isothermal heat treatment.pdf

7/27/2019 Isothermal heat treatment.pdf

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a well-differentiated, nontextured ferritepearlite structure is the optimum structure for

machinability of these steels. If low-carbon steels are soft annealed, they give long shavings

when turned and a bad surface appearance (sometimes called smearing or tearing)

because of the accumulation of the material on the tools cutting edge. On the other hand,

nonannealed workpieces, having harder structural constituents like bainite, result in heavywear of the cutting edge when machined.

An isothermally annealed structure should have the following characteristics:

1. High proportion of ferrite

2. Uniformly distributed pearlite grains

3. Fine lamellar pearlite grains

4. Short pearlite lamellae

5. Coarse ferrite grains

Figure 6.65 shows the structure of a thin-wall die forging made of low-alloy steel for

carburizing (DIN 16MnCr5) after a normalizing anneal (Figure 6.65a) and after an isother-

mal annealing process (Figure 6.65b). The desired ferritepearlite structure originates during

an isothermal annealing, the principle of which is explained by Figure 6.66. This figure shows

an IT diagram of a low-alloy steel for carburizing (DIN 15CrNi6) with superimposed coolingcurves for different cooling rates at continuous cooling. The slowest cooling rate of 3 K/min

relates to a furnace cooling, and the fastest cooling rate of 3000 K/min relates to a quenching

process. From the diagram in Figure 6.66 it can be clearly seen that bainite formation can be

avoided only by very slow continuous cooling, but with such a slow cooling a textured

(elongated ferrite) structure results (hatched area in Figure 6.66). There is only one way to

avoid both the formation of bainite and the formation of a textured structure (see the open

arrow in Figure 6.66), and this is the isothermal annealing process, which consists of

10 30 75

150

300

600

Diam.=

1000m

m

Ac3

Ac1

P

A 100

35 93

75

20

53 5B

M

796

Ms

Hardness HV 870

900

800

700

600

500

400

300

200

100

01 101 102

102 103 104

min1011Time, s

103 104 105 106s

796

772 743 454 363 370 285

753

782

786

Temperature,

C

FIGURE 6.64 CCT diagram of the alloyed steel DIN 55NiCrMoV6 (austenitizing temperature 9508C),

with superimposed cooling curves measured in the core of round bars of different diameters cooled in

air. (From G. Spur and T. Stoferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Warmebehandeln,

Carl Hanser, Munich, 1987.)

2006 by Taylor & Francis Group, LLC.


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austenitizing followed by a fast cooling to the temperature range of pearlite formation

(usually about 6508C (12008F)), holding at this temperature until the complete transform-

ation of pearlite, and cooling to room temperature at an arbitrary cooling rate. The tempera-turetime diagram of an isothermal annealing is given in Figure 6.67. The metallurgical

mechanism of a good isothermally annealed structure depends on the austenitizing conditions

as well as on the temperature and time of the isothermal transformation and on cooling from

the austenitizing temperature to the isothermal transformation temperature.

The austenitizing temperature and time should be high enough to completely dissolve all

carbides, to homogenize the austenite matrix, to stabilize the austenite structure, and achieve

a coarse-grained ferritepearlite structure after cooling. The undesired textured structure

originates by preeutectoid ferrite precipitation along stretched phases acting as germs, for

instance manganese sulfides, carbon segregations, or aluminum nitride precipitations. These

phases have been stretched as a consequence of a preliminary hot-forming process.

To avoid the textured structure the steel has to contain as little sulfur, nitrogen, and

aluminum as possible, and during austenitizing a complete dissolution of nitride precipita-

tions and carbides should be achieved. Therefore the austenitizing temperature is adequately

high, i.e., about 1008C (2128F) above Ac3, and the holding times are usually about 2 h.

FIGURE 6.65 Structure of a forging made of low-carbon steel for carburizing (DIN 16MnCr5) (a) after

normalizing and (b) after isothermal annealing. Magnification 200. (From G. Spur and T. Stoferle

(Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Warmebehandeln, Carl Hanser, Munich, 1987.)

Field of textured structure

1000

500

P

400

M

A

B

F3000

300

303 K/min

Isothermalannealing

320 250 170 HV

0102 101 102 103101

Time, min

T

emperature,

C

FIGURE 6.66 The principle of isothermal annealing. TTT diagram of the low-alloy steel for carburizing

DIN 15CrNi6. (From J. Wunning, Harterei-Tech. Mitt. 32:4349, 1977, pp. 4349 [in German].)



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Another very important condition to avoid a textured structure is to realize a minimum

cooling rate between the austenitizing temperature (%9508C (%17508F)) and the isothermal

transformation temperature (%6508C (12008F)). Thus, about 3008C (5728F) decrease should

pass through at a minimum cooling rate of 2040 K/min. This means that the whole batch of

treated workpieces should be cooled from about 9508C (17508F) to about 6508C (12008F) inless than 10 min. During this cooling process an undercooling below the chosen isothermal

transformation temperature must be avoided to prevent the formation of bainite.

The physical mechanism that accounts for the manner and magnitude of ferrite precipi-

tation is the carbon diffusion during cooling from the austenitizing temperature. To achieve a

good structure after isothermal annealing, all measures that reduce the carbon diffusion rate

or restrict the diffusion time for carbon atoms during cooling are useful.

Figure 6.68 shows three structures after isothermal annealing of the low-alloy steel DIN

16MnCr5. It can be seen that cooling too slowly from the austenitizing temperature to the

transformation temperature results in an undesirable textured structure of ferrite and pearlite,

and if during this cooling process an undercooling takes place (i.e., the transformation

temperature has been chosen too low) before the pearlite formation, then bainite will be

present in the structure, which is not allowed.

Big companiesusually have internal standards to estimate the allowabledegreeof texturing

of the isothermally annealed structures, with respect to machinability, as shown in Figure 6.69.

The transformation temperature and the necessary transformation time for the steel in question

may be determined by means of the appropriate IT diagram. Figure 6.70 shows such a diagram

for the steelDIN 17CrNiMo6.Ascanbe seen, the lowerthe transformation temperaturechosen,

1000Ac3

Ac1

Temperature,

C800

600

400

200

0Time

FIGURE 6.67 Temperaturetime cycle of isothermal annealing. (From G. Spur and T. Stoferle (Eds.),

Handbuch der Fertigungstechnik, Vol. 4/2, Warmebehandeln, Carl Hanser, Munich, 1987.)

FIGURE 6.68 Different structures after isothermal annealing of the low-alloy steel DIN16MnCr5 (left).

Well-distributed ferritepearlite; correct annealing (center). Textured ferritepearlite structure; too slow

cooling from the austenitizing to the transformation temperature (right). Ferrite pearlitebainite;

undercooling before pearlite transformation. (From J. Wunning, Harterei-Tech. Mitt. 32:4349,

1977, pp. 4349 [in German].)



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thesooner thetransformationstarts,up toa temperature (the so-calledpearlite nose) atwhich the

shortest time to start the transformation is achieved. Below this temperature, longer times are

again necessary to start the transformation. In the range of the pearlite nose temperature, fine

lamellarpearlite will be formed, and the time to complete pearlite transformation is the shortest.

For unalloyed steels, the pearlite nose temperatures are between 550 and 5808C (1022 and

10768F), while for alloyed steels they are between 640 and 6808C (1184 and 12568F). The

optimum isothermal annealing temperature is 10208C (50688F) above the pearlite nose

temperature.The necessary transformation time depends on the alloying elements in the steel. In the

practice of isothermal annealing the holding time at the transformation temperature includes

an adequate reserve because of compositional tolerances in different steel heats. Usually for

low-alloy steels for carburizing and structural steels for hardening and tempering the trans-

formation times are below 2 h.

From the technical standpoint, when a batch of workpieces has to be isothermally

annealed, the biggest problem is to realize sufficiently fast cooling from the austenitizing

FIGURE 6.69 Internal standard of the German company Edelstahlwerke Buderus A.G.-Wetzlar forestimation of the allowable degree of texturing of the structure after isothermal annealing. Magnifica-

tion 100. (From G. Spur and T. Stoferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Warmebe-

handeln, Carl Hanser, Munich, 1987.)



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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 ANNEALING (SPHEROIDIZING ANNEALING)

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 ferritepearlite (hypoeutectoid steels), pearlite (eutectoid steels), or

cementitepearlite (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 machinabilitymedium 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 ofoperations is hotworking, soft annealing, cold forming, hardening, andtempering.

The required degree of spheroidization (i.e., 8090% 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

Temperature,

C

Bainite

Hardness HRC

Hardness HRB

Pearlite 959381918433

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

pearlitetransformation

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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