石油学会誌

In dissimilar weld joints such as ferritic steel and austenitic steel, properties of heat affected zone are by heat treat ment, but it accelerates diffusion of carbon producing decarburized and carburized zone around the weld bond. Then, we studied relations between conditions of post weld heat treatment and the characteristic of decarburized zone by means of dissimilar weld joint consisting of 21/4 Cr-1 Mo, 0.5 Mo and D309 Mo steel. Some of the results are summarized as below.
(1) Width of decarburized zone in weld bond was calculated from heating temperature and time according to diffusion formula.
(2) Properties of weld bond was much affected by the heating temperature than the holding time. Sharpy impact energy was lowered at 700°C×3hr in 21/4-1 Mo and 650°C×8hr in 0.5 Mo.

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Many kinds of gas purification processes are used in various industries to remove acid gases from other gas streams. In a few processes, however, it has been known that stress corrosion cracking (S.C.C.) occurred.
Recently, in the G.-V. CO₂ removal process using activated hot potassium carbonate solution, cracks considered as a kind of S.C.C. were likewise experienced in the equipment, i.e., regenerator, heat exchanger and piping, which were made of carbon steel. These cracks were typically transgranular, while the cracks in other gas treating processes using amine or ammonium solution were usually intergranular as is well known in the case of caustic or nitrate solution.
The following were the results of the plant tests;
1) the cracks were easily duplicated on the ring weld specimens installed in the regenerator,
2) in the absorber, the duplicated cracks were relatively small,
3) high tensile strength steel was more sensitive to the cracking than mild steel.
Although it is difficult to deduce the mechanism of the cracking in G.-V. solution, the cracking might be related with the unstable steel surface. In order to prevent such a cracking, the stress relief annealing is an effective measure.

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