There are many factors and variables involved in the optimization of friction stir processing or welding. These include FSP tool design and tool material, the tool rotating speed, travel speed and processing or plunging force. With friction stir processing or welding of steel, one of the most important inputs is the tool material. The two most common families of materials that have historically been used are poly-crystalline boron nitride (PCBN) based or refractory materials, such as tungsten–rhenium (W–Re) or lanthanated tungsten (W–La). While each of the material families have advantages and disadvantages, in the present study, the PCBN tools was used for FSP treatment due to their ability to have more intricate geometry, which aid in the mixing of the processed materials. There are also multiple grades of PCBN tools materials. For the present study, it was felt that toughness of the tool material was of overriding importance. Thus, Q60 grade was selected for its optimal toughness. The tool has a convex shoulder and stepped spiral pin geometry as shown in Fig. 1