• One should however keep in mind that unlike tensile properties, creep is not characterized by a simple value. The slight differences observed after 300 hrs. under 30 MPa may become important at a longer time or under a different stress. We our- selves were led to replace the former method (σ 0.1/100) based on 100 hrs. tests because it did not differentiate alloys as effectively as the present one with 300 hrs. tests, especially when the transition from secondary to tertiary creep is considered.1
d) thermal Conductivity
The solidification rate, at least within the explored range 15 sec. – 60 sec. – 180 sec, proved to have no effect. The eu- tectic modification also showed a very small influence (less than 2% in relative terms), if any.
The important factors are: • Temperature • Heat treatment. T7 increases the conductivity by 20% at RT and 10% at 300C (572F), with the F condition being the reference (Figure 13).
• The 100 hrs. soaking at 300C (572F). It increases the conductivity by 10% at RT. This means that, in a real cylinder head, the RT heat conductivity of the hottest areas of the combustion chamber will progressively increase during the first hours of use. Similar but less strong and slower effects are to be expected in cooler zones of the heads.
Conclusions
The original objectives of this study have been at- tained: 1. A limited addition of Mg, 0.10% to 0.20%, is sufficient to strongly improve the room temperature TYS (+ 100 MPa) and LCF (life duration∗10), this at the cost of a significant but tolerable reduc- tion of the elongation.
2. The effect of Mg on LCF as well as on the tensile properties is surprisingly non- linear with a steep variation between 0.05 and 0.10% followed by a plateau between 0.10% and 0.20%. This should also make the industrial use of the alloy easier as no narrow Mg specification will be required.
3. This addition of Mg also proved to have an unexpected favorable effect on the creep strength, which was already high in the Mg-free alloy.
4. The decisive effect of V on 300C (572F) creep strength has been confirmed, but it
International Journal of Metalcasting/Summer 2011
Figure 11. Effects of V on 300C (572F) creep strength under 30 MPa Load at two Mg levels.
23
is possible to reduce the V level notably from the original 0.25%. Down to 0.17%, the creep strength remains constant, and it is only moderately affect- ed by a further diminution to 0.10% or less.
Figure 9. 300C Tensile Properties “NLV” (after 100 hrs. at 300C (572F).
Figure 10. Room Temperature Wöhler Curves at R=-1, and 0, 0.05, and 0.10% Mg.
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