Metallography: Principles and Practices - ASM International

Metallography: Principles and Practice (#06785G)
Author(s): George F. Vander Voort
Surface ^
61
Center 35
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MACROSTRUCTURE 39
Bar diameter, in
2 1± 1 5
Hardness, HRC
^ S
62 62
32.5 38 58 61.5
Figure 1-31 Macroetching (10% aqueous HN03) was used to reveal the extent of hardening in these
AISI 1060 carbon steel round bars.
1-3.13 Response to Heat Treatment
Macroetching can also be used to determine the hardenability of various steel bars
subjected to known heat treatment conditions. This procedure, coupled with
hardness testing, was widely used prior to the adoption of hardenability analysis.
As an illustration, Fig. 1-31 shows discs cut from round bars of AISI 1060 carbon
steel ranging in size from a diameter of 3 A to 2V2 in. The two smallest sizes were
through-hardened, that is, the center region contains more than 50% martensite,
and the etch pattern was uniform. The other three sizes exhibit a case and core
pattern, since the central region was unhardened. For this test, all bars were
austenitized at 1525°F (829°C), brine quenched, and then tempered at 300°F
(149°C). The bar length was twice the diameter, and the etched section was taken
from the center.
Cold etching is also useful in studying the results of surface-hardening
treatments. Figure 1-32 shows the results of induction hardening of gear teeth
made from AISI 1055 carbon steel. The areas hardened and the depth of the
hardened zone are quite apparent.
1-3.14 Flame Cutting
Figure 1-33 illustrates the use of the cold etch to reveal the extent of the heataffected
zone developed during flame cutting of two AISI S5 gripping cams. The
etched discs clearly show the effect of different heat inputs on the depth of the
heat-affected zone.