Abstract
Based on the studies of electrochemical stability of Mo carbides, a new method for state analysis of M2C 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) M2C in Cr-Mo steel was electrochemically less stable than carbides of M3C, M7C3 and M23C6, but almost as stable as M6C. The electrochemical stability of carbides in Cr-Mo steel is in the following order M2C≅M6C<M3C<M7C3≅M23C6.
2) M2C 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. M6C is also decomposed at the same condition.
3) The amount of M2C can be determined by the dissolved amount of Mo after the secondary electrolysis. When M6C is present in primary electrolytic residues, the amount of M2C is calculated by subtracting the amount of Mo as M6C, amount of which is calculated from the dissolved amount of Fe (Fe as M6C) after the secondary electrolysis, from the dissolved amount of Mo by the same treatment.
4) The amount of M2C 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 M2C in these specimens.
