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Austempered ductile irons—their properties and significance
Austempered Ductile I r o n s - their Properties and Significance 1"
I. C. H. HUGHES Director, BCIRA, Alvechurch, Birmingham, B48 7QB. The advantages of austempered irons are briefly described Introduction Cast iron with a spheroidal or nodular graphite structure, also known as ductile iron, was first introduced some 35 years ago and has since become established as the material in the cast iron family with the most advanced range of properties competitive with many other materials for exacting engineering applications. Ductile irons are normally used in the as-cast condition or after relatively simple annealing or normalishag heat treatment. However, as generally supplied they are nowhere near to achieving the full potential of the materiaL Many more applications, competitive with other high-quality materials, could be achieved and new markets opened up by the use of austempering heat treatments. These are able to produce a whole new class of ductile irons with outstanding and exceptional properties which, until recently, were not well appreciated or exploited. To put these exciting and promising new materials quickly into perspective in terms of traditional properties, consider the range of tensile strengths and elongations achievable, compared with those types of ductile cast iron currently in well-established use (Fig. I). It can be seen that not only is the range of properties in a much higher band than those of as-cast, normalised or annealed irons, but it also exceeds that of hardened and tempered irons which are sometimes considered for specially exacting requirements.
Fi& 1
Treatment To obtain the best results only the bestquality, defect-free castings must be made, under good quality control Customers who are offered specially high properties cannot be expected to tolerate any chance of failure from defective castings.
Austempering heat treatment is not complicated. As shown diagrammatically in Fig. 2, it consists of first soaking at the austenitising temperature of 850 to 950°C. Castings are then rapidly transferred to a bath of liquid medium at a controlled temperature, usually in
124
1600
Austernpered
Q&T
600 -cast or
400 i
0
I
5
I
I
10 15 Elongation %
I
20
Tensile strengths and elongations in austempered ductile iron compared with other ductile irons. the range 235 to 425 °C and held at that temperature for a period of up to 4
Based on a lecture given at a BCIRA Seminar on Austempered Ductile Irons December 1984
MATERIALS & DESIGN Vol. 6 No. 3 JUNE/JULY 1985
A
B
---~
High quo[ity costing
Austenitize 850-950"C
! I I ~ ~
i ~
D Cool to roon temperoture
C Quench at 235 -425 °C Hold1,/2-4 hours 1
The oustempering process
Schematic arrangement of the austempering process.
Fig. 2
Fig. 3
2 3 4 Relotive weor resistonce
5
6
Wear resistance of austempered ductile irons compared with other materials.
,.,,, 5C 0.41
,red 600°C
4ci
i30
i o+ 0
I
103
Fig. 4
104
I
105 Reversols to foilure
I
106
Fatigue properties ofaustempered ductile irons compared with other iductile irons.
hours. During this time austempering occurs - to transform the iron to a structure referred to as bainite. The castings are then cooled to room temperature and are ready for service. Properties Advantages of austempering heat treatments compared with hardening and tempering - the only previous alternative treatment for achieving very high strength include: (a) lower alloying-element requirements, (b) less chance of cracking, (c) less chance of distortion. The latter means that rough or even finish machining may be carded out prior to heat treatment. Some typical conventional properties achievable in austempered ductile irons, compared with other nodular irons of high quality and with some comparable
oon," .0
107
Fig. 5
I
Contact-fatigue strength ofaustempered ductile iron gears compared with other materials.
Tensile Strength N/mm'
Yield Strength N/mm'
Pearlitic and ferritic
400-960
2 5 0 - 610
130-300
28-3
Austempered
800-1600
600-1400
250- 480
16-1
Hardened and tempered
600-1300
500-1100
300- 400
5-1
700-1800
450-1450
210-510
25-8
Material
Hardness Elongation HB %
Ductile Irons
Steels Hardened and tempered
Table 1 Properties of austempered ductile irons compared with other materials.
MATERIALS & DESIGN Vol. 6. No. 3 JUNE/JULY 1985
125
Typical Unnotched Impact Values for Nodular Irons CONDITION
IMPACT VALUE J/cm 2 100°C
50°C
25°C
0°C
As-Cast Pearlitic
24
Normalised Hardened Et Tempered
_40°C
20
7
3
1.5
26
24
22
22
13
22
22
22
22
22
Annealed Ferritic
100
100
100
100
100
Bainitic
103
100
98
93
62
Table 2 Typical impact values for ductile irons.
Typical Fracture Toughness of Nodular Irons (Unalloyed) CONDITION
COD
(p,m)
EQUIV KIc ( M N I m 3/2)
24-35
As Cast Pearlitic Normalised Pearlitic
12--22
35-50
Bainitic (Austempered 325*C)
35--52
82 -- 97
Wrought Steels (Hardened Et Tempered)
50--125
Table 3 Fracture toughness values of austempered ductile irons compared with other ductile irons and wrought steels.
strain limits, which is a most exacting type of fatigue test. Many different measurements of wear and fatigue resistance are possible, dependant on the application and, as may be seen from Fig. 5, tests related to gears also show that contact fatigue strength of austempered ductile irons is comparable with that of other gear-making materials and far superior to that of other cast irons. Toughness of materials is traditionally evaluated in terms of an impact test and Table 2 compares the unnotched-impact values measured on austempered ductile irons and ductile irons having pearlitic and ferritic matrices. Austempered ductile irons have impact values much higher than pearlitic ductile irons, and comparable with those of ferritic ductile irons. There is a transition range of temperature over which the impact value falls slowly. This occurs over a lower temperature range to that for pearlitic irons. They do not, however, have ductile to brittle transition temperatures in the unnotched impact test as low as those of ferritic irons, which remain fully ductile down to about -50°C. Modern applications will increasingly require the use of modern tests of particular relevance to the service performance. Fatigue properties will play an important and increasing part in the acceptance of these irons and fracture toughness is an increasingly important measure of service performance. Fracture toughness information is currently being accumulated and is showing a remarkably higher range of values than those for other ductile cast irons, comparable with the values obtained on engineering steels, as indicated in Table 3.
Applications steels, are shown in Table 1. Dependent on the temperature of austempering treatment, high-strength/high-hardness/ limited ductility or lower-strengtldlowerhardness/high ductility combinations can be achieved with the austempered materials. Machinability is generally good, and (in the more ductile grades) comparable with that of other ductile irons or low-alloy steels. It may, however, be an advantage in the higherhardness materials to do most of the
126
machining prior to heat treatment. An important property of these materials is wear resistance (Fig. 3) which, because of the bainitic structure, is greater than that of other ductile irons and some steels, though not in the same class as white cast iron. For a similar reason the fatigue properties are also very much better than those of other ductile irons. Fig. 4 shows fatigue tests which have been carded out in compression and tension cycling between fixed
Austempered ductile irons are potentially attractive materials for gears, for which other special tests are required. Applications of the most exacting kinds sometimes depend upon quite unexpected properties. For example, in gearboxes low noise may be important; the high damping-capacity of cast irons leads to less vibration and transmission noise than occurs with steels. This is a property that will undoubtedly assist in the wider use of these materials.
MATERIALS & DESIGN Vol. 6 No. 3 JUNE/JULY 1985
I. C. H. HUGHES Director, BCIRA, Alvechurch, Birmingham, B48 7QB. The advantages of austempered irons are briefly described Introduction Cast iron with a spheroidal or nodular graphite structure, also known as ductile iron, was first introduced some 35 years ago and has since become established as the material in the cast iron family with the most advanced range of properties competitive with many other materials for exacting engineering applications. Ductile irons are normally used in the as-cast condition or after relatively simple annealing or normalishag heat treatment. However, as generally supplied they are nowhere near to achieving the full potential of the materiaL Many more applications, competitive with other high-quality materials, could be achieved and new markets opened up by the use of austempering heat treatments. These are able to produce a whole new class of ductile irons with outstanding and exceptional properties which, until recently, were not well appreciated or exploited. To put these exciting and promising new materials quickly into perspective in terms of traditional properties, consider the range of tensile strengths and elongations achievable, compared with those types of ductile cast iron currently in well-established use (Fig. I). It can be seen that not only is the range of properties in a much higher band than those of as-cast, normalised or annealed irons, but it also exceeds that of hardened and tempered irons which are sometimes considered for specially exacting requirements.
Fi& 1
Treatment To obtain the best results only the bestquality, defect-free castings must be made, under good quality control Customers who are offered specially high properties cannot be expected to tolerate any chance of failure from defective castings.
Austempering heat treatment is not complicated. As shown diagrammatically in Fig. 2, it consists of first soaking at the austenitising temperature of 850 to 950°C. Castings are then rapidly transferred to a bath of liquid medium at a controlled temperature, usually in
124
1600
Austernpered
Q&T
600 -cast or
400 i
0
I
5
I
I
10 15 Elongation %
I
20
Tensile strengths and elongations in austempered ductile iron compared with other ductile irons. the range 235 to 425 °C and held at that temperature for a period of up to 4
Based on a lecture given at a BCIRA Seminar on Austempered Ductile Irons December 1984
MATERIALS & DESIGN Vol. 6 No. 3 JUNE/JULY 1985
A
B
---~
High quo[ity costing
Austenitize 850-950"C
! I I ~ ~
i ~
D Cool to roon temperoture
C Quench at 235 -425 °C Hold1,/2-4 hours 1
The oustempering process
Schematic arrangement of the austempering process.
Fig. 2
Fig. 3
2 3 4 Relotive weor resistonce
5
6
Wear resistance of austempered ductile irons compared with other materials.
,.,,, 5C 0.41
,red 600°C
4ci
i30
i o+ 0
I
103
Fig. 4
104
I
105 Reversols to foilure
I
106
Fatigue properties ofaustempered ductile irons compared with other iductile irons.
hours. During this time austempering occurs - to transform the iron to a structure referred to as bainite. The castings are then cooled to room temperature and are ready for service. Properties Advantages of austempering heat treatments compared with hardening and tempering - the only previous alternative treatment for achieving very high strength include: (a) lower alloying-element requirements, (b) less chance of cracking, (c) less chance of distortion. The latter means that rough or even finish machining may be carded out prior to heat treatment. Some typical conventional properties achievable in austempered ductile irons, compared with other nodular irons of high quality and with some comparable
oon," .0
107
Fig. 5
I
Contact-fatigue strength ofaustempered ductile iron gears compared with other materials.
Tensile Strength N/mm'
Yield Strength N/mm'
Pearlitic and ferritic
400-960
2 5 0 - 610
130-300
28-3
Austempered
800-1600
600-1400
250- 480
16-1
Hardened and tempered
600-1300
500-1100
300- 400
5-1
700-1800
450-1450
210-510
25-8
Material
Hardness Elongation HB %
Ductile Irons
Steels Hardened and tempered
Table 1 Properties of austempered ductile irons compared with other materials.
MATERIALS & DESIGN Vol. 6. No. 3 JUNE/JULY 1985
125
Typical Unnotched Impact Values for Nodular Irons CONDITION
IMPACT VALUE J/cm 2 100°C
50°C
25°C
0°C
As-Cast Pearlitic
24
Normalised Hardened Et Tempered
_40°C
20
7
3
1.5
26
24
22
22
13
22
22
22
22
22
Annealed Ferritic
100
100
100
100
100
Bainitic
103
100
98
93
62
Table 2 Typical impact values for ductile irons.
Typical Fracture Toughness of Nodular Irons (Unalloyed) CONDITION
COD
(p,m)
EQUIV KIc ( M N I m 3/2)
24-35
As Cast Pearlitic Normalised Pearlitic
12--22
35-50
Bainitic (Austempered 325*C)
35--52
82 -- 97
Wrought Steels (Hardened Et Tempered)
50--125
Table 3 Fracture toughness values of austempered ductile irons compared with other ductile irons and wrought steels.
strain limits, which is a most exacting type of fatigue test. Many different measurements of wear and fatigue resistance are possible, dependant on the application and, as may be seen from Fig. 5, tests related to gears also show that contact fatigue strength of austempered ductile irons is comparable with that of other gear-making materials and far superior to that of other cast irons. Toughness of materials is traditionally evaluated in terms of an impact test and Table 2 compares the unnotched-impact values measured on austempered ductile irons and ductile irons having pearlitic and ferritic matrices. Austempered ductile irons have impact values much higher than pearlitic ductile irons, and comparable with those of ferritic ductile irons. There is a transition range of temperature over which the impact value falls slowly. This occurs over a lower temperature range to that for pearlitic irons. They do not, however, have ductile to brittle transition temperatures in the unnotched impact test as low as those of ferritic irons, which remain fully ductile down to about -50°C. Modern applications will increasingly require the use of modern tests of particular relevance to the service performance. Fatigue properties will play an important and increasing part in the acceptance of these irons and fracture toughness is an increasingly important measure of service performance. Fracture toughness information is currently being accumulated and is showing a remarkably higher range of values than those for other ductile cast irons, comparable with the values obtained on engineering steels, as indicated in Table 3.
Applications steels, are shown in Table 1. Dependent on the temperature of austempering treatment, high-strength/high-hardness/ limited ductility or lower-strengtldlowerhardness/high ductility combinations can be achieved with the austempered materials. Machinability is generally good, and (in the more ductile grades) comparable with that of other ductile irons or low-alloy steels. It may, however, be an advantage in the higherhardness materials to do most of the
126
machining prior to heat treatment. An important property of these materials is wear resistance (Fig. 3) which, because of the bainitic structure, is greater than that of other ductile irons and some steels, though not in the same class as white cast iron. For a similar reason the fatigue properties are also very much better than those of other ductile irons. Fig. 4 shows fatigue tests which have been carded out in compression and tension cycling between fixed
Austempered ductile irons are potentially attractive materials for gears, for which other special tests are required. Applications of the most exacting kinds sometimes depend upon quite unexpected properties. For example, in gearboxes low noise may be important; the high damping-capacity of cast irons leads to less vibration and transmission noise than occurs with steels. This is a property that will undoubtedly assist in the wider use of these materials.
MATERIALS & DESIGN Vol. 6 No. 3 JUNE/JULY 1985