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 A c1 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 A c3 A c1 M s Austenite Martensite Time, s Start of transformation Temperature, °C Bainite Hardness HRC Hardness HRB Pearlite 95 93 81 91 84 33 35 31 46 900 700 880 600 500 400 300 200 100 0 1 10 10 2 10 3 1 2 4 1 2 4 1 8 2 3 days 10 5 h 24 8 15 min 60 10 4 10 5 10 6 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. Sto ¨ferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Wa ¨rmebehandeln, Carl Hanser, Munich, 1987.) ß 2006 by Taylor & Francis Group, LLC.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
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