Investigation of Microstructure and Mechanical Properties for API 51 X70 Steel Welded by Robotic Mig Welding

International Journal of Mechanical Engineering

© 2025 by SSRG - IJME Journal

Volume 12 Issue 12

Year of Publication : 2025

Authors : Nitin Gudadhe, Mridul Singh Rajput, Sanjeev Kumar

10.14445/23488360/IJME-V12I12P105

How to Cite?

Nitin Gudadhe, Mridul Singh Rajput, Sanjeev Kumar, "Investigation of Microstructure and Mechanical Properties for API 51 X70 Steel Welded by Robotic Mig Welding," SSRG International Journal of Mechanical Engineering, vol. 12,  no. 12, pp. 45-53, 2025. Crossref, https://doi.org/10.14445/23488360/IJME-V12I12P105

Abstract:

In the present research, the investigation of the heat-affected zone behavior and weld metal characteristics in API 5L X70 pipeline steel welded using robotic Metal Inert Gas (Robo-MIG) process is studied. The microstructural evolution and its influence on mechanical properties are also analyzed for the same metal. Nine weld samples were fabricated using a Taguchi L9 orthogonal array experimental design. The input parameters, such as current (140-180 A), voltage (24-28 V), and welding speed (2.7-3.6 mm/s), varied systematically to perform various testing and characterization. This mechanical characterization consists of tensile testing, Vickers microhardness measurements, and Charpy V-notch impact testing at both room temperature and sub-zero conditions (-40°C to -100°C). Microstructural analysis is done using optical microscopy. This shows that the distinct phase transformations occur across the base metal, HAZ, and weld zone. It was observed that welding current exerts the most significant influence on mechanical properties. With an increase in the welding current from 140 A to 180 A, the hardness increases from 161 HV to 190 HV. The result shows that a current of 180 A, voltage of 26 V, and speed of 2.7 mm/s are the optimal input parameters, which yield output parameters as maximum tensile strength of 478 MPa and average hardness of 190 HV at a heat input of 2.9 kJ/mm. Due to the formation of Widmanstätten ferrite, bainite, martensite, and Martensite-Austenite (MA), the HAZ shows the highest hardness among all zones. Impact toughness evaluation revealed that the HAZ maintained adequate toughness (28 J average) at -40°C, meeting ASME requirements for cold-service applications. It is observed from the Microstructural analysis that lower heat input conditions produced finer grain structures with higher bainite content and pronounced acicular ferrite. This optimizes the strength-toughness balance. Higher heat input resulted in coarser grains with predominant Widmanstätten ferrite and elevated MA content. These findings reveal the detailed information for optimizing robotic MIG welding parameters to ensure structural integrity and reliable performance of API 5L X70 steel pipelines in oil and gas transmission applications.

Keywords:

API 5L X70 steel, Robotic MIG welding, Microstructure, Impact toughness.

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