Abstract
Microstructures observed by analytical scanning and transmission electron microscopy in the overlap region of a double-sided friction-stir-welded microalloyed steel (EH46) were recorded in detail. They are compared with microstructures in the thermomechanically affected region of the weld and with the base material. The differences suggest that the overlap region has been stirred in the single-phase ferrite, and consists mainly of small equiaxed ferrite grains with strain-induced precipitates, while the thermomechanically affected zone was processed in the austenite-ferrite-phase field, resulting in a mixture of bainite lath packets and ferrite grains. The almost complete absence of pearlite or cementite in the overlap region has led to the suggestion that it dissolved during friction stir welding, providing carbon for strain-induced precipitation. Also, in the complex microstructures of the overlap region, ferrite grains containing a high density of cell-like structures were observed, some of them having precipitates nucleated on their intersections. This implies that strain-induced continuous dynamic recrystallization has occurred.
Highlights
FRICTION stir welding (FSW) is a mature procedure for joining the light metals and alloys of aluminum, magnesium, and titanium.[1,2] More recently, attention has been paid to steels, through work initially on stainless steels[1,3,4,5] and on mild and microalloyed steels.[5,6,7,8,9] This has been achieved through improvements in tools, in particular polycrystalline boron nitride (PCBN) pins for use at higher temperatures, in the range 1273 K to 1473 K (1000 °C to 1200 °C)
The conditions used in the current study considered alongside those of Xue et al.[27] support the assertion that the peak temperature at the base of the pin during FSW was below Ac1
A study has been undertaken using analytical scanning electron microscopy (SEM) and Transmission electron microscopy (TEM), to understand the evolution of microstructure resulting from double-sided FSW of a 14.5-mm-thick shipbuilding grade EH46, niobium-vanadium microalloyed steel
Summary
FRICTION stir welding (FSW) is a mature procedure for joining the light metals and alloys of aluminum, magnesium, and titanium.[1,2] More recently, attention has been paid to steels, through work initially on stainless steels[1,3,4,5] and on mild and microalloyed steels.[5,6,7,8,9] This has been achieved through improvements in tools, in particular polycrystalline boron nitride (PCBN) pins for use at higher temperatures, in the range 1273 K to 1473 K (1000 °C to 1200 °C). The majority of this research has been concentrated on alloys with plate thicknesses £ 10 mm, where the weld can be accomplished in a single pass. The plate thickness which can be welded is limited by the length of the tool, which for PCBN pins, often the pin of choice for FSW of steels, is ~ 8 mm. The plates must be turned after the first pass has cooled to ambient, and a second pass made to complete the weld.[10] The
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