Scholarly article on topic 'Testing of New Materials and Computer Aided Optimization of Laser Beam Welding of High-Strength Steels'

Testing of New Materials and Computer Aided Optimization of Laser Beam Welding of High-Strength Steels Academic research paper on "Materials engineering"

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{"Laser beam welding" / "cold-resistant steel" / "high strength steel" / "computer aided optimization" / "mechanical properties"}

Abstract of research paper on Materials engineering, author of scientific article — M. Sokolov, A. Salminen, E.I. Khlusova, M.M. Pronin, M. Golubeva, et al.

Abstract The objective of this research is to investigate the performance and potential high power laser beam welding of high strength cold resistant low carbon quenched and tempered steel F620W with the use of new computer aided optimization methods. The research was divided into three parts. First, in order to identify the optimal thermal cycle, dilatometric and thermokinetic investigations were performed using a Bahr Thermoanalise DIL 805 dilatometer. To achieve recommended cycle during laser beam welding, the process was simulated using LaserCAD software resulting in optimal process parameters. Based on the simulation results an experiment was performed in bead on plate setup with use of continuous wave fiber laser IPG YLS-10000. Mechanical properties of the weld were studied by subjecting the specimens to a number of destructive tests, namely hardness, tensile strength and impact toughness testing at -40°C and -60°C.

Academic research paper on topic "Testing of New Materials and Computer Aided Optimization of Laser Beam Welding of High-Strength Steels"

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Physics Procedía 78 (2015) 255 - 264

15th Nordic Laser Materials Processing Conference, Nolamp 15, 25-27 August 2015,

Lappeenranta, Finland

Testing of New Materials and Computer Aided Optimization of Laser Beam Welding of High-Strength Steels

M. Sokolova*, A. Salminena, E.I. Khlusovab, M.M. Proninb, M. Golubevab,

M. Kuznetsovc

aLaboratory of Laser Materials Processing, Lappeenranta University of Technology, Lappeenranta 53850, Finland, bFederal State Unitary Enterprise Central Research Institute of Structural Materials "PROMETEY", Saint-Petersburg 191015, Russia cInstitute of Laser and Welding Technologies, Peter the Great Saint-Petersburg Polytechnic University, Saint-Petersburg 195251, Russia

The objective of this research is to investigate the performance and potential high power laser beam welding of high strength cold resistant low carbon quenched and tempered steel F620W with the use of new computer aided optimization methods. The research was divided into three parts. First, in order to identify the optimal thermal cycle, dilatometric and thermokinetic investigations were performed using a Bahr Thermoanalise DIL 805 dilatometer. To achieve recommended cycle during laser beam welding, the process was simulated using LaserCAD software resulting in optimal process parameters. Based on the simulation results an experiment was performed in bead on plate setup with use of continuous wave fiber laser IPG YLS-10000. Mechanical properties of the weld were studied by subjecting the specimens to a number of destructive tests, namely hardness, tensile strength and impact toughness testing at -40 °C and -60 °C.

© 2015 The Authors.PublishedbyElsevierB.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the Lappeenranta University of Technology (LUT)

Keywords: Laser beam welding, cold-resistant steel, high strength steel, computer aided optimization, mechanical properties

* Corresponding author. Tel.: +358-451-189-808; fax: +358-451-189-808. E-mail address: mikhail.sokolov@lut.fi

1875-3892 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the Lappeenranta University of Technology (LUT) doi:10.1016/j.phpro.2015.11.036

Abstract

1. Introduction

The use of cold-resistant High Strength Steels (HSS) in shipbuilding and offshore structures provides new horizons for development of the Arctic region. To date, studies investigating laser beam welding (LBW) have produced univocal results claiming that laser beam welding is effective for thick section welding of both low-alloyed and HSS steels. (Sokolov and Salminen, 2012), (Vanska et al., 2013), (Sokolov and Salminen, 2014), (Farrokhi et al., 2015)

Steels for Arctic and offshore applications have to fulfill requirements in impact toughness, high strength and ductility at low temperatures of - 40°C and below. However, little is known about the characteristics of HSS joints welded by fiber lasers, especially when it comes to operations in cold environments.

It was reported that LBW of HSS with the use of high power fiber lasers results in high quality butt joints with superior tensile properties within a wide range of low to moderate line energies. However, the amount of materials, investigated for above mentioned applications with high power LBW is very limited. In recent years, a few authors have reported on successful testing of laser welded joints including DP600 (Farabi et al., 2011), DP980 (Xu et al. 2013), AISI 316 Steel (Yilbas and Akhtar, 2013), Optim 960 QC (Farrokhi et al., 2015). What is not yet clear is what materials are most effectively used in LBW for Arctic region. The traditional design of experiments methods require multiple experimental sets to build "windows" for weldability and desired mechanical properties. Number of experiments can be significantly reduces with the use of modern computer aided optimization methods.

Early examples of research into computer aided optimization of laser processing include stationary models developed by Kaplan (1994), Beyer et al. (1995) and dynamic models developed by Matsunawa et al. (1998). Recent research by Kazemi, & Goldak (2009) and Weidinger et al. (2014) give an example of efficient use of modern simulation software to optimize the welding process parameters and reduction of the amount of necessary experiments.

The current work presents predevelopment steps and demonstration of the method of testing the material's behavior at different thermal cycles to determine the optimal welding parameters to get the acceptable cooling rates in order to achieve metal structure that provides acceptable impact toughness and ductility for special climate conditions.

2. Experimental procedure

2.1. Test Materials

The test material used in the experiments was commercially available high strength quenched and tempered steel F620W GOST 52927 - 2008 of 12 mm thickness, commonly used in shipbuilding and offshore structures. Chemical composition and mechanical properties are given in Table 1.

Table 1. Nominal alloying composition and mechanical properties of F620W.

Chemical composition, wt%

C Si Mn P S Cu + Ni Cr + Mo Al Nb N

0.09 0.6 0.2 - - 0.9 0.8 0.02 0.02 -

0.11 0.8 0.4 0.01 0.005 2.2 1.05 0.05 0.05 0.008

Mechanical Properties

R0.2 MPa Rm MPa Ae % KV-40 J KV-60 J

620 720 15 - 70

According to EN ISO 15614-11: 2002 steels plates were waterjet cut to 150 mm x 300 mm pieces, edge surface roughness was about Ra 5 ^m. The samples were tightly fixed flat on the jig such that the welding optics was perpendicular to the welding direction.

1.1 Welding Equipment

Single pass laser beam welding was performed in bead on plate setup with use of continuous wave fiber laser YLS-10000 with a wavelength of 1070 nm. The laser was guided through an optical fiber of 200 ^m to a Scansonic BO-SF welding head mounted on Kawasaki FS45 robot. Focusing lens focal length was 250 mm and focal point diameter of about 200 ^m. Air cross-jet nozzles were located below the focusing lens, argon shielding gas was delivered in the welding zone and in the root zone via copper tubes at the rate of 25 l/min and 10 l/min respectively. Laser welding setup is shown in Fig. 1.

Fig. 1. Laser welding setup: (1) welding head and collimator; (2) air cross-jet system; (3) copper tubes for shielding gas

1.2 Testing Equipment

Dilatometric investigations were performed using a Bahr Thermoanalise DIL 805 dilatometer. Test specimen is schematically shown in Fig. 2. Cylindrical specimens were induction heated from room temperature up to 900 °C and 1200 °C at rates 3 °/s and 500 °/s. The samples were subsequent cooled to room temperature in Nitrogen environment.

Fig. 2. Test specimen for dilatometric investigation of phase transformations

Metallography was performed using a metallographic microscope "AxioObserverA1M", quality assurance was performed in accordance with ISO 13919-1: 2001. Microhardness was measured using DM-8 tester with 100 gf load in accordance with ISO 22826: 2005. Before testing samples were polished and etched with nital (3% HNO3 + alcohol). Preheating was performed using VETS-Flexible resistance preheating element by Heatmasters up to 1000 C.

Tensile tests were performed according to ISO 4136: 2012. Charpy V impact tests were performed at -40 °C and -60 °C as per ISO 148-1: 2009 standard to evaluate the toughness of the joints at sub-zero temperatures.

1.3 Simulation software and research method

LBW process is characterized by the simultaneous impact of multiple physical effects. The numerical modelling enables detailed analysis of the resulting physical behavior of the material during welding. One of the most important aspects is the detailed knowledge of material properties, which are required as input data for the numerical simulation. LaserCad is a LBW dynamic simulation model that can be represented as six equation modules: heat transfer in liquid

and solid metal, hydrodynamics of the melt flow, gas dynamics of metal vapor, formation of laser-induced plasma, beam-plasma and beam-material interaction. Connections between these modules and input information are presented in Fig. 3.

Fig. 3. Scheme for LaserCad equation modules: solid lines for boundary conditions and dash lines for the coefficients in the equations

(Turichin et al., 2013)

LaserCad simulate a shape and size of melt pool as well as temperature distribution in weld bath and HAZ during welding. Material properties, received from thermokinetic and dilatometric investigations of material's phase transformations, are uploaded to the system and LBW is simulated with different welding parameters to fulfil thermal cycle recommendations formulated at previous phase of the study and achieve high process efficiency. The research method is set out in Fig. 4.

Fig. 4. Structure of the research

2 Simulation Results

2.1 Material testing

Laser beam welding have the lowest line energy compared to laser arc hybrid welding or arc welding processes but having the highest heating and cooling rates among those processes. High heating rates lead to austenite-martensite transformation process in non-equilibrium conditions. Temperature zones of 1200 °C and 900 °C were chosen for simulation as they correspond to overheated and grain-refined sections (Tufyakov, 1973) as shown in Fig. 5.

tit< 2 t< 3 t< 4 t Heat-affacted zone (HAZ) section

Fig. 5. Diagram of the heat-affected zone (HAZ): (1) overheated section > Tr, (2) grain-refined (normalized) section Tr - Ac3, (3) partly grain-

refined section Ac3 - Ac1, (IV) recrystallized section Ac1 - T1

Transformation diagrams are presented in Fig. 6 show that high-speed heating up to 1200 0C with cooling rates at 100-150 0C/sec (a) results in bainite-martensite structure and increase in martensite proportion with cooling rate higher than 150 0C/sec. Therefore, hardness increases from 465 HV10 to 550 HV10 with increase of the cooling rates from 100 0C/sec to 250 0C/sec. High-speed heating up to 900 0C (b) results in bainite-martensite structure with a relatively small proportion of ferrite with cooling rates below 150 0C and increase in martensite proportion with cooling rates higher than 150 0C/sec resulting in hardness increase from 445 HV10 to 500 HV10.

1 10 1 10

Time, s Time, s

Fig. 6. Transformation diagram: F620W, a) 23 °C to 1200 °C, heating rate 500 °C/s b) 23 °C to 900 °C, heating rate 500 °C/s

2.2 Computer aided development of the welding process

In current research a number of simulations was performed to get the estimated cooling rates at different welding parameters and recommend the most optimal ones. The simulation was run within limits of used equipment and results from previous studies (Vanska et al., 2013), (Sokolov and Salminen, 2014), (Farrokhi et al., 2015): laser power level (PL) = 4.5 - 6 kW, welding speed (vW) = 0.9 - 1.2 m/min; focal point position (fpp) = -2 mm (below surface); focal point diameter (dOF) = 200 ^m. The results of the simulation analysis are set out in Table 2. Selection criteria were higher penetration depth and lower cooling rates.

Table 2. LaserCad simulation results

No Pl, kW vW, m/min Vcool, 0C/sec Weld depth,

1 4.5 0.9 291.3 7.29

2 4.5 1.2 428.6 6.77

3 4.8 0.9 288.5 7.59

4 4.8 1.2 422.5 7.03

5 5.1 0.9 275.2 7.85

6 5.1 1.2 411.0 7.29

7 5.4 0.9 267.9 8.15

8 5.4 1.2 394.7 7.55

9 5.7 0.9 258.6 8.70

10 5.7 1.2 379.7 7.76

11 6.0 0.9 270.0 8.87

12 6.0 1.2 379.7 8.15

Based on the analysis of simulation results, next welding parameters were selected for the experimental testing: material thickness: 12 mm; laser power level = 5.7 kW; welding speed = 0.9 m/min, simulation number 9 in Table 2.

Geometry of the weld and thermal cycles in fusion zone (FZ) heat affected zone (HAZ) were simulated, results are shown in Fig. 7.

Fig. 7. LaserCad simulation: F620W; t = 12 mm; PL = 5.7 kW; vw = 0.9 m/min; fpp = -2mm; dOF = 200 jjm, a) Simulated weld geometry: red -HAZ, yellow - FZ; b) Weld geometry simulation and experiment: penetration depth and width on the surface; c) thermal cycles in the distance from the weld axis (-4 mm below the surface): 0.75 mm (red), 1 mm (orange), 1.6 (yellow).

3 Experimental results

Macrosections and weld structure in heat affected zone (HAZ) and base metal (BM) are shown in Fig. 8. To achieve a full penetration weld a second pass from the bottom side of the weld was performed. Use of preheating increased the HAZ overheated zone width from 0.14 to 0.17 mm and grain-refined zone width from 0.2 mm to 0.32 mm and decrease in penetration depth from 8.3 to 7.8 mm.

Fig. 8. Macrosection and material structure on the boundaries of FZ, HAZ and BM: x200 magnification. a) without preheating; b) with preheating up to 100 °C F620W; t = 17 mm; PL = 9.7 kW; vw = 1 m/min; fpp = -2mm; dOF = 200 ^m.

As shown in Fig. 9 average hardness in overheated zone is lowered with the use of preheating from 536 HV10 to 493 HV10 and in grain-refined from 525 HV10 to 469 HV10. Possible further hardness reduction in HAZ can be achieved by use of additional preheating with induction techniques or electric arc or by reduction of the welding speed.

S90 Mil 49» 4411 3911 .140 39» 140

0 <u 0,4 0,6 0.8 1 1,3

11 2 . 3 4

J * • ' • * ♦ • *

•* % ' +

• * 1 * 361 •

HU10 590

540 490

1 2 3 4

• • • ■ •

• • P 340

390 340 290 140

0 0,1 0.4 0,< 0.8 1 IJ

Fig. 9. Hardness measurement results in HAZ by sections: a) without preheating; b) with preheating up to 100 °C

Tensile strength tests results are shown in Fig. 10. Results at -40 °C and -60 °C do not vary much, however, preheating results in 20-25 MPa increase. All tensile tests, both preheated and not preheated, resulted in fracture by the main metal in 15-20 mm from the weld metal zone. The weld itself was much stronger than the base metal.

Fig. 10. Ultimate tensile strength: results at -40 °C and -60 °C (a) fracture behavior at -60 °C (b)

The main criteria of the weld quality from the point of applicability for Arctic conditions were impact tests at low temperatures. Charpy V experiments were performed at sub-zero temperatures: -40 °C and -60 °C and results are presented in Fig. 11. Both welding with preheating and without showed very good impact properties at the required range of temperatures. Preheating reduced the difference between the weld's fusion line and centerline. It is important to notice that both samples shown in Fig. 11 have fracture line at base metal.

Fig. 11. Absorbed energy at -40 °C and -60 °C (a) fracture behaviour at -60 (b) CL: weld centerline; FL: weld fusion line

The present results are significant in at least two major respects: it is safe to say that welds at both -40 °C and -60 °C temperature levels have passed ultimate tensile strength and impact toughness tests, but hardness level in HAZ are beyond the limits for shipbuilding applications (400 HV), according to Lloyd's Register of Shipping. Future research should concentrate on the investigation of possible reduction of hardness level. In next investigations, it might be possible to use a wider range of welding parameters for simulation to achieve cooling rates below 150 °C/sec and add a stage of additional experiment to validate the results. Overall, these test results indicate that F620W has a good potential for use in Arctic and offshore applications, but further optimization of welding parameters required.

More research on this topic needs to be undertaken within a closed loop modification of the research process before making final conclusions about the applicability of F620W cold resistant high strength steel for LBW. Further simulation and experimental investigations with the use of updated simulation model are required to ascertain the material behavior at heating and cooling modes close to laser beam welding thermal dynamics. Updated version of the research structure (Fig. 4) is presented in Fig. 12 and will be used in future research.

Fig. 12. Updated research structure

4 Conclusions

The present research was designed to demonstrate the new approach in testing of new materials for laser beam welding on the example of F620W high strength cold resistant low carbon quenched and tempered steel. The aim of the study was to test the material's behavior at different thermal cycles and based on the results, reverse design the welding parameters to get the acceptable cooling rates in order to achieve metal structure that provides acceptable tensile strength and impact toughness.

High quality joints with superior tensile and impact properties in extreme conditions of -60 °C can be obtained by the application of high power laser beam welding. However, the gap between the base and the weld metal hardness is high and may be reduced by decreasing the cooling rates during welding.

Using of suggested research methods allows to significantly reduce the number of laser beam welding experiments to evaluate the applicability of the steels for Arctic and offshore applications.

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