Carbide Diagrams and Precipitation of Alloying Elements during Aging of Low-Alloy Steels
- PDF / 499,555 Bytes
- 5 Pages / 597.28 x 777.28 pts Page_size
- 20 Downloads / 239 Views
Transition Velocities in Fe-3.4 Pet C-0.25 Pet Si Alloys with Al203 Tube End
Rate Sequence Rate in/zm/s (Run Distance in mm) 150(20)-100(20)-50(15)-10(15)-5(15)-2.5(10)-1(5) 5(5)-50(20)-100(20)-50(20)-20(15)-10(15)-5(10) 100(25)-50(25)-10(15)-5(15)-2.5(15)-1(10)
Vg.w > 150 Vg_w > 100 Vg.w > 10
10 pct H2 + Ar 10 pct H2 + Ar pure Ar
able that this effect reduces the growth kinetics of the gray flakes and thereby promotes the white iron growth. In discussing the work of Dawson,t3.4.5] showing the effectiveness of H in promoting the ability of Te additions to induce white iron, it was argued that the probable reason for the effect was that the H reduced Te compounds, such as oxides, thereby allowing more Te to remain in solution within the liquid and enhancing its adsorption at the graphite/liquid interfaces. Ironically, it was stated that the H effect was probably not important in the reported experiments,tll but the present results indicate that there was probably a small partial pressure of H present because of the epoxy seal and that this H played a crucial role in controlling the transition velocities. To further study the H effect, an experiment was run at a constant rate of 1/xrn/s for a distance of 85 mm. The first 14 mm was run under Ar, the next 60 mm under Ar + 10 pct H, and the final 11 mm under Ar. Vacuum purging was employed to change the atmospheres. The initial 14 mm under Ar produced type-A graphite. At a point located 35 mm from the location where Ar + 10 pct H was introduced, the structure changed to type D, and at a point 2.1 nun from where Ar was introduced for the final 11 mm, the structure reverted to type A. The results are consistent with the action of H being the reduction of oxides of Te in the melt produced by low levels of oxygen in the Ar atmosphere. The much longer lag time for the initial change can be attributed to the much larger volume of liquid present at the first gas interchange coupled with the longer liquid column from the growth front to the top of the liquid. Contrary to the above proposed mechanism, it is possible that the reduction of the transition velocities is caused by the action of H itself without an effect on the Te. To check this possibility, experiments were made on an alloy of Fe4.3 pct C-0.25 pct Si without addition of Te. The Si content was reduced to enhance the possibility of forming white iron, and experiments were run with and without H present. The results are given in Table IV, and it is seen that the H by itself does not promote white iron but, in fact, seems to promote gray iron. In conclusion, we have made the following discoveries. 1. Small additions of H to the atmosphere present in directional solidification experiments of Te-doped cast irons have a large effect on the gray-white and white-gray transition velocities. 2. Addition of 10 pct H to the atmosphere allows growth of white cast iron at rates down to 1 and 2.5/zm/s in irons containing 1 and 2 wt pct Si, respectively. 3. It appears that the action of the H is to reduce compounds of Te, most l
Data Loading...
