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FEATURE
A D V A N C E D
M A T E R I A L S
&
P R O C E S S E S |
M A R C H
2 0 1 6
4 7
13
more than color. As a result, the spectrophotometer was not
useful for its intended function. However, differences in re-
flectivity proved meaningful, showing that bright-appearing
samples can exhibit more alpha case than non-bright sam-
ples. Figure 1 summarizes these results. The two coupons
heated to 1450°F show a yellowish tint. The two coupons
heated to 1750°F are matte gray. The coupon of Cycle 5 ap-
pears as bright as the virgin piece. Based on appearance, the
sample from Cycle 5 might suggest that there is no alpha
case. However, Test Piece 5 exhibits considerablymore alpha
case than the yellow tinted samples from Tests 2 and 4.
RGA data reveals that water vapor is the primary oxi-
dizing residual gas resulting in alpha case. As temperature
rises, the partial pressure of carbon monoxide and carbon
dioxide increase to contribute to alpha case formation. Yet
ultimately, the trends show a continuous decrease in partial
pressure of furnace gases due to the strong gettering effect
of titanium (Fig. 2).
Metallographic results show that visual inspection can-
not reliably be used as an indication of alpha case. Varying
the ramp rate or incorporating intermittent temperature
holds did not affect the amount of alpha case formed in the
low temperature cycles (Fig. 3) or high temperature cycles
(Fig. 4). Comparison of Cycles 3 and 5, which were one-step
fast ramps to 1750°F, shows that increased surface area re-
duces the amount of alpha case formation (Fig. 5).
Evaluation of three different metallographic etchants
indicate that 2% HF best distinguishes the delineation be-
tween alpha case and base metal (Fig. 6). However, etch-
ing time is all-important in generating an accurate reading
Fig. 1 —
Summary of test runs and resulting alpha case and color.
Fig. 2 —
RGA trends show strong gettering effect of titanium.
Fig. 3 —
Metallographic photos of samples heated to 1450°F at
different ramp rates.
Fig. 4 —
Metallographic photos of samples heated to 1750°F at
different ramp rates.
Fig. 5 —
Metallographic photos show that increased surface area
reduces alpha case.