Page 137 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment

Pearlite + Pearlite +
carbide carbide
Ferrite + pearlite Ferrite + pearlite
200
A E )` 100A
160 fu E 80 Oo 3
g 3 geo, éosd, Q
Gee Q0
Y\%x¢<\ $00 120 _ga gg 09% 6* 50 E
gi
80 03' 0 5 5 00" 2
.ae _,a t 8 5, 4° *QQ
K
~4\®°%\ 40 5 ° o
0 O
0 0.2 0.4 0.6 0.8 1.0 %C 0 0.2 0.4 0.6 0.8 1.0 %C
0 3 6 9 12 15 % Fe3C O 3 6 9 12 15 % Fe3C
O 25 50 75 100 97 % Pearlite 0 25 50 75 100 97 °/0 Pearlite
(H) (D)
FIGURE 4.I9 Mechanical properties of annealed steels as a function of composition and
microstructure. Note in (a) the increase in hardness and strength, and in (b), the decrease in
ductility and toughness, with increasing amounts of pearlite and iron carbide.

4.8.1 The End-quench Hardenability Test
In this commonly used _lominy test (after WE. ]ominy, 1893-1976), a round test bar
100 mm long, made from the particular alloy, is austenitized-that is, heated to the
proper temperature to form 100% austenite. It is then quenched directly at one end
(Fig. 4.20a) with a stream of water at 24°C. The cooling rate thus varies throughout
the length of the bar, the rate being highest at the lower end, which is in direct con-
tact with the water. The hardness along the length of the bar is then measured at var-
ious distances from the quenched end.
As expected from the discussion of the effects of cooling rates in Section 4.7,
hardness decreases away from the quenched end of the bar (Fig. 4.20b). The greater
the depth to which the hardness penetrates, the greater the hardenability of the
alloy. Each composition of an alloy has its particular hardenability band. Note that
the hardness at the quenched end increases with increasing carbon content; note also
that 1040, 4140, and 4340 steels have the same carbon content (0.40%) and thus
they have the same hardness (57 HRC) at the quenched end.
Because small variations in composition and in grain size can affect the shape
of hardenability curves, each lot of an alloy should be tested individually. The data
may be plotted as a band, rather than as a single curve. Hardenability curves are
essential in predicting the hardness of heat-treated parts (such as gears, cams, and
various other components) as a function of their composition.
4.8.2 Quenching Media

The fluid used for quenching the heated alloy also has an effect on hardenability.
Quenching may be carried out in water, brine (salt water), oils, molten salts, or air;
caustic solutions, polymer solutions, and gases are also used. Because of the differ-
ences in thermal conductivities, specific heats, and heats of vaporization of these
media, the rate of cooling of the alloy (severity of quench) is also different. In relative
terms and in decreasing order, the cooling capacities of several quenching media are as
follows: agitated brine, 5; still water, 1; still oil, 0.3; cold gas, 0.1; still air, 0.02.
Agitation is also a significant factor in the rate of cooling. The more vigorous
the agitation, the higher is the rate of cooling. In tool steels, the quenching medium

(HB)
Hardness
l\)
-P
O