Understanding the changes in viscosity due to the different oxidation states of iron ions is important for simulating phenomena related to molten slags and also for understanding the structure of iron-oxidecontaining silicate melts. However, these viscosity changes are not well understood. Here, we show the viscosity changes of R2O-SiO2-FexO (R = Li, Na, or K) melts due to changes in the oxidation states of the iron ions by systematically varying the oxygen partial pressure using several Ar-based gases at 1 773 K. The initial compositions of the samples were 30R2O-60SiO2-10Fe2O3 (mol%), and the ratio of Fe 3+ to Fe 2+ in the R2O-SiO2-FexO melts increased with increasing oxygen partial pressure in all samples. Meanwhile, the viscosity of all the R2O-SiO2-FexO melts decreased with increasing Fe 2+ to total-Fe ratio. The data indicate that the increase in the amount of Fe 2+ ions, which behave as network modifiers, would result in depolymerization of the silicate melts. It should also be noted that the viscosity of the melts increased in the order of alkali cationic radius (K > Na > Li) when the ratios of Fe 2+ to Fe 3+ in the melts were comparable. This was due to the change in the coordination structure of Fe 3+ in the melts. Not only the oxidation state of the iron ions but also the coordination structure of Fe 3+ may be important for understanding the viscosity.KEY WORDS: viscosity; alkali ferrosilicate melts; redox equilibria of Fe; oxygen partial pressure.

In the present study, the growth rate of optically dark area (ODA) was investigated for two types of bearing steels using a fracture mechanics approach. Ultrasonic fatigue tests were performed for the specimens with hydrogen pre-charging followed by exposing in the atmosphere for a week in order to investigate the effect of hydrogen trapped by inclusions on the growth of ODA. Together with the constant amplitude loading test, twostep and repeated two-step variable amplitude loading tests were performed in order to determine the fatigue crack growth rate by beach marks. The results revealed that fatigue strengths of the both materials were reduced by the hydrogen pre-charging. The size of ODA increased with decreasing the stress amplitude at fracture so that the stress intensity factor range at the end of the ODA growth became a constant value ranging between 4.5 and 5.5 MPa m independent of the materials and the fatigue lives. ODA formation behavior is enhanced by trapped hydrogen by inclusions, which reduces the fatigue strength in the very high cycle fatigue regime.

Now at Maruichi Steel Tube LTD. Viscosity changes of RO-SiO 2-Fe x O (R = Ca, Sr, Ba) melts due to changes in the oxidation states of the iron ions by systematically varying the oxygen partial pressure were measured at 1773 K. The initial compositions of the samples were 30RO-60SiO 2-10Fe 2 O 3 (mol%), and the ratio of Fe 3+ to Fe 2+ in the RO-SiO 2-Fe x O melts increased with increasing oxygen partial pressure in all samples. Meanwhile, the viscosity of all the RO-SiO 2-Fe x O melts decreased with increasing Fe 2+ to total-Fe ratio (Fe 2+ /t-Fe). The data indicate that the increase in the amount of Fe 2+ ions, which behave as network modifiers, would result in depolymerization of the silicate anions. In addition, the viscosity of the melts increased in the order of alkaline-earth cationic radius (Ba > Sr > Ca) when the Fe 2+ /t-Fe values of the melts were comparable. This would be due to the change in the coordination structure of Fe 3+ in the melts.