Bhadeshia H.K.D.H., Honeycombe R. Steels: Microstructure and Properties

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228 CHAPTER 10 THERMOMECHANICAL TREATMENT OF STEELS

(a)

(b)

(c)

(d)

Fig. 10.16 Evolution of room-temperature microstructure as a function of time during

isothermal transformation to bainite. (a) Low-silicon steel Fe–0.16C–0.38Si–1.3Mn wt%.

(b) High-silicon steel Fe–0.29C–1.41Si–1.42Mn wt%. (c) Scanning electron micrograph of

low-silicon alloy isothermally transformed to bainite for 1800 s. Much of the austenite has

decomposed to bainitic ferrite and cementite. (d) Corresponding micrograph for high-silicon

steel transformed to bainite for 900 s with plenty of austenite evident (courtesy of Pascal

Jacques).

10.5 TWIP STEELS 229

MnO begins to cover the surface leading to a deterioration in the ability of the

liquid zinc to wet the surface. The aluminium-containing steels are therefore

better suited to continuous galvanizing lines.

It is clear that silicon and manganese must diffuse to the surface to form

oxides. Some of the diffusion ﬂux is via grain boundaries. When phosphorus

is present, its segregation to the grain boundaries reduces the boundary

diffusion-ﬂux, thereby reducing the extent to which oxides form. Such steels

are more amenable to wetting by molten zinc (in contrast, the same effect

makes it more difﬁcult to form iron–zinc compounds during the galvannealing

process).

10.5 TWIP STEELS

There are three essential modes by which steels can be permanently deformed

at ambient temperature, without recourse to diffusion. Individual dislocations

whose Burgers vectors correspond to lattice vectors can glide, leading to a

change in shape without altering the crystal structure or volume. In contrast, a

displacive transformation (e.g. martensite or bainite) results not only in a plastic

strain, but also a change of crystal structure and density; this is the phenomenon

exploited in the TRIP steels.

The third mode of deformation is mechanical twinning, in which the crystal

structure of the steel is preserved but the twinned region is reoriented in the

process. Mechanical twinning results in a much larger shear strain s =1/

√

compared with displacive transformations where s is typically 0.25. There is a

particular class of extraordinarily ductile alloys of iron, known as the TWIP

steels, which exploit mechanical twinning to achieve their properties.

TWIP stands for twinning-induced plasticity. The alloys are austenitic and

remain so during mechanical deformation, but the material is able to accommo-

date strain via both the glide of individual dislocations and through mechanical

twinning on the {111}

< 11

2 >

system. The alloys typically contain a

large amount of manganese, some aluminium and silicon (e.g. Fe–25Mn–3Si–

3Al wt%) with carbon and nitrogen present essentially as impurities. Larger

concentrations of carbon may be added to enhance strength. At high man-

ganese concentrations, there is a tendency for the austenite to transform into

ǫ-martensite (hexagonal close packed) during deformation. ǫ-martensite can

form by the dissociation of a perfect a/2 < 011>

dislocation into Shockley

partials on a close packed {11

plane, with a fault between the partials. This

faulted regionrepresents a three layer thick plate of ǫ-martensite.A reduction in

the fault energy therefore favours the formation of this kind of martensite. The

addition of aluminium counters this because it raises the stacking fault energy

of the austenite. Silicon has the opposite effect of reducing the stacking fault

energy, but like aluminium, it leads to a reduction in the density of the steel;

the combination of Al and Si at the concentrations used typically reduces the

overall density from some 7.8 g cm

−3

to about 7.3 g cm

−3

230 CHAPTER 10 THERMOMECHANICAL TREATMENT OF STEELS

(a)

(b)

Fig. 10.17 (a) Typical stress–strain curve for a TWIP steel. (b) Optical microstructure of

a TWIP steel following deformation, showing profuse twinning (image and data courtesy of

Frommeyer, G., Brüx, U. and Neumann, P).

The alloys have a rather low yield strength at 200–300MN m

−2

but the

ultimate tensile strength can be much higher, in excess of 1100 MN m

−2

. This

is because the strain-hardening coefﬁcient is large, resulting in a great deal

of uniform elongation, and a total elongation of some 60–95%. The effect

of mechanical twinning is two-fold. The twins add to plasticity, but they also

have a powerful effect in increasing the work-hardening rate by subdividing the

untwinned austenite into ﬁner regions (Fig. 10.17).

One major advantage of TWIP steels is that they are austenitic and they

maintain attractive properties at cryogenic temperatures (−150

◦

C) and high

strain rates, e.g., 10

−1

. They therefore have great potential in enhancing the

safety of automobiles by absorbing energy during crashes.

10.6 STEELS SUBJECTED TO THERMOMECHANICAL TREATMENTS 231

10.6 INDUSTRIAL STEELS SUBJECTED TO THERMOMECHANICAL

TREATMENTS

Micro-alloyed steels produced by controlled rolling are a most attractive prop-

osition in many engineering applications because of their relatively low cost,

moderate strength, and very good toughness and fatigue strength, together with

theirability to bereadilywelded.They have,to a considerabledegree,eliminated

quenched and tempered steels in many applications.

These steels are most frequently available in control-rolled sheet, which is

then coiled over a range of temperatures between 750

◦

C and 550

◦

C.The coiling

temperature has an important inﬂuence as it represents the ﬁnal transformation

temperature, and this inﬂuences the microstructure. The lower this temperature,

under the same conditions, the higher the strength achieved. The normal range

of yield strength obtained in these steels varies from about 350 to 550 MN m

−2

(50–80 ksi). The strength is controlled both by the detailed thermomechanical

treatment, by varying the manganese content from 0.5 to 1.5 wt%, and by using

the micro-alloying additions in the range 0.03 to above 0.1 wt%. Niobium is used

alone, or with vanadium, while titanium can be used in combination with the

other two carbide-forming elements. The interactions between these elements

are complex,but in general terms niobium precipitates more readily in austenite

than does vanadium as carbide or carbo-nitride, so it is relatively more effective

as a grain reﬁner. The greater solubility of vanadium carbide in austenite under-

lines the superior dispersion strengthening potential of this element shared to a

lesser degree with titanium. Titanium also interacts with sulphur and can have

a beneﬁcial effect on the shape of sulphide inclusions. Bearing in mind that the

total effect of these elements used in conjunction is not a simple sum of their

individual inﬂuence, the detailed metallurgy of these steels becomes extremely

complex.

One of the most extensive applications is in pipelines for the conveyance

of natural gas and oil, where the improved weldability due to the overall lower

alloying content (lower hardenability) and, particularly, the lower carbon levels

is a great advantage. Furthermore, as the need for larger diameter pipes has

grown, steels of higher yield stress have to be used to avoid excessive wall

thicknesses. In practice, wall thicknesses of 10–12.5 mm have been found to be

the most convenient. Typical compositions (wt%) to achieve a yield stress of

around 410 MN m

−2

(60 ksi):

C 0.12 S 0.012 Mn 1.35 Nb 0.03

C 0.12 S 0.006 Mn 1.33 Nb 0.02 V 0.04

for higher yield strengths (450 MN m

−2

C 0.06 S 0.006 Mn 1.55 Nb 0.05 V 0.10

However, it should be emphasized that often higher yield stresses are

achieved by control of the fabrication variables such as the temperature at which

232 CHAPTER 10 THERMOMECHANICAL TREATMENT OF STEELS

Table 10.1 Typical compositions of micro-alloyed vanadium steels

Element Typical composition (%)

Yield strength 550 (MN m

−2

) (80 ksi)

345 (MN m

−2

) (50 ksi)

Carbon 0.08–0.12 0.12–0.17

Manganese 0.75–1.10 1.20–1.55

Phosphorus 0.008–0.013 0.008–0.013

Sulphur 0.007–0.020 0.007–0.020

Silicon 0.05–0.15 0.30–0.55

Aluminium 0.03–0.06 0.03–0.06

Vanadium 0.03–0.07 0.10–0.14

Nitrogen 0.006–0.012 0.015–0.022

Cerium 0.02–0.06 0.02–0.06

rolling is ﬁnished and the temperature used for coiling the sheet. Nitrogen is

often deliberately used as an alloying element. One successful range of steels

relies on vanadium to form carbo-nitride precipitates for grain size control and

dispersion strengthening. In some steels, rare earth additions are made to con-

trol the inclusion shape.Typical compositions at lower and higher strength levels

are given in Table 10.1.

At the higher strength levels, micro-alloyed steels are used for heavy duty

truck frames, tractor components, crane booms and lighting standards, etc. The

control of sulphide inclusions gives the steels a high degree of formability in cold

fabrication processes. This recent development has allowed the use of HSLA

steels for many applications involving substantial cold forming which previously

led to cracking in the absence of rare earth additions.

A steel is said to have been ausformed when martensite is produced from

plastically deformed austenite Ausforming has provided some of the strongest,

toughest steels so far produced, with the added advantage of very good fatigue

resistance. However, they usually have high concentrations of expensive alloy-

ing elements and must be subjected to large deformations which impose heavy

work loads on rolling mills. Nevertheless, these steels are particularly useful

where a high strength to weight ratio is required and where cost is a secondary

factor. Typical applications have included parts for undercarriages of aircraft,

special springs and bolts.

The 12 wt% Cr transformable steels respond readily to ausforming to the

extent that tensile strengths of over 3000 MN m

−2

(>200 tsi) can be obtained in

appropriate compositions.A 0.4C–6Mn–3Cr–1.5Si steelhas beenausformed to a

tensile strength of 3400 MN m

−2

, with an improvement in ductility over the con-

ventional heat treatment. Similar high strength levels with good ductility have

been reported for0.4C–5Cr–1.3Mo–1.0Si–0.5V wt% steel (H11)(Fig. 10.18).All

FURTHER READING

Bhadeshia, H. K. D. H., TRIP-Assisted Steels? ISIJ International, 42, 1059, 2002.

Cooman, B. C. de, Structure–properties relationship in TRIP steels containing carbide-free

bainite, Current Opinion in Solid State and Materials Science 8, 285, 2004.

Davenport, A. T. (ed.), Formable HSLA and Dual-phase Steels, The Metallurgical Society of

AIME, USA, 1979.

Davies, G., Materials for Automobile Bodies, Elsevier, London, 2003.

Frommeyer, G., Brüx, U. and Neumann, P., Supra-ductile and high-strength manganese-

TRIP/TWIP steels for high energy absorption purposes, ISIJ International 43, 438, 2003.

Gladman,T., Physical Metallurgy of Microalloyed Steels, Institute of Materials, London, 1997.

HSLA Steels – Metallurgy and Applications. Proceedings of an International Conference,

Beijing, 1985, Chinese Society of Metals, ASM International.

International Conference on Processing, Microstructure and Properties of Microalloyed and

Other Modern Low Alloy Steels, Pittsburgh, 1991.

Jacques, P. J., Transformation-induced plasticity for high strength formable steels, Current

Opinion in Solid State and Materials Science 8, 259, 2004.

234 CHAPTER 10 THERMOMECHANICAL TREATMENT OF STEELS

Kozasu, I., Processing – thermomechanical controlled processing in Materials Science and

Technology (eds Cahn, R. W., Haasen, P. and Kramer, E. J.), Vol. 7, Constitution and

Properties of Steels (ed. Pickering, F. B.), VCH,Weinheim, Germany, 1992.

Krauss, G. (ed.), Deformation, Processing and Structure, American Society for Metals, Ohio,

USA, 1984.

Maki, J., Mahieu, J., de Cooman, B. C. and Claessens, S., Galvanisability of silicon free CMnAl

TRIP steels, Materials Science and Technology 19, 125, 2003.

Physical Metallurgy of Thermo-mechanical Processing of Steels and other Metals (Thermec

88), Iron and Steel Institute of Japan, 1988.

Yokota, T., Garcia-Mateo, C., and Bhadeshia, H. K. D. H., Transformation-induced plasticity

for high strength formable steels, Scripta Materialia 51, 767, 2004.

THE EMBRITTLEMENT AND FRACTURE

OF STEELS

11.1 INTRODUCTION

Most groups of alloys can exhibit failure by cracking in circumstances where

the apparent applied stress is well below that at which failure would normally

be expected. Steels are no exception to this, and probably exhibit a wider var-

iety of failure mechanisms than any other category of material. While ultimate

failure under excessive stress must occur and can be reasonably predicted by

appropriate mechanical tests, premature failure is always dangerous, involving

a considerable element of unpredictability. However, a detailed knowledge of

structure and of the distribution of impurities in steels is gradually leading to a

much betterunderstanding of the origins and mechanisms of the various types of

cracks encountered. Furthermore, the now well-established science of fracture

mechanics allows the quantitative assessment of growth of cracks in various

stress situations, to an extent that it is now frequently possible to predict the

stress level to which steel structures can be conﬁdently subjected without the

risk of sudden failure.

11.2 CLEAVAGE FRACTURE IN IRON AND STEEL

Cleavage fracture is familiar in many minerals and inorganic crystalline solids

as a crack propagation frequently associated with very little plastic deformation

and occurring in a crystallographic fashion along planes of low indices, i.e. high

atomic density. A low temperatures zinc cleaves along the basal plane, while

body-centred cubic (bcc) iron cleaves along {100} planes (Fig. 11.1), as do all

bcc metals. This behaviour would appear to be an intrinsic characteristic of iron,

but it has been shown that iron, highly puriﬁed by zone reﬁning and containing

minimal concentrations of carbon, oxygen and nitrogen, is very ductile even at

235

236 CHAPTER 11 THE EMBRITTLEMENT AND FRACTURE OF STEELS

Fig. 11.1 Cleavage fracture in pure iron–0.04 P at 55

◦

C (courtesy of Shell) ×60.

extremely low temperatures. For example, at 4.2 K reductions in area in ten-

sile tests of up to 90% have been observed with iron specimens of the highest

available purity. As the carbon and nitrogen content of the iron is increased, the

transition from ductile to brittle cleavage behaviour takes place at increasing

temperatures, until in some steels this can occur at ambient and above-ambient

temperatures. Clearly, the signiﬁcant variables in such a transition are of great

basic and practical importance.

The propagation of a cleavage crack in iron and steel requires much less

energy than that associated with the growth of a ductile crack. This is easily

shown by carrying out impact tests in a pendulum apparatus (Charpy, Izod and

Housﬁeld type tests) over a range of temperature. The energy absorbed by the

specimen from the pendulum when plotted as a function of temperature usually

exhibits a sharp change in slope (Fig. 11.2) as the mode of fracture changes from

ductile to brittle. These impact transition curves are a simple way of deﬁning

the effect of metallurgical variables, e.g. heat treatment (Fig. 11.2) on the frac-

ture behaviour of a steel from which a fairly precise transition temperature, T

can be readily obtained for a particular heat treatment. However, it should be

emphasized that T

is not an absolute value and it is likely to change appre-

ciably as the mode of testing is altered. It nevertheless provides a simple way of

comparing the effects of metallurgical variables on the fracture behaviour.

More sophisticated tests have been developed in which it is recognized that

the propagation of the fracture is the important stage. These fracture toughness

11.3 FACTORS INFLUENCING THE ONSET OF CLEAVAGE FRACTURE 237

Fig. 11.2 Effect of heat treatments on the impact transition temperature of a pure

iron–0.12C wt% alloy (after Allen et al., Journal of Iron and Steel Institute 174, 108, 1953). AC, FC

and WQ represent air-cooling, furnace-cooling and water-quenching, respectively.

tests used notched and pre-cracked specimens, the cracks being initiated by

fatigue. The stress intensity factor, K, at the root of the crack is deﬁned in terms

of the applied stress σ and the crack size c:

K = σ(πc)

1/2

When a critical stress intensity K

is reached, the transition to rapid fracture

takes place.

11.3 FACTORS INFLUENCING THE ONSET OF CLEAVAGE

FRACTURE

There are several factors, some interrelated, which play an important part in the

initiation of cleavage fracture:

1. The temperature dependence of the yield stress.

2. The development of a sharp yield point.

3. Nucleation of cracks at twins.

4. Nucleation of cracks at carbide particles.

5. Grain size.

All bcc metals including iron shown a marked temperature dependence of

the yield stress, even when the interstitial impurity content is very low, i.e. the

stress necessary to move dislocations, the Peierls–Nabarro stress, is strongly