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temperature to the chosen transformation temperature without any undercooling. This cool-
ing process depends on several factors, and the main factors include the workpiece cross-
sectional size, the loading arrangement, the temperature difference between the austenitizing
temperature and the temperature of the cooling medium, and the heat transfer coefficient
between the workpieces’ surface and the ambient.
6.2.4 SOFT A NNEALING (SPHEROIDIZING A NNEALING)
Soft or spheroidizing annealing is an annealing process at temperatures close below or close
above the Ac1 temperature, with subsequent slow cooling. The microstructure of steel before
soft annealing is either ferrite–pearlite (hypoeutectoid steels), pearlite (eutectoid steels), or
cementite–pearlite (hypereutectoid steels). Sometimes a previously hardened structure exists
before soft annealing. The aim of soft annealing is to produce a soft structure by changing all
hard constituents like pearlite, bainite, and martensite (especially in steels with carbon
contents above 0.5% and in tool steels) into a structure of spheroidized carbides in a ferritic
matrix.
Figure 6.71 shows the structure with spheroidized carbides (a) after soft annealing of a
medium-carbon low-alloy steel and (b) after soft annealing of a high-speed steel. Such a soft
structure is required for good machinability of steels having more than 0.6% C and for all cold-
working processes that include plastic deformation. Whereas for cold-working processes the
strength and hardness of the material should be as low as possible, for good machinability
medium strength or hardness values are required. Therefore, for instance, when ball bearing
steels are soft annealed, a hardness tolerance is usually specified. In the production sequence,
soft annealing is usually performed with a semiproduct (after rolling or forging), and the
sequence of operations is hot working, soft annealing, cold forming, hardening, and tempering.
The required degree of spheroidization (i.e., 80–90% of globular cementite or carbides) is
sometimes specified. To evaluate the structure after soft annealing, there are sometimes
internal standards, for a particular steel grade, showing the percentage of achieved globular
Start of ferrite transformation
Ac3
Ac1
Ms
Austenite
Martensite
Time, s
Start of transformation
Tem
pera
ture
, 8C
Bainite
Hardness HRC
Hardness HRB
Pearlite95
9381918433
3531
46
900
700
880
600
500
400
300
200
100
01 10 102 103
1 2 4
1 2 4
1
8
2 3days
105
h24
8 15
min
60
104 105 106
End of transformation
Start of
pearlite
transformation
FIGURE 6.70 Isothermal transformation (IT) diagram of the steel DIN 17CrNiMo6. Austenitizing
temperature 8708C. (From G. Spur and T. Stoferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2,
Warmebehandeln, Carl Hanser, Munich, 1987.)
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cementite, as shown in Figure 6.72 for the ball bearing steel DIN 100Cr6. The degree of
spheroidization is expressed in this case as percentage of remaining lamellar pearlite.
The physical mechanism of soft annealing is based on the coagulation of cementite
particles within the ferrite matrix, for which the diffusion of carbon is decisive. Globular
cementite within the ferritic matrix is the structure having the lowest energy content of all
structures in the iron–carbon system. The carbon diffusion depends on temperature, time, and
the kind and amount of alloying elements in the steel. The solubility of carbon in ferrite, which
is very low at room temperature (0.02% C), increases considerably up to the Ac1 temperature.
At temperatures close to Ac1, the diffusion of carbon, iron, and alloying atoms is so great that
it is possible to change the structure in the direction of minimizing its energy content.
The degree of coagulation as well as the size of carbides after soft annealing is dependent
also on the starting structure before annealing. If the starting structure is pearlite, the spher-
oidization of carbides takes place by the coagulation of the cementite lamellae. This process can
be formally divided into two stages. At first the cementite lamellae assume a knucklebone
shape, as shown in Figure 6.73. As annealing continues, the lamellae form globules at their ends
and, by means of boundary surface energy, split up into spheroids, hence the name spheroidiz-
ing. During the second stage, some cementite (carbide) globules grow at the cost of fine carbide
particles, which disappear. In both stages, the rate of this process is controlled by diffusion. The
thicker the cementite lamellae, the more energy necessary for this process. A fine lamellar
pearlite structure may more easily be transformed to a globular form.
In establishing the process parameters for a soft (spheroidizing) annealing, a distinction
should be drawn among hypoeutectoid carbon steels, hypereutectoid carbon steels, and
alloyed steels. In any case the value of the relevant Ac1 temperature must be known. It can
be taken from the relevant IT or CCT diagram or calculated according to the formula
Ac1 ¼ 739 ÿ 22(% C) þ 2(% Si) ÿ 7(% Mn) þ 14(% Cr) þ 13(% Mo) þ 13(% Ni)
þ 20(% V), [�C] (6:36)
The temperature range for soft annealing of unalloyed carbon steels may be taken from the
iron–carbon diagram as shown in Figure 6.74. The holding time at the selected temperature is
approximately 1 min/mm of the workpiece cross section.
For alloyed steels, the soft annealing temperature may be calculated according to the
empirical formula
(a) (b)
FIGURE 6.71 Structures of (a) a medium-carbon low-alloy steel DIN 50CrMoV4 after soft annealing at
720–7408C and (b) a high-speed steel annealed at 8208C. Magnification 500�. (From G. Spur and
T. Stoferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Warmebehandeln, Carl Hanser, Munich,
1987.)
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FIGURE 6.72 Internal standard of the German company Edelstahlwerke Buderus A.G.-Wetzlar for
evaluation of the degree of spheroidization after soft annealing of grade DIN 100Cr6 steel. Magnifica-
tion 500�. Amount of lamellar pearlite remaining 1, 0%; 2, 8%; 3, 20%; 4, 35%; 5, 60%; 6, 80%. (From
G. Spur and T. Stoferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Warmebehandeln, Carl Hanser,
Munich, 1987.)
FIGURE 6.73 Schematic presentation of the process of transforming cementite lamella to spheroids
during soft annealing. (From K.E. Thelning, Steel and Its Heat Treatment, 2nd ed., Butterworths,
London, 1984.)
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T ¼ 705 þ 20(% Si ÿ % Mn þ % Cr ÿ % Mo ÿ % Ni þ % W) þ 100(% V) [�C] (6:37)
This formula is valid only up to the following values of the alloying elements: 0.9% C; 1.8% Si;
1.1% Mn; 1.8% Cr; 0.5% Mo; 5% Ni; 0.5% W; and 0.25% V. If the steel has higher amounts
of alloying elements, only these indicated maximum values are to be taken into account.
Figure 6.75 shows possible temperature–time regimes for soft annealing. The swinging
regime (Figure 6.75c) is used to accelerate the transformation of cementite lamellae to globular
form. Increasing the temperature aboveAc1 facilitates the dissolution of cementite lamellae. At
subsequent cooling below Ac1 this dissolution process is interrupted and the parts broken off
(which has less resistance to boundary surface energy) coagulate more easily and quickly.
On the basis of the investigations of Kostler, a degree of spheroidization e has been
established that gives the amount of globular cementite compared to the total amount of
G
P
O
K
E1050
1000
Tem
pera
ture
, 8C
950
0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
Carbon content, wt%
1.2 1.3 1.4 1.5 1.6
900
850
800
750
700
650
S
FIGURE 6.74 Temperature range for soft annealing of unalloyed steels having carbon contents of
0.6–1.35% C. (From G. Spur and T. Stoferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Warme-
behandeln, Carl Hanser, Munich, 1987.)
Ac1
Ac1
Ac1
800
Tem
pera
ture
, 8C
500
400
600
(a)
800
Tem
pera
ture
, 8C
500
400
600
700
700
(b)
800
700
Tem
pera
ture
, 8C
500
400
600
(c)
FIGURE 6.75 Temperature–time regimes at soft annealing. (a) Annealing at 208C below Ac1, for
unalloyed steels and for alloyed steels with bainitic or martensitic starting structure; (b) annealing at
108C above Ac1 (start) and decreasing temperature to 308C below Ac1 for alloyed steels; (c) swinging
annealing +58C around Ac1 for hypereutectoid steels. (From G. Spur and T. Stoferle (Eds.), Handbuch
der Fertigungstechnik, Vol. 4/2, Warmebehandeln, Carl Hanser, Munich, 1987.)
ß 2006 by Taylor & Francis Group, LLC.
cementite in a steel after soft annealing. e ¼ 1 means that 100% of the globular cementite (i.e.,
no lamellar cementite) has remained. Because the degree of spheroidization depends on the
time and temperature of the soft annealing process, diagrams may be established that
correlate the degree of spheroidization with the time and temperature of soft annealing.
Figure 6.76 shows such a diagram for the unalloyed steel DIN C35.
The degree of spheroidization, especially above 80% (e ¼ 0.8), has considerable influence on
ultimate tensile strength, yield strength, and elongation, as shown inFigure 6.77 for the unalloyed
eutectoid steel DIN C75. The hardness after soft annealing depends on the time and temperature
of spheroidization, as shown in Figure 6.78 for an unalloyed steel with 0.89% C.
The machinability of steels with more than 0.6% C can be increased by soft annealing as
shown in Figure 6.79, from which it can be seen that decreasing tensile strength and increasing
the degree of spheroidization allows a higher turning speed (v60) in m/min.
The cooling after soft annealing should generally be slow. Depending on the steel grade,
the cooling should be performed as follows:
For carbon and low-alloy steels up to 650 8C (12008F), with a cooling rate of 20–25 K/h
(furnace cooling)
Degree of spheroidization “e”1000
0.40
0.20
0.1 0.2 0.4 0.6 0.8 1 4 6 8 102
0.50 0.60 0.80 0.95 960
920
880
840
800
Annealing time, h
Annealin
g tem
pera
ture
, K
FIGURE 6.76 Time–temperature diagram for soft annealing of the unalloyed steel DIN C35 (previously
deformed 50%), to achieve the required degree of spheroidization. (After Kostler; see H.J. Eckstein
(Ed.), Technologie der Warmebehandlung von Stahl, 2nd ed., VEB Deutscher Verlag fur Grundstoffin-
dustrie, Leipzig, 1987.)
Rm
28850
750
650
550
450
350
250
Tensile
str
ength
(R
m)
Yie
ld s
trength
(R
e),
MP
a
A (L
0 =
80 m
m)
Elo
nga
tion, %
Degree of spheroidization, %
25
22
19
16
13
10100806040200
Re
A
FIGURE 6.77 Change of ultimate tensile strength, yield strength, and elongation with increasing
spheroidization of an unalloyed eutectoid steel, DIN C75. (From H.J. Eckstein (Ed.), Technologie der
Warmebehandlung von Stahl, 2nd ed., VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig, 1987.)
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For medium-alloy steels up to 6308C (11668F), with a cooling rate of 15–20 K/h (furnace
cooling)
For high-alloy steels up to 6008C (11128F),with a cooling rate of 10–15K/h (furnace cooling)
Further cooling below the temperatures indicated is usually performed in air
6.2.5 RECRYSTALLIZATION ANNEALING
Recrystallization annealing is an annealing process at temperatures above the recrystalliza-
tion temperature of the cold-worked material, without phase transformation, that aims at
regeneration of properties and changes in the structure that exists after a cold-forming process
6008C
130
110
90
700 50 100 150 200
Time, h
Hard
ness, H
RB
6258C
6508C6758C7008C
FIGURE 6.78 Hardness of an unalloyed steel with 0.89% C after soft annealing, depending on the
spheroidization time and temperature. (From H.J. Eckstein (Ed.), Technologie der Warmebehandlung
von Stahl, 2nd ed., VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig, 1987.)
ab
200
Turn
ing s
pee
d (v
60),
m/m
in
Tensile strength Rm, N/mm2
150
100
50500 600 700 900800 1000
c
FIGURE 6.79 Influence of the ultimate tensile strength and degree of spheroidization on machinability
of steels for carburizing and structural steels for hardening and tempering, expressed as 1 h turning
speed (v60) in m/min. (a) Spheroidization degree less than 30%; (b) spheroidization degree between 40
and 60%; (c) spheroidization degree greater than 70%. (From G. Spur and T. Stoferle (Eds.), Handbuch
der Fertigungstechnik, Vol. 4/2, Warmebehandeln, Carl Hanser, Munich, 1987.)
ß 2006 by Taylor & Francis Group, LLC.