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This paper describes an investigation on the weldability of a double normalized and tempered 5% Ni steel, using weld simulation to estimate the heat-affected zone (HAZ) ductility at cryogenic temperatures. A series of large scale tensile tests have also been carried out on 5% Ni butt welds, the results of which are reported in Part 2 of this WRC Bulletin. Charpy-V specimens were subjected to various weld simulation cycles and heat treatments and subsequently broken at a range of cryogenic temperatures. Very high peak temperatures result in a sharp drop in impact strength, especially if high heat input welding is simulated. Very low base metal and HAZ ductilities can result from artificial aging, such as a plastic deformation followed by heating in the 200-400C temperature range; this suggests that care must be taken when cold deformation is applied or distortions are corrected in structures made of 5% Ni steel. The ductility can be restored by heat treating the specimens for instance for 1 hr in a very narrow temperature range at around 650C.