hrs. at the test temperature in a separate oven and later re- heated to the desired temperature on the test machine and allowed to stabilize for 30 minutes before the test began.
Tensile testing was done according to Standards EN 10 002- 1 at room temperature and 10 002-5 at elevated temperature. A comparison between both was run at elevated temperature and yielded interesting information (See phase 2).
Fatigue tests were carried out in fully reversed tension (R = -1) on a Servo Hydraulic MTS machine at a frequency of 50 Hz. The test-piece is toroïdal with a 5 mm minimum diameter and is machined from the AFNOR diameter 18 mm blank.
Creep tests consisted of applying a constant stress of 30 MPa for up to 300 hrs and continuously monitoring the progres- sive deformation. The test was automatically interrupted if the sample reached 4% deformation. The test piece, also ma- chined from the AFNOR blank, is cylindrical with a 4 mm diameter.
The heat conductivity measurements were done by the LNE (Laboratoire National d’Essais, Trappes, France) who used a method based on the direct measurement of: a (thermal diffusivity) ρ (density)
c ( specific heat)
From this data, the thermal conductivity λ is then calculated: λ = a ∗ ρ ∗ c
Measurements were made at room temperature, 150C (302F), 200C (392F), 250C (482F) and 300C (572F). For all elevated temperature measurements, the samples were preheated 100 hrs. at the consid- ered temperature prior to being shipped to the LNE, where they were briefly reheated before the test. One measure- ment was made at room temperature on a sample which had been preheated and soaked for 100 hrs. at 300C (572F), in order to estimate the evolution which occurs in the combustion chamber of a cylinder head.
results and discussion
a) tensile properties: influences of mg, Cu and fe
Phase 1 yielded the following results at room temperature, 250C (482F) and 300C (572F): • Mg has a strong and surpris- ingly non-linear effect between 0 and 0.10% on the TYS, which increases by 100MPa in this
20
very narrow domain. The effect on UTS is equally non-linear and still quite notable: + 50 MPa. Con- versely, the elongation decreases from roughly 10% to 6%.
• V has no significant effect at room temperature. (See Figure 3)
• With regard to the elevated temperature proper- ties, the effect of Mg is very limited, causing slight gains in TYS and E% and losses in UTS, which constitutes a rather unusual combination.
• V has no notable effect even at 250 (482F) – 300C (572F). This justifies the decision to separately study its effect on creep and, in phase 2, to inves- tigate more closely the tensile properties at a con- stant level of V, which is 0.20+/- 0.01%. (See Fig- ure 4 & 5)
Phase 2: As the previous phase had shown a very steep variation of the room temperature TYS and UTS between Mg 0.05% and 0.10%, the main objective of phase 2 was to study this effect up to 0.20% (0.19% being the achieved value). In this phase, HIPed and non-HIPed properties were measured. The graphs hereafter show the influences of Mg and Cu at room temperature, 250C (482F) and 300C (572F):
room temperature properties: influence of mg and Cu:
In this graph, the results of Phase 1 have been included to obtain a complete view of the effect of Mg from 0 to 0.20%. The non-linearity is striking, the curves (especially that of TYS) resembling sigmoïds. The major observation seems to be that a plateau of properties occurs between roughly Mg 0.10% and 0.20% (Figure 6).
Figure 3. Room temperature properties at Mg 0.00%, 0.05% and 0.10%. International Journal of Metalcasting/Summer 2011
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