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In this thesis, the heat affected zone (HAZ) of Gleeble simulated welds, the weldments and the creep specimens for several types of 9%-12% Cr ferritic steels were studied by focusing on the Type IV cracking in the fine grained zone (FZ). The field emission gun transmission electron microscopy (FEGTEM) and scanning electron microscopy (SEM) were used to measure the phosphorus segregation on the grain boundary (GB) and the creep fracture morphologies respectively. Meanwhile the well-developed grain boundary segregation and precipitation (GBSP) model was applied to simulate the experimental results. The experimental results have showed that the HAZ zone was characterised by softening and Type IV cracking. All the high Cr ferritic steel welds gave a microstructure of mainly tempered martensite and M23C6 precipitates after the post weld heat treatment (PWHT). There was no δ-ferrite observed in the HAZ. The Type IV cracking exhibited a mixed cracking mechanism in which the intergranular grain boundary separation is dominant due to the crack initiation by voids and the faster M23C6 growth with the service time. A new model on the mechanism of the Type IV cracking is established. The FEGTEM research has also showed obvious non-equilibrium phosphorus segregation at the grain boundaries, which is affected significantly by the quenching temperature. The phosphorus GB segregation deteriorates the weak grain boundaries. The experimental results were well in agreement with the GBSP modelling.