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

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298 CHAPTER 13 WELD MICROSTRUCTURES

microstructure and mechanical properties change more rapidly at low carbon

concentrations.

13.3 THE HAZ

The HAZ is the portion of the material which has not been melted, but whose

microstructure and mechanical properties are altered by the heat of welding.

13.3.1 Heat flow

All welding processes involve a source of heat, the prime purpose of which is

to cause melting. Subsequent solidiﬁcation should lead to the formation of an

integral joint. Much of the heat manages to diffuse from the fusion zone into

the adjacent solid regions. As a consequence, those regions experience a heating

and cooling cycle, the severity of which depends on the distance from the fusion

boundary (Fig. 13.10). The peak temperature and the heating rate decrease with

distance away from the fusion boundary. The cooling rate, on the other hand, is

less sensitive to this distance, and can be stated as the time t

8−5

taken to cool

over the range 800–500

◦

C. For many weldable steels, this deﬁnes the tempera-

ture range within which austenite decomposes by solid-state transformation.

The nature of the thermal cycle at any position within the HAZ can be

characterized by two parameters, the peak temperature T

and the time period

t

8−5

. Both of these parameters increase with the heat input q:

∝

Fig. 13.10 Temperature–time curves representing typical thermal cycles experienced in the

HAZ of a weld (adapted from data published in theWelding Handbook, editor C.Weisman,Vol. 1,

American Society for Welding, Florida, USA, 1981).

13.3 THE HAZ 299

t

8−5

∝ q

where r is the distance from the fusion boundary and n has a value (1 or 2)

which depends on whether the component being welded is thick compared with

the size of the weld bead. The relative thickness determines whether the ﬂow

of heat is two- or three-dimensional (Fig. 13.11). The heat input q is per unit

length of weld, and typically is in the range about 1–5 kJ mm

−1

13.3.2 Microstructural zones

There is a well-deﬁned gradient of microstructure in the HAZ, as a function of

the distance from the fusion boundary (Fig. 13.12):

1. Those regions immediately adjacent to the fusion boundary are heated to

very high temperatures and hence transform completely to austenite. During

continuous heating, austenite begins to form at a temperature Ac

≃800

◦

and the samples become fully austenitic at Ac

≃950

◦

C.These temperatures

are different from the corresponding equilibrium temperatures Ae

and Ae

because they increase with the heating rate. The peak temperatures in the

HAZ close to the fusion boundary are well in excess of the Ac

temperature

of weldable steels. Consequently, the austenite that forms is annealed during

heating beyond Ac

, giving rise to a very coarse grain structure. This forms

the coarse-grained austenite zone.

2. The austenite grain size decreases sharply with distance from the fusion

boundary. It is necessary to distinguish this as the ﬁne-grained zone because

its mechanical properties tend to be superior to those of the coarse

zone.

3. As the peak temperature decreases, regions of the HAZ further away from

the fusion boundary become only partially austenitic during the heating

part of the thermal cycle. The austenite that forms has a rather high car-

bon concentration, due to the increase in the solubility of carbon in γ with

decreasing temperature. The part that does not transform into austenite

becomes tempered.

Fig. 13.11 Schematic illustration of two- and three-dimensional heat-flow conditions.

300 CHAPTER 13 WELD MICROSTRUCTURES

Fig. 13.12 Schematic illustration of the microstructural variation to be expected in the HAZ

of steel welds.

4. When the peak temperature becomes less than the Ac

temperature, the

only effect of the heat input is to temper the microstructure, the extent of

tempering decreasing with distance from the fusion boundary.

The individual microstructures are illustrated in Fig. 13.13, and discussed in

detail in the sections that follow (Table 13.2).

13.3.3 The coarse-grained austenite

The formation of austenite during heating is in many ways different from trans-

formations which occur during cooling below the equilibrium temperature. As

discussed in Chapter 6, the formation of ferrite follows a C-shaped curve kinetic

13.3 THE HAZ 301

Fig. 13.13 The gradient of microstructure in the HAZ of a mild steel plate (courtesy of

C. Davis). (a) The plate microstructure far away from the weld, completely unaffected by

welding.The bands of ferrite/pearlite are typical of many structural steels which are chemically

inhomogeneous.

behaviour on a time–temperature–transformation (TTT ) diagram; the overall

transformation rate, therefore, goes through a maximum as a function of the

supercooling below the equilibrium temperature. This is because of two oppos-

ing effects; the diffusion coefﬁcient decreases as the temperature falls, but the

driving force for transformation increases.

During heating, however, both the diffusion coefﬁcient and the driving

force increase with temperature. The overall rate of transformation, therefore,

increases continuously as the transformation temperature is raised, Fig. 13.14.

For practical purposes, the formation of austenite during heating can be

represented by a continuous heating transformation (CHT) diagram, analo-

gous in concept to the continuous cooling transformation (CCT) diagrams so

useful in illustrating the formation of ferrite (Fig. 13.15a). The CHT diagram is

displaced to longer times when compared with the isothermal transformation

diagram for austenite formation. For typical heating rates encountered in the

region adjacent to the fusion boundary, the formation of austenite should be

completed when the temperature has exceeded the Ac

temperature by about

100

◦

C. Since the peak temperature in this zone is much higher than Ac

, the

302 CHAPTER 13 WELD MICROSTRUCTURES

Fig. 13.13 (Continued). (b) The tempered region. (c) The partially reaustenitized region.

13.3 THE HAZ 303

Fig. 13.13 (Continued). (d) The fully austenized region. (e) High magnification image of the

partially austenitized region.

304 CHAPTER 13 WELD MICROSTRUCTURES

Table 13.2 Characteristic temperature ranges for the variety of

microstructural regions within the HAZ of steel welds

HAZ microstructure Temperature range

Coarse-grained austenite 1500

◦

C > T

> 1200

◦

Fine-grained austenite 1200

◦

C >T

> Ac

Partially austenitized zone Ac

> T

> Ac

Tempered regions Ac

> T

Fig. 13.14 A comparison of theTTT curves for the γ →α transformation, and for the reverse

α →γ transformation. G represents the driving force for transformation and D is the diffusion

coefficient.

austenite grains coarsen rapidly as T

is approached. In steels which are microal-

loyed, it may necessary for the grain boundary pinning particles (e.g. niobium

carbonitrides) to dissolve before substantial grain coarseningoccurs. In any case,

once the coarsening begins, it proceeds very rapidly because the effect of tem-

perature increases exponentially during heating.The austenite grain growth can

be expressed conveniently in the form of grain growth diagrams (Fig. 13.15b)

which contain contours of equal grain size as a function of the peak temperature

and t

8−5

The importance of the coarse-grained austenite zone is in the mechanical

properties which develop as the austenite transforms during the cooling part of

the thermal cycle. The coarse grain structure leads to an increase in hardenabil-

ity, because it becomes easier to avoid intermediate transformation products, so

that untempered martensite or other hard phases can form during cooling. The

welding process introduces atomic hydrogen into the weld metal, which is able

to diffuse rapidly into the HAZ. Hard microstructures are particularly suscepti-

ble to embrittlement by hydrogen, the fracture occurring shortly after the weld

13.3 THE HAZ 305

Fig. 13.15 (a) The TTT and CHT diagrams for the beginning of austenite growth in a

Fe–0.15C–0.5Si–1.5Mn wt% alloy (courtesy of Suzuki). (b) Schematic austenite grain growth

diagram for a microalloyed steel welded using a preheat of 200

◦

C (after Ashby and Easterling,

1982).

has cooled to room temperature. This hydrogen-induced phenomenon is called

‘cold-cracking’. This is why the CE of the steel has to be kept low enough to

prevent the hardness in the coarse-grained region from becoming unacceptably

large.

13.3.4 Fine-grained austenite zone

This region is typiﬁed by austenite grains some 20–40 µm in size. The grain struc-

ture and hardenability are, therefore, not very different from those associated

with control-rolling operations during the manufacture of the steel. The ﬁne

306 CHAPTER 13 WELD MICROSTRUCTURES

austenite grains thus transform into more desirable ferritic phases, with lower

hardness values and higher toughness.

13.3.5 Partially austenitic regions and local brittle zones

At a sufﬁciently large distance from the fusion boundary, the peak temperature

is such thatthe steel cannottransform completelyto austenite.The smallamount

of austenite that does form has a larger carbon concentration. Thisis because the

solubility of carbon in austenite, which is in equilibrium with ferrite, increases

as the temperature decreases. The subsequent transformation behaviour of this

enriched austenite is then quite different, since it has a higher hardenability.

If the cooling rate is sufﬁciently large, then the carbon-enriched austenite

transformspartially into hardmartensite,the remainingaustenite beingretained

to ambient temperature. These minute regions of hard martensite are known

as ‘local brittle zones’. They are located in much softer surroundings consisting

of tempered ferrite. Consequently, they do not cause a general reduction in

toughness, but lead to an increase in the scatter associated with toughness tests.

This is because the test specimen only sometimes samples the local brittle zone,

in which case the recorded toughness will be poor. On other occasions, the

measured toughness can be very high, presumably because the test region does

not include a local brittle zone. Such scatter in mechanical property data is not

only disconcerting, but also makes design difﬁcult because of the existence of a

few very low values.

When the cooling rate in this region is not high enough to induce martensitic

transformation, the carbon-enriched austenite can decompose into a mixture

of coarse cementite and ferrite. The cementite particles again constitute local

brittle zones and increase the variability in mechanical properties.