There are many different steel grades available for various purposes and applications. For semi-finished workpieces with different, specific local requirements with regard to mechanical or chemical properties, a strong joint between the different steel grades is necessary. Additionally, workpieces subject to forming require a sufficient plasticity in the area of the joint. In thermal welding of different metals, a new alloy always appears in the weld itself. This means, for example, that due to different properties, grain growth and interfaces with graded transitions, and induced negative or restrictive consequences, the tendency for crack initiation may increase. Therefore, heat input and the molten area of the weld have to be kept as small as possible. The main part of the work deals with butt welding of round bars with a diameter of 30 mm made of C22 (1.0402) and 41Cr4 (1.7035). For this application, a laser welding process using a high power disk laser is an appropriate tool. First, a controlled melting ratio can be achieved by varying the beam position relating to the gap. Further, the problem of cracking, due to the inhomogeneous alloy formation in the weld, is countered by the introduction of an ultrasonic vibration into the joining zone. With low amplitudes and a frequency of 18 kHz, the homogeneity and the microstructure formation could be influenced. Therefore, the risk of cracking can be reduced. The impact of this approach is tested using metallographic methods.