Achieving ultrafine-grained ferrite structure in friction stir processed weld metal

• A weld metal of high strength low alloy steel was friction stir processed.
• Ultrafine-grained (UFG) ferrite structure was obtained in the processed zone (PZ).
• The grain sizes were 500 and 300 nm in upper and lower parts of PZ, respectively.
• This study provides an effective strategy to prepare UFG ferrite in steel weld metal.

Ultrafine-grained (UFG) ferrite phase was the desired structure in the weld metal of high strength low alloy (HSLA) steel joints. In this study, submerged arc weld metal of a HSLA steel was subjected to friction stir processing (FSP) under a very low rotation rate of 80 rpm. The original coarse bainite structure was changed into UFG ferrite structure after FSP, and the grain sizes were refined to about 500 and 300 nm in the upper and lower parts of the processed zone, respectively. This study provides an effective strategy to preparing UFG ferrite structure in the weld metal of HSLA steel joints, and also a potential welding method.