Materials and Design 56 (2014) 345–352

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Materials and Design

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Investigation on gas metal arc weldability of a high strength tool steel

0261-3069/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.matdes.2013.11.042

⇑ Corresponding author. Tel.: +39 0103532679; fax: +39 010317750.E-mail addresses: chiara.mandolfino@unige.it (C. Mandolfino), e.lertora@unige.it

(E. Lertora), luca.davini@aen.ansaldo.it (L. Davini), gambaro@diptem.unige.it(C. Gambaro).

C. Mandolfino a,⇑, E. Lertora a, L. Davini b, C. Gambaro a

a University of Genoa, Department of Mechanical Engineering, Via All’ Opera Pia 15, 16145 Genoa, Italyb Ansaldo Energia S.p.a, Via Nicola Lorenzi 8, 16152 Genoa, Italy

a r t i c l e i n f o

Article history:Received 8 July 2013Accepted 19 November 2013Available online 28 November 2013

Keywords:Tool steelWeldabilityGas metal arc weldingWelding procedure qualification

a b s t r a c t

In this paper, gas metal arc weldability results of a particular advanced tool steel are presented. Indeed,the study was focused on the weld profile, microhardness and microstructure of the joints. The aim wasto identify an appropriate filler material and optimize the process parameter.

The validation of results started with a careful metallographic analysis of the joints, in order to verifythat the metallurgical properties of the material were not compromised by the welding process. In thefollowing step, all the non-destructive and mechanical tests, imposed by procedure qualification, wereperformed in order to have a complete characterization of the joints. For all the wires used, hardness testshighlighted that the use of low heat input and a high number of beads causes an increase in the HeatAffected Zone (HAZ) hardness up to values equal to or exceeding the limits imposed by the Europeanstandard on the process qualification. To avoid this problem, it was therefore necessary to adopt highelectric parameters and thus high heat inputs. The filler material that gave the best results, in terms ofuniformity of mechanical properties, is the rutile flux wire.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Tool steels are mainly used for machining and finishing materi-als, through operations such as turning and milling, and for mouldrealization. Because of the strict working conditions to which theyare subjected, they must have excellent mechanical properties,such as hardness, toughness, wear and deformation resistance,and it is necessary to maintain these properties even at high tem-peratures [1].

Special high strength steels for tools are the Toolox� series,produced by SSAB Oxelosund (Sweden). They are quenched andtempered steels, produced with an integrated process, able toguarantee a constant and accurate control of chemical composi-tion. Toolox� is based on the concept of metallurgical low carboncontent and rapid cooling during quenching: the result is a steelcharacterized by a particular morphology and by a much greatertoughness compared to steels of similar hardness. These character-istics ensure high wear resistance and an increased productivityrate for tools made with this material, even after long periods athigh temperatures [1,2].

These kinds of steels are widely studied concerning their wearresistance and the mechanical characteristics in high temperatureprocesses, including their microstructural modification [1,3–8].

Zhang et al., for example, investigated the microstructural evo-lutions of a martensitic hot-work tool steel during tempering andservice, in order to control the tool lifetime. They also proposed atempering ratio to describe the evolution of different types of hard-ness with temperature and time, during the tempering of martens-itic steel [3].

Concerning the abrasive wear behavior, Colaço and Vilar stud-ied the relationship between the microstructure of a martensiticstainless tool steel and the abrasive wear coefficient. Their workshows that at lower loads, the material with a microstructureformed of martensite and carbide particles presents the higherwear resistance. On the contrary, at higher loads, a microstructureformed of martensite and 15–25% of retained austenite presents ahigher wear resistance [4].

Medvedeva et al. and Firrao et al. correlated the microstructureof tool steels for different applications with both their static anddynamic properties, especially in high temperature conditions[5,6].

Luo et al. in two of their works [7,8] investigated in depth thebehavior of non-quenched prehardened steel for a large sectionplastic mould with particular attention to microstructure andhardness uniformity, machinability and few references to its Tung-sten Inert Gas (TIG) weldability. From these studies it could beestablished that hardness is one of the main features to take intoaccount while considering the machining, including welding, ofthese materials.

Translating these excellent characteristics from machining toolsto other kinds of ‘‘tools’’, such as forks, knives and buckets for earth

Fig. 1. Toolox 33� martensitic microstructure, 100�.

346 C. Mandolfino et al. / Materials and Design 56 (2014) 345–352

moving machines, its metallurgical, mechanical and wear-resistantcharacteristics make Toolox� an excellent choice for heavy dutyapplications, such as high temperature environments. For all thesereasons, its use in the manufacturing of these and other weldedstructures makes for a very interesting possibility.

Furthermore, other high strength steels are effectively weldedthrough gas metal arc welding techniques, and there are severalstudies in the literature that show the effect of process parameterchanging on the quality of the welded joint [9–15].

Lazic et al. in three works emphasize the importance of theselection of the optimal procedure and technology for welding highstrength steels, with particular reference to Weldox� 700, devel-oped by the same producer of the Toolox� series. After a detailedanalysis of the properties of the base metal and the evaluation ofthe main aspects related to its weldability, they selected the opti-mal combination of filler materials, methods and technologies ofwelding, as well as the conduction of a model and other standardtests, establishing the optimal technology of welding, which wasthen applied to a very secure welded structure [9–11].

The welding techniques are also largely and effectively em-ployed to ‘‘functionalised’’ steels, for example realizing tool steelhardfacing deposits, as studied by Gualco et al. and by Coronadoet al. [12,13]. In this case too, the investigation into weldingparameters, heat input and shielding gas is essential to realize adurable deposit.

Magudeeswaran et al. analyzed in detail the effect of weldingconsumables and processes on tensile, impact and fatigue proper-ties of armour grade quenched and tempered joints fabricated byflux cored arc welding (FCAW) processes. In particular they statedthat the use of low hydrogen ferritic steel consumables is found tobe beneficial to enhance the tensile properties and fatigue resis-tance of these steel joints, also compared to the joints fabricatedby conventional austenitic stainless steel consumables, which aremuch more expensive [14,15].

On the contrary, only little information on the weldability oftool steel is reported, because welding processes occur mainly inrepair operations [16].

In order to find detailed information about industrial applica-tions of welded components with this particular material, usingGas Metal Arc (GMA) process, a careful experimental campaignwas carried out.

In particular, the aim was the identification of an appropriatefiller material and the process parameters ensuring the best qualityof the joints.

The validation of results started with a careful metallographicanalysis of the joints, in order to confirm that the metallurgicalproperties of the material were not compromised by the weldingprocess.

In the following step, all the non-destructive and mechanicaltests, imposed by procedure qualification, were performed to havea complete characterization of the joints and verify industrialapplicability.

2. Materials and methods

2.1. Base metal and filler material

Toolox 33� is a quenched and tempered steel, marked by verylow residual stress and good dimensional stability. It offers lowcarbon content and its production process is marked by a rapidcooling: this leads to a particular carbide morphology whichensures limited wear and high production rate, even after regularuse at elevated temperatures. In spite of the elevated hardness,Toolox 33� preserves very high toughness values, especiallycompared to steels with the same hardness, making it an effectiveand better choice [1,2].

Fig. 1 shows Toolox 33� martensitic microstructure and Fig. 2the fine carbide dispersion in its matrix.

Due to the high content of alloying elements, this steel has acarbon equivalent, evaluated with the CEV (Carbon EquivalentValue) index [18,19], which is quite high. The specific casting usedfor the tests (Table. 1), for example, has a CEV index equal to 0.62,for which some precautions during welding were suggested[2,17,18]:

� preheat temperature of about 170 �C;� low-hydrogen filler materials (max 5 ml/100 g);� heat input in order to have Dt8/5 between 10 and 20 s;� minimum interpass temperature of 170 �C;� post-heating treatment of about 200 �C maintained for

120 min.

For the welding tests, 14-mm-thick sheets of Toolox 33� wereused for the fabrication of single ‘V’ butt joint configuration. Table 2reports the specimen dimensions and the indication of bevel angleand the side edge of root face.

Three types of wire were used as filler material:

� metal cored wire, a tubular electrode that consists of a metalsheath and a core of various powdered materials, primarilyiron.

� rutile flux cored wire, which gives a remarkable fluidity tothe weld pool and a good finishing of the bead. Its arc stabil-ity is quite high and usually the joints are free of spatter andits slag is easily removable. This flux, however, does not pro-vide any purifying action of the weld metal.

� basic flux cored wire, which is filled with iron oxides, ferro-alloys of manganese and silicon, silicates and carbonates,especially calcium and magnesium. Fluorite (calcium fluo-ride) is usually added to make the arc ignition easier. Indeed,calcium and magnesium carbonates are used as purifiers buttheir melting temperature is quite high [17–19].

As a general statement, filler material with yield strength (Rp0,2)up to 700 MPa has a CEV index lower than base material while veryhigh yield strength filler materials have CEV index higher than basematerial. In the latter case, it is necessary to pay attention to thethermal cycle, since this kind of material is sensitive to high inter-pass temperatures [17–19].

In Table 3, the mechanical properties of the specific filler mate-rials selected are reported. All the data are referred to supplierInspection Certificate, class 3.1.

Fig. 2. Carbide dispersion in Toolox 33�, 1000�.

Table 2Specimen dimensions.

Length(mm)

Width(mm)

Thickness(mm)

Bevel angle(�)

Side edge of root face(mm)

300 55 14 60 3

Table 3Geometrical and mechanical characteristics of filler materials used for welding tests.

Type of filler material

Metal cored Rutile Basic

Diameter (mm) 1.2 1.2 1.2Rp0.2 (N/mm2) 495 786 562Rm (N/mm2) 570 806 602Strain (%) 26 17 26kV (J)�20 �C 90�30 �C 60 84�40 �C 84 118

C. Mandolfino et al. / Materials and Design 56 (2014) 345–352 347

2.2. Welding tests and joint characterization

The joints were realized using the filler materials selected, withgas metal arc welding technique, using an Ar–CO2 blend as shield-ing gas and spray-arc as metal transfer mode.

The experimental campaign was carried out in two phases. Thefirst focused on research and optimization of welding parameters,both electrical (voltage and current) and thermal (preheating,interpass and post heating temperatures). This investigation madeit possible to obtain the information necessary for the choice ofsize and number of beads required to fill the welding gap, andidentification of the filler materials most suitable for theapplication.

Visual examination, macrographic and micrographic analysesand hardness tests were conducted to assess absence of macro-scopic imperfections, degree of surface finish and mechanical char-acteristics, especially in the heat affected zone.

For each filler material, the joints were realized in two differentways using, when possible:

� low heat input and therefore greater number of beads;� high heat input and thus fewer beads.

Table 4 shows some details of the welding performed in the firstphase, reporting the welding sequence, the number of beads, thewelding parameters and the values of average heat input usedfor each joint.

All joints were realized with a preheating to 175 �C and, afterwelding, a post-heating for 2 h at 200 �C, followed by slow coolingin the furnace to room temperature, in order to facilitate hydrogenemission and avoid crack occurrence. The maximum interpasstemperature was maintained at 225 �C [2].

In the second phase, a joint with parameters, filler material andthermal cycle that had guaranteed the best results in the first stagewas realized. This joint was completely investigated throughnon-destructive testing, mechanical and structural characteriza-tion, obtaining a qualified procedure, according to UNI EN ISO15614-1 [20].

Table 1Chemical composition [% weight] of the specific casting of Toolox 33�.

C Si Mn P S Cr Ni Mo

0.22 1.07 0.79 0.008 0.001 1.04 0.06 0.19

3. Results and discussion

3.1. First phase joints

After welding, the specimens were first examined by visualinspection, all resulting free from imperfections. In order to inves-tigate the influence of heat input and type of wire on the micro-structure, they were cut from transverse sections and the cutsurfaces prepared for metallographic inspection by polishing andetching using a 2% Nital solution, to display weld shape and micro-structure. Microhardness was evaluated using a Vickers hardnesstester at a load of 10 N.

The macrographic analysis of the specimens revealed perfect fu-sion, full penetration and complete absence of porosity and inclu-sions. Fig. 3 shows the macrographic investigation of the samples,in which the various zones of the joint and the bead distributionare clearly visible.

The type of wire used influenced the possibility of using bothhigh and low heat input approach, or only one of them. In fact, cal-cium carbonates and fluorite in the basic flux wire, make arc stabil-ity at low parameters really difficult [17,19]. Indeed, with this kindof wire it was not possible to realize the comparison between thetwo approaches and the parameters adopted were the highestamong all wires, to ensure good arc stability and a correct realiza-tion of the joint.

Always referring to Fig. 3, the HAZ is generally greater and moreevident for higher heat input values adopted, as could be imagined[17–19].

Fig. 4 highlights a considerable difference of the HAZ micro-structure in Toolox 33� (close to the finishing bead, both takenfrom the right side) between two specimens: the first underwenta heat input of 1 kJ/mm (A) and the second experienced a doublevalue (B). A finer grain structure of low heat input samples allowsus to predict higher mechanical properties in this area of the joint[19,21].

From the operative point of view, metal cored wire is very easyto manage. Rutile cored wire, however, does not need to enlarge

V Ti Cu Al Nb B N

0.098 0.014 0.01 0.013 0.017 0.002 0.003

Table 4Number of beads and heat input values used for the first phase of test campaign.

Welding sequence Filler material Bead number Current (A) Voltage (V) Average heat input (KJ/mm)

Metal cored 1–8 203 27.0 0.649–10 210 27.7

Metal cored 1, 2, 4 235 28.0 13, 5 203 27.0

Rutile 1–8 206 25.0 0.91

Rutile 1, 2, 4, 5, 6 235 25.5 1.33 206 25.0

Basic 1, 4 220 27.0 2.052, 3, 5, 6, 7 240 28.0

348 C. Mandolfino et al. / Materials and Design 56 (2014) 345–352

weld pool, to avoid slag cooling and its solidification at bead sides,increasing the risk of inclusions. This problem is offset by the factthat joints realized with this type of wire, unlike other joints, donot show dendritic structure in high heat input beads: micro-graphic analysis confirms that the structure is very fine in everypart of the fusion zone (Fig. 5), promising higher strength valuescompared to other fillers. The image was taken from the middleof the fusion zone of the joint realized with rutile wire and highheat input. Indeed it is possible to observe the effect of grainrefinement of the last beads on the first.

To confirm the microstructure remarks, microhardness testsVickers HV1 [22] were carried out, collecting three lines of sam-pling for each joint. Standard sampling method is reported in Fig. 6.

In Fig. 7, the microhardness values of joint finishing beads (Pos.1) are compared to the average hardness value of the base material.

The realization of the two types of passes (fewer and greaternumber of beads) has allowed us to compare the different ap-proaches, in order to verify how a large heat input affects the finalcharacteristics of the joint and whether there are obvious reasonsto prefer one method over the other.

Unlike what commonly happens to quenched and temperedsteel [8,13,14], the most marked result is that all joints

experimented the same tendency: hardness, compared to the basematerial, increases in the HAZ.

This is even clearer on the right side of the joint (indentation10–11–12), in which the low heat input samples experimentedthe highest values. This is due to the fact that the heat affectedzones corresponding to these indentations underwent a heattreatment produced by the realization of final beads on the leftside, which causes the effect of grain refining already reportedin Fig. 4A. This refinement is connected to an increase ofmechanical properties and therefore of the zone hardness[17,19].

This might become a problem for the process qualification: infact, it is necessary to keep hardness increasing in HAZ undercontrol, in order not to reach maximum levels permitted byUNI EN ISO 15614-1 standard, which limits the maximum hard-ness to 450 HV10 for this class of material, not heat treated afterwelding.

On the other hand, the fusion zone experimented a softening,since, thanks to the extraordinary Toolox 33� characteristics, it isvery difficult to guarantee performance of the same level usingconventional filler materials, without employing super alloys thatwould increase production costs excessively [17,18,20].

Fig. 3. Macrographic investigation on Toolox 33� joints, etching Nital 2%.

Fig. 4. Microstructures in the Toolox 33� HAZ produced with different parameters and types of wire, etching Nital 2% (50�).

Fig. 5. Fusion zone (FZ) microstructure of Toolox 33� using high strength rutile wire, etching Nital 2%.

C. Mandolfino et al. / Materials and Design 56 (2014) 345–352 349

In any case, the best results were given by rutile flux wire andno marked difference between the two approaches occurred. Forthe joint realized with this wire, the performance gap betweenbase material, HAZ and the weld metal remains quite small, espe-cially for the high heat input one, giving a more regular behavior tothe material.

Considering the discrete user friendliness and the highperformance revealed, the rutile flux cored wire was considered

the best choice for the implementation of the second testcampaign.

3.2. Welding procedure qualification

In this second phase, a joint realized with the rutile-flux wire,using high heat input-fewer beads approach and all the parametersthat guaranteed the best results in the previous stage, was realized.

Fig. 6. Indentation position for hardness test.

Fig. 7. Hardness trend of joint finishing beads and average HV1 value of base material.

350 C. Mandolfino et al. / Materials and Design 56 (2014) 345–352

This joint was completely investigated through non-destruc-tive testing, mechanical and structural characterization, obtain-ing a qualified procedure, according to UNI EN ISO 15614-1 [20].

Visual and penetrant test [23,24], showed no evidence of surfaceimperfections, good finishing and bead regularity. The same goodresult was confirmed by the third non-destructive test, the X-rayexamination [25], by which the joint is considered acceptable.

In the same way, macrographic investigation [26] shows a verysymmetrical and regular transverse section, with very well distrib-uted beads, absence of lack of penetration and slag inclusions, anda very flat finishing bead (Fig 8).

The finishing pass was realized in a unique bead, to avoid theirregular hardness behavior registered in the first part of the testcampaign.

The absence of internal imperfections is also confirmed by thesuccessful results of bending test [27], performed reaching an an-gle slightly smaller than 180�. No principle of failure occurred, alsothanks to the excellent behavior of both base and filler material(Fig. 9).

Fig. 8. Macrograph of welded joint, obtained using rutile-flux wire as filler material,Nital 5% etching.

Tensile tests required for process qualification [28] were con-ducted on two specimens, with a deformation rate of 0.007 1/s.They gave excellent results, as expected, even though the failure al-ways occurred in the fusion zone, which represents the weakestarea of the whole joint. As already mentioned, in the case of veryhigh-strength base material, it is especially difficult for the fusionzone to have the same performance as the base material usingconventional filler materials. This issue is not always negative,but has some advantages. Indeed, Lazic et al. in several studies re-ported that by application of filler materials with lower strengthcompared to the base metal, it is possible to achieve the highertoughness of the welded joint, a higher resistance to cold cracksand lower residual stresses in the welded joint, especially in HAZ[10,11].

In any case, the value of the welded joint tensile strength isquite high, considering that the certified value by the supplier re-ported a Rm equal to 806 MPa, in tests carried out on the fillermaterial only.

The efficiency of the joint was also calculated, comparing thevalue to which the products break with the base material Rm. Asshown in Table 5, the joint efficiency reaches good levels, between80% and 90%.

The joint realized showed, in addition to a good ductility, also areasonable toughness, evaluated by impact tests [29], performed at20 �C, providing high energy values summarized in table 6. As auseful comparison, it has to be reported that the Charpy energyof base material is 96 J [2]. The samples are identified by the termsVWT, if notched in the melted zone, and VHT, if notched in the heataffected zone.

Finally, Fig. 10 reports Vickers test results. They were performedwith a 100 N load, according to the standard [30]. The samplingmethod was the same as reported in Fig. 6.

Fig. 9. Results of bending tests on the specimens. Detail of the plastic deformation of the molten zone (B – C).

Table 6Results of Charpy test at 20 �C.

Charpy energy (J)

VWT 65.5 61.5 61.4VHT 168.1 161.0 150.5

Table 5Tensile test report.

Sample Rm basematerial (MPa)

Rm weldedjoint (MPa)

Failure zone Efficiency

F – 1 962 837 Fusion zone 87%F – 2 841 Fusion zone 87%

C. Mandolfino et al. / Materials and Design 56 (2014) 345–352 351

The trend of the curves is quite irregular but it confirms whathas been seen previously. From Fig. 10, it is possible to see howthe fusion zone settles the expected high values and how the pres-ence of only one finishing bead contributes to the limited hardnessincreasing in HAZ on both sides.

Fig. 10. Hardness HV10 values on

4. Conclusions

The weldability of a particular tool steel, Toolox 33�, using GMAprocess was investigated, carrying out a careful experimentalcampaign.

Homogeneous joints using three kinds of filler materials andtwo different heat input approaches were realized. The develop-ment of the optimal parameters for each filler wire – heat inputcouple was the first step.

The results of macrographic and micrographic analysis andhardness test on the first samples allowed the scheduling of a sec-ond test campaign, using the filler material that gave the bestresults.

From the final outcomes, the following conclusions can bedrawn.

(1) Regarding the operational characteristics of filler materials,the metal cored was the easiest to manage in a wide rangeof parameters. On the contrary, calcium carbonates and fluo-rite, in basic wire, make arc stability at low parameters reallydifficult. In fact, to ensure a good arc stability and a correct

the rutile-flux wire specimen.

352 C. Mandolfino et al. / Materials and Design 56 (2014) 345–352

realization of joints with this wire, the parameters had to bemaintained the highest among all wires. Using rutile wire,however, is better not to enlarge weld pool, to avoid slagcooling and its solidification at bead sides, increasing therisk of inclusions.

(2) For all the wires used, hardness tests highlight that the useof low heat input and a high number of beads causes anincrease in the HAZ hardness up to values equal to orexceeding the limits imposed by the European standard onthe process qualification. To avoid this problem, it is there-fore necessary to adopt high electric parameters and thushigh heat inputs.

(3) The filler material that gave the best results, in terms of uni-formity of mechanical properties, is the rutile flux wire. Infact, considering also the good user friendliness, it was con-sidered the best choice for the implementation of the secondtest campaign.

(4) The second stage of this study established that GMA processassociated with a suitable filler material can be successfullyused in Toolox 33� welding, with high productivity rate anduser-friendliness, ensuring process quality imposed byrecent standards.

Acknowledgements

The authors wish to thank Lameter S.r.l and Ansaldo Energia fortheir technical support, the material supply and all their assistancein carrying out this investigation.

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