Fractographic Investigation of Cleavage Initiation in Steel Friction Stir Welds

There have been significant efforts over the last decade to advance friction stir welding (FSW) technology for joining steel.  A considerable amount of development work has focused on understanding the effect of FSW process parameters and tool materials on weld defects, microstructure and mechanical properties such as strength and toughness.  Friction stir welds on steel typically produce good strength properties, but poor toughness can result.  In the literature, there is limited information available regarding the causes of poor toughness properties in these welds.  In this study, a large number of crack tip opening displacement (CTOD) tests were conducted on several grades of linepipe steel using a variety of FSW parameters.  These CTOD tests produced a wide range of toughness values, some of which were lower than typical acceptance standards.  This paper presents a fractographic investigation that was conducted to study the factors leading to poor CTOD toughness.  Scanning electron microscopy (SEM) was used to examine specimen fracture surfaces and locate inclusions and other features at cleavage initiation sites.  Some of these features were suspected to be martensite-austenite (M-A) particles, but this could not be confirmed using energy dispersive spectroscopy (EDS) because the composition of M-A is similar to the surrounding matrix.  To further investigate these features, focused ion beam (FIB) techniques were used to extract transmission electron microscopy (TEM) samples from specific areas of interest.  The foils were then analyzed using TEM imaging and electron beam diffraction techniques to identify the phases at the area of interest.  This detailed analysis enabled the positive identification of M-A particles at cleavage initiation sites.  The results provide valuable insight into the relationship between microstructure and toughness in steel FSW welds.  The results also demonstrate the utility of the characterization techniques employed in understanding the features that contribute to cleavage initiation in steel.