Controlling hard zone formation in friction stir processed HSLA steel

Abstract

a b s t r a c tThe present study was undertaken to determine the effects of friction stir process conditions on mitigating the "hard zone" often present in FSW HSLA steels. The primary parameters investigated were travel speed, heat input, and cooling rates. The hard zone consists of primarily fine lath ferrite. There are strong linear correlations between the ferrite lath width, hardness of the hard zone, and weld cooling rate. The hard zone was eliminate at cooling rates below 20 • C s −1 in FSW HSLA-65.

Modeling of thermal cycles and microstructural analysis of pipeline steels processed by friction stir processing

“…However, according to our numerical simulation, it is unlikely that such a significant difference in cooling rates exists in such a small distance. In addition, Nelson et al [15] reported that cooling rates below 20°C s −1 eliminated the presence of the hard zone, while we found the same microstructures up to around 45°C s −1 . Difference between our FSW modeling results and the open literature [14,15] is related to chemical composition of the steels, thermal and strain history before processing or welding, and welding parameters.…”

“…In addition, Nelson et al [15] reported that cooling rates below 20°C s −1 eliminated the presence of the hard zone, while we found the same microstructures up to around 45°C s −1 . Difference between our FSW modeling results and the open literature [14,15] is related to chemical composition of the steels, thermal and strain history before processing or welding, and welding parameters. Therefore, taking into account the material flow and thermal history dependency on the metallurgical aspects of the workpiece [30], tool design, and type of material [4] is important to better interpret the produced microstructures and its evolution.…”

“…Post-welding cooling rates have been reported as the main factor controlling microstructures through the SZ [14,15]. However, based on the numerical simulation, cooling rate difference between HZ and HAZ-RS was only 6.3°C s −1 , which seems not enough to cause substantial changes in the microstructure at the different zones.…”

“…Cooling rates have been reported between 20 and 240°C s −1 for the stir zone (SZ) and hard zone (HZ) [14]. HZ is usually found in the advancing side and the top side close to the centerline of the joint [15]. HZ exhibits higher hardness than the rest of the SZ; therefore, it is supposed that HZ presents higher peak temperatures and cooling rates than the rest of the SZ [15,16].…”

“…HZ is usually found in the advancing side and the top side close to the centerline of the joint [15]. HZ exhibits higher hardness than the rest of the SZ; therefore, it is supposed that HZ presents higher peak temperatures and cooling rates than the rest of the SZ [15,16]. Peak temperatures above 1150°C with heating rates of 10 and 100°C s −1 caused austenite coarsening of approximately 20 μm in a X80 pipeline steel [17].…”