Abstract
The evolution of microstructure and mechanical properties in AISI 8630 low-alloy steel subjected to inertia friction welding (IFW) have been investigated. The effects of three critical process parameters, viz. rotational speed, friction and forge forces, during welding of tubular specimens were explored. The mechanical properties of these weld joints, including tensile and Charpy V-notch impact were studied for determining the optimum welding parameters. The weld joints exhibited higher yield strength, lower hardening capacity and ultimate tensile strength compared to base metal (BM). The maximum strength and ductility combination was achieved for the welds produced under a nominal weld speed of ~ 2900–3100 rpm, the highest friction force of ~ 680–720 kN, and the lowest axial forging load of ~ 560–600 kN. The measured hardness distribution depicted higher values for the weld zone (WZ) compared to the thermo-mechanically affected zone (TMAZ), heat-affected zone (HAZ) and BM, irrespective of the applied welding parameters. The substantial increase in the hardness of the WZ is due to the formation of microstructures that were dominated by martensite. The observed microstructural features, i.e. the fractions of martensite, bainite and ferrite, show that the temperature in the WZ and TMAZ was above Ac3, whereas that of the HAZ was below Ac1 during the IFW. The fracture surface of the tensile and impact-tested specimens exhibited the presence of dimples nucleating from the voids, thus indicating a ductile failure. EBSD maps of the WZ revealed the formation of subgrains inside the prior austenite grains, indicating the occurrence of continuous dynamic recrystallisation during the weld. Analysis of crystallographic texture indicated that the austenite microstructure (i.e. FCC) in both the WZ and TMAZ undergoes simple shear deformation during IFW.
Highlights
A key element for successful exploitation of newly discovered oil and gas fields in challenging geographical locations, both onshore and offshore, is transportation to depots by the aid of pipelines [1]
Previous report on other microalloyed steels shows that in the upper bainite (UB), the carbide precipitates are predominantly observed at the lath boundaries, whereas in the lower bainite (LB), they were inside the laths [19]
The major findings of this study are summarised as follows: 1. The post-inertia friction welding (IFW) microstructural observations showed the formation of microstructure dominated by martensite in the weld zone (WZ), whereas the presence of ferrite, bainite and carbide precipitates, in addition to martensite, was confirmed in the thermo-mechanically affected zone (TMAZ)
Summary
A key element for successful exploitation of newly discovered oil and gas fields in challenging geographical locations, both onshore and offshore, is transportation to depots by the aid of pipelines [1]. Considering the enormous difference in environmental conditions such as high pressures and low temperatures throughout the transportation process, the application of high strength steels has been encouraged to avoid premature failures in service [2]. An excellent candidate material is AISI-8630, designated as SAE-AISI 8630 (G8630) Ni–Cr–Mo steel owing to its optimal combination of strength and ductility over a wide temperature range. The most prevalent usage of AISI-8630 is structural components for subsea applications such as in oil extraction and transportation [3]. The conventional fusion welding process results in a localised heterogeneity in the microstructure at the fusion zone, which makes the weld more susceptible to cracking and premature failure under extreme environmental conditions [4].
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