1985 年 3 巻 3 号 p. 567-574
The dissolution of TiN in a matrix during synthetic weld thermal cycling was investigated in detail and at the same times, the relationship between the amount of soluble Ti, soluble N, and TiN in a thermal cycled specimen and were investigated quantitatively in this study. The results obtained are as follows;
(1) The amounts of insoluble Ti (soluble Ti) and insoluble N (soluble N) are not affected by Al content.
(2) It is considered that TiN particles dissolve depending upon a solbility product of TiN. (3) And the temperature dependence is denoted by the next equation,
where, 0.01≤Ti≤0.03 wt%, 0.002≤N≤0.008 wt% and 1200≤T≤1410°C. In case of specimens containing a few amount of Ti like less 0.006 wt%, the another smaller solubility product of TiN was obtained and the value was 4.9×10-6 at 1350°C. (4) As the precipitation of TiN was not observed during the cooling process of the thermal cycle, the amounts of soluble Ti and soluble N depend upon the solubility product of TiN at the maximum temperature of the thermal cycle. (5) Transition temperature, vTrs of synthetic weld heat affected zone depends strongly upon the forms of Ti and N, that is shown the following equation,
vTrs=0.229 Sol.Ti+0.0991 Sol.N+0.047 Insol.Ti-75.1
It is very significance for toughness improvmeent to decrease soluble N content. (6) Fraction of fine grained ferrite-pearlite structures increases with the increasing Ti and N content in specimens keeping the stoichiometric ratio of Ti/N=3.42. (7) However, the notch toughness deteriorates a little with the increasing Ti and N content in spite of a constant amount in soluble Ti and soluble N. (8) It is made clear from observation of TEM and SEM that TiN particles of above 0.2 μm in size increase with the increasing Ti and N content in case of Ti/N=3.42, and also TiN particles of above 0.5μm in size were observed on an unit facet. It is considered that these large TiN particles cause the initiation and the propagation of brittle cracks and therefore deteriorates the notch toughness.