The erosion/corrosion damage symptom was experienced in a part of units made of carbon steel in nuclear power plants. The damage was erosion/corrosion one caused by the wet steam stream containing little dissolved oxygen. Therefore, erosion/corrosion tests were executed about various kinds of steel having different additional elements. As the results of the tests, it has been concluded that 11/4Cr-1/2Mo steel has excellent erosion/corrosion resistance. This material was applied to actual plants to solve the above problem for the time being.
However, since this 11/4Cr-1/2Mo steel is heat-resisting steel for hot pressure vessels primarily, it contains chromium and molybdenum in addition to about 0.16% of carbon. It is necessary for the prevention of cracking in the welds of this steel to keep preheat and interpass temperature at 200 to 350°C during welding and perform dehydrogen heating process at 400°C for 30 min after completion of welding. These procedure is very much complicated, as compared with the procedure for the welding of carbon steel. Furthermore, the material's strength is not so much important at the elevated temperature higher than 400°C, because the maximum temperature of the steam is 282°C in the units concern. Therefore, the reduction of carbon content has been investigated, for the improvement of the weldability of 11/4Cr-1/2Mo steel. The effect of carbon content has been examined on tensile properties, Charpy absorved energy, the resistance against weld cracking and erosion/corrosion properties. As the results of the investigations, (1) the carbon content can be reduced to 0.07 to 0.10% in case of annealed type of steel, securing its tensile properties and improving weldability, (2) the annealed type of steel can be welded free from cracking without dehydrogen heating process, keeping the interpass temperature more than 225°C, and (3) the hydrogen diffusion parameter τ=Σi(Di/Δti) can be used to prevent multi-pass weld cracking.
