In this thesis the effect of single and multipass welding on the microstructure and hardness of X65 linepipe steel of four different chemical compositions was investigated. The welding was simulated for three different heat inputs and two second pass peak temperatures. The thermal simulation has been achieved by simulating the CGHAZ with a Gleeble? thermal-mechanical simulator, peak temperatures used for this simulation were derived from dilatometer experiments. Significant overheating of 50 ?C above Ac1 was observed at high heating rates, comparable heating rates of welding. The peak temperature of the first weld cycle was 1350 ?C and the second cycle either 810 ?C or 860 ?C. The overall hardness of the samples was measured as was the micro hardness of M-A constituents. The microhardness of M-A constituents was found to be significantly lower than in previous research, 310 Hv versus 800 Hv. The hardness of the single pass CGHAZ was parametrized for the cooling rate and the chemical composition with an R2 value 0.98. The parametrization shows increased hardenability due to increased Ni content. The CGHAZ microstructures were examined by optical and scanning electron microscopy as well as EBSD, revealing microstructures consisting of martensite, lower bainite, upper bainite, granular bainite, acicular ferrite and M-A constituents. A number of samples were quantitatively described by use of the IQ distribution from EBSD measurements and the M-A constituents were quantified by Lepera’s etchant for all multipass welded samples. A fraction of M-A constituents was found to be higher than 1.6 %all reheated CGHAZ samples and the fraction is found to be dependent on the heat input and hardenability of the steel. It was found that Lepera’s etchant reveals a higher fraction of M-A constituents than what is observed by EBSD measurements. The M-A constituents as revealed by Lepera’s etchant were found to not completely consist martensite and austenite in the steels considered in this research. Therefore the M-A constituents are suspected to be of less importance for the fracture toughness than discussed in previous research.