Texture analysis of welded 304L pipeline steel

Soumia Hamza, Zakaria Boumerzoug, Anne-Laure Helbert, Francois Brisset,

Thierry Baudin

To cite this version:

Soumia Hamza, Zakaria Boumerzoug, Anne-Laure Helbert, Francois Brisset, Thierry Baudin. Texture
analysis of welded 304L pipeline steel. Journal of Metals, Materials and Minerals, 2019, 29 (3), pp.32-
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 Texture analysis of welded 304L pipeline steel

Soumia HAMZA1, Zakaria BOUMERZOUG1*, Anne-Laure HELBERT2 , François

BRISSET2, and Thierry BAUDIN2

1
Mechanical Engineering Department, LMSM, Biskra University, B.P. 145, Biskra, 0700,
Algeria
2
ICMMO, SP2M, Univ Paris-Sud, Université Paris-Saclay, UMR CNRS 8182,91405 Orsay
Cedex, France.
*
Corresponding author e-mail: z.boumerzoug@univ-biskradz Phone: +213 (0) 775759694,

Fax: +213 (0) 33543148

Abstract

This present paper studied the effect of arc welding process on the crystallographic texture of

pipeline 304L steel. The Electron Backscatter Diffraction (EBSD) technique was the main

technique used to illustrate the effect of welding on grain orientation in the fusion zone, in the

heat affected zone, and in the base metal. Moreover, the effect of isothermal heat treatment at

400 °C on welded joint has been studied. It was found the same orientation in the base metal

and the heat affected zone, however different and heterogeneous structures were observed in

fusion zone. The applied heat treatments at 400°C on welded material had a slight effect on

crystallographic texture, but it had an effect on grain size in fusion zone.

Keywords: stainless steel; welding; EBSD; texture; heat treatments

1. Introduction

Austenitic stainless steels constitute an important class of engineering materials that have

been used widely in a variety of industries and environments due to their high corrosion and

oxidation resistance [1-4]. Among the many 300 series austenitic stainless steel grades, AISI

1
304L stainless steel is extensively used in industries due to its superior low temperature

toughness and high corrosion resistance [5].

In the fabrication of equipment made from stainless steels such as pipe, automotive

exhaust gas system, chemical industrial equipment, ..., arc welding using shielding gas is

often used [6]. Tungsten inert gas (TIG) welding is the most reliable method to weld stainless

steels [5]. The welded joint microstructure in any installation can differ significantly from the

parent metal. Differences in both welding consumables and the welding process may affect

the final weld composition and due to competitive crystal growth which occurs during

solidification, preferred crystallographic texture are always observed [8]. Heat generated

during welding induces an important temperature gradient in and around the welded area or

fusion zone (FZ). The microstructure that develops in the FZ varies noticeably from region to

region. The zone outside the fusion zone that is thermally affected by the welding treatment is

known as the heat-affected zone (HAZ) [9]. In addition, the understanding of the grain

structure development in the FZ of polycrystalline welds is limited. This is because many

competitive transformations have been observed.

However, the most previous works on welding of 304L stainless steel were focused on

microstructure and corrosion behavior of welded material [1, 10-12] and a limited published

works were devoted to the texture investigation in welded 304L stainless steel [14].

Furthermore, investigating and controlling texture are necessary because it affects mechanical

properties. In this context, the aim of the present work is to investigate the crystallographic

texture in welded 304L stainless steel and also to study the heat treatment effect on the

crystallographic texture which has not been studied before.

2
2. Experimental

In this study, the base metal (BM) was AISI 304L austenitic stainless steel used for

transport gas pipeline applications. Table 1 presents the chemical composition of the AISI

304L which has been determined by the EDS technique.

Table 1

To meet strength and safety requirements and to produce good quality welds, V-shaped butt

welds were prepared using two consecutive passes with gas tungsten arc welding (GTAW)

method and ER-308L filler was used as electrode. The chemical composition of the electrode

is presented in table 2.

Table 2.

In addition, argon gas was used during welding to avoid a penetration into weld region of

some undesirable elements (N2, O2, H2). Figure 1 presents the schematic illustration of a butt

weld specimen. After welding, a transverse cross section (perpendicular to the welding

direction) of samples was cut for EBSD analyses.

Figure 1.

In order to study the heat treatment effect on welded specimens, an isothermal annealing was

performed in electrical furnace at 400°C during 30 min. Then, detailed texture, grain size and

morphology studies were carried out in the FZ, HAZ, and BM region. EBSD specimens were

prepared in the standard manner (mechanical polishing with SiC paper and electro polishing

with the A2 Struers solution during 12s in 40V flux 12). EBSD measurements were

performed on the sample transverse cross section using a Scanning Electron Microscope

(SEM) with a Field Emission Gun (FEG) SUPRA 55 VP operated at 20 kV with a TSL

orientation imaging system (using OIMTM software). EBSD maps covered sample area of

about 7 mm x 7 mm of cross-section of the weld joint (on the transverse plane perpendicular

3
to the weld direction) (Fig. 2) with a step size of 2 μm. To study at least half of the welded

joint by the EBSD technique, 20 maps of welded material, and 27 maps of welded and heat

treated at 400 °C have been studied.

Figure 2

The pole figures and the orientation distribution function (ODF) are calculated, using the

harmonic series expansion (rank L= 22) method, from the orientations measured by EBSD.

Each orientation is modeled by Gaussian function with a 5° half width. Even if the EBSD

maps are measured in the (ND, RD) plane, the crystallographic orientation are described in

the (RD, TD) sheet rolling plane.

3. Results and discussion

3.1. Texture in welded joint

Figure 3 shows EBSD map of the half part of the welded joint of 304L stainless steel which

contains the BM, HAZ and the half part of FZ. The color of individual grains describes the

{hkl} crystallographic plane parallel to the observation plane. This EBSD map gives a

general idea about the grain morphology and orientation in each zone. The microstructure of

either BM or HAZ is characterized by finer equiaxed-grains, however the FZ adjacent to the

HAZ is totally different from other zones (HAZ, BM), and composed of different sub-zones

with different grain morphologies.

Figure 3.

3.1.1 The texture of the base metal

It is well known that the microstructure of austenitic stainless steel is mainly composed of

austenite under the condition of equilibrium solidification. Concerning the texture of the BM,

4
its EBSD map indicates a polygonal shape of the grains (Fig. 4a). On the basis of three poles

figures of planes {001}, {011} and {111} (Fig. 4b) and the ODF (Fig. 4c), the texture of the

base metal corresponds to the major {110}<001> Goss texture. The minor near {110}<111>

orientation is also observed (Fig. 4b and c).

Figure 4.

3.1.2 The texture of the heat affected zone

The HAZ is a narrow zone and it represents a transition zone between FZ and the base metal.

HAZ is generally composed of an austenite matrix and interspersed ferrite precipitates,

because, during the non-equilibrium rapid solidification conditions, such as in welding, the

high cooling rate will result in incomplete  → δ transformation and small amounts of δ-

ferrite should be remained unavoidably in the weld microstructure at room temperature. The

retained δ-ferrite is known to prevent solidification and hot cracking and to improve ductility,

toughness and corrosion resistance. However, it is also reported that excess δ-ferrite (usually

more than 10 vol. %) can decrease the hot workability [15].

As presented in Figure 5, the texture of HAZ corresponds to the {110}<001> Goss

orientation. The minor near {110} <111> orientation is still present such as in the BM. In this

zone, the texture acuity is slightly lower than in BM. This difference is probably due to the

effect of heat input during welding process.

Figure 5.

3.1.3 The texture of the fusion zone

Concerning the microstructure of FZ, the majority of the grains in fusion zone have

columnar structure with preferred directions. During growth of the solid in the weld pool, the

shape of the solid-liquid interface controls the development of microstructural features [16].

5
This columnar microstructure is due to the preferred mechanism of solidification after

welding process which has been observed in previous works [17-19].

For clarity, four distinct sub-zones were considered in FZ (sub-zones 1, 2, 3, and 4). The

texture of FZ varies according the sub-zones. The difference of the microstructure inside the

FZ is due to the difference of cooling rates which were caused by the heat flow during

welding process. For example, the microstructure of sub-zone 1 exhibits an isotropic

polycrystalline structure (Fig. 6). In general, development of equiaxed grain structure induces

best mechanical properties of weld.

Figure 6.

The microstructure of sub-zone 2 presents a columnar structure with elongated grains and

its texture is near a fiber close to {hkl}<001> (Fig. 7). Columnar grains generally grow

perpendicular to the fusion boundary in direction opposite to the heat flow [20]. This result

indicates that the columnar region exhibits a specific solidification texture which has its

origin in the phenomenon of preferred crystal growth [21-23]. It has been found that a

solidification texture depends on the local heat flow directions and competitive grain growth

in one of the six <100> preferred growth directions in face centered cubic alloys [24]. Ugla

[25] observeded the columnar dendritic arms in FZ of welded AISI 304L stainless steel and

he attributed its formation to the cooling rate direction after welding process.

Figure 7.

The third adjacent sub-zone (Sub-zone 3) presents an equiaxed-grains microstructure.

This area presents a near {100}<001> Cube texture. This component is rotated around ND

and tilted along the (Fig. 8). The presence of other texture components (not

6
ideal ones) is thus a result of complementary crystal rotations taking place within the same

orientation region [21].

Figure 8.

Finally, the sub-zone 4 exhibits a partial {100}<uvw> fiber that is slightly rotated around

ND (A1). In this fiber, the Cube component is preferentially developed (Fig. 9). These

oriented grains form as extensions of grains of sub-zone 3. This sub-zone texture corresponds

to the solidification growth direction after the second pass of welding because the austenitic

grains grow with a coarse columnar structure along the preferential growth direction. Let us

note that such coarse columnar structure is susceptible to hot cracking [26].

Figure 9.

For brevity, Table 3 presents the crystallographic texture on the BM, HAZ, and FZ in

welded 304L stainless steel.

Table 3

It is convenient to consider the FZ as a mini-casting and fundamental solidification

mechanisms developed primarily for cast metals have been successfully applied to the welds

[26]. Based on the developed structures in fusion zone of welded 304L stainless steel and

above considerations, a schematic illustration of solidification mechanism is proposed (Fig.

10). Therefore, the microstructures formed in FZ of welded 304L stainless steel contain three

successive main sub-zones. First, a sub-zone formed by finer grains, adjacent to the HAZ is

followed by large columnar grains and stopped by an equiaxed grains area formed in third

sub-zone. This grain distribution is identical to the grain distribution in steel ingots because

7
most castings have three zones: the chill zone, columnar zone, and equiaxed zone [27]. The

chill zone corresponds to the finer grain zone which is due to the rapid nucleation that occurs

because the molten metal comes into contact with the base metal.

Figure 10.

3.2 Effect of heat treatment on texture evolution in welded join

Fig. 11 presents a general view of the welded joint (EBSD maps) after heat treatment at

400°C during 30 minutes of welded 304L stainless steel. The finer grains observed in sub-

zone 3 of the unheated welded steel disappear inside the FZ, i.e., there was a grain growth

reaction during this isothermal annealing.

Figure 11.

Except the disappearance of the sub-zone 1, the heat treatment does not modify the

microstructure and the texture of the weld.

4. Conclusions

The investigation by the EBSD technique of welded of AISI 304L stainless steel has

showed different grain morphologies and textures. In summary, three distinct zones were

observed in the welded joint which are the base metal, the heat affected zone and the fusion

zone. In addition:

 The BM presents an equiaxe grain structure and its texture is composed of

a major Goss orientation and minor component close to {110}<111>.

 In the HAZ, the microstructure and the texture are similar to those

observed in the BM.

 The microstructure and the texture in FZ depend on each selected sub-zone

inside this region. However the main zones are described by a columnar

8
 microstructure and a <100> or {100} fiber for the first and the second pass

of welding, respectively. In the first join, two areas of equiaxe grains are

found. The first one, with finer grains is isotropic and situated near the

HAZ. The second one, in the middle of the weld, shows a near Cube

orientation with a large spread plus some other orientations not easily

identifiable.

The isothermal heat treatment (30 min at 400°C) leads to the disappearance of the finer

grains in the join, but the texture is not affected.

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Figure captions

Figure 1. Schematic illustration of the butt weld specimen with the sheet coordinate system:

Rolling Direction (RD), Transverse Direction (TD) and Normal Direction (ND).

Figure 2. Macrographic view of welded 304L steel. The red square indicates approximately

the analyzed area by EBSD.

12
Figure 3. EBSD map of BM, HAZ and FZ (with selected sub-zones 1, 2, 3 and 4) in welded

304L stainless steel. The color code is given on the standard triangle. (First pass

contains three sub-zones 1, 2 and 3. Second pass contains sub-zone 4)

Figure 4. a) EBSD map (step size = 0.5 µm), b) {001}, {011} and {111} pole figures, and c)

ODF of BM in the welded 304L stainless steel. ( □ Goss orientation and ○

{110}<111> orientation).

Figure 5. a) {001}, {011} and {111} pole figures and b) ODF of HAZ in the welded 304L

stainless steel. ( □ Goss orientation and ○ {110}<111> orientation)

Figure 6. a) {001}, {011} and {111} pole figures and b) Orientation Density Function (ODF)

of fusion zone, sub-zone 1 in the welded 304L stainless steel.

Figure 7. a) {001}, {011} and {111} pole figures and b) ODF of FZ sub-zone 2 in the

welded 304L stainless steel.

Figure 8. a) {001}, {011} and {111} pole figures and b) ODF of FZ sub-zone 3 in the

welded 304L stainless steel. ( ◊ {100}<001> Cube orientation)

Table captions

Table 1. Chemical composition of the base metal (Wt.%).

Table 2. Chemical composition of the electrode (Wt.%).

Table 3. Microstructure and textures in BM, HAZ and FZ of welded 304L stainless steel.

13
Figures

Figure 1.

Figure 2.

14
Figure 3.

15
Figure 4.

16
Figure 5.

17
Figure 6.

18
Figure 7.

19
Figure 8.

20
Figure 9.

21
Figure 10.

22
Figure 11.

23
Tables

Fe C Mn Si P S Ni Cr

Balance 0.026 1 .07 0.40 0.038 0.001 8.11 18.50

Table 1

Fe C Mn Si P S Ni Cr

Balance 0.026 1 .07 0.40 0.038 0.001 8.11 18.50

Table 2

BM and HAZ FZ

Sub-zone 1: Equiaxe grains

Microstructure Equiaxe grains Sub-zone 2: columnar grains

Sub-zone 3: Equiaxe grains
Sub-zone 4: columnar grains

Sub-zone 1: Isotropic texture

Texture Major {110}<001> Sub-zone 2: Fiber close to

Goss component {hkl}<001>
with minor
orientation close to Sub-zone 3: Near {100}<001> Cube
{110}<111> orientation plus other not ideal
orientations
Sub-zone 4: Fiber close to
{100}<uvw> with a major Cube
component

Table 3

24