Based on the studies of electrochemical stability of Mo carbides, a new method for state analysis of M
2C in Cr-Mo steel was established. By use of this method, the precipitation behavior of carbides during creep rupture test was examined.
Results obtained are as follows:
1) M
2C in Cr-Mo steel was electrochemically less stable than carbides of M
3C, M
7C
3 and M
23C
6, but almost as stable as M
6C. The electrochemical stability of carbides in Cr-Mo steel is in the following order M
2C≅M
6C<M
3C<M
7C
3≅M
23C
6.
2) M
2C can be selectively decomposed by the secondary electrolysis of primary electrolytic residues, prepared in the porous-graphite electrode, at the anode potential of +0.65 V vs. SCE in 4% methyl salicylate-1% salicylic acid-2% LiCl-methanol. M
6C is also decomposed at the same condition.
3) The amount of M
2C can be determined by the dissolved amount of Mo after the secondary electrolysis. When M
6C is present in primary electrolytic residues, the amount of M
2C is calculated by subtracting the amount of Mo as M
6C, amount of which is calculated from the dissolved amount of Fe (Fe as M
6C) after the secondary electrolysis, from the dissolved amount of Mo by the same treatment.
4) The amount of M
2C precipitated during the creep rupture test is found to roughly correspond to the change in the master rupture strength of the specimens. The creep rupture strength is increased with increasing the precipitation amount of M
2C in these specimens.
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