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.)

ß 2006 by Taylor & Francis Group, LLC.

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.)

ß 2006 by Taylor & Francis Group, LLC.

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.)

ß 2006 by Taylor & Francis Group, LLC.

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.)

ß 2006 by Taylor & Francis Group, LLC.

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.