Friction stir welding of hypereutectoid steel (SK5) below eutectoid temperature

The welded joints of the hypereutectoid steel (SK5, AISI-SAE W1-8), which is a high carbon steel with 0·85 wt-%C, were studied by friction stir welding below and above the A 1 point (eutectoid temperature 723°C). Defect free joints can be successfully fabricated, and the joint structure and the mechanical properties were investigated for both conditions. The microstructures of the above A 1 joints mainly consisted of martensite. Therefore, the microhardness of the above A 1 joints is significantly increased to more than 1000 HV. The tensile tests results are scattered in which some of the joints are fractured at the stir zone while the others are fractured at the base metal. On the other hand, the microstructures of the below A 1 joints consist of a ferrite with globular cementite, and grain refinement occurred because no phase transformation to the martensite structures occurred. The microhardness of the below A 1 joints is slightly greater than the hardness of the base metal. All of the below A 1 joints are fractured at the base metal during the tensile test.