Bhadeshia H.K.D.H. Bainite In Steels. Transformations, Microstructure and Properties

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It is believed that in ferritic steels, irradiation induces the formation of two

kinds of dislocations, one of which is strongly biased whereas the other is

neutral. This provides sinks for both interstitials and vacancies, resulting in a

smaller excess of void-forming vacancies. There are differences in the void

swelling resistance of creep-resistant ferritic steels because of variations in

their microstructures. For a given vacancy concentration, a larger number

density of vacancy traps leads to a smaller tendency for swelling. Lath bound-

aries or oxide particles are good vacancy traps.

It is for their swelling resistance that ferritic steels have been considered for

structural applications in the construction of the ®rst wall and blanket

structures of fusion reactors. But in addition, there is a search for speci®c alloys

whose radioactivity decays most rapidly once they are removed from the

radioactive environment. These are the so-called reduced activation alloys

which have minimal concentrations of Mo, Ni, Nb, Cu and nitrogen, all of

which have long-lived radioactive isotopes (Abe et al., 1990; Klueh et al.,

1995). Some of these elements are key constituents of creep-resistant steels,

but can be eliminated by using tungsten instead of molybdenum and by

substituting vanadium and tantalum for niobium. Some examples of steels

which have been studied speci®cally for their reduced activation are listed in

Table 12.4.

Mechanical Properties

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Table 12.4 Chemical compositions (wt%) of reduced activation steels (Klueh et al.,

1995). All of these steels are bainitic with the exception of the 9Cr and 12Cr steels which

are martensitic. The chemical compositions of the ®rst group of steels are nominal.

Steel C Si Mn Cr V W Ta B

Cr±V 0.1 2.25 0.25

Cr±1WV 0.1 2.25 0.25 1

Cr±2W 0.1 2.25 2

Cr±2WV 0.1 2.25 0.25 2

Cr±2WV 0.1 5 0.25 2

9Cr±2WV 0.1 9 0.25 2

9Cr±2WVTa 0.1 9 0.25 2 0.07

12Cr±2WV 0.1 12 0.25 2

Cr±2WVTa 0.1 0.12 0.40 2.41 0.24 2.03 0.05

Cr±2WVTB 0.090 0.12 0.38 2.37 0.24 2.04 0.005

Cr±2WVTaB 0.093 0.12 0.38 2.36 0.24 2.04 0.05 0.005

2.6Cr±2WVTa 0.11 0.11 0.39 2.59 0.25 2.02 0.05

2.6Cr±2WVTaB 0.11 0.11 0.39 2.60 0.25 2.07 0.05 0.004

Cr±2W 0.11 0.15 0.39 2.48 1.99

Cr±2WV 0.11 0.20 0.42 2.41 0.24 1.98

9Cr±2WVTa 0.10 0.23 0.43 8.72 0.23 2.09 0.07

Early studies revealed that of the ®rst group of steels listed in Table 12.4, the

9Cr±2WVTa alloy in its martensitic condition has a high strength, good

toughness and is able to retain its impact properties after irradiation

(Fig. 12.28). On the other hand, the low-chromium bainitic steels do not require

a post-weld heat treatment which can be dif®cult to implement in complex

fabricated structures.

Although the high Cr steels show only modest shifts in the ductile-brittle

transition temperature following irradiation (Fig. 12.28), dif®culties arise when

the displacement damage is accompanied by the production of helium by the

transmutation of traces of nickel. The helium is insoluble in the matrix and

precipitates as small bubbles which embrittle the material. Suppose that the

bubble has a radius r, then for an ideal gas the pressure inside the bubble is

P  2=r where  is the interface energy per unit area. A small bubble can

therefore hold a lot more gas under high-pressure than a large bubble.

Typically, a 100 A

bubble holds gas at a pressure of 10

atmospheres whereas

the pressure is only 1 atmosphere in a bubble which is 10

in size. Swelling

can obviously be minimised by increasing the number density of bubble

nucleation sites (Thompson, 1969). Carbide particles are bubble nucleation

sites but the high chromium steels form coarse M

particles in contrast to

the larger numbers of ®ne MC or M

C particles in the lower chromium alloys,

Bainite in Steels

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Fig. 12.28 The ductile±brittle transition temperature for normalised and tempered

3 mm diameter bar samples which have been irradiated to about 14 dpa. The 12Cr

steel contains some -ferrite in addition to martensite. Some of the bainitic steels

may also contain allotriomorphic ferrite. After Klueh and Alexander (1999).

making the latter more suitable for fusion reactor applications. On the other

hand, the microstructures of the bainitic steels are much more sensitive to the

cooling rate from the austenitic condition. Care has to be taken to ensure that

the required ®ne distribution of carbides is obtained in practice.

Some of the steels listed in Table 12.4 are designed with low chromium

concentrations to enhance the impact toughness in the irradiated condition.

Klueh and co-workers found that the hardenability could be increased with a

slight increase in the chromium concentration supported by additions of B and

Ta; this resulted in a re®ned bainitic microstructure with improved toughness

and tempering resistance. Indeed, the 2.6Cr±2WVTaB alloy is found to have

mechanical properties comparable to those of the best 9Cr steels.

A ®ne distribution of carbides promotes toughness whether or not the steel

is irradiated. The iron carbides which form in the early stages of tempering can

be re®ned by increasing the silicon or aluminium concentration. It is these

carbides which set the scene for the subsequent formation of alloy carbides,

so there should be a consequential re®nement of the entire carbide micro-

structure. This idea would be worth exploring in the context of reduced-

activation steels.

12.11 Steels with Mixed Microstructures

Mixed microstructures consisting of bainite and martensite are usually a con-

sequence of inadequate heat-treatment or the use of steels with insuf®cient

martensitic hardenability.

Early research suggested that bainite in an otherwise martensitic microstruc-

ture leads to a deterioration in ductility, toughness and strength (Bailey, 1954;

Hehemann et al., 1957). The impairment of properties becomes less severe as

the bainite forms at lower transformation temperatures, and is related to the

difference in strength between martensite and bainite. As this difference

decreases, so does the reduction in properties (Hehemann et al., 1957).

Tempering homogenises the strength so bainite in a tempered martensite

microstructure has less of an effect on the overall properties (Triano and

Klinger, 1952; Hehemann et al., 1957). For the same reason, a mixture of mar-

tensite and lower bainite has better properties than one consisting of upper

bainite and martensite. The strength of lower bainite more closely matches that

of martensite.

There are, however, circumstances in which mixed microstructures are ben-

e®cial. Edwards (1969) observed that after tempering, mixtures of lower bainite

and martensite were tougher than either martensite or bainite. There are con-

siderable recent data that following tempering, the presence of bainite in a

predominantly martensitic microstructure leads to a higher strength and

toughness relative to the single phase samples, Fig. 12.29 (Tomita and

Mechanical Properties

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Okabayashi, 1983a,b 1985a,b, 1987, 1988; Tomita, 1991, 2000). This is because

the bainite partitions the austenite, thus re®ning the size of the martensite

packets that form subsequently (Mutui et al., 1977). The re®ned martensite is

stronger, and furthermore, the strength of the bainite is increased by constraint

from the strong martensite. But a further point that should be taken into

account is that the martensite grows from austenite which is enriched in carbon

due to the bainite transformation; it will therefore be expected to be harder.

It is particularly interesting that the strength of a tempered mixture of lower

bainite and martensite can exceed that of the martensite alone, and yet can be

tougher (Fig. 12.30).

12.12 Summary

The anisotropic, thin-plate shape of bainite ensures that the mean free path for

slip is comparable to the plate thickness rather than to the plate length. This

means that the major microstructural contribution to strength is via the ®ne

sub-unit size, rather than the sheaf or austenite grain size which are of minor

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Fig. 12.29 Variation in the 0.2% proof stress as a function of the volume fraction of

lower bainite in a mixed, tempered microstructure of lower bainite and marten-

site. The different curves represent data collected at the temperatures indicated on

the diagram.

importance as far as the strength is concerned. The sub-unit size is so small,

that the Hall±Petch relation does not apply; yield is instead controlled by the

stress required to expand dislocation loops. This gives a linear relationship

between the yield strength and the inverse of the sub-unit size.

Bainitic steels usually exhibit continuous yielding behaviour, with proof

stress to UTS ratios which can be much smaller than 0.8. This gradual yielding

is a consequence of mobile dislocations, the presence of heterogeneities in the

microstructure and residual stresses due to transformation. The ratio can be

increased by annealing at low temperatures, although there is not much of a

change if the bainite transformation temperature is comparable to the anneal-

ing temperature.

There is no doubt that higher carbon concentrations lead to a deterioration of

ductility, primarily because of the void nucleating tendency of cementite par-

ticles. The presence of large regions of untempered martensite in the micro-

structure can reduce ductility, whereas retained austenite can in some

circumstances enhance it.

The impact toughness of upper bainite deteriorates as its strength increases,

but that of lower bainite which has much ®ner carbides is superior at the same

strength level. An interpretation in terms of fracture toughness theory has

demonstrated that it is the coarsest carbides in the microstructure which con-

trol toughness. Consistent with theory, the fracture strength correlates directly

with the reciprocal of the square root of the coarsest carbide size. The scatter in

toughness data increases as the microstructure becomes more heterogeneous;

Mechanical Properties

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Fig. 12.30 Plot of toughness versus strength for a variety of microstructures in an

ultra-high strength steel (data from Tomita, 1988).

mixed microstructures of bainite and martensite show greater scatter than fully

bainitic samples. During fracture, the cleavage facet dimensions are found to

be comparable to those of bainite packets. A re®nement of the austenite grain

size also reduces the packet size, and consequently leads to an improvement in

toughness. All of the impurity controlled temper embrittlement phenomena

normally associated with martensite, are found in bainitic steels.

The endurance limit during the fatigue testing of smooth samples correlates

well with the UTS of bainitic steels. The threshold stress intensity range for

fatigue crack growth is reduced by the presence of retained austenite, because

its transformation to martensite prevents the reversal of plastic strain during

cyclic deformation. Stable austenite on the whole leads to an improvement in

fatigue properties via the ductility it confers to the microstructure.

Retained austenite can lead to an improvement in the stress corrosion

cracking resistance, by hindering the diffusion of hydrogen. However, com-

parable bene®ts can be achieved by any method which increases the number

density of hydrogen traps in the microstructure.

Creep-resistant alloys containing strong carbide-forming elements represent

one of the most successful industrial applications of bainitic microstructures.

They achieve their creep strength via solid solution strengthening and with the

help of ®ne dispersions of alloy carbides. The most modern of these steels is

such that it can be welded without the need for post-weld heat treatments.

Finally, there are now considerable data to suggest that mixed micro-

structures of bainite and martensite can offer higher strength and toughness

than fully martensitic alloys. The mechanism behind this remains to be

established.

Bainite in Steels

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13 Modern Bainitic Steels

Steels with yield strengths in excess of 1000 MPa are important in certain

applications, but the biggest commercial markets are for lower strength vari-

eties, where the total alloy content rarely exceeds 2 wt%. The alloy design

therefore has to be careful with a proper balance of hardenability in the context

of large scale steel production technologies. Lean steels tend to transform into

mixtures of allotriomorphic ferrite and bainite, whereas any attempt to

improve hardenability usually leads to partly martensitic microstructures.

The solution therefore lies in low-alloy, low-carbon steels, containing small

amounts of boron and molybdenum to suppress allotriomorphic ferrite forma-

tion, principles established originally by Irvine and Pickering. Boron increases

the bainitic hardenability. Other solute additions can, in the presence of boron,

be kept at suf®ciently low concentrations to avoid the formation of martensite.

Steels like these (Alloy 1, Table 13.1), when normalised, are found to transform

almost completely into bainite with only small fractions of martensite or other

residual phases.

The other alloys listed in Table 13.1 are examples of some of the latest

commercial bainitic steels. A striking feature of the list is that they don't

look particularly different in chemical composition. This impression is mis-

leading because each steel has distinct mechanical properties. Trace element

concentrations are vital in determining the details of the microstructure as are

the thermomechanical processing routes used in their manufacture.

Any alloy development or `advanced material' needs to be considered in the

context of what already exists. For this reason, we ®rst discuss the incredibly

successful ferrite±pearlite microalloyed steels which set the standard by which

any new alloy system must be assessed.

13.1 Alternatives to the Ferrite±Pearlite Microstructure

The most popular microstructure in the context of structural steels has

undoubtedly been a mixture of ferrite and pearlite. There are hundreds of

millions of tonnes of this manufactured annually. Typical chemical composi-

tions are given in Table 13.2 together with an indication of the usual mechan-

ical properties. Figure 13.1 shows that the main effect of microalloying and the

associated thermomechanical processing is to re®ne the microstructure. The

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Table 13.1 Typical compositions of advanced bainitic steels (wt%). Notice that the steels

are fairly similar in composition with respect to the major alloying additions, although

there are signi®cant differences with respect to carbon and trace element concentrations.

The fabrication procedure is also of vital importance in giving a large variation in proper-

ties in spite of the similarities of chemical composition. In the inoculated steels, the tita-

nium oxide-bearing steel is the most effective.

No. C Si Mn Ni Mo Cr Nb Ti B Al N Others Steel Type

1 0.100 0.25 0.50 ± 0.55 ± ± ± 0.0030 ± ± ± Early bainitic

2 0.039 0.20 1.55 0.20 ± ± 0.042 0.015 0.0013 0.024 0.0030 ± Rapidly cooled bainitic

3 0.081 0.25 1.86 0.20 0.09 ± 0.045 0.016 ± 0.025 0.0028 ± Rapidly cooled bainitic

4 0.110 0.34 1.51 ± ± ± 0.029 ± ± ± ± ± Rapidly cooled bainitic

50.1000.251.00±±±±± ± ±± ± Bainiticdual phase

60.040±0.40±±±±± ±0.05± ± Triplephase

7 0.150 0.35 1.40 ± ± ± 0.022 0.011 ± 0.035 ± ± Bainitic dual phase

80.1201.501.50±±±±± ±0.0450.0035 ± TRIP-assisted

9 0.020 0.20 2.00 0.30 0.30 ± 0.050 0.020 0.0010 ± 0.0025 ± ULCB

10 0.028 0.25 1.75 0.20 ± 0.30 0.100 0.015 ± 0.030 0.0035 Cu 0.3, ULCB

Ca 0.004

11 0.080 0.20 1.40 ± ± ± ± 0.012 ± 0.002 0.0020 O 0.0017 acicular ferrite, TiO

12 0.080 0.20 1.40 ± ± ± ± 0.008 0.0015 0.038 0.0028 ± acicular ferrite, TiB

13 0.080 0.20 1.40 ± ± ± ± 0.019 ± 0.018 0.0050 ± acicular ferrite, TiN

14 0.15 0.80 1.40 ± 0.20 ± ± ± ± ± ± V 0.15 Forging (high strength)

15 0.09 0.25 1.00 0.50 1.00 ± 0.10 0.02 0.002 0.04 0.006 ± Forging (100% bainitic)

16 0.09 0.40 1.40 ± ± ± 0.07 ± ± 0.04 0.010 V 0.06 Forging (Nb + V)

17 0.09 0.25 1.40 ± ± ± 0.07 0.02 0.002 0.04 0.006 ± Forging (Nb + B)

18 0.012 ± 1.60 ± ± ± 0.08 ± 0.004 ± ± ± Cold-heading

Table 13.2 Typical ferrite±pearlite structural steels in both the standard and niobium

microalloyed conditions.

Type Composition, wt.%

ÐÐÐÐÐÐÐÐÐÐÐ

CSiMnNb

Stress, MPa

ÐÐÐÐÐÐÐÐÐ

UTS Elongation

(20 8C),

Standard 0.11 0.21 1.24 285 320 480 36 55±90

Microalloyed 0.11 0.30 1.40 0.034 395 430 525 32 100±190

Type Ferrite % Pearlite % Ferrite grain size Pearlite interlammellar

spacing

Standard 76 24 15 mm0.22mm

Microalloyed 78 22 9 mm0.22mm

niobium carbonitrides that form also strengthen the ferrite by interphase

precipitation or by strain-induced precipitation (Pickering, 1978, 1992;

Honeycombe and Bhadeshia, 1995; Gladman, 1997).

Some of the major applications of structural steels include: oil and gas pipe-

lines (

 350485 MPa), drilling rigs and production platforms (345±415

MPa), ships (315±450 MPa), pressure vessels and tubing, earth-moving

equipment (345±550 MPa), heavy goods vehicles and automobile components,

high-rise buildings (415±530 MPa), bridges (345±485 MPa), transmission

towers, fuel and other storage tanks and reinforcement bars for concrete

(Pickering, 1992; Bodnar et al., 1997). A few of the attributes that are consistent

with these applications include the low cost, ability to produce a variety of

forms, weldability, fabricability, reliability under extreme conditions (such as

®re and earthquakes and other force majeures).

It seems unlikely in this context that the dominant position of the ferrite±

pearlite steels will ever be challenged by alternative steel microstructures, let

alone other materials! Nevertheless, we shall describe rivals based on bainite

because such steels can be produced using a similar production route and

without any additional heat treatments.

13.2 Strength

Strength is a basic engineering speci®cation; steels can be made stronger by

transforming the austenite at ever decreasing temperatures (Fig. 12.4) or by

increasing hardenability. There are many other strengthening mechanisms

available. Unfortunately, toughness does not necessarily increase with

strength. The weldability may also deteriorate because the excessive use of

alloying elements can cause the formation of untempered hard-phases in the

austenitised regions of the heat-affected zone or there may be excessive soft-

ening in the tempered regions of the HAZ.

Modern Bainitic Steels

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Fig. 13.1 Optical micrographs of banded ferrite±pearlite microstructures in (a)

standard, (b) niobium microalloyed structural steel.

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Fig. 13.2 The range of bainitic steels currently available on a commercial basis.

TRIP stands for transformation-induced plasticity.