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A D V A N C E D

M A T E R I A L S

P R O C E S S E S | F E B R U A R Y

2 0 1 6

ow temperature, carbide free bain-

itic steels are capable of achieving

tensile strengths to 2500MPawhile

maintaining reasonable values of duc-

tility and fracture toughness

[1,2]

. Their

mechanical properties are derived from

a very fine scale of bainitic ferrite plates

with thicknesses of 20-50 nm. Plates are

formed at low temperatures between

200°C to 300°C within steels where the

precipitation of cementite is inhibit-

ed by alloying additions of roughly

1.5%Si or Al, thus enriching the retained

austenite with carbon. This provides

sufficient stability for it to undergo a

stress/strain induced transformation

into martensite, which retains the work

hardening capacity and is known as

the transformation induced plasticity

(TRIP) effect

[3,4]

To ensure low transformation tem-

peratures, these alloys contain concen-

trations of carbon close to 0.8%. When

TABLE 1—STEEL COMPOSITION (WT%)

C Si

Mn Mo Cr

V Al

0.82 1.2 2.5 0.6 1.8 0.22 1.5

1 240°C 51°C

such steels are welded, they are suscep-

tible to cold cracking, due to formation

of brittle martensite within the weld

and reaustenitized heat affected zones

(HAZ)

[6]

. Therefore, these steels must be

preheated before welding, after which

prolonged isothermal holding treat-

ments are applied to regenerate the

bainitic microstructure within the weld

and reaustenitized regions. These treat-

ments can last from several hours to a

few days. The endeavor of welding such

steels is nevertheless encouraging, as

it has been shown that the welds can

achieve tensile strengths comparable

to those of the base materials

[7]

The authors’ research has led to

development of a newgrade of low tem-

perature bainitic steels—named Kineti-

cally Activated Bainite (KAB) steels—

with exceptionally rapid transformation

kinetics at temperatures below 200°C.

This is achieved by creating kinetical-

ly suitable conditions at a higher tem-

perature via a precipitation reaction. If

such a rapid formation of bainite can be

obtained during welding, determining

how welding parameters influence the

microstructure and mechanical proper-

ties of the weld is of great interest.

MATERIALS AND METHODS

The alloy’s chemical composition,

along with the critical temperatures of

the bainite start (Bs) and martensite

start (Ms), are shown in Table 1. Upon

air cooling to room temperature af-

ter a hot rolling treatment, the newly

developed steel forms a microstructure

of carbide-free lower bainite (Fig. 1). Mi-

crostructures were investigated by etch-

ing with 7% aqueous Na

, which is

known to respond by coloring bainitic

regions blue and martensite brown,

whereas retained austenite etches white

or in a slight purple

[8]

. In this condi-

tion, the steel has a Vickers hardness of

660 HV, coinciding with a yield strength

of 1800 MPa and tensile strength of

2800 MPa. Because no suitable fillers

for welding this steel are commercially

available, welds were produced using

the activated tungsten inert gas (A-TIG)

process with variable heat inputs. The

region of interest is the one that experi-

ences the highest heat input and is indi-

cated with a square in Fig. 2.

RESULTS

The initial trial involved a rela-

tively high heat input. Weld 1 develops

a high hardness and exhibits a coarse

microstructure (Fig. 4), while the mi-

crostructural features of interest are

shown in greater detail in Fig. 5. The

presence of coarse austenite and so-

lidification porosity is thought to be

the main cause of the modest 850 MPa

tensile strength. Heat input was then

reduced during formation of Weld 2, in

which a more refined microstructure

develops (Fig. 6). A more detailed view

is provided in Fig. 7(a), where appre-

ciable segregations within the HAZ are

still visible, but to a lesser extent than

Weld 1. The grain boundary enriched re-

gion was additionally etched with Vilel-

la’s reagent, as shown in Fig. 7(b). This

etchant delineates a thinner austenite

layer with additional bainitic sheaves

growing into the enriched region.

Figure 8 presents a mosaic image of

Detail (b) of Weld 2, showing coarse re-

tained austenite regions.

The tensile properties of Weld 2

are appreciably higher, reaching values

up to 1200 MPa. Heat input was fur-

ther decreased during the third weld-

ing test. The microstructure of Weld 3

indicates that films of austenite at the

grain boundaries no longer occur. How-

ever, some martensite is present within

the segregated region in the HAZ. The

parameters used for the production of

Fig. 1 —

Polarized LOM of initial bainitic

microstructure before welding shows fine

sheaves of bainitic ferrite separated by

regions of retained austenite. Etched with

7% aqueous Na

, mag. 1000×.

Fig. 2 —

Polarized LOMmosaic image of a weld with investigated region marked by a square.

Dotted lines represent points where hardness measurements were performed. A marked segre-

gation appears at the edge of the fusion zone boundary, indicated by color change.