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Optics & Laser Technology 74 (2015) 125–131

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Optics & Laser Technology

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journal homepage: www.elsevier.com/locate/optlastec

Heat treatment of welded joints of steel 0.3С–1Cr–1Si producedby high-power fiber lasers

S.V. Kuryntsev n, A.Kh. GilmutdinovDepartment of Laser Technologies, KNRTU-KAI, Kazan, Russian Federation

a r t i c l e i n f o

Article history:Received 15 February 2015Received in revised form3 June 2015Accepted 4 June 2015Available online 11 June 2015

Keywords:Medium carbon steelLaser weldingHeat treatmentMechanical propertiesMicrostructure

x.doi.org/10.1016/j.optlastec.2015.06.00492/& 2015 Elsevier Ltd. All rights reserved.

esponding author.ail address: [email protected] (S.V. Kurynt

a b s t r a c t

The effect of heat treatment on the welded joints of steel grade 0.3С–1Cr–1Si produced by 30 kW powerfiber lasers was investigated in the paper. The speed of the welding process was 20 mm/s. Heat treatmentwas carried out on two levels, quenching with subsequent middle tempering and high tempering. Thesamples were examined before and after heat treatment, macro- and microstructure were studied usingSEM, UTS, three points bent test, microhardness. The effect of heat treatment was significant: it allowedreduction of the weld hardness of considerably and enhancement of its ductility.

& 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Currently, many studies have been devoted to processes of laserwelding of low-carbon and austenitic steels of large thickness [1,15].This paper describes the experimental studies on welding and sub-sequent heat treatment of welded joints of medium carbon, low-alloy steel 0.3С–1Cr–1Si (chemical composition is presented inTable 1). This steel is a widely used construction material, which iswell-quenched and has limited weldability [16,17]. The basic defectsin electric arc welding of these steels are hot and cold cracks, theyare formed as a result of polymorphic and phase transformations(Austenite–Martensite, Austenite–FerriteþCementite, Austenite–Bainite) [1–8,16–21] due to the change of volume, at high speeds ofcooling after the thermal influence of welding. To reduce the coolingrate of the welded joint, pre-heating and related heating [22], post-weld heat treatment [23–29] are used, besides it is necessary toprevent cooling of the welded joint to the room temperature. Con-sidering the foregoing, it is obvious that the technology of electricarc welding of these steels is rather complicated, as complex che-mical, physical and thermodynamic processes of high-temperaturephases [17,19] take place in the weld joint and in the heat affectedzone. When welding steels of this class energy sources, ensuringminimal overheating of molten metal and the HAZ should be used.

In the last decade, one of the most popular and advancedwelding processes is laser welding and its variations: original laser

sev).

welding, hybrid laser-arc welding, laser welding with cold fillermaterial, dual beam laser welding, and defocused beam laserwelding [1,2,11,31]. Laser welding is used for welding of austeniticsteels of various classes [2,8], duplex [6,9], zinc-coated [10],microalloyed [30] steels, and a large number of works is devotedto hybrid welding with various technological methods for weldingof thick stock materials [4,8–10].

However, laser welding is characterized by higher weld coolingrate [2], since the metal heating and melting by highly concentratedsource of energy. This feature, in most cases, [1,2,6,8–12], has positiveeffect on the properties of the compounds-low HAZ, decrease orabsence of thermal deformations. But during welding of hardenedsteels of perlitic and martensitic grades it naturally leads to formationof hardening structures-martensite and bainite in the weld and HAZ[17–19,21,34–36]. Formation of these structures may lead to increasedstrength and brittleness and reduced ductility of the weld. To reduceor neutralize this effect heat treatment of welded joints was carriedout on two regimes in the study, with quenching and subsequentmedium tempering (QMT) and high tempering (HT) Fig. 1. The aim ofthis study is to investigate the effect of heat treatment carried outaccording to standard regimens for this grade of steel on the weldhaving abnormally high hardness for the steel grade.

2. Experiment

This paper describes the study of the effect of heat treatmenton the welded joints of steel 0.3S–1Cr–1Si (Table 1) obtained by

Table 1Chemical composition of specimen.

Composition C Si Mn S P Cr Ni Ti As Cu

Base metal 0.28–0.34 0.9–1.2 0.8–1.1 o0.025 o0.025 0.8–1.1 – – o0.08 o0.3

Fig. 1. Schematic illustration of heat treatment modes: QMT and HT.

Table 2Modes of welding process.

Sample/welding parameters P (kW) Vweld (m/s)

1 NHT 10 0.022 QMT 10 0.023 HT 10 0.02

The front side of the weld

The root side of the weld

Fig. 2. Appearance of the weld.

S.V. Kuryntsev, A.Kh. Gilmutdinov / Optics & Laser Technology 74 (2015) 125–131126

high-power fiber lasers (30 kW) at high speeds (20 mm/s). Threesamples were welded, as for geometrical sizes the samples beforewelding were 10 mm thick, 50 mm wide, and 200 mm long.

Welding works were performed on LS-30 of “IPG-Photonics”(USA) device equipped with a robot KUKA KR 120 R 2700 extra HA,welding head laser focusing head KUGLER GmbH, LK-690. Thewavelength of radiation is 1070 nm [13,15], focal length is 450 mm.The defocusing distance of laser beam to the surface of work pieceis 0.0 mm, laser spot diameter in focus is 200 μm. 99.99% Argonwas used as a shielding gas to protect the top part of molten pool,flow rate of the shielding gas is 17 l/min. The process stability wasprovided by the use of non-extreme modes of laser radiation up to10 kW [14,15,32].

Welding modes are shown in Table 2. Heat treatment was per-formed after cooling of the welded joints until room temperature asper the modes shown in Fig. 1, two electric ovens SNOL 7.2/1300were used. The thermal processing was aimed at increase of plasti-city [20], with maintaining high strength properties of the weldedjoint having a martensite structure [21–36] with hardness of 8500–9000 MPa. For studying the mechanical properties and micro-structure, the samples were prepared from the welded blank No.1(no heat treatment-NHT), No. 2 (quenchingþmiddle tempering-QMT), No. 3 (high tempering-HT). To investigate the microstructurethin sections were prepared, 4% solution of nitric acid in alcohol wasapplied as reagent. Photos of the microstructure were obtained onSEM Carl Zeiss AURIGA CrossBeam (FIB-SEM) Zeiss NVision 40. For

mechanical testing (bending, UTS) tensile machine Shimadzu AG-5kNX was used, microhardness was measured by manual equipmentRemet HX 1000 with the load of 100 gr. Before bending test thespecimens were machined from the root and from the surface of theweld to obtain defect-free surface. The tensile specimens were cut atBuehler AbrasiMatic 300, the face and the root were not treated, testswere conducted to identify the differences outside the standard.

It was assumed that after heat treatment carried out in two dif-ferent modes it will be possible to identify the most effective mode ofthermal processing in terms of the spent time. And in general tosatisfy oneself of the applicability of classical approaches of materialscience [17,18,20,21,34–36] to the welded joints of steel 0.3С–1Cr–1Si produced at high speeds using high energy sources [14,15,32].

3. Results and discussion

3.1. Visual control

Fig. 2 shows the appearance of the face and root sides of theweld joint obtained under the above-mentioned modes. Since thewelding was performed in the lower spatial position, the weld hasa shrinkage on the front side and the root bead.

3.2. Macro-sections and ductility

Table 3 shows the results of three-point bending test and weldmacrostructure after etching by 4% solution of nitric acid in alco-hol. Bending tests were carried out before destruction of thesample, ductile fracture was observed only on the 2nd sample(QMT), so in this case the tests were carried out before the firstdefect (crack). The maximum deformation was in sample 2 (QMT),the minimum was in sample 1 (NHT). It also shows that afteretching the samples have a different appearance, it is due to achange in local etched heat of the treated samples compared withthe state after welding. The photograph of the macrostructure ofthe first sample 1 (NHT) clearly shows HAZ (darker part) and theweld (less dark part), the maximum HAZ in the center, it is con-nected with the lowest heat removal in this part.

3.3. Microstructure

Fig. 3 shows SEM photographs of the microstructure of the weld,HAZ and base metal before and after heat treatment. Certainlystructure of the weld, the HAZ and base metal of samples QMT andHT changed significantly after heat treatment. Fig. 3 shows that themicrostructure of the weld joint, HAZ and the base metal of all thesamples have been modified after thermal treatment. NHT sample inthe weld has a structure of fine needled martensite without retainedaustenite, HAZ – of high bainite, base metal – of perlite. QMT samplehas a homogenized microstructure presumably it is of sorbite struc-ture. The microstructure of HT sample in the weld has a structure ofmartensite, HAZ – of troostite, base metal – of ferrite perlite.

Fig. 4 shows pictures of the microstructure of the weld metal at40,000 magnification before and after heat treatment. In the caseof NHT fine needled martensite without carbide is observed.After the heat treatment under the QMT regime structure ofsorbite is observed, which has fairly large particles of carbides.After thermal processing under HT regime (at 640 °С 120 min)

Table 3Photo of macrosections and samples after bend tests.

Heat treatment Deformation (mm) Strain (kN) Macrostructure Samples after three-point bend test

NHT 13–14.2 9.8–10.5

QMT 20–20.5 17–17.9

HT 17.5–18 16.5–17

S.V.Kuryntsev,A

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NHT QMT HT

Fine needled martensite Sorbite Tempered martensiteSeam

High bainite Sorbite TroostiteHAZ

Perlite Sorbite Ferrite perliteBase metal

10 µm10 µm10 µm

Fig. 3. Photographs of the microstructure of various parts of the weld joint before and after heat treatment.

S.V. Kuryntsev, A.Kh. Gilmutdinov / Optics & Laser Technology 74 (2015) 125–131128

tempered martensite structure is observed, usually after quite longexposure to the given temperature carbides are smaller in size, asshown in Fig. 4 (HT).

3.4. Microhardness

Microhardness measurements were carried out in three areas:on the top of the weld, in the middle and at the root part as shownin Fig. 5, the results are presented in Fig. 6(a)–(c). The measure-ments show that the microhardness of NHT weld sample corres-ponds to martensite, the HAZ hardness corresponds to martensiteand high bainite, and microhardness of base metal corresponds toperlite in accordance with Fig. 2. Microhardness of the weld, HAZ

and base metal of QMT sample does not exceed 3500, whichcorresponds to sorbite. Microhardness of the weld of HT sample iswithin the range 3000–4300, which corresponds to temperedmartensite structure in the weld, troostite in the HAZ and ferriteperlite in the base metal. The microhardness of the root part in allcases is higher than the middle and the top, due to high coolingrates, this fact should be considered when choosing the directionof deformation of welded product.

3.5. Ultimate tensile strength and elongation

3 samples were prepared and tested for each mode (NHT, QMT,and HT), the results of mechanical tests UTS and elongation are

NHT

QMT

HT

Fig. 4. Pictures of the microstructure of the weld metal magnification 40,000.

Fig. 5. Schematic illustration of microhardness measurement areas.

Fig. 6. Microhardness of samples, cap, middle and root part of the weld. a – NHT, b– QMT, c – HT.

Fig. 7. Mechanical properties of the weld joints before and after heat treatment.

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shown in Fig. 7. It can be seen that the samples of NHT series havethe highest rates of UTS (100%) and elongation (100%), a series ofQMT samples have the rate of UTS (�0.14%) and elongation

(�19.7%), a series HT samples have the rate of UTS (�10.5%) andelongation (�11.3%).

Fig. 8 shows samples from each series after UTS tests. It isnatural that destruction of the samples in NHT series happened on

Sample Photographs of the samples after the break

NHT

QMT

HT

Fig. 8. Photographs of the samples after the break.

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the base metal, as the joint is very strong, a closer look shows thatthe weld metal is not deformed by stretching samples of QMT andHT series were deformed along the weld or the weld and HAZ.

4. Conclusions

- Welded joints produced by laser welding using high powerand high speed have high hardness and low ductility. Applicationof heat treatment results in improvement of mechanicalproperties.

- Mechanical properties of HT samples have higher elongation(�18%) and acceptable tensile strength (�623MPa) compared toQMT samples having elongation (�16%), tensile strength (�695MPa).However, QMT samples have the same tensile strength (�695MPa) asNHT samples (�696MPa), but greater ductility at three point bendingof QMT (deformation is 20–20.5 mm), of NHT (deformation is 13–14mm). It should be noted that the processing mode for HT (120 min)takes longer time and is more energy-consuming in comparison withQMT mode (40 min).

- It is proved that abnormally high hardness of the weld can bereduced by heat treatment in the entire volume of a workpiece(workpiece thickness being 10 mm) according to standard modesfor this grade of steel. This result establishes that the developedtechnology of welding and heat treatment can be used to producea defect-free weld with high strength and ductility.

- More thorough study is required to identify the structuralcomponents of the welded joint, and to choose the heat treatmentmodes in accordance with the desired performance characteristics.

Acknowledgments

The present research was conducted within project by theresolution No. 220, Contract No. 14z50.31.0023 with financialsupport of the Ministry of Education and Science of the RussianFederation.

Appendix A. Supplementary material

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.optlastec.2015.06.004.

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