate and optimize laser welding and post-weld heat treatment (PWHT) parameters for P91 steel using Response Surface Methodology (RSM).
The study was conducted in three distinct phases. Phase I focused on optimizing laser power (LP), welding speed (v), and focal position (FP) to achieve desirable weld bead geometry under argon and carbon dioxide shielding atmospheres. Analysis of variance (ANOVA) identified welding speed as the most influential parameter affecting penetration depth, bead width, and HAZ width. Optimum welding conditions yielded maximum penetration depths of 5.5 mm (argon) and 6.6 mm (CO₂) at low heat inputs of 0.077–0.094 kJ/mm, corresponding to minimal HAZ widths of 0.36 mm and 0.20 mm, respectively.
Phase II investigated the influence of tempering temperature, holding time, and heating rate during PWHT on the hardness and microstructure of the fusion zone. Hardness decreased with increasing temperature and time, while FESEM–EDS analyses confirmed the formation of a uniform tempered microstructure characterized by the redistribution of M₂₃C₆ and MX-type precipitates and the reduction of internal strain.
Phase III evaluated the mechanical performance of as-welded and PWHT joints. The PWHT specimens exhibited significant improvements in impact toughness (61% for argon and 41% for CO₂ shielding), alongside acceptable tensile strength and bend ductility. Fractography revealed ductile fracture features, and residual stress analysis showed a shift from compressive to tensile nature after PWHT.
Overall, the optimized fiber laser welding and PWHT conditions produced deep-penetration, low-heat-input welds with narrow HAZ and homogenized microstructure, thereby enhancing mechanical performance and demonstrating their suitability for high-temperature service in power plant applications.
Recommended Citation
S, Rakesh Mr, "Experimental Investigation and Evaluation of Mechanical Properties and Microstructural Characterization on Chromium Molybdenum Steel Processed through LASER based Welding" (2024). Theses and Dissertations. 85.
https://knowledgeconnect.sastra.edu/theses/85