“Heat input effect in the microstructure on High Strength...

MEMORIAS DEL XXV CONGRESO INTERNACIONAL ANUAL DE LA SOMIM 18 al 20 DE SEPTIEMBRE DE 2019 MAZATLÁN, SINALOA, MÉXICO

Tema A2a Materiales: Soldadura híbrida, Hybrid Welding.

“Heat input effect in the microstructure on High Strength Low Alloy Steel welded by Laser-GMAW Hybrid Welding”

Ana Soriaa, Melvyn Álvareza, Miguel Carrizaleza

aCorporación Mexicana de Investigación en Materiales S.A. de C.V., Ciencia y Tecnología No.790, Saltillo Coah. C.P.25290, México

*Contacto: [email protected]

R E S U M E N

Los ciclos de temperatura utilizados durante los procesos de soldadura tienen efecto en la evolución de la microestructura

y propiedades mecánicas de la unión. El objetivo del estudio es evaluar la microestructura obtenida en el acero HSLA550

de 8 mm soldado por el proceso de soldadura híbrida láser-GMAW en función de la entrada de calor del proceso. Se

evaluaron la microdureza en las áreas superior, media e inferior de la soldadura transversal y la resistencia a la tensión. Se

usó microscopía electrónica de barrido para analizar los cambios microestructurales en la zona de fusión (ZF) y la zona

afectada por el calor (ZAC). La microestructura observada en la zona del láser fue mayormente martensita en listones,

mientras que en la ZAC se encontró una mezcla de martensita en placa y listones y en la ZF se encontraron algunas áreas

de ferrita acicular y ferrita widmastäten secundaria.

Palabras Clave: Soldadura híbrida de laser- arco, acero de alta resistencia y baja aleación, microestructura.

A B S T R A C T

The temperature cycles used during the welding processes have an effect in the evolution of the microstructure, which is

mainly responsible for the mechanical properties of the joint. The aim of this study is to evaluate the microstructure

obtained in the 8 mm HSLA 550 steel welded by the laser-GMAW hybrid welding process depending on the heat input of

the process. The microhardness was evaluated in the upper, middle and lower areas of the transversal welding and tension

strength. Scanning electron microscopy was used to analyze the microstructural changes in the fusion zone (FZ) and heat

affected zone (HAZ). The microstructure observed in the laser zone was mostly lath martensite, while in HAZ a mixture

of martensite in plate and lath was found and in FZ some areas of acicular ferrite and secondary widmastäten ferrite were

found.

Keywords: Hybrid Laser Arc Welding, High Strength Low Alloy Steel, Microstructure.

1. Introducción

High strength and low alloy steels (HSLA) are used in the

construction, transportation and energy industry; some

grades of API HSLA steels, contain small amounts of

hardener elements such as Ti, Nb and V and low carbon

content, which provides good weldability [1]. High strength

steels are manufactured by advanced technologies achieving

a microstructure of fine ferrite grains and perlite bands

aligned in the direction of the deformation during the

manufacturing process providing high strength and good

toughness [2,3].

Welding processes are necessary for the fabrication of

structures, fusion welding employs thermal cycles that

modify the microstructure of the components and can affect

the mechanical properties of the joint as well as promoting

defects such as solidification cracking, hydrogen

embrittlement or segregations [4]. However, the effect of

each type of welding provides different heat input, welding

speed and cooling speed.

The effect of laser welding (LW), gas tungsten arc

welding (GTAW) and gas metal arc welding (GMAW) on

the microstructure and mechanical properties of high-

strength steel joints has been investigated [5,6]. In LW and

GTAW processes, greater hardness was obtained in the FZ

than in the base metal; while in GMAW the FZ showed

lower hardness than in the base metal [7]. In addition, in

laser welding, the change in the microhardness in the joint

was consistent, with hardening increments and softening

zones [8]. The microstructure resulting from a ultra-narrow

gap laser process and GMAW in a high strength steel S960

is also compared; obtaining in the FZ martensite and

martensite tempered in LW, the fracture resistance of the

welded metal and the base metal are similar. While in the

GMAW process the microstructure of the FZ is formed of

ferrite with some martensite, the welded metal was reduced

by 100 MPa compared to the base metal; as well showing a

softening in the HAZ [9].


The differences between arc and laser welding are due to

the particularities of each process; laser welding is

characterized by producing narrow and deep welds while arc

welding induces high heat input, that could generate

distortion [10]. A novel process for the industry is hybrid

laser-arc welding (HLAW), which is the combined process

where laser and arc welding are used simultaneously

interacting in the same welding pool [11]. Narrower welds

and small HAZ are obtained in the HLAW process. These

welds minimize the residual tension and distortion, due the

greater penetration depth of the laser allowed by the arc

interaction in the weld pool [12]. In a comparison between

HLAW and laser welding, the microstructure of HLAW FZ

was found acicular ferrite. Meanwhile in laser process the

FZ show the presence of bainite and martensite phases. In

addition, microhardness in laser welding was 40% higher

than in HLAW, due less temperature gradient within the

HLAW process [13].

In the present study the microstructure of the HAZ of 8

mm thick HSLA 550 steel plates joined by the HLAW

process was evaluated, the welds were made with a power

variation of 7 and 8 kW and wire feeding rate of 10 and 12

m/min. The microstructure was analyzed in optical

microscopy and scanning electron microscopy, in addition,

the microhardness of the weld and the tensile strength were

evaluated to determine the effect of the hybrid welding

process in the joint.

2. Experimental materials and procedures

HSLA 550 steel of 50 mm x 50 mm and 8 mm thickness

were used as the base metal. Table 1 lists the chemical

composition of HSLA 550.

Table 1 – Chemical composition of the HSLA 550. (wt.%).

Fe C V Nb Ti Si Mn Al

98.3 0.12 0.0034 0.0055 0.045 0.25 1.13 0.049

The butt joints were welded by the laser-GMAW hybrid

welding process by the TRUMPF equipment consisting of a

disk laser and a GMAW torch (Fig. 1). The welding

parameters used are shown in Table 2.

Samples were prepared metallographically with grinding

and polishing, were etched with 5% Nital for 5 seconds and

Picral 2% for 8 seconds. Penetration deep and dilution

measurements as well as HAZ, and FZ areas were observed

by the Nikon SMZ-745T stereoscope. The metallographic

characteristics of the base material, FZ and HAZ of the

samples were observed using the scanning electron

microscope TESCAN MIRA3 at 2000X.

Vickers microhardness test was carried out with Wilson

Hardness Tukon 2500, thirty indentations with distance of

0.3 mm between each indentation and 500 gf of preload were

obtained. In addition, tensile tests were performed on a

Tinius-Olsen H300KU-0049 equipment, based on the

ASTM E8 / E8M-16a standard.

Table 2 – Hybrid laser-GMAW welding parameters used.

Weld

Laser

Power

(kW)

Welding

speed

(mm/s)

Wire

feeding

rate

(m/min)

Voltage

(V)

Current

(A)

Heat

input

(J/mm)

a 8.0 30 12 28.8 194 452.90

b 7.0 30 12 29.2 186 414.37

c 7.0 30 10 36.0 286 576.53

d 8.0 30 10 36.0 287 611.06

Figure 1 – Hybrid laser-GMAW welding equipment.


3. Results and discussions

3.1. Weld cross section

The cross sections of the joints are shown in the Fig. 2 for

the penetration and width of the weld beads and Fig. 3 for

the HAZ and FZ areas. The numerical data are summarized

in Tables 3 and 4.

Figure 2 – Measurement of the weld’s widths and penetration

depths.

Table 3 – Welds width and depth penetration of HLAW. (mm).

Welds Depth Length FZ width HAZ width

a 8.92 4.40 6.98

b 9.36 4.07 5.57

c 8.98 4.19 6.22

d 9.16 4.55 6.07

Table 4 – Welds areas of HAZ and FZ in HLAW. (mm2).

Welds HAZ-1 FZ HAZ-2

a 8.13 16.47 7.97

b 6.98 20.98 6.97

c 8.91 16.40 8.02

d 7.88 16.33 7.59

Figure 3 – Measurement of the weld’s areas of HAZ and FZ.

In Fig. 2 (a) - (d) the full penetration of the welds is

shown. In the welds (a) (b) and (c) there is an excessive filler

metal on the top of the weld, however in Fig. 2 (d)

incomplete filling is observed in the upper part of the weld.

An increase in the area of heat affected and FZ is

observed by applying greater heat input during welding [14].

However, in this study no difference was found in the

different measurements of the FZ and HAZ in Fig. 3.

3.2. Microstructural analysis

Fig. 4 shows the optical microscopy image of the HSLA 550

base metal microstructure containing ferrite and perlite.

In welding of low carbon steels (less than 0.2% C)

microstructures commonly form is the martensite in shape

of laths grouped into shaves or packets. As the steel carbon

content increase, the martensite tends to form into individual

plates [15].

The weld metal in bottom zone revealed mainly lath

martensite (LM) showed in Fig.5. Coarse grain in heat

affected zone (CGHAZ) is shown in Fig. 6, exposed a

mixture of plate martensite (PM) and LM. In the Fig. 7, the

fine grain heat affected zone (FGHAZ) also shown PM but

with a grain refinement.


Figure 4 – HSLA 550 microstructure, F: ferrite, P: perlite.

Figure 5 – SEM microstructure of Lath Martensite (LM).

Figure 6 – SEM microstructure of Plate Martensite (PM) of CGHAZ.

CGHAZ occur due the high temperature reached for an

austenitic grain growth but insufficient for the fusion of the

grain. In the other hand, the grain refinement in FGHAZ, the

temperature reached is close to the isotherm of eutectoid

transformation, so it limits the growth of austenitic grain.

[16].

Figure 7 – SEM microstructure of Plate Martensite (PM) in FGHAZ.

In Fig. 8 shows the SEM an image of the FZ, it shows

localized zones of acicular ferrite (AF); the intergranular

inclusions generate the transformation of the AF phase. The

AF consists of microphases of primary ferrite (PF),

secondary widmanstäten ferrite (IIWF) and intergranular

bainite (IB) and its morphology depends on the chemical

composition and characteristics of the steel or inclusions

[15]. Fig. 8, also shows areas of (IIWF). IIWF is formed

from the austenitic grain boundary with parallel

arrangements, although it requires less transformation

energy than bainite or martensite, it is often confused with

them due to its morphology [15].

Figure 8 – SEM microstructure of Secondary Widmanstäten Ferrite

(IIWF) y Acicular Ferrite (AF).

3.3. Microhardness and tensile test

In Fig. 9, the variation of the microhardness test is shown.

The indentations were obtained from the cross section of

each weld at three different positions; the top section where

arc welding is observed; the middle area where a mixture of

laser and arc welding can be found; the lower area where it

is only affected by laser welding.


Figure 9 – Transverse microhardness profile of the weld

The upper welding zones were wider than the

intermediate and lower zones. In Fig. 9 (a) 514 HV in

FGHAZ was obtained, while in the CGHAZ the average

hardness was 486 HV, in addition, a softening zone is

observed in the FZ. Fig. 9 (c) shows a behavior similar to

Fig. 9 (a) but the arc welding zone is narrower. In Fig. 9 (b)

the GMAW area obtained less hardness than in the

intermediate zone and the lower one; the welding of (b)

received less heat input. On the other hand, the weld (d) was

the one that received the largest heat input, uniformity was

observed in the hardness profiles in the three zones.

For the tension test, two specimens of each weld were

performed. the welding samples examined in the stress test

fractured in the base metal, the stress-strain curves for each

specimen are showed in the Fig.10. The average ultimate

tensile strength is between 543 and 559 MPa and the

Young´s modulus 143 and 188 GPa. According to the

analysis carried out by SEM, Fig.11 shows a fracture of the

ductile type where dimples was observed on the surfaces.

This morphology is representative of ductile fractures [17].

Figure 10 – Stress-strain curve from tensile test in HSLA 550 steel.

Figure 11 – SEM microstructure of ductile fracture in HSLA 550

steel.

4. Conclusion

Plates of HSLA 550 steels were hybrid laser-GMAW

welded. The effect of the heat input on the microstructure

and mechanical properties was investigated. The results are

summarized below:

• Weld d had the higher heat input (611.06 J/mm), this

specimen shows narrow FZ and HAZ.


• The lower zones were most affected by laser welding,

the microstructure obtained was lath martensite. The

upper and intermediate zones present a mixture of

Widmastäten Ferrite, Acicular Ferrite and Martensite

due to the most heat input supplied by the arc in the upper

welding, obtaining less temperature gradient. The

microstrctures along of the joint formed by the weld

process produces a difference in microharness.

• The microhardness profiles vary according to the

properties and characteristics of the obtained

microstructures. The grain refinement of the FGHAZ

shows a higher hardness than CGHAZ due to the growth

of austenitic grain during welding.

• Resistance strength was not affected by the weld process

due to the fracture didn’t occur in the FZ or HAZ.

Acknowledgements

This work has been conducted under the financial support of

CONACYT during the Master´s Degree studies of the first

author Ana Soria at Corporación Mexicana de Investigación

en Materiales S.A. de C.V.


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