Steel Heat Treatment - Metallurgy and Technologies 2nd ed - G. Totten (CRC_ 2007) 8 pdf

Increased ductility and toughness as well as increased bendability and fatigue life are the
strongest reasons to apply austempering instead of hardening and tempering. Figure 6.150
shows the relation of impact toughness and Brinell hardness (HB) of a Cr–Mn–Si steel after
conventional hardening and tempering and after austempering, as a function of tempering
temperature and austempering temperature, respectively. The most important difference is
that a good combination of hardness and toughness after conventional hardening and
tempering is possible only at high tempering temperatures, which means low hardness,
whereas at austempering a good combination of hardness and impact toughness may be
achieved at high hardness values.
Another comparison of impact toughness of a carbon steel after hardening and tempering
and after austempering, as a function of hardness, is shown in Figure 6.151. It is evident that
austempering yields much better impact toughness, especially at high hardness, around 50
HRC. It is necessary to emphasize that high toughness after austempering is possible only
under conditions of complete transformation of austenite to bainite. Table 6.7 shows a
comparison of some mechanical properties of austempered and of hardened and tempered
bars made of AISI 1090 steel. In spite of having a little higher tensile strength and hardness,
austempered specimens have had remarkably higher elongation, reduction of area, and
fatigue life.
Figure 6.152 shows the fatigue diagram of DIN 30SiMnCr4 steel after conventional
hardening and tempering and after austempering. The increase in fatigue resistance values
is especially remarkable for notched specimens.
Regarding bendability, Figure 6.153, from an early work of Davenport [30], shows the
results of bending a carbon steel wire austempered and hardened and tempered to 50 HRC.
When selecting a steel for austempering, IT diagrams should be consulted. The suitability
of a steel for austempering is limited first of all with minimum incubation time (the distance of
the transformation start curve from the ordinate). Another limitation may be the very long
transformation time. Figure 6.154 shows the transformation characteristics of four AISI
grades of steel in relation to their suitability for austempering. The AISI 1080 steel has only
limited suitability for austempering (i.e., may be used only for very thin cross sections)
Austempering
Austempering temperature, ЊC

Hardness HB
Impact toughness, kg/cm
2
600
550
500
450
400
350
300
250
200
14
12
10
8
6
4
2
0
250 300 350 400
Impact
toughness
Hardness
Hardening and tempering
Tempering temperature, ЊC
300 400 500 550 600
Hardness
Impact
toughness

FIGURE 6.150 Impact toughness and hardness (HB) of five heats of a Cr–Mn–Si steel after conven-
tional hardening and tempering and after austempering, as a function of tempering temperature and
austempering temperature, respectively. (From F.W. Eysell, Z. TZ Prakt. Metallbearb. 66:94–99, 1972
[in German].)
ß 2006 by Taylor & Francis Group, LLC.
Increased ductility and toughness as well as increased bendability and fatigue life are the
strongest reasons to apply austempering instead of hardening and tempering. Figure 6.150
shows the relation of impact toughness and Brinell hardness (HB) of a Cr–Mn–Si steel after
conventional hardening and tempering and after austempering, as a function of tempering
temperature and austempering temperature, respectively. The most important difference is
that a good combination of hardness and toughness after conventional hardening and
tempering is possible only at high tempering temperatures, which means low hardness,
whereas at austempering a good combination of hardness and impact toughness may be
achieved at high hardness values.
Another comparison of impact toughness of a carbon steel after hardening and tempering
and after austempering, as a function of hardness, is shown in Figure 6.151. It is evident that
austempering yields much better impact toughness, especially at high hardness, around 50
HRC. It is necessary to emphasize that high toughness after austempering is possible only
under conditions of complete transformation of austenite to bainite. Table 6.7 shows a
comparison of some mechanical properties of austempered and of hardened and tempered
bars made of AISI 1090 steel. In spite of having a little higher tensile strength and hardness,
austempered specimens have had remarkably higher elongation, reduction of area, and
fatigue life.
Figure 6.152 shows the fatigue diagram of DIN 30SiMnCr4 steel after conventional
hardening and tempering and after austempering. The increase in fatigue resistance values
is especially remarkable for notched specimens.
Regarding bendability, Figure 6.153, from an early work of Davenport [30], shows the
results of bending a carbon steel wire austempered and hardened and tempered to 50 HRC.
When selecting a steel for austempering, IT diagrams should be consulted. The suitability
of a steel for austempering is limited first of all with minimum incubation time (the distance of

the transformation start curve from the ordinate). Another limitation may be the very long
transformation time. Figure 6.154 shows the transformation characteristics of four AISI
grades of steel in relation to their suitability for austempering. The AISI 1080 steel has only
limited suitability for austempering (i.e., may be used only for very thin cross sections)
Austempering
Austempering temperature, ЊC
Hardness HB
Impact toughness, kg/cm
2
600
550
500
450
400
350
300
250
200
14
12
10
8
6
4
2
0
250 300 350 400
Impact
toughness
Hardness

Hardening and tempering
Tempering temperature, ЊC
300 400 500 550 600
Hardness
Impact
toughness
FIGURE 6.150 Impact toughness and hardness (HB) of five heats of a Cr–Mn–Si steel after conven-
tional hardening and tempering and after austempering, as a function of tempering temperature and
austempering temperature, respectively. (From F.W. Eysell, Z. TZ Prakt. Metallbearb. 66:94–99, 1972
[in German].)
ß 2006 by Taylor & Francis Group, LLC.
7
Heat Treatment with Gaseous
Atmospheres
Johann Grosch
CONTENTS
7.1 General Introduction 415
7.2 Fundamentals in Common 417
7.3 Carburizing 422
7.3.1 Introduction 422
7.3.2 Carburizing and Decarburizing with Gases 422
7.3.2.1 Gas Equilibria 423
7.3.2.2 Kinetics of Carburizing 426
7.3.2.3 Control of Carburizing 428
7.3.2.4 Carbonitriding 431
7.3.3 Hardenability and Microstructures 432
7.4 Reactions with Hydrogen and with Oxygen 440
7.5 Nitriding and Nitrocarburizing 446
7.5.1 Introduction 446
7.5.2 Structural Data and Microstructures 448

7.5.2.1 Structural Data 448
7.5.2.2 Microstructures of Nitrided Iron 450
7.5.2.3 Microstructures of Nitrided and Nitrocarburized Steels 452
7.5.2.4 Microstructural Specialties 456
7.5.3 Nitriding and Nitrocarburizing Processes 457
7.5.3.1 Nitriding 457
7.5.3.2 Nitrocarburizing 460
7.5.3.3 Processing Effects on the Nitriding and Nitrocarburizing Results 461
7.6 Properties of Carburized and Nitrided or Nitrocarburized Components 463
References 469
7.1 GENERAL INTRODUCTION
Heat treatment of components is to date mostly accomplished in gaseous atmospheres, the
more so if plasma and vacuum are regarded as special cases of gaseous atmospheres. In
comparison, heat treatment in solid or liquid media is negligible in numbers. Heat treatment
in gaseous atmospheres falls into two categories: processes wi th the aim of avoiding a mass
transfer between the gaseous atmosphere and the material, and processes with the aim of
achieving just such a transfer. Mass transfer occurs when there is a difference in the potential
between the constituents of a gaseous atmosphere and tho se of the microstructure of a
component. The direction of such a mass transfer is determined by the potential difference,
which leaves two fund amental possibilities with regard to the component. One is the intake
ß 2006 by Taylor & Francis Group, LLC.
Temperature (ЊC)
Coefficient of diffusion (m
2
/s)
1500 400
01 2 3 4
10
−29
10

−24
10
−19
10
−14
10
−9
10
−4
5
200 100 25 0 50
Hydrogen
Interstitial atoms N, C
Substitutional atoms
10
3
T
1
K
FIGURE 7.1 Diffusion coefficients of hydrogen and of interstitial and substitional elements in a-iron.
(From E. Hornbogen, Werkstoffe, 2nd ed., Springer-Verlag, Berlin, 1979.)
0 1.0 2.0 3.0 4.0
10
−26
10
−24
10
−22
10
−20

10
−18
10
−16
10
−14
10
−12
10
−10
10
−8
10
−6
10
−4
10
−28
10
−26
10
−24
10
−22
10
−20
10
−18
10
−16

10
−14
10
−12
10
−10
10
−8
10
−6
D
O
and D
C
(α-Fe) (m
2
/s)
D
N
(α-Fe) (m
2
/s)
0 in α-Fe
10
3
T
1
K
FIGURE 7.2 Diffusion coefficients of C, N, and O in a-iron. (From Th. Heumann, Diffusion in
Metallen, Springer-Verlag, Berlin, 1992.)

ß 2006 by Taylor & Francis Group, LLC.