Page 308 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 11.8 Melting Practice and Furnaces 28

f’\

Gas \\ Crucible
Induction coil
Strip
l Melt

Copper disk

(H) (D)

FIGURE ll.27 (a) Schematic illustration of melt spinning to produce thin strips of
amorphous metal. (b) Photograph of nickel-alloy production through melt spinning. Source:
Courtesy of Siemens AG.

sufficient time to crystallize (see also Fig. 1.10). Rapid solidification results in a sig-
nificant extension of solid solubility, grain refinement, and reduced microsegrega-
tion (see Section 1().2.3), among other effects.
In a common method called melt spinning (Fig. 11.27), the alloy is melted by
induction in a ceramic crucible. It is then propelled under high gas pressure against
a rotating copper disk (chill block), which chills the alloy rapidly (splat cooling).

I |.7 Inspection of Castings

The control of all casting stages-from mold preparation to the removal of castings
from molds or dies-is essential to maintaining good quality. Several methods can
be used to inspect castings to determine their quality and the presence and types of
any possible defects. Castings can be inspected visually, or optically, for surface de-
fects. Subsurface and internal defects are investigated using various nondestructiz/e
techniques (Section 36.10). In destructive testing (Section 36.11), specimens are re-
moved from various sections of a casting and tested for strength, ductility, and other
mechanical properties and to determine the presence, location, and distribution of
porosity and any other defects.
Pressure tightness of cast components (valves, pumps, and pipes) usually is de-
termined by sealing the openings in the casting and pressurizing it with Water, oil, or
air. (Because air is compressible, its use is very dangerous in such tests because of the
possibility of a sudden explosion due to a major flaw in the casting.) For extreme
leaktightness requirements in critical applications, pressurized helium or specially
scented gases with detectors (sniffers) are used. The casting is then inspected for
leaks While the pressure is maintained. Unacceptable or defective castings are
remelted for reprocessing.

I l.8 Melting Practice and Furnaces

The melting practice is an important aspect of casting operations, because it has a
direct bearing on the quality of castings. Furnaces are charged with melting stock,
consisting of metal, alloying elements, and various other materials (such as flux and