## 1 Introduction  
1 Cr11 Ni2W2 MoV heat-resistant steel is commonly used in aero-engine high-temperature components. Welding is the technique most used to fabricate complex shapes. However, the traditional fusion welding methods cause many solidification defects and bad mechanical properties when used to join this type of steel. [PERSON] and [PERSON] [1] reported that the heat affected zone (HAZ) width of a tungsten inert gas (TIG) welded 1 Cr11 Ni2W2 MoV joint was 14 mm, while the weld width was 4.8 mm. In addition, the HAZ showed a high reduction in the fatigue limit and tensile strength. [PERSON] et al. [2] studied the microstructure and mechanical properties of flash welded 1 Cr11 Ni2W2 MoV steel. They found that the weld microstructure contains martensite and delta ferrite. Moreover, quenching and tempering heat treatments were required to improve joint plasticity and impact toughness. On the other hand, [PERSON] et al. [3] used diffusion bonding to join 1 Cr11 Ni2W2 MoV steel. They reported that the surface was required to be finished by 2000# gri SiC paper produce good joint quality. However, the process was very time consuming, and the joint size was limited to the available equipment. Consequently, finding new welding techniques for 1 Cr11 Ni2W2 MoV heat resistant steel is a crucial issue. Friction stir welding (FSW) is a solid-state welding technique invented in 1991 by The Welding Institute (TWI) for the main purpose of joining aluminum alloys [4]. The heat generated due to the plastic deformation and friction between the tool and workpiece material plasticizes the material below the tool shoulder and around the pin. Thus, the plasticized material is welded as the tool traverses in the solid state under the tool pressure and stirring. The heat input and peak temperature during the process are lower than those produced during the traditional fusion welding methods. Thus, the microstructure changes away from the nugget zone are smaller after the FSW. The welds are manufactured with low distortion, residual stress and solidification defects [5]. Moreover, FSW can reduce the energy consumed compared to fusion welding by up to 60% [6]. The FSW parameters such as tool rotation, welding speed, plunge depth, plunge rate and tool tilt angle should be selected carefully to avoid or reduce the welding defects. It is well known that tool rotation and welding speed play the most significant role in the FSW process. The rotation speed is mainly responsible for generating the heat and stirring the material at the tool-workpiece interface, while the welding speed controls the cooling rate after welding [7]. To create a good joint, a suitable combination of tool rotation and welding speed is crucial. To date, FSW becomes one of the most powerful techniques in joining low softening temperature alloys such as aluminum and magnesium alloys [8; 9; 10; 11]. However, there is a great interest in the last decade in using FSW to join high softening temperature materials such as steel [12; 13]. The major challenges in the welding of steels such as carbon steel (CS), high strength steel (HSS) and stainless steel (SS) by FSW are the expensive tool material and choosing suitable welding parameters [14]. Many tool materials are utilized to weld steel by FSW. The polycrystalline boron nitride (PCBN) [15; 16] and tungsten-based refractory metals [17; 18; 19] are the FSW tool materials most commonly used for steel. Several studies have been performed to join stainless steel by FSW. The FSW of territ stainless steel [13; 20] and austenitic stainless steel [21; 22] have been carried out. However, no investigations have studied the feasibility of using FSW to join 1 Cr11 Ni2W2 MoV martensitic heat-resistant steel due to its high hardness and the severe tool wear that might occur during welding. Numerical simulation is an advantageous and successful method to study the FSW process.