6.3 HARDENING BY FORMATION OF MARTENSITE

6.3.1 A USTENITIZING

Austenitizing is the first operation in many of the most important heat treatment processes

(hardening, carburizing, normalizing) on which the properties of heat-treated parts depend.

Let us assume the bulk heat treatment of real batches of workpieces and consider the

metallurgical and technological aspects of austenitizing.

6.3.1 .1 Meta llurgi cal Aspec ts of Austeni tizing

The way austenite is formed when a certain steel is heated depends very much on the steel’s

starting microstructure. Let us take as an example an unalloyed eutectoid steel with 0.8% C

and follow the process of its austenitization using the schemes shown in Figure 6.90. At room

temperature the cementite (Fe3C) plates of the pearlite are in direct contact with ferrite ( a-Fe,

see Figure 6.90a). The carbon atoms from cementite have a tendency to diffuse into the ferrite

lattice. The higher the temperature, the greater this tendency is. Upon heating, on reaching

the Ac1 temperature (7238C (13338F)), the transformation of ferrite into austenite (g-Fe)

(b)(a)

FIGURE 6.88 Development of coarse-grained structure during recrystallization of soft iron. (a) Micro-

structure before cold working and (b) microstructure after cold working with very low degree of deform-

ation (10%) and subsequent recrystallization annealing at 7008C. Magnification 500�. (From G. Spur and

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

1987.)

Gra

in s

ize

Degree of deformation at cold working, %

0 10 20 30 40 50 60 70 80 90 100

FIGURE 6.89 Grain growth in the range of the critical degree of deformation (at 10%) for a steel with

0.06% C. Recrystallization temperature, 7008C. (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.

starts immediately adjacent to the cementite plates (see Figure 6.90b). After that the cementite

plates start to dissolve within the newly formed austenite, becoming thinner and thinner

(Figure 6.90c and Figure 6.90d). So two processes take place at the same time: the formation

of austenite grains from ferrite and the dissolution of cementite plates in the austenite lattice.

Experiments have shown that the process of ferrite-to-austenite transformation ends before

all the cementite has been dissolved. This means that after all the ferrite has transformed into

austenite, small particles of cementite will remain within the austenite grains (Figure 6.90e).

Figure 6.91 shows the formation of austenite in a microstructure of eutectoid steel. Areas of

Fe3C γ-Fe

α-Feα-Fe α-Fe

(a) (b) (c) (d) (e) (f)

AusteniteAustenite

FIGURE 6.90 Transformation of a pearlitic structure to austenite when heating an unalloyed eutectoid

steel of 0.8% C.

FIGURE 6.91 Formation of austenite (light patches) from pearlite as a function of time. (From G. Krauss,

Steels: Heat Treatment and Processing Principles, ASM International, Materials Park, OH, 1990.)

ß 2006 by Taylor & Francis Group, LLC.

austenite formation are visible as white patches within the lamellar pearlitic structure. Some of

the cementite persists in the form of spheroidized particles (the small dark spots in the

white areas). They dissolve only with longer holding times at temperature. Once these cementite

particles completely dissolve, the structure consists of only one phase—austenite (see Figure

6.90f). In this state, however, there are still differences in carbon concentration among particu-

lar austenite grains. In spots where cementite plates were previously to be found, the carbon

concentration is high, while in other spots far from cementite plates it is low.

Equalizing of the carbon concentration proceeds gradually by diffusion, resulting in a

homogeneous austenite structure at the end of this process. The holding time at austenitizing

temperature necessary for this process is called the homogenization time. During pearlite–

austenite transformation, several austenite grains are formed from one pearlite grain, i.e., the

newly formed austenite is fine-grained.

Nucleation sites for austenite formation depend on the starting microstructure as

shown in Figure 6.92. In ferrite the nucleation sites are situated primarily at grain bound-

aries. In spheroidized structures nucleation starts on carbide particles, whereas in pearlitic

structures it starts primarily at the intersection of pearlite colonies but also at cementite

lamellae.

6.3.1 .1.1 Kinet ics of Transf ormatio n during Aus tenitizin gFigure 6.93 shows the volume percent of austenite formed from pearlite in eutectoid steel as a

function of time at a constant austenitizing temperature. From the beginning of austenitization

(a) (b) (c)

a1

22

2

1

1

3

Austenite

Carbide

3a2

a3

a3

a2 a2a1a1

FIGURE 6.92 Nucleation sites for austenite formation in microstructures of (a) ferrite; (b) spheroidite;

(c) pearlite. (From G. Krauss, Steels: Heat Treatment and Processing Principles, ASM International,

Materials Park, OH, 1990.)

100

75

50

25

00 5

Volu

me o

f au

ste

nite

, %

10

Time, s

15 20 25 30

FIGURE 6.93 Volume percent austenite formed from pearlite in eutectoid steel as a function of time at a

constant austenitizing temperature. (From G. Krauss, Steels: Heat Treatment and Processing Principles,

ASM International, Materials Park, OH, 1990.)

ß 2006 by Taylor & Francis Group, LLC.

a certain incubation time is necessary to form the first nuclei, and then the process proceeds at a

more rapid rate as more nuclei develop and grow. At higher temperatures the diffusion rate

increases and austenite forms more rapidly, as shown in Figure 6.94.

The duration of austenitizing process depends on the austenitizing temperature and the

steel composition. The influence of time at austenitization can best be explained by the

diagrammatic illustrations shown in Figure 6.95. From Figure 6.95a and Figure 6.95b,

which apply to eutectoid carbon steel of 0.8% C, one can see that if an austenitizing

temperature of 7308C (13468F) is maintained (after a rapid heating to this temperature), the

transformation will start in about 30 s. If instead an austenitizing temperature of 7508C

(13828F) is chosen, the transformation will begin in 10 s, and if a temperature of 8108C

(14908F) is selected, in about 1 s. The transformation of pearlite to austenite and cementite is

in this case completed in about 6 s. If the steel is to be fully austenitic (all carbides dissolved,

hatched area), it must be held at this temperature for about 2 h (7 � 103 s).

Figure 6.95c and Figure 6.95d apply to a hypoeutectoid plain carbon steel of 0.45% C.

They show that in this case at an austenitizing temperature of 8108C (14908F) the transform-

ation from pearlite to austenite starts in about 1 s. In about 5 s the pearlite has been

transformed and the structure consists of ferrite, austenite, and cementite. About 1 min

later the carbon has diffused to the ferrite, which has thereby been transformed to austenite.

Residual particles of cementite remain, however, and it takes about 5 h at this temperature to

dissolve them completely.

Figure 6.95e and Figure 6.95f apply to a hypereutectoid steel containing 1.2% C. If this

steel is austenitized at 8108C (14908F), the pearlite starts to transform in about 2 s, and in

about 5 s the structure consists only of austenite and cementite. It is not possible for the

cementite to be completely dissolved at this temperature. To achieve complete solution of the

cementite, the temperature must be increased above Acm, in this case to at least 8608C

(15808F).

The holding time at austenitizing (hardening) temperature depends on the desired degree

of carbide dissolution and acceptable grain size, taking into account that the grain growth

increases with higher austenitizing temperatures and longer holding times. Since the amount

of carbide is different for different types of steel, the holding time (from the metallurgical

point of view) depends on the grade of steel. However, carbide dissolution and the holding

time are dependent not only on the austenitizing temperature but also the rate of heating to

7518C

100

Au

ste

nite

, %

Time, s

80

60

40

20

01 10 100 1000

7308C

FIGURE 6.94 Effect of austenitizing temperature on the rate of austenite formation from pearlite in a

eutectoid steel. (From G. Krauss, Steels: Heat Treatment and Processing Principles, ASM International,

Materials Park, OH, 1990.)

ß 2006 by Taylor & Francis Group, LLC.

this temperature. Varying the rate of heating to this temperature will have an effect on the rate

of transformation and dissolution of the constituents.

The influence of the role of heating (and correspondingly of the holding time) on carbide

dissolution, grain growth, and hardness after hardening for various grades of steel has been

studied in detail and published in Refs. [18,19]. These time–temperature–austenitizing dia-

grams (Zeit-Temperatur-Austenitisierung Schaubilder in German) have been produced either as

isothermal diagrams (the steel specimens were heated rapidly at the rate of 1308C/s (2668F/s)

to the temperature in question and held there for a certain predetermined time) or as continuous

heating diagrams (the steel specimens were heated continuously at different heating rates).

A

8C900

600

50010−1 1

P

(b)

10 102 103 104 105 s

10−1 1 10 102 103 104 105 s

0

(a)

(c) (d)

10–1 1 10 102 103 104 105 s

(f)

0.2 0.4 0.6 0.8 1.0 1.2 % C

0 0.2 0.4 0.5 0.8 1.0 1.2 % C

(e) 0 0.2 0.4 0.5 0.8 1.0 1.2 % C

700

8C

900

Temperature

Temperature

A3

A1

Acm

Acm

A3

A3

A1

A1

800

600

500

700

8C

900

800

600

500

700

Temperature8C

900

800

600

500

700

A + C

A +P

800

8C

900

600

500

700

800

8C900

600

500

700

800

A + C

F + P

A + C

P + C

F+P+A

A+

P + C

F+A +C

A

Acm

A

FIGURE 6.95 Structural transformations during austenitizing steels containing (a, b) 0.8% C; (c, d)

0.45% C; (e, f) 1.2% C. A, austenite; C, cementite; F, ferrite; P, pearlite. (From K.E. Thelning, Steel and

Its Heat Treatment, 2nd ed., Butterworths, London, 1984.)

ß 2006 by Taylor & Francis Group, LLC.

Consequently, isothermal diagrams may be read only along the isotherms, and the continuous

heating diagrams may be read only along the heating rate lines.

Figure 6.96 shows an isothermal type of time–temperature–austenitizing diagram of grade

DIN 50CrV4 steel. From this type of diagram one can read off, for instance, that if the steel is

held at 830 8C (1526 8F), after about 1 s, pearlite and ferrite will be transformed to austenite,

but more than 1000 s is necessary to completely dissolve the carbides to achieve a homoge-

neous austenite.

In practice, the continuous heating diagrams are much more important because every

austenitizing process is carried out at a specified heating rate. Figure 6.97 shows a time–

temperature–austenitizing diagram of the continuous heating type for grade DIN Ck45 steel.

The continuous heating was carried out at various constant rates ranging from 0.05 to

24008C/s (32.09 to 43528F). If the heating rate was extremely slow (e.g., 0.228C/s (32.48F/s))

to about 7758C (14278F), on crossing the Ac3 temperature after about 1 h all pearlite

and ferrite would have been transformed to inhomogeneous austenite. At a heating rate

1300

1200

1100

1000

900

800

830

7000.01 0.1 1 10

Time, s

Tem

pera

ture

, 8C

102 103

Austenite + carbide

Ferrite + pearlite + austenite

Ferrite + pearliteHeating rate to hardening temp. 1308C/s

ACC

AC3

Homogenuous austenite

AC2

AC1

FIGURE 6.96 Isothermal time–temperature–austenitizing diagram of the steel grade DIN 50CrV4

(0.47% C, 0.27% Si, 0.90% Mn, 1.10% Cr). (From J. Orlich and H.J. Pietrzenivk (Eds.), Atlas zur

Warmebehandlung der Stahle, Vol. 4, Zeit-Temperatur-Austenitisierung-Schaubilder, Part 2, Verlag

Stahleisen, Dusseldorf, 1976 [in German].)

ß 2006 by Taylor & Francis Group, LLC.

of 108C/s (508F/s) the pearlite and ferrite would have been transformed to inhomogeneous

austenite after crossing the Ac3 temperature at about 8008C (14728F) after only 80 s.

A remarkable feature of such diagrams is that they show precisely the increase of Ac1 and

Ac3 transformation temperatures with increasing heating rates. This is especially important

when short-time heating processes like induction hardening or laser beam hardening, with

heating rates ranging to about 10008C/s (18328F/s), are applied for surface hardening. In such

a case this diagram should be consulted to determine the required austenitizing temperature,

which is much higher than in conventional hardening of the same grade of steel. For the steel

in question, for example, the conventional hardening temperature would be in the range of

830–8508C (1526–15628F), but for induction or laser beam hardening processes the hardening

temperatures required are between 950 and 10008C (1742 and 18328F). When heating at a rate

of 10008C/s (18328F/s) to the austenitizing temperature of 10008C (18328F), only 1 s is

necessary, and the above-mentioned short heating time processes operate in approximately

this time range. As Figure 6.97 shows, much higher temperatures are necessary to achieve the

Heating rate, 8C/s

2400

1300

1200

1100

1000

900A

c3

Ac1

Ac2

800

70010−1 1 10

Time, s

Tem

pera

ture

, 8C

102 103 104 105

1000 300 100 30 10 3 1 0.22 0.05

Austenite

Ferrite + pearlite

Inhomogeneous

austenite

Homogeneous austenite

Ferrite + pearlite

FIGURE 6.97 Time–temperature–austenitizing diagram for continuous heating of the steel grade DIN

Ck45 (0.49% C, 0.26% Si, 0.74% Mn). (From J. Orlich, A. Rose, and P. Wiest (Eds.), Atlas zur

Warmebehandlung der Stahle, Vol. 3, Zeit-Temperatur-Austenitisierung-Schaubilder, Verlag Stahleisen,

Dusseldorf, 1973 [in German].)

ß 2006 by Taylor & Francis Group, LLC.

homogeneous austenite structure. In such a case one is, of course, concerned with the grain

growth.

Figure 6.98 shows the grain growth (according to American Society for Testing and

Materials [ASTM]) when grade DIN Ck45 steel is continuously heated at different heating

rates to different austenitizing temperatures. Figure 6.99 shows the achievable Vickers hard-

ness after hardening for grade DIN Ck45 steel austenitized at various heating rates to various

temperatures. It shows, for example, that maximum hardness would be achieved upon

austenitizing the steel at 850 8C (1562 8F) for about 900 s (or heating at a heating rate of

18C/s (33.88F/s)), which corresponds to the field of homogeneous austenite (see Figure 6.97).

The hardness after quenching, which depends on the amount of carbide dissolution, is also

dependent on the initial structure of the steel. This is illustrated in Figure 6.100. Figure 6.100a

shows that a structure of spheroidized cementite (after soft annealing) of the hypoeutectoid

DIN Cf53 carbon steel will attain the maximum hardness of 770 HV when heated at a rate

of 18C/s (33.88F/s) to 8758C (16098F) (holding time 855 s or 14 min). The hardened and

Heating rate 8C/s

Tem

pera

ture

, 8C

24001300

1200

1100

1000

900

800

700

10−1 1 10

Time, s

103102 104 105

1000 300 100 30 10 3 1 0.22

Ac3

Ac1

0.05

11 to 10

4 to 3

4

6

8

9

10

Gra

in s

ize (

AS

TM

):

FIGURE 6.98 Time–temperature–austenitizing diagram for continuous heating showing the grain

growth of steel grade DIN Ck45. (From J. Orlich, A. Rose, and P. Wiest (Eds.), Atlas zur Warmebe-

handlung der Stahle, Vol. 3, Zeit-Temperatur-Austenitisierung-Schaubilder, Verlag Stahleisen, Dussel-

dorf, 1973 [in German].)

ß 2006 by Taylor & Francis Group, LLC.

tempered structure (tempered martensite) of the same steel, as shown in Figure 6.100b,

will attain the maximum hardness of 770 HV, however, if heated to 875 8C (1609 8F) at the

rate of 1000 8C/s (1832 8F/s) (holding time less than 1 s). For this reason, when short-time

heating processes are used, the best results are achieved with hardened and tempered initial

structures.

For eutectoid and hypereutectoid steel grades, which after quenching develop substantial

amounts of retained austenite, the attainment of maximum hardness after quenching is more

complicated. Figure 6.101 shows the hardness after quenching for the ball bearing hypereu-

tectoid grade DIN 100Cr6 steel (1.0% C, 0.22% Si, 0.24% Mn, and 1.52% Cr). The maximum

hardness of 900 HV after quenching is attained on heating to a very narrow temperature

range, and furthermore this temperature range is displaced toward higher temperatures as the

heating rate is increased. If this steel is quenched from temperatures that exceed the optimum

range, the resulting hardness is reduced owing the presence of an increasing amount of

retained austenite.

Heating rate 8C/s

Tem

pera

ture

,8C

24001300

1200

1100

1000

900

800

7000.1 1 10 102 103 104 105

Time, s

1000 300 100 30 10 3 1 0.22 0.05

780

Ac1

Ac3

840

840

820

800780

840

800

Hard

ness a

fter

quenchin

g (

HV

):

FIGURE 6.99 Time–temperature–austenitizing diagram for continuous heating showing the achievable

hardness after hardening steel grade DIN Ck45. (From J. Orlich, A. Rose, and P. Wiest (Eds.), Atlas zur

Warmebehandlung der Stahle, Vol. 3, Zeit-Temperatur-Austenitisierung-Schaubilder, Verlag Stahleisen,

Dusseldorf, 1973 [in German].)

ß 2006 by Taylor & Francis Group, LLC.

For plain carbon and low-alloy structural steels, which contain easily dissolved carbides, a

holding time of 5–15 min after they have reached the hardening temperature is quite enough

to make certain that there has been sufficient carbide dissolution. For medium-alloy struc-

tural steels this holding time is about 15–25 min. For low-alloy tool steels, it is between 10 and

30 min; and for high-alloy Cr steels, between 10 min and 1 h.

6.3.1.2 Technological Aspects of Austenitizing

In heating metallic objects to their austenitizing (hardening) temperature, there are two kinds

of heating rates to be distinguished: those that are technically possible and those that are

technologically allowed.

The technically possible heating rate is the heating rate the heating equipment could

realize in actual use. It depends on

1. The installed heating capacity of the equipment

2. The heat transfer medium (gas, liquid, vacuum)

3. The temperature difference between the heat source and the surface of the heated

objects (workpieces put in a hot or cold furnace)

4. The mass and shape of the workpiece (the ratio between its volume and superficial

area)

5. The number of workpieces in a batch and their loading arrangement

Ha

rdn

ess a

fte

r q

ue

nch

ing

, H

V1 900

800

700

600700 800 900 1000

Temperature, 8C

1100 1200 1300

1

1000

900

800

700

600700 800 900 1000 1100 1200 1300

100101

Rate of heating in 8C/s

Rate of heating in 8C/s

Temperature, 8C

Hard

ness a

fter

quenchin

g,

HV

1

1000100

10

(a)

(b)

FIGURE 6.100 Hardness after quenching as a function of the rate of heating and austenitizing tem-

perature for grade DIN Cf53 steel (hypoeutectoid carbon steel) (a) for soft-annealed condition and (b)

for hardened and tempered condition. (From K.E. Thelning, Steel and Its Heat Treatment, 2nd ed.,

Butterworths, London, 1984.)

ß 2006 by Taylor & Francis Group, LLC.

The technologically allowed heating rate is the maximum heating rate that can be applied

in actual circumstances, taking into account the fact that thermal stresses that develop within

the workpiece must not exceed the critical value because this could cause warping or cracking,

since sections having different dimensions heat up at different speeds and large temperature

gradients can arise between the surface and the core of the workpiece. This heating rate

depends on

1. The mass and shape of the workpiece (the ratio between its volume and superficial

area)

2. The chemical composition of the material

3. The initial microstructure

When workpieces of heavy sections or of complicated shapes are heated, temperatures

between 250 and 6008C (482 and 11128F) are particularly dangerous, because in this tem-

perature range the steel does not have enough plasticity to compensate for thermal stresses.

If the heating of an object is asymmetrical, the object will warp. If thermal stresses are

developed that overstep the strength of the material (which is substantially lower at higher

temperatures), cracks will result.

If the heating rate is too high through the transformation temperature range (between Ac1

and Ac3), warping may occur because of volume change of the structure lattice. The tendency

of a steel to crack during heating depends on its chemical composition. Carbon content has

the decisive influence. The higher the carbon content, the greater the sensitivity to cracking.

The complex influence of carbon and other alloying elements is expressed by the following

empirical formula termed the C equivalent (Cekv):

Cekv ¼C þ Mn

5þ

Cr

4þ

Mo

3þ

Ni

10þ

V

5þ

Si ÿ 0:5

5þ

Ti

5þ

W

10þ

Al

10(6:40)

where the element symbols represent wt% content. This formula is valid up to the following

maximum values of alloying elements.

Temperature, 8C

Hard

ness a

fter

quenchin

g, H

V1

1 10 1000

1000

900

800

700

600

500700 800 900 1000 1100 1200 1300

Rate of heating in 8C/s

100

FIGURE 6.101 Hardness after quenching as a function of the rate of heating and austenitizing tem-

perature for grade DIN 100Cr6 steel initially soft annealed. (From K.E. Thelning, Steel and Its Heat

Treatment, 2nd ed., Butterworths, London, 1984.)

ß 2006 by Taylor & Francis Group, LLC.

The values of the alloying elements actually present are put into the formula in wt%. If

the amount of an alloying element exceeds the limit given above, then the indicated maximum

value should be put into the formula.

The higher the calculated Cekv value, the greater the sensitivity of the steel to cracking. For

instance,

Cekv � 0.4: The steel is not sensitive to cracking (it may be heated quite rapidly).

Cekv ¼ 0.4–0.7: The steel is medium sensitive to cracking.

Cekv � 0.7: The steel is very sensitive to cracking (when heating up a preheating

operation should be included).

The initial microstructure also has some influence on the technologically allowed heating

rate. A steel with a homogeneous microstructure of low hardness may be heated more rapidly

than a steel of high hardness with inhomogeneous microstructure.

The thermal gradients and consequently the thermal stresses developed when heating to

austenitizing temperature can usually be diminished by preheating the workpiece to tempera-

ture lying close below the transformation temperature Ac1 and holding it there until

temperature equalizes throughout the cross section.

The theoretical time–temperature diagram of the austenitizing process is shown in Figure

6.102. Practically, however, there is no such strict distinction between the heating and soaking

C � 0.9% V � 0.25%

Mn � 1.1% Si � 1.8%

Cr � 1.8% Ti � 0.5%

Mo � 0.5% W � 2.0%

Ni � 5.0% Al � 2.0%

1—preheating 2—heating up3—heating through (thermal soaking)4—structure homogenizing (metallurgical soaking)Ta—austenitizing temperature ta —austenitizing time

Ta

1

2

3

4

T 8C

ta

t h

Surf

ace Core

FIGURE 6.102 Austenitizing process (theoretically).

ß 2006 by Taylor & Francis Group, LLC.

times. Contrary to the generally widespread belief that the surface of the steel reaches the

preset temperature considerably earlier than the center, the closer the temperature of the steel

approaches the preset temperature, the smaller the temperature difference between surface

and core, as shown in Figure 6.103. It can therefore be assumed that when the surface has

reached the preset temperature, part of the soaking time (depending on the cross-sectional

size) has already been accomplished. Certainly, one has to be aware of the corner effect—

corners, sharp edges, and thin sections reach the preset temperature much earlier than the

core of the workpiece.

The most important parameters of every austenitizing process are:

1. The austenitizing temperature

2. The heat-up and soak time at austenitizing temperature

For each grade of steel there is an optimum austenitizing (hardening) temperature

range. This temperature range is chosen so as to give maximum hardness after quenching

and maintain a fine-grained microstructure. It can be determined experimentally as shown

in Figure 6.104 and Figure 6.105. From Figure 6.104 it is clear that the lowest possible

hardening temperature for the steel in question is 850 8C (1562 8F). A lower hardening

temperature would result in the formation of bainite and even pearlite with inadequate

hardness.

When the hardening temperature is increased (see Figure 6.105), the grain size and

the amount of retained austenite increase. At 920 and 970 8C (1688 and 1778 8F) the retained

austenite may be discerned as light angular areas. On the basis of these experiments,

the optimum hardening temperature range for the steel in question has been fixed at 850–

880 8C (1562–1616 8F). The optimum hardening temperature range for unalloyed steels can be

determined from the iron–carbon equilibrium diagram according to the carbon content

of the steel. This range is 30–50 8C (86–1228F) above the Ac3 temperature for hypoeutectoid

steels and 30–50 8C (86–1228F) above Ac1 for hypereutectoid steels, as shown in Figure 6.106.

Because the curve S–E in this diagram denotes also the maximum solubility of carbon

∅1 × 4 in. I ∅ 25 × 100 mm

∅2 × 7 in. II ∅ 50 × 175 mm

∅4 × 8 in. III ∅ 100 × 200 mm

I II

III

Temperature

min

10001832

1652

1472

1292

1112

932

752

572

392

212

32

800

900

700

600

500

400

300

200

100

0

0 2 4 86 10 12 14 16 18 20 22 24 26 28

Heating-up time

8F 8C

FIGURE 6.103 Time–temperature curves for steel bars of different diameters heated in a salt bath at

10008C. Full line, measured temperature at surface; dashed line, measured temperature at center. (From

K.E. Thelning, Steel and Its Heat Treatment, 2nd ed., Butterworths, London, 1984.)

ß 2006 by Taylor & Francis Group, LLC.

in austenite, it is clear that the higher the austenitizing (hardening) temperature, the

more carbon can be dissolved in austenite. For alloyed steels the optimum austenitizing

(hardening) temperature range depends on the chemical composition, because different

alloying elements shift the A1 temperature to either higher or lower temperatures. For

FIGURE 6.104 Microstructures of a steel having 1% C, 1.5% Si, 0.8% Mn, and 1% Cr, hardened from

hardening temperatures between 800 and 8508C. Dimensions of test pieces: 30-mm diameter� 100 mm.

Magnification 400�. (a) Hardening temperature 8008C, hardness 55 HRC; (b) hardening temperature

8258C, hardness 61.5 HRC; (c) hardening temperature 8508C, hardness 66 HRC. (From K.E. Thelning,

Steel and Its Heat Treatment, 2nd ed., Butterworths, London, 1984.)

ß 2006 by Taylor & Francis Group, LLC.

these steels, therefore, data from the literature on the optimum hardening temperature range

have to be consulted.

It should also be mentioned that increasing the austenitizing temperature causes the

following effects.

FIGURE 6.105 Microstructures of steel having 1% C, 1.5% Si, 0.8% Mn, and 1% Cr, hardened from

hardening temperatures between 870 and 9708C. Dimensions of test pieces: 30-mm diameter� 100 mm.

Magnification 400�. (a) Hardening temperature 8708C, hardness 62.5 HRC, retained austenite 12%; (b)

hardening temperature 9208C, hardness 62 HRC, retained austenite 20%.; (c) hardening temperature

9708C, hardness 61 HRC, retained austenite 28%. (From K.E. Thelning, Steel and Its Heat Treatment,

2nd ed., Butterworths, London, 1984.)

ß 2006 by Taylor & Francis Group, LLC.

1. It increases the hardenability of the steel because of the greater amount of carbide

going into solution and the increased grain size.

2. It lowers the martensite start temperature ( Ms). Owing to the more complete carbide

dissolution, the austenite becomes more stable and starts to transform upon quench-

ing at lower temperature.

3. It increases the incubation time, i.e., the time until the isothermal transformation to

pearlite or bainite starts. This is expressed as a shift in the start of transformation

curves in an IT diagram to later times.

4. It increases the amount of retained austenite after quenching due to stabilization of

the austenite, which at higher temperatures is more saturated with carbon from

dissolved carbides.

Heat-up and soak time at austenitizing temperature is a very important parameter for

bulk heat treatment because it not only determines the furnace productivity and economy

(consumption of energy) but may also affect the properties of the treated workpieces. Until

recently there was no reliable, objective method for accurately predicting heat-up and soak

times for heat treatment cycles that took into account all workpiece characteristics, variations

in furnace design, and load arrangement. Current determinations of heat-up and soak time

are based on either a very conservative and general rule (e.g., 1 h/in. of cross section) or some

empirical method, the results of which [20] differ substantially.

By heat-up and soak time we mean the time it takes for the heated workpiece to go from

starting (room) temperature to the preset temperature in its core. The main factors that

influence heat-up and soak time are diagrammed in Figure 6.107.

On the basis of experiments with 26 specimens (cylinders, round plates, and rings of

various dimensions) made of unalloyed and low-alloy structural steels, Jost et al. [20] found

from core temperature measurements that the heat-up and soak time depends substantially on

the geometry of the heated workpiece and its mass. They found the heat-up and soak time to

be directly proportional to the mass/surface area ( m/A, kg/m2) ratio, as shown in Figure 6.108.

By regression analysis for their conditions (the specimens were heated in an electrically heated

chamber furnace of 8 kW capacity and 240 � 240 � 400 mm working space, to the hardening

Iron−carbon equilibrium diagram

α' + pearlite

Pearlite

Austenite

T 8C

1148E

912

727

0.77 2.11C, wt%

The range of optimun hardening temperatures

A cm

γ

γ + Fe3C0

α + γ

S

A1

A3

0

G

Pearlite + Fe3C

FIGURE 6.106 Optimum hardening temperature range for unalloyed steels, depending on the carbon

content.

ß 2006 by Taylor & Francis Group, LLC.

temperature, 8708C (15988F)), they found that the heat-up and soak time (t) can be calculated

using the equation

t ¼ 0:42(m=A)ÿ3:7 (6:41)

The regression coefficients 0.42 and 3.7 are, of course, valid for their experimental conditions

only. Comparison with their experimentally obtained results (see the points in Figure 6.108)

showed a standard deviation of s2 ¼ 1.4 min2, or s ¼ +1.2 min, indicating that this way of

predicting heat-up and soak time in specific circumstances may be quite precise.

The Jost et al. [20] approach may be used generally for prediction of heat-up and soak

times according to the general expression

t ¼ a(m=A) þ b (6:42)

provided that for a given situation the straight line of regression and relevant values of the

regression coefficients a and b are fixed by means of some preliminary experiments. It should

be stressed, however, that the described results of this investigation are valid for single

workpieces only.

In another investigation [21], a method enabling heat treaters to accurately determine the

heat-up and soak times for different loads treated in batch-type indirect fired furnaces was

Workpiece

Heat-up and soak time

Shape

size

Surface area

Mass

Heat conductivity

Temperature

Number of work-

pieces

Loading arran-

gement

Trays

Type

Working space

Heating mode and

installed capacity

Heat transfer-

medium

Temperature

distribution

Actualconditions

Furnace

FIGURE 6.107 The main factors that influence the heat-up and soak time. (From S. Jost, H. Langer,

D. Pietsch, and P. Uhlig, Fertigungstech. Betr. 26(5):298–301, 1976 [in German].)

40

30

20

t, m

in

10

030 40 50 60 70 80 90 100 110 120

50

m/A , Kg/m2

FIGURE 6.108 Dependence of the heat-up and soak time on the mass/surface area ratio, (m/A). (From

S. Jost, H. Langer, D. Pietsch, and P. Uhlig, Fertigungstech. Betr. 26(5):298–301, 1976 [in German].)

ß 2006 by Taylor & Francis Group, LLC.

developed. To develop the method, a statistical and experimental investigation of load

temperature conditions was performed. A computer-aided mathematical model of heat and

mass transfer throughout the furnace and load was developed. The computer model accur-

ately predicts the suitable heat-up and soak times for various types of furnace loads, load

arrangements, workpiece shapes, and thermal properties. The treated loads were divided into

several groups in terms of workpiece allocation and aerodynamic patterns of the furnace

atmosphere, as shown in Figure 6.109.

The experiments with six different loads were conducted in indirectly fired batch furnaces,

the working space of which was of length 915–1680 mm, width 610–1420 mm, and height 610–

1270 mm. The furnaces were equipped with four burners firing into the trident burner tubes

located on the side walls, with a circulating fan located on top of the furnace as shown in Figure

6.110. The thermocouples were located in different parts of the load (measuring always the

surface temperature of the workpieces)—on the top and bottom, in the core, at the corners, and

on the surfaces facing radiant tubes—to determine temperature variations across the load.

As can be seen from Figure 6.110, the heat and mass transfer in the furnace and load are very

complicated and are characterized by nonlinear three-dimensional radiation and convection

and by nonlinear heat conduction within the workpieces. In this case, the mathematical

Monolayer, horizontally oriented, ordered loads

Monolayer, horizontally oriented, random loads

Multilayer ordered and random loads

Vertically oriented loads

Packed

(a)

(b)

(c)

(d)

Spaced

Packed Spaced Bulk

FIGURE 6.109 Load characterization. (a) Monolayer, horizontally oriented, ordered loads; (b) mono-

layer, horizontally oriented, random loads; (c) multilayer ordered and random loads. (d) Vertically

oriented loads. (From M.A. Aronov, J.F. Wallace, and M.A. Ordillas, System for prediction of heat-up

and soak times for bulk heat treatment processes, Proceedings of the International Heat Treatment

Conference on Equipment and Processes, April, 18–20, 1994, Schaumburg, IL, pp. 55–61.)

ß 2006 by Taylor & Francis Group, LLC.

model to describe the heat and mass exchange is a system of integral and differential nonlinear

equations. The input parameters to the computer program were as follows:

Geometrical data of the furnace and load: Furnace working space dimensions,

radiant tube diameter and layout in the furnace, dimensions of the baskets, number

of trays in the basket, workpiece characteristic size

Type of load (according to load characterization, see Figure 6.109)

Type of steel (carbon, alloyed, high-alloy)

Load thermal properties

Load and furnace emissivities

Temperature conditions (initial furnace and load temperature)

Fan characteristic curve parameters

Composition of protective atmosphere

As an example, maximum and minimum steel part temperatures for a test (heating of

shafts) together with the calculated data are shown in Figure 6.111. The experimental data

show that the temperature curve of the load thermocouple usually reaches the set furnace

temperature well within the soak time requirements. The experimentally determined soak

time is seen to be considerably shorter than the soak time defined by the heat treater. It was

found that the discrepancy between soak times determined from the test data and calculations

does not exceed 8%, which is acceptable for workshop practice.

The developed computer model was used for simulation of temperature conditions for

different load configurations, and a generalized formula and set of graphs were developed.

The generalized equation for the soak time determination is

ts ¼ tsb k (6:43)

where ts is the calculated soak time, min; tsb is soak time for baseline temperature conditions,

min; and k is a correction factor for the type of steel.

The basic soak time ( tsb) is obtained from graphs derived from the computer simulation.

Such a graph for packed loads is shown in Figure 6.112. Other load shapes and configurations

Radianttubes

Radiation from the radiant tubesConvection from the radiant tubesRadiation from the walls Convection from the walls Convection from the furnace gasesRadiation between partsConduction through the partsLoad

qwr

qpc

qtr

qtc

qwr

qwc

qgc

qpr

qpc

qgc

qpr

qtc

qtr

qwc

Roof fan

FIGURE 6.110 Heat transfer in the used furnace and load. (From M.A. Aronov, J.F. Wallace, and

M.A. Ordillas, System for prediction of heat-up and soak times for bulk heat treatment processes,

Proceedings of the International Heat Treatment Conference on Equipment and Processes, April, 18–20,

1994, Schaumburg, IL, pp. 55–61.)

ß 2006 by Taylor & Francis Group, LLC.

require different graphs. The correction factor k depends on the type of steel. The generalized

equation (Equation 6.43) for the heat-up and soak time determination was set into a user-

friendly computer package that incorporates charts for the calculation. This resulted in a

straightforward way of determining the soak time without the use of charts while allowing for

a quick and accurate soak time calculation.

6.3.2 QUENCHING INTENSITY MEASUREMENT AND E VALUATION B ASED ON HEAT F LUX DENSITY

In designing the method for practical measurement, recording, and evaluation of the quench-

ing and cooling intensity in workshop conditions, in contrast to the Grossmann H value

concept, which expresses quenching intensity by a single number, the main idea of Liscic was

to express the quenching intensity by continuous change of relevant thermodynamic functions

during the whole quenching process. Instead of recording only one cooling curve (as in

laboratory-designed tests) in the center of a small (usually 1/2 in.) cylindrical specimen, the

900

Tem

pera

ture

, 8C

800

700

600

500

400

3000 15 30 45 60 75 90

Time, min

Furnace temp. + Load max. exper. Load min. exper.Load max. calcul. Load min. calcul.

105 120 135

FIGURE 6.111 Computer simulation for heating of shafts. (From M.A. Aronov, J.F. Wallace, and

M.A. Ordillas, System for prediction of heat-up and soak times for bulk heat treatment processes,

Proceedings of the International Heat Treatment Conference on Equipment and Processes, April, 18–20,

1994, Schaumburg, IL, pp. 55–61.)

N = 3 N = 2

N = 4

N = 1

N = Number

of trays

N > 4

300

Load characteristic size, in.

So

ak tim

e (

t sb),

min

250

200

150

100

50

00 0.5 1 1.5 2 32.5 3.5 4 4.5 5 5.5 6 6.5 7

FIGURE 6.112 Thermal soak time for a packed load. (From M.A. Aronov, J.F. Wallace, and M.A.

Ordillas, System for prediction of heat-up and soak times for bulk heat treatment processes, Proceedings

of the International Heat Treatment Conference on Equipment and Processes, April, 18–20, 1994,

Schaumburg, IL, pp. 55–61.)

ß 2006 by Taylor & Francis Group, LLC.