PURPOSE:To prevent brittle fracture or cracking of the material of the pressure vessel caused by hydrogen by employing the constitution which can reduce the amount of hydrogen diffused in the material. CONSTITUTION:The inner body 3 of chrome-molybdenum steel with the stainless steel padding-welded on the inner surface is provided inside of the body material 1 of chrome-molybdenum steel in which a few hydrogen diffusing holes 4 are bored. Being thus constituted, hydrogen diffused from the inner surface of the pressure vessel during high temperture-high pressure operation is discharged to outside from the diffusing holes 4 through the padded welding 2, the inner body, and the space between the inner body 3 and the body material 1. Then, as the inner body 3 is thin, the residual rate of peak hudrogen is far below the dangerous value of brittle fracture and cracking and becomes a safe value, so that the danger of brittle fracture or cracking of the material is completely avoided.
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