Two different hot rolled strip steel qualities where investigated with regard to their weldability. Because of the heat entry during the welding process, mechanical properties like hardness, yield strength, tensile strength, and notch toughness are changing. This is a consequence of the tempering and transformation processes which affect the martensitic matrix. The heat affected zone (HAZ) of a weld seam can be divided into three different areas: the tempered zone, the partly austenited zone and the fully austenited zone. So the mechanical properties and the microstructure are evaluated and characterised as a function of the maximum temperature, which occurs in each of the different parts in the HAZ. The maximum temperature was realised by employing a HAZ-simulator. The HAZ-simulator is able to realise precise time-temperature cycles on a sample, which occur in a HAZ during welding. To confirm the results of the HAZ-simulating process, the mechanical properties are also investigated at real welded seams. Further more, dilatometric measurements are used to characterise the transformation behaviour during quenching. The amount of retained austenite was determined by magnetic measurements with the magnet yoke method. Light microscopy and electron microscopy were used to characterise the micro structure. A theoretical model was applied to estimate the hardness in the tempering zone and another model was used to calculate the prior austenitic grain size.