Trial production of wrapper tubes of advanced material (IIB)

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The second trial production of wrapper tubes was made on the carbide precipitation hardened ferritic steel, followed by the selection of the suitable chemical compositions and heat treatment obtained from the plates. The addition of B was of particular interest in the chemical compositions of 0.1C-11Cr-0.4Mo-2W-0.2V-0.05Nb-0.05N steel. The test results are summarized as follows. (1)In the system of 0.1C-11Cr-0.4Mo-2W-0.2V-0.05Nb-0.05N steel, the addition of B with 20ppm was found ineffective in terms of such mechanical properties as tensile strength and hardness, and rather resulted in degradation in charpy impact properties. (2)The suitable heat treatment condition was chosen that could meet the requirement of elevated temperature tensile strength with (650C) 40kgf/mm and (650C)30kgf/mm which was thougth difficult to satisfy among others. When 1050 C was selected for normalization temperature, tempering temperature was required below 725C, while normalization temperature higher than 1100C could allow the tempering temperature up to 750C. (3)The requirement in impact properties (As normalized and tempered : DBTT20C, 650C5 year service : DBTT150C) could be met even without tempering when normalized at 1050 and 1100C. While, normalized at 1200 and 1300C, the requirement could not be met, even when tempering was given at 750C (4)The chemical compositions (0.1C-11Cr-0.4Mo-2W-0.2V-0.05Nb-0.05N) and heat treatment condition (1050C normalized and 700C tempered), which was established on the plates, was extended to trial production of wrapper tubes and they were evaluated in terms of quality and properties. The results are as follows. (a)Dimensions and defects met the specification. (b)Microstructure was full martensitic and grain size was fine. ...