Effect of Heat Input

TECHNICAL ARTICLE

Effect of Welding Heat Input on Microstructure and MechanicalProperties of HSLA Steel Joint

Honggang Dong Xiaohu Hao Dewei Deng

Received: 27 November 2013 / Revised: 25 February 2014 / Accepted: 10 March 2014 / Published online: 29 March 2014

Springer Science+Business Media New York and ASM International 2014

Abstract Gas tungsten arc welding of HSLA steel was

conducted with different welding heat inputs, and the

influences of welding heat input on the microstructure,

Vickers hardness, and impact toughness of heat-affected

zone (HAZ) in prepared joints were systematically inves-

tigated. The microstructure in HAZ with low welding heat

input mainly consisted of martensite, and the microhard-

ness of coarse grain HAZ was measured higher than that of

fine grain zone. The results show that increasing the

welding heat input could suppress the formation of mar-

tensite and reduce the microhardness of HAZ. However,

the impact toughness of HAZ was not monotonously

improved with the increase of welding heat input. It is

deemed that the coarsened grain, the formation of upper

bainite, and the non-uniform distribution of carbides and

inclusions in HAZ could degrade the impact toughness.

Tests demonstrate that the optimum comprehensive prop-

erties of HAZ were obtained when the welding heat input

was 0.67 kJ/mm.

Keywords Welding heat input Microstructure Hardness Impact toughness HSLA steel

Introduction

Due to the excellent mechanical properties and weldability,

HSLA steels have been widely used in various fields,

especially in the manufacturing of large-scale naval vessels

and air compressors [1]. As a result, welding of HSLA steels

is indispensable in these large-scale structures. Generally,

high welding heat input is employed because the HSLA

steel structures to be joined usually serve in large size.

However, Viano et al. [2] pointed out that high welding heat

input was often accompanied by wide heat-affected zone

(HAZ), and Li et al. [3] found that the impact toughness of

HAZ was deteriorated when high welding heat input was

applied. So it is necessary to pay more attention to the

influence of welding conditions, especially, the welding

heat input, on the mechanical properties of HAZ.

Extensive investigations have been conducted on the

influence of welding heat input on the microstructure and

mechanical properties of HSLA steel joints. Shome [4]

investigated the relationship between the heat input and the

austenite grain size, and noticed that the prior austenite grain

size was controlled by the peak temperature (Tp) in the

welding thermal cycle which was related to the welding heat

input. Spanos et al. [5] studied the effect of grain size on

hardness, and found that small austenite grain size could

result in higher hardness. Shome and Mohanty [6] also

reported that the toughness decreased with increasing the

austenite grain size. Moreover, Morito et al. [7] revealed that

close relation existed between the strength of lath martensite

in HAZ and the austenite grain size. However, few

researchers comprehensively discussed the relation among

the welding heat input, austenite grain size, hardness, and

impact toughness of HAZ, especially for HSLA steel.

Besides the prior austenite grain size, the properties of

HAZ are heavily dependent on the microstructure which is

determined by the phase transformation after welding. Dhua

et al. [8] found that the welding heat input played a sig-

nificant role in the microstructure of the joints. However,

more attention should be paid to the influence of welding

heat input on the properties of HAZ, as the weakest region,

rather than the weld metal. Mukherjee and Pal [9]

H. Dong (&) X. Hao D. DengSchool of Materials Science and Engineering, Dalian University

of Technology, Dalian 116085, Peoples Republic of China

e-mail: [email protected]

123

Metallogr. Microstruct. Anal. (2014) 3:138146

DOI 10.1007/s13632-014-0130-z

investigated the effect of welding heat input on the micro-

structure and properties of ferritic stainless steel, and they

found that higher welding heat input led to more martensite

and higher toughness. Ion et al. [10] discussed the variation

of the microstructure and hardness of HAZ with the welding

heat inputs, and proposed the HAZ microstructure/hardness

diagrams to guide the welding process. But the impact

toughness of HAZ was not considered which was decisive

for the properties of the joints. Bhadeshia et al. [11] built a

model of the microstructural transformation and believed

that the different phase transformation products were con-

trolled by the cooling rate. Zhang et al. [12] studied the

influence of welding heat input on the toughness of coarse

grain HAZ. They found that when the welding heat input

was increased from 30 to 100 kJ/cm, self-tempered mar-

tensite with fine lath bainite in HAZ transformed to lath

bainite with ferrite, and the impact toughness of coarse

grain heat-affected zone (CGHAZ) decreased. Moreover,

Babu [13] demonstrated that the acicular ferrite produced a

beneficial effect on the properties of the joints. On the other

hand, as indicated by Bhadeshia et al. [11], the formation of

the acicular ferrite was mainly determined by the austenite

grain size, inclusions, and the cooling rate. Also, Lan et al.

[14] found that the medium cooling rate produced larger

volume fraction of acicular ferrite. Bhole et al. [15] reported

that the addition of Mo could promote the formation of

acicular ferrite, and the increase of acicular ferrite led to the

improvement of impact toughness in API HSLA-70 steel.

Furthermore, Harrison and Farrar [16] pointed out that the

high angle grain boundaries responded to the improvement

of impact toughness by acicular ferrite. However, there

were few reports about the formation of acicular ferrite and

the effect of welding heat input on the phase transformation

in HAZ of HSLA steel joints.

In practice, microcracks often initiate in HAZ of the

HSLA steel joints and cause severe economic losses,

especially when the welded structures are subjected to

working conditions with high pressure and large load.

Therefore, more attention should be paid to the properties of

HAZ in HSLA steel joints. In this paper, different welding

heat inputs were applied to gas tungsten arc welding

(GTAW) of HSLA steel plates in order to systematically

investigate the influence of welding heat input on the

microstructure and mechanical properties of HAZ. Sub-

sequent tests and analysis were carried out to explore the

relationship between the welding heat input and the variation

of microstructure, hardness, and impact toughness of HAZ.

Experimental

The nominal composition of the as-rolled HSLA steel is

given in Table 1, and the dimensions of HSLA steel plates

used in this paper are 200 mm 9 100 mm 9 6 mm. The

oxide film on the surface of HSLA steel plate was removed

with stainless steel brush and the grease was cleaned with

acetone. Since the aim of this paper is to investigate the

microstructure and corresponding properties of HAZ in the

joint, the effect of filler metal and groove design was not

considered for simplification, although the accepted sheet

thickness was 6 mm. Automatic autogenous GTAW of

HSLA steel was carried out, as shown in Fig. 1. Argon was

used as the shielding gas with a flow rate of 10 L/min. The

arc length was kept constant at 3 mm, corresponding to the

welding voltage of 11 V. The welding heat input (E) was

calculated with the formula: E = gIU/m, where I, U, v, andg are the welding current, welding voltage, welding speed,and arc efficiency, respectively. Here, the arc efficiency gwas set at 0.7 based on the simulation results by Mousavi

and Miresmaeili [17]. Dt8/5 is commonly used as thecooling rate index to represent the cooling time from 800 to

500 C, and it could be calculated by the empirical equa-tion: Dt8/5 = 5gE [18]. The other welding parameters,resultant welding heat inputs, and Dt8/5 are listed inTable 2. The specimen was etched with 4% nital solution

and microstructure observation was carried out with optical

microscopy (OM). At about 1 mm below the surface of the

weld, Vickers hardness test was conducted on the cross

section of the joints with a load of 300 g for 15 s to obtain

the distribution of microhardness in the HAZ and weld.

The distance between every two adjacent test points was

200 lm. Specimens were extracted by wire cutting forstandard Charpy notch (U-notch) impact test to evaluate

the impact toughness of HAZ. Figure 2 shows the location

Table 1 Nominal compositionof HSLA steel in wt%

C Si Mn Cr Mo Cu S P Fe

0.130.18 0.170.37 0.500.80 2.202.50 0.770.90 \0.30 \0.03 \0.035 Bal.

Fig. 1 Schematic for gas tungsten arc welding of HSLA steel platewithout filler metal. The welding was performed by a travel

mechanism automatically

Metallogr. Microstruct. Anal. (2014) 3:138146 139

123

of U-notch in the joint for impact test. Then the fractured

surface was examined with a scanning electron microscope

(SEM).

Results and Discussion

Microstructure

It can be seen in Fig. 3 that the microstructure of HSLA steel

base metal was typically granular bainite (GB) which con-

sisted of carbides (black) and stripy ferrite (white). The

microstructure of HAZ under welding heat inputs of 0.25 and

0.67 kJ/mm are revealed in Figs. 4 and 5, respectively. The

weld penetration and weld width increase with increasing the

welding heat input, as shown in Table 2 and Figs. 4(a) and

5(a). It was found that the microstructure in fine grain HAZ

(FGHAZ) of HSLA steel mainly consisted of fine grain lower

bainite. Small amount of martensite appeared in FGHAZ

when the welding heat input was 0.25 kJ/mm and it changed

to acicular ferrite and lower bainite after increasing the heat

input to 0.67 kJ/mm. In contrast, the microstructure in

CGHAZ is more complicated.

As reported by Lan et al. [14], lath martensite (M), lath

bainite (B), retained austenite, and various ferrites (F) with

different morphologies could be observed in HAZ of HSLA

steel joints. When different welding heat inputs were used,

the HSLA steel base metal near the fusion line experienced

different thermal cycles, and different microstructures were

formed in HAZ. When the welding heat input was only

0.25 kJ/mm, the cooling time (Dt8/5) was about 0.88 s, andthis fast cooling rate easily produced brittle and hard phases

in HAZ. It can be seen in Fig. 4(b, c) that lath martensite

dominated in the narrow CGHAZ, but lower bainite with a

little martensite formed in FGHAZ. The microstructure in

HAZ with welding heat input of 0.40 kJ/mm consisted of

lath martensite and some bainite, and Dt8/5 was up to 1.4 scorresponding to the increase of heat input. The increase of

Dt8/5 restrained the phase transformation of martensite andproduced some lath bainite in HAZ. This mixed micro-

structure of lath martensite and bainite was similar to that

observed by Shome et al. [19] who believed that martensite

was the main microstructure in HAZ of HSLA-80 steel and

HSLA-100 steel-welded joints. Although martensite could

improve the strength, Thewlis [20] stated that it usually led

to high hardness and was harmful to the impact toughness of

HAZ. And similar conclusions were obtained by Zhang

Fig. 2 Illustration of the location of the U-notch in the joint

Fig. 3 Microstructure of the as-rolled HSLA steel base metal. GBgranular bainite, F ferrite

Table 2 Welding parameters,welding heat input, weld

penetration, weld width, and

Dt8/5 for GTAW of HSLA steel

Sample

no.

Welding

current,

I (A)

Welding

speed,

v (mm/s)

Welding

heat input,

E (kJ/mm)

Weld

penetration

(mm)

Weld

width

(mm)

Dt8/5(s)

1 180 3 0.46 1.8 8.0 1.61

2 220 3 0.57 2.5 11.0 2.00

3 260 3 0.67 2.7 12.3 2.35

4 260 5 0.40 2.2 9.0 1.40

5 260 8 0.25 1.7 7.3 0.88

6 300 3 0.77 2.8 13.4 2.70

140 Metallogr. Microstruct. Anal. (2014) 3:138146

123

et al. [12]. With increasing the welding heat input, the

cooling time Dt8/5 became longer, and the amount of mar-tensite in HAZ decreased accompanied by an opposite

change of bainite. When the welding heat input was

0.67 kJ/mm, the microstructure in HAZ was mainly fine

lower bainite with some acicular ferrite, as shown in

Fig. 5(b, c). In lower bainite, the carbides existed within

and between the ferrites. While Thewlis [21] demonstrated

that in upper bainite, the carbides could only be observed

between the ferrites. In Fig. 5(c), the carbides were refined

further, and the distribution of carbides was more homo-

geneous. Moreover, Lan et al. [14] revealed that the mod-

erate cooling rate was beneficial for the phase

transformation of acicular ferrite. When the welding heat

input was 0.67 kJ/mm, Dt8/5 was about 2.35 s, which pro-moted the formation of acicular ferrite and lower bainite.

The microstructure in CGHAZ under the welding heat

input of 0.77 kJ/mm consisted of lower bainite, small

amount of upper bainite, and coarsened acicular ferrites.

Especially, the grain size in CGHAZ became very large

when the welding heat input was 0.77 kJ/mm, and even

some extraordinary large grains about 250 lm in lengthcould be observed in CGHAZ, as shown in Fig. 6. The

formation of extraordinary large grain could be attributed

to the carbides or other inclusions dissolved under the high

welding heat input, and also the weakened block effect of

second phase particles or inclusions on the grain growth.

The columnar crystal formed in the fusion zone (FZ),

and the microstructure consisted of various ferrites with

carbides. The increase of welding heat input changed the

morphology of ferrite in FZ. In Fig. 4(d), the microstruc-

ture was acicular ferrite (AF) and primary ferrite (PF).

Increasing the welding heat input to 0.67 kJ/mm, primary

ferrite (PF) appeared at the grain boundary. The acicular

ferrite in FZ was coarsened and then ferrite side plate (FS)

formed, as shown in Fig. 5(d).

Furthermore, higher welding heat input also enlarged

the grain size of CGHAZ. It can be seen that the grain size

of CGHAZ under the welding heat input of 0.67 kJ/mm

was larger than that under 0.25 kJ/mm, because higher

welding heat input produced a higher peak temperature in

the welding thermal cycle and promoted the grain growth

in CGHAZ.

The chemical composition and welding thermal cycle

are the most important factors that affect the phase

bFig. 4 Microstructure in HSLA steel joint with welding heat input of0.25 kJ/mm (a) cross section, (b) FGHAZ, (c) CGHAZ, and (d) FZ.B bainite, M martensite, AF acicular ferrite, PF primary ferrite, BM

base metal, FGHAZ fine grain heat-affected zone, CGHAZ coarse

grain heat-affected zone, FZ fusion zone


123

transformation in HAZ. Based on the formula for calcu-

lating the carbon equivalent recommended by the Inter-

national Institute of Welding [22]: Ceq = C ? Mn/

6 ? (Cu ? Ni)/15 ? (Cr ? Mo ? V)/5, the carbon

equivalent of HSLA steel was about 0.831.05%. Such a

high carbon equivalent could lead to severe harden

quenching tendency and cause the formation of brittle

phases, such as lath martensite. Besides, the heat input

plays a critical role in controlling the microstructure.

Summarizing the above-mentioned description, it reveals

that the welding heat input influenced the microstructure

in HAZ through two aspects. First, the increase of

welding heat input caused the growth of prior austenite

grains and then enlarged the grain size of CGHAZ. Sec-

ond, the welding heat input affected the cooling rate and

then controlled the phase transformation in HAZ. When

the temperature fell into the range of 800500 C afterwelding, the austenite decomposed to different phases.

Babu [13] reported that allotriomorphic ferrite formed

initially at the austenite grain boundary in the high tem-

perature range. With further cooling, acicular ferrite

would nucleate around the inclusions in austenite. While

cooled to lower temperature, the remaining austenite

would transform to bainite and martensite partially or

completely. The cooling rate was very fast when the

welding heat input was lower, and it caused the

bFig. 5 Microstructure in HSLA steel joint with welding heat input of0.67 kJ/mm (a) cross section, (b) FGHAZ, (c) CGHAZ, and (d) FZ.LB lower bainite, FS ferrite side plate, AF acicular ferrite, FS ferrite

side plate, BM base metal, FGHAZ fine grain heat-affected zone,

CGHAZ coarse grain heat-affected zone, FZ fusion zone

Fig. 6 The extraordinary large grain in CGHAZ under welding heatinput of 0.77 kJ/mm. BM base metal, FGHAZ fine grain heat-affected

zone, CGHAZ coarse grain heat-affected zone, FZ fusion zone


123

martensitic phase transformation. This could be proved by

the microstructure in HAZ with welding heat input of

0.25 kJ/mm as shown in Fig. 4. On the contrary,

increasing the welding heat input to 0.57 or 0.67 kJ/mm,

the corresponded low cooling rate was beneficial for the

formation of bainite. When the welding heat input was

further increased to 0.77 kJ/mm, the ferrite was coarsened

and some upper bainite formed in CGHAZ.

Fig. 7 The distribution of hardness in different zones with welding heat input of (a) 0.25 kJ/mm, (b) 0.40 kJ/mm, (c) 0.46 kJ/mm, (d) 0.57 kJ/mm, (e) 0.67 kJ/mm, and (f) 0.77 kJ/mm. BM base metal, FZ fusion zone, CG coarse grain HAZ, FG fine grain HAZ


123

Microhardness

The Vickers hardness curves versus different welding heat

inputs are illustrated in Fig. 7, and the average microh-

ardness value in each zone is shown on the top of the figure

for different welding heat inputs. The microhardness in

HAZ and FZ were higher than that in base metal (about

370HV) for all welding heat inputs, but the average hard-

ness changed with the variation of welding heat inputs. The

average hardness of HAZ and FZ with the low heat input

(0.25 kJ/mm) was about 458 and 474 HV, respectively.

However, the average hardness of HAZ with larger heat

inputs (0.570.77 kJ/mm) was relatively lower as around

425 HV.

The microhardness in the HAZ and weld greatly

depended on the microstructure. From the discussion in

Microstructure section, it suggested that the high hard-

ness of the HAZ and weld was attributed to the formation

of lath martensite and bainite, and the decrease of mi-

crohardness was caused by the transformation of micro-

structure. High cooling rate corresponded to the low

welding heat input resulted in the formation of martensite

and increased the microhardness of the HAZ and weld. As

the welding heat input increased, the cooling rate became

slower and martensite disappeared gradually with the for-

mation of bainite. So the average hardness of HAZ was

decreased with increasing the welding heat input due to the

microstructure transformation from martensite to bainite.

In addition, the volume fraction of ferrite enlarged with the

increase of the welding heat input. Arivazhagan et al. [23]

reported that more ferrite would reduce the microhardness.

Moreover, Gharibshahiyan et al. [24] found that the mi-

crohardness also decreased due to the coarsening of grains.

However, Mohandas et al. [25] found a softening zone in

CGHAZ which was opposite to the common observation

and the extent of softening varied with welding methods as

well as the welding heat input.

It was also found that when using lower welding heat

input (0.25 kJ/mm), the average hardness of FGHAZ was

lower than that of CGHAZ, but it was a little higher than that

when higher welding heat inputs (0.67 and 0.77 kJ/mm)

were applied. It revealed that different strengthening

mechanisms affected the microhardness when welded with

different heat inputs. Generally, the microhardness of

FGHAZ was higher than that of CGHAZ due to the fine

grain strengthening. However, the formation of martensite in

CGHAZ under low welding heat input resulted in the higher

hardness than that of FGHAZ in HSLA steel joints.

Impact Toughness

Figure 8 shows the comprehensive variation of the impact

toughness and microhardness in HAZ versus welding heat

inputs. It was found that the impact toughness of HAZ was

higher than that of HSLA steel base metal (107.5 J/cm2),

and it was improved by the increase of welding heat input

accompanied with the decrease of microhardness. The

impact toughness was 182.5 J/cm2 when welded under the

welding heat input of 0.67 kJ/mm and then slightly

declined to 177.5 J/cm2 when the welding heat input was

0.77 kJ/mm. Figure 9 displays the SEM images taken from

the fractured surface. The percentage of shear fracture in

the base metal reached 14%, but that in the rest samples

was around 10%. In Fig. 9(a), river-like patterns could be

seen in the fractured surface on base metal, showing the

feature of quasi-cleavage fracture which was in agreement

with the relatively lower impact toughness of base metal.

However, many dimples could also be found in the frac-

tured surface in Fig. 9(bf), which indicated that the

toughness of HAZ was better than that of base metal. This

phenomenon suggests that within a certain range, the

welding heat input might produce a beneficial effect on the

impact toughness of HAZ in HSLA steel joints.

The effect of welding heat input on the impact toughness

lies in the influence of heat input on the microstructure in

HAZ. When the welding heat input was low, low carbon lath

martensite formed in HAZ, leading to the high microhardness

and low toughness. It is easy for the propagation of micro-

cracks along the parallel martensite lath. With the increase of

welding heat input, martensite transformed to lower bainite

gradually. The carbides distributing within and/or between

ferrites and the interface between martensite and bainite

impeded the propagation of microcracks in lower bainite,

exhibiting better toughness. However, when the heat input

was 0.57 kJ/mm, the toughness of HAZ dropped a little, as

shown in Fig. 8. It could be seen that the size of dimple in

Fig. 8 Vickers hardness and impact toughness versus welding heatinput


123

Fig. 9(d) was very uneven. It was well known that the dimple

was usually formed where there were inclusions or second

phase particles. Therefore, the drop of impact toughness under

the welding heat input of 0.57 kJ/mm could be attributed to

the dissolution and segregation of inclusions or second phase

particles. The increase of the distance between adjacent

inclusions or particles could reduce the resistance to the

propagation of microcracks, and then decrease the impact

toughness. When the distribution of inclusions was homoge-

neous again, the impact toughness improved, as indicated in

Fig. 8. The microstructure of HAZ with the welding heat

input of 0.67 kJ/mm was fine lower bainite with some acicular

ferrite, which is more difficult for the propagation of micro-

cracks and results in higher impact toughness. However, when

the welding heat input was further increased to 0.77 kJ/mm,

the large grain size of HAZ led to the reduction of total grain

boundary area, and then decreased the impact toughness.

Viano et al. [2] also indicated that high welding heat input

adversely affected the impact toughness of HSLA-80 steel

joint. Besides, the impact toughness was deteriorated due to

the appearance of upper bainite in HAZ, which was in

agreement with the opinion obtained by Abbaszadeh et al.

[26], who reported that the toughness of lower bainite was

much better than that of upper bainite.

As indicated in Fig. 8, the optimum comprehensive

mechanical properties of HAZ in HSLA steel joints were

achieved at a medium welding heat input, 0.67 kJ/mm. The

HAZ welded with the welding heat input of 0.67 kJ/mm

exhibited higher impact toughness (182.5 J/cm2) and lower

microhardness (425 HV). Too low or too high welding heat

input is adverse for the mechanical properties of HAZ.

Conclusions

The effects of welding heat input on the microstructure and

mechanical properties of HAZ in HSLA steel joints were

investigated. Increasing the welding heat input restrained the

formation of martensite and promoted the transformation of

martensite to bainite. When the welding heat input was

0.67 kJ/mm, the microstructure in HAZ was fine lower bainite

with some acicular ferrite. Upper bainite was produced in

HAZ when the welding heat input was 0.77 kJ/mm. The

Vickers hardness of HAZ and FZ of HSLA steel joints was

much higher than that of base metal. The average hardness of

HAZ decreased with increasing the welding heat input. The

hardness of CGHAZ was higher than that of FGHAZ when

using a lower welding heat input. The impact toughness of

HAZ was not monotonously improved with the increase of

welding heat input. The optimum comprehensive properties

of HAZ were obtained when the welding heat input was

0.67 kJ/mm.

Fig. 9 SEM images of the impact fractures in base metal (a) and in HAZ under different welding heat inputs of (b) 0.25 kJ/mm, (c) 0.46 kJ/mm,(d) 0.57 kJ/mm, (e) 0.67 kJ/mm, and (f) 0.77 kJ/mm


123

Acknowledgments This work was financially supported by theNational Key Basic Research and Development Program of China

(Grant No. 2011CB013402), the National Natural Science Foundation

of China (Grant No. 51374048), and the Fundamental Research Funds

for the Central Universities (Grant No. DUT13ZD209).

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Effect of Welding Heat Input on Microstructure and Mechanical Properties of HSLA Steel JointAbstractIntroductionExperimentalResults and DiscussionMicrostructureMicrohardnessImpact Toughness

ConclusionsAcknowledgmentsReferences

Effect of Heat Input

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