PROLAS - Luleå tekniska universitet, LTU/file/PROLAS tekninen...PROLAS Process Optimization of...

UN IV E R S IT Y O F OU L U P a j a t i e 5 P u h . 04 0 08 4 3 0 5 0 Ou l u S o u t h e r n I n s t i t u te 8 55 0 0 N IV A LA F ak s i : ( 08 ) 55 3 2 0 2 6 De p a r tm e n t o f Me c ha n i c a l E n g i n ee r i n g F i n l a n d www. o u l u . f i / fm t

F u t u r e M a n u f a c t u r i n g T e c h n o l o g i e s r e s e a r c h g r o u p m e . o u l u . f i

1(45)

PROLAS

Process Optimization of Laser Welding and Fatigue Behaviour of High Strength Steels using High Power Fibre and Disc Lasers



2(45)

1 Introduction

The quality of laser weld depends on the quality of seam edge and also the precise of the beam moving. Now-adays the beam diameters become smaller so that the diameter can be even under 0,1 mm. In practice it is quite difficult to weld the good welds by using 0,1 mm beam because of precision requirement of the beam moving and also the requirement of seam edge quality. The power density of the beam must be under the 10

6

W/mm2 in order to avoid the vaporization of the material. This means that when using 0,1 mm beam diameter

the 8 kW power can vaporizate the steel so that the laser welding transforms to remote laser cutting.

2 The properties of lasercut seam edges

The quality of the seam edge of laser welds has a relevant consequence to the quality of the laser weld. In case of laser welding without filler material the rough edge can cause insufficiency to the weld. On the other hand the higher welding energy must be used to get the weld when the seam edges are rough. In this study the surface roughness of the laser cut edge and also the hardness profile of the lasercut seam edge was measured.

2.1 Surface roughness of the seam edges

The seam edge roughness of the lasercut seam edges of different steels was measured by Mitutoyo surftest measuring equipment. The roughnesses are given in table 1.



3(45)

Table 1. The surface roughness of lasercut seam edges of different steels

Steel Gauge length, mm 0,25 0,80 2,50

Rz Ra Rz Ra Rz Ra

AISI 201 LN mean, μm 4,95 1,23 8,02 1,70 11,08 2,06

2,27 mm std dev. μm 2,06 0,45 3,44 0,52 3,87 0,68

mean dev., μm 1,54 0,35 2,80 0,36 2,95 0,51

EN 1.4509 mean, μm 2,55 0,59 6,33 1,38 9,83 1,82

1,5 mm std dev. μm 0,89 0,15 2,37 0,37 3,57 0,61

mean dev., μm 0,61 0,11 1,79 0,30 2,43 0,42

EN 1.4521 mean, μm 2,52 0,59 6,52 1,45 16,24 2,11

1,5 mm std dev. μm 0,90 0,16 2,35 0,41 7,35 0,76

mean dev., μm 0,59 0,12 1,55 0,33 5,76 0,57

LDX mean, μm 2,58 0,58 5,67 1,27 10,62 2,01

1,5 mm std dev. μm 1,11 0,22 1,74 0,20 3,92 0,35

mean dev., μm 0,70 0,15 1,01 0,14 2,60 0,26

EN 1.4307 mean, μm 2,39 0,60 5,75 1,26 10,75 2,28

1,5 mm std dev. μm 0,94 0,23 2,65 0,39 4,32 1,54

mean dev., μm 0,64 0,18 1,94 0,30 2,94 1,09

Miilux 500 mean, μm 5,10 2,56 14,61 3,46 15,38 2,75

6,5 mm std dev. μm 5,84 4,23 7,25 1,80 6,36 0,76

mean dev., μm 3,62 3,08 5,70 1,45 4,84 0,63

2.2 Hardness profiles of the seam edges

The hardness profiles of Heat affected Zones (HAZ) of lasercut edges is given in fig. 1.



4(45)

100

120

140

160

180

200

220

240

260

280

300

320

340

360

0 0,2 0,4 0,6 0,8 1 1,2

Ha

rdn

es

s, H

V 2

00

g

Distance from the lasecut edge, mm

The hardness profile of (N2) lasercut edges of different stainless steels

201LN TR

"LDX"

"1.4307"

"1.4509"

"1.4521"

Fig. 1 Hardness profiles of lasercut seam edges

In case of lasercut edges there are two kind of changes which affects to the hardness of the HAZ. One is the heat influence and other is the composition influence to the hardness. Especially when the N2 is used as lasercut gas of austenitic stainless steel the seam edge seems to be harder than the base material. This is a influence of the N2 diffusion to the austenite as interstitial atom which hardens the material. In case of lasercut of work hardened austenite AISI 201LN TR the higher N2 content can compensate the decrease of the hard-ness caused by heat influence.

2.3 Nitrogen content of the seam edge

The nitrogen content profile of the seam edge were measured by using EPMA WDS in order to fulfill the theory about higher hardness of the seam edge of austenitic stainless steelsi. In fig. 2 is seen the hardness and nitro-gen content of lasercut seam edge of austenitic stainless steel EN 1.4307. The nitrogen content seems to cor-relate with hardness of the seam edge profile thus the measuring with EPMA WDS system were more like qual-itative.



5(45)

0

0,05

0,1

0,15

0,2

0,25

0,3

100

120

140

160

180

200

220

240

260

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

N2

Co

nte

nt,

%

Har

dn

ess

, H

V 2

00

g

Distance from the lasecut edge, mm

The hardness profile and N2 content of (N2) lasercut edges of EN 1.4307 type austenitic stainless steel

"1.4307"

N2 content

Fig. 2 The hardness profile and nitrogen content of lasercut seam edge of 1,5 mm EN 1.4307 material

3 Welding parameters

The welding parameters of different steel grades used in this study are given in table 2



6(45)

Table 2 Welding parameters of different steel grades

testnr Type

Thickness,

mm Power, kW

Optic, mm

Speed, m/min

focus le-vel, mm

Shielding gas, l/min

root gas

Energy input, J/mm

1 LDX 2101® 1,50 3 300 9,0 -1 Ar, 30 Ar 20,0

2 LDX 2101® 1,50 3 300 9,0 -1 N2, 30 N2 20,0

3 EN 1.3407 1,50 3 300 7,0 -1 Ar, 30 Ar 25,7

4 EN 1.3407 1,50 3 300 7,0 -1 N2, 30 N2 25,7

5 EN 1.4509 1,50 3 300 8,0 -1 Ar, 30 Ar 22,5

6 EN 1.4521 1,50 3 300 8,0 -1 Ar, 30 Ar 22,5

7 AISI 201LN TR 2,27 4 300 8,0 -1 Ar, 30 Ar 30,0

8 AISI 201LN TR 2,27 4 300 10,0 -1 Ar, 30 Ar 24,0

9 AISI 201LN TR 2,27 4 200 10,0 -1 Ar, 30 Ar 24,0

10 AISI 201LN TR 2,27 4 200 14,0 -1 Ar, 30 Ar 17,1

11 AISI 201LN TR 2,27 4 300 7,0 -1 N2, 30 N2 34,3

12 AISI 201LN TR 2,27 4 300 7,0 -1 Ar, 30 Ar 34,3

13 AISI 201LN TR 2,27 4 300 8,0 -1 N2, 30 N2 30,0

14 Optim 960 QC 6,00 4 300 1,8 -1 Ar, 30 Ar 133,3

15 EN 1.4318 TR 2,00 4 300 9,0 -1 Ar, 30 Ar 26,7

16 EN 1.4318 TR 2,00 4 300 9,0 -1 N2, 30 N2 26,7

17 Miilux 500 6,50 4 300 1,5 -1 Ar, 30 Ar 160,0

18 EN 1.4521 1,50 1 300 1,5 -1 Ar, 30 Ar 40,0

19 EN 1.4521 1,50 2 300 1,5 6 Ar, 30 Ar 80,0

20 EN 1.4521 1,50 3 300 1,5 9 Ar, 30 Ar 120,0

21 EN 1.4521 1,50 3 300 8,0 -1 Ar, 30 Ar 22,5

22 EN 1.4521 1,50 2 300 4,5 -1 Ar, 30 Ar 26,7

23 EN 1.4521 1,50 1,8 300 3,5 -1 Ar, 30 Ar 30,0

24 EN 1.4521 1,50 1,75 300 3,0 -1 Ar, 30 Ar 35,0

25 EN 1.4521 1,50 3 300 8,0 -1

22,5

26 EN 1.4521 1,50 2 300 4,5 -1

26,7

27 EN 1.4521 1,50 1,8 300 3,5 -1

30,0

28 EN 1.4521 1,50 1,75 300 3,0 -1

35,0

29 LDX 2101® 1,50 3 300 4,5 -1 Ar, 30 Ar 40,0

30 LDX 2101® 1,50 2 300 1,5 6 Ar, 30 Ar 80,0

31 LDX 2101® 1,50 3 300 1,5 9 Ar, 30 Ar 120,0

32 LDX 2101® 1,50 3 300 4,5 -1 N2, 30 N2 40,0

33 LDX 2101® 1,50 2 300 1,5 6 N2, 30 N2 80,0

34 LDX 2101® 1,50 3 300 1,5 9 N2, 30 N2 120,0



7(45)

4 The properties of laser welds

4.1 The hardness profiles of the welds

The influence of welding parameters to the hardness profile of the AISI 201 LN TR ¼-hard laserwelds is shown in fig. 3. The hardness profile of laser welds of AISI 201 LN TR steel depends slightly on the energy input so that the hardness of the melted areas are a bit higher in the welds which were welded with lower heat input. The width of the softer areas of the welds were narrower in case of welds welded with 200 mm focal length op-tics compared with welds welded with 300 mm focal length optics. The melted area was a bit softer than the base material. Still the minimum hardness was harder than the annealed (2D) base material. One hypothesis is that the N2 content is higher in melted area because of higher N2 content of seam edges (see fig.2). In practice it was too difficult to weld together two lasercut plates by using 0,2 mm beam (200 mm optics) and lowest possible energy input. Therefore there was some unjoined areas in welds which were welded with Ø 200 mm focal length optics by using 17,14 J/mm (weld nr 10).

Fig 3. The hardness profiles of Welds nr 7,8,10,11



8(45)

Fig. 4 The hardness profiles of Welds nr 15 and 16

The hardness profiles of laserwelds of different steel grades are shown in figs. 7-11.

0

50

100

150

200

250

-0,8 -0,6 -0,4 -0,2 0 0,2 0,4 0,6 0,8

Hard

ness

, HV

20

0g

Distance from the centerline of the weld, mm

The hardness profile of 1,5 mm EN 1.4521 laser weld

Hardness of the base material

Fig. 5 The hardness profile of 1,5 mm EN 1.4521 laser weld. Weld nr 6



9(45)

0

50

100

150

200

250

-1 -0,8 -0,6 -0,4 -0,2 0 0,2 0,4 0,6 0,8 1

Har

dn

ess

, HV

20

0g


The hardness profile of 1,5 mm 1.4509 laser weld

Fig. 6 The hardness profile of 1,5 mm EN 1.4509 laser weld. Weld nr 5

Fig. 7 The hardness profile of 1,5 mm LDX 2101® laser weld. Welds nr 1 and 2



10(45)

According to fig. 10 the weld which has welded with nitrogen shielding gas instead of argon is a bit harder.

0

50

100

150

200

250

-1 -0,8 -0,6 -0,4 -0,2 0 0,2 0,4 0,6 0,8 1

Har

dn

ess

, HV

20

0g


The hardness profile of 1,5 mm 1.4307 laser welds

Argon

N2

Fig. 8 The hardness profile of 1,5 mm EN 1.4307 laser weld. Welds nr 3 and 4

Fig . 9 The hardness profile of 6,0 mm Optim 960 QC laser weld. Weld nr 14



11(45)

Fig . 10 The hardness profile of 6,5 mm Miilux 500 laser weld. Weld nr 17



12(45)

4.2 Tensile test results

All test welds were welded in rolling direction. Therefore the tensile test bars were taken transversely com-pared to rolling direction just like it was done in quality testing of the manufacturer of the steel /1/.

In case of work hardened AISI 201 LN TR steel the Rp0,2 strength of laserwelds were about 90 % of the base material as seen in table 2. All other laser welds of 2B and 2K finished materials were broken in the base ma-terial so that the tensile test results were correspond to the quality testing of the manufacturer of the steel.

Table 3. Tensile test results of the laser welds and base materials

Material, Test nr Rp0.2 Rm Ag A50

Base material Rp0.2

Base material Rm

Base material A50

MPa MPa % % MPa MPa %

LDX 2101®, 1 615,00 802,00 19,75 28,65 602 797 31

mean deviation 2,50 1,00 0,23 0,37

EN 1.4307, 3 321,75 618,50 46,98 53,55 E 315 610 54

mean deviation 2,75 4,50 0,35 0,40

EN 1.4509, 5 376,00 476,25 16,72 30,18 E 387

A368

470

469

30

mean deviation 1,50 2,13 0,10 0,68 31

EN 1.4521, 6 428,75 533,75 15,20 26,10 E 369

A 372

524

523

29

mean deviation 0,75 0,75 0,10 0,10 30

AISI 201 LN TR Ar, 7 624,00 810,75 20,16 23,70* E 702

A 685

870

871

32

mean deviation 23,00 6,88 4,71 6,25 32

AISI 201 LN TR Ar, 12

mean deviation

677,07 819,65 22,21 28,01 E 702 870 32

1,38 1,45 0,73 1,27 A 685 871 32

AISI 201 LN TR ¼ N2, 11

mean deviation

679,50 820,83 22,24 21,77 E 702 870 32

4,53 2,17 0,31 0,33 A 685 871 32

Miilux 500, 17 1298,00 1534,00 1,98 3,10

mean deviation 1,0 2,0 0,06 0,60

EN 1.4318 TR Ar, 16

mean deviation

629,25 907,00 26,47 27,48 E 627 944 35

14,88 14,00 1,88 2,13 A 635 951 36

Optim 960 QC, 14

mean deviation

1037,67 1120,67 1,40 5,73

9,78 1,78 0,06 0,24

*A80, A= beginning of the coil E= end of the coil

4.3 X-ray radioscopy

The x-ray pictures were done by using equipment. The performance of the equipment was not enough good for the radioscopy of thicker than 1,5 mm materials.



13(45)

Fig. 11 The X-ray pictures of laserwelds of different kind of 1,5 mm stainless steels

4.4 Fatigue tests

The fatigue tests were done by using compression-tension equipment. The fatigue test parameters are shown in fig.13 and table 3.

Fig. 12 The parameters of fatigue tests

The compression-tension test results are shown in table 4.

Δσ

σa

Stress

Time



14(45)

Table 4. fatigue test results of laser welds

Test material,test nr nr of tests Δσ [Mpa] R σa [MPa]

σm [MPa]

Frequency [Hz]

Average n

Mean dev.

n Mean dev. % n

LDX 2101® Ar,1 8 984,00 -1 492,00 0 5 5209 804 15,42

LDX 2101® N2,2

3 984,00 -1 492,00 0 5 2072 472 22,75

3 615,00 -1 307,50 0 15 226179 105426 46,61

EN 1.4509,5 9 601,60 -1 300,80 0 12 127501 54868 43,03

EN 1.4521,6

8 686,00 -1 343,00 0 15 33509 13123 39,16

1 514,80 -1 257,40 0 15 183855

EN 1.4507 Ar,3 10 514,80 -1 257,40 0 15 52201 16188 31,01

EN 1.4507 N2,4

3 449,68 -1 224,84 0 15 186341 44849 24,07

3 514,80 -1 257,40 0 15 20220 5570 27,55

AISI 201LN TR N2,11

10 1086,00 -1 543,00 0 10 439 61 13,79

5 514,00 -1 257,00 0 15 387675 143373 36,98

AISI 201LN TR Ar,12

6 1086,00 -1 543,00 0 10 526 112 21,38

5 514,00 -1 257,00 0 15 167358 90920 54,33

EN 1.4318 TR Ar,15

5 514,80 -1 257,40 0 10 196668 38947 19,80

5 1006,80 -1 503,40 0 5 1152 245 21,67

EN 1.4318 TR N2,16

Polished bars

Polished bars

4 514,80 -1 257,40 0 10 477179 170166 35,66

5 1006,80 -1 503,40 0 5 1973 220 11,17

1 1006,80

503,40 0 5 23569

1 800,00 -1 400,00 0 15 >3*10

6

Optim 960 QC Ar,14

4 1536,00 -1 768,00 0 5 1644 154 9,00

3 800,00 -1 400,00 0 10 60162 4892 8,13

Miilux 500 Ar, 17

3 1298,00 -1 649,00 0 5 5872 1408 23,99

3 1536,00 -1 768,00 0 5 3410 560 16,44

3 2076,80 -1 1038,40 0 3 679 195 28,67



15(45)

4.4.1 Duplex stainless steel

Fig. 13 Fatigue strength of LDX 2101® laser welds compared to base material.

4.4.2 Ferritic stainless steel

Fig. 14 Fatigue strength of EN 1.4509 laser welds compared to base material.



16(45)

Fig. 15 Fatigue strength of EN 1.4521 laser welds compared to base material. Also the bending fatigue test results

4.4.3 Austenitic stainless steel



17(45)

Fig. 16 Fatigue strength of EN 1.4307 laser welds.

Fig. 17 Fatigue strength of AISI 201LN laser welds compared to base material.

Fig. 18 Fatigue strength of EN 1.4318 TR laser welds 15 and 16 compared to base material. Also two tests with laser welded and polished fatigue bar.



18(45)

4.4.4 Ultra high strength structural steel

Fig. 19 Fatigue strength of Optim 960 QC laser welds compared to base material.

4.4.5 Wear proof steel

Fig. 20 Fatigue strength of Miilux 500 laser welds compared to base material.



19(45)

4.5 Erichsen tests

The laser welds of Duplex- and ferritic stainless steels can be brittle depending on the welding parameters. The forming properties does not absolutely come up in the tension tests, which was noticed during research. Therefore the erichsen test must be done for the LDX 2101® and EN 1.4521 steels.

4.5.1 EN 1.4521

The welds were placed in the center of the forming area

Fig 21. The Erichsen test result of weld nr 6



20(45)

Fig 22 The Erichsen test result of weld nr 18




21(45)


Fig 25 Comparison of the Erichsen test results of different EN 1.4521 laser welds and base material



22(45)

4.5.2 LDX 2101®

The welds were placed in 5 mm from the center of the forming area





23(45)





24(45)





25(45)

Fig 32 Comparison of the Erichsen test results of different LDX 2101® laser welds and base material

4.6 Bending tests

Bending tests were done by using hand-operated bending machine. The welds were aligned to the bending point

4.6.1 EN 1.4521

Table 5. Maximum Bending angles of EN 1.4521 steel laser welds nr 18-21

Shielding gas

22,5 J/mm 40 J/mm 80 J/mm 120 J/mm

Ar, max bending angle,° 180 40-180 30-180 30-180

According to table 4 the energy input should be under 40 J/mm to get ductile weld. Therefore the new welds were welded in order to find the maximum heat input between the 22,5 – 35 J/mm. The welds were welded with argon shielding gas and without shielding gas. One weld was welded by using Nitrogen shielding gas in order to compare the influence of shielding gases. The results are seen in figs. 33-36.



26(45)

Fig 33 The bended welds of EN 1.4521. Energy input 22,5 J/mm

Fig 34 The bended welds of EN 1.4521. Energy input 26,70 J/mm

Without shielding gas Argon shielding gas


top side root side

root side

top side

top side

top side

top side

top side

top side

top side

root side

root side

root side

root side root side

root side



27(45)

Fig. 35 The bended welds of EN 1.4521. Energy input 30,90 J/mm

Fig. 36 The bended welds of EN 1.4521. Energy input 35,0 J/mm

Argon shielding gas Without shielding gas


top side

top side top side

top side

top side

top side

top side

top side

root side

root side

root side

root side

root side

root side

root side

root side



28(45)

Table 6. The maximum bending angles of welded samples of EN 1.4521 steel

Energy input, J/mm 22,50 26,70 30,90 35,00

Wihtout top side, ° 22,00 39,33 31,33 19,33

shielding gas root side, ° 131,33 139,33 113,67 95,67

Argon top side, ° 180,00 180,00 180,00 180,00

shielding gas root side, ° 172,67 180,00 165,00 92,33

Nitrogen

shielding gas

top side, °

root side, °

180,00

53,67

4.6.2 LDX 2101®

Table 7. . Maximum Bending angles of LDX 2101® steel laser welds

Shielding gas

22,5 J/mm 40 J/mm 80 J/mm 120 J/mm

N2, max bending angle,° 180 180 180 180

Ar, max bending angle,° 30-40 180 180 180

Fig. 37 The austenite content of LDX 2101 Steel compared to energy input and shielding gas type



29(45)

4.7 Metallography 4.7.1 Cross section pictures

Fig. 38 EN 1.4521 1,5 mm. Energy input 22,5 J/mm



30(45)

Fig. 39 EN 1.4509 1,5 mm. Energy input 22,5 J/mm.



31(45)

Fig. 40 LDX 2101® 1,5 mm welded with argon shielding gas. Energy input 20 J/mm



32(45)

In the fig. 41 is seen difference of welds which were welded by using different shielding gases. The samples were etched with NaOH by using 2,5V voltage for 15 seconds. The austenite is the white unetched area.

a) b)

c) d)

Fig. 41 a) weld nr 1. 1,3 % austenite b) Weld nr 2. 8,1 % austenite

c) Weld nr 30. 16,5 % austenite d) Weld nr 33. 28,8 % austenite

Argon

20 J/mm

Argon

80 J/mm

Nitrogen

20 J/mm

Nitrogen

80 J/mm



33(45)

Fig. 42 AISI 201 LN TR ¼ hard 2,27 mm, Energy input 30 J/mm. welded by using Ar as shielding gas



34(45)

Fig. 43 EN 1.4318 TR 2,0 mm, Energy input 26,7 J/mm welded by using Ar as shielding gas



35(45)

Fig. 44 EN 1.4318 TR 2,0 mm, Energy input 26,7 J/mm welded by using Nitrogen as shielding gas



36(45)

Fig. 45 Weld nr 14. Ruukki Optim 960 QC 6 mm Energy input 133,3 J/mm.



37(45)

Fig. 46 Miilux 500 wear proof steel. weld nr 17. Plate thickness 6,5 mm. Energy input 160 J/mm

4.7.2 Fracture surfaces

The figs 47 and 48 were taken from the bended weld sample of 1,5 mm EN 1.4521 laser weld. The fracture in

fig 47 was occurred very fast at bend angle of 19 °. The fracture in fig. 48 was on the surface of the 180° bend-ed sample.



38(45)

Fig. 47 Brittle cleavage fracture at Weld nr 28 cross section and surface



39(45)

Fig 48 Ductile fracture on the surface of the Weld nr 22 cross section and surface

In the fig.49 there is a very brittle cleavage fracture at 30° bending angle in the LDX 2101® weld, which was

welded by using low heat input and argon as shielding gas. In fig. 50 the parameters are the same as in fig. 49, but the shielding gas is nitrogen instead of argon.



40(45)

Fig. 49 Brittle cleavage fracture at 30° bending angle. Weld nr 1 cross section and surface



41(45)

Fig. 50 The 180° bended weld nr 2

5 Discussion

5.1 Hardness tests

The nitrogen as shielding gas seems to harden the austenitic weld due to interstitial atom effect. The use of Nitrogen as shielding gas can compensate the lower hardness of the weld of work hardened AISI 201LN TR austenitic stainless steel (Fig 3.), but in case of EN 1.4318 TR stainless steel laser weld there seems to be no influence of shielding gas on hardness profile (Fig 4.). The hardness profile of wear proof steel laser welds is higher than base material at melted area and austenitized area because of high cooling rate. The softer region is in the tempered area of the weld. The hardness profile of UHS steel Optim 960 QC is quite like the same, but the hardness level is lower due to lower carbon equivalent.

5.2 Tension tests

The yield strength of work hardened austenitic stainless steel laser welds were almost as high as in base mate-rial, but the ultimate strengths were 6 % lower comparing the base material. Still the ultimate strengths of AISI 201LN TR laser welds exeeds the C800 standard. The use of nitrogen as shielding gas can slightly strengthen the laser weld of workhardened AISI 201 LN steel.

The tension properties of LDX 2101®, EN 1.4307, EN 1.4521 and EN 1.4509 laser welds were same as in base

material.

The tension strength of Optim 960QC laser welds fulfilled the standard levels of the steels. The yield strength of Miilux 500 laserweld were higher than in minimum standard level, but the ultimate strength was a bit lower than in base material.



42(45)

The heat input of laser weld is often too low for the wear proof steels, so there can be a possibility for the cold cracking. Therefore the hydrogen level of the weld seam must be as low as possible and there should not be any residual stress in the welded structure.

5.3 Radioscopy

According to fig. 11 there was not pores in the laser welds of 1,5 mm materials. Instead there was seems to be

some undercut (EN 1.4521) and incomplete weld (LDX 2101® and EN 1.4509). The best weld was Austenitic

EN 1.4307 due to influence of higher thermal expansion coefficient on the weld fillfactor.

5.4 Fatigue tests

According to fig the compression-tension fatigue test with stress ratio R=-1 is about 10 times tougher than the bending fatigue test. The fatigue test results of high strength steels were better at higher stress amplitudes but were almost the same as lower stress amplitudes. The one reason for this is the geometry of the weld which has a notch effect on the fatigue bar. It is seen in fig.18 , where the grinded and polished bar has 10 times bet-ter fatigue strength than the bars which has geometry of the weld. The laser welded and polished EN 1.4318 TR bar was as good as milled base material bar of the same material fig 18.

5.5 Erichsen tests

5.5.1 EN 1.4521

The best Erichsen test result was in weld which was welded by using 80 J/mm energy input. The weakest re-sult was in weld which was welded by using 120 J/mm energy input. There was one test weld which was broke down very fast and brittle.

5.5.2 LDX 2101®

The Erichsen tests showed that Nitrogen shielded welds were had higher punch strokes and drawing forces than argon shielded welds. The welds which were welded by using lower heat input had also higher punch strokes and drawing forces.

5.6 Bending tests

5.6.1 EN 1.4521

According to figs. 33-36 and table 6 the best welding parameters were achieved when the energy input was under 30 J/mm at 1,5 mm butt weld, which is differing a bit from the Erichsen test results. Welds which were welded without shielding gas were always brittle. The using of shielding gas seems to be very important in or-der to achcieve formable and ductile welds. The geometry of the welds were quite like same so that there seems to be difference between metallurgy of the shielded and non-shielded weld seams. The one theory is that argon as shielding gas removes the Nitrogen away from the welding area, but the nitrogen shielded welds

seems to be better than the weld without shielding gas. The welds were heat treated at 200 °C for 6 hours in



43(45)

order to remove the possible hydrogen from the weld. The heat treated welds were as brittle as the welds which were not heat treated. So the hydrogen does not cause the brittle weld. The oxygen is the only element which causes the poor weld. The oxidation of titanium can prevent the TiN and TiC formation thus caus-ing the martensite formation and also sensitization in the weld material. The difference between fractures of Argon shielded and non shielded welds is seen in figs 47 ad 48. The fracture surface of non shielded weld is cleavage and transgranular whereas the fracture of the argon shielded weld includes dimples like ductile frac-ture surface.

5.6.2 LDX 2101®

The influence of nitrogen shielding gas on formability of weld is seen in fig. 37. The weld which was welded with lowest heat input was very brittle as argon shielded, but was even good as nitrogen shielded. The reason for this is seen in figs. 41 a and b, where the argon shielded weld has almost 100 % ferrite and nitrogen shielded weld has also a bit austenite. The austenite content is just enough to solute the nitrogen, so that the amount of chromium nitrides at the grain boundaries decreases. The higher energy input increases the aus-tenite content so that the formability of the welds are in good stage even at argon shielded welds. The reason for this is lower cooling rate so that there is more time for diffusion and further austenite formation at solid state. The austenite content should be 50 % to get good pitting corrosion and toughness properties /2,9/. The 50 % austenite content of the weld microstructure is difficult to achieve in keyhole welding without filler material.

5.7 Metallography

Both ferritic stainless steels are Ti-stabilized so that the microstructure should be ferritic at all temperatures dur-ing welding process.

According to figs. 38-39. the solidification structure of the ferritic stainless steels is very coarse due to pure ferritic solidification. The solidification edges meet others at centerline of the weld so that the length of the so-lidified grain is 0,5 * width of the melted area. There seems to be no grain growth of HAZ in case of 22,50 J/mm energy input welding.

The duplex stainless steel weld solidifies as ferrite. Then the austenite becomes in solid state during cooling sequence by contribution of diffusion. The ferrite content of solidified area of LDX 2101

® stainless steel laser

weld is about 95 % due to high solidification rate. Depending on high cooling rate of laser weld there is too short time to diffusion and further austenite formation from the solidified area. According to fig 37 there seems to be the highest austenite content at 80 J/mm energy input. The nitrogen content of nitrogen shielded weld metal depends of time and temperature according to fig. 51, where the dissolution of the nitrogen and the de-pletion of the nitrogen are the competitive reactions.



44(45)

Fig 51. The nitrogen content of the nitrogen shielded weld during the welding. d=dissolution e=depletion /1,2/

The dissolution of the nitrogen takes place quite fast during the melt sequence then the high solidification speed can increase the nitrogen content of the weld metal. On the other hand the high solidification speed in-creases the ferrite content of the weld. Therefore there can be a certain energy input where the austenite con-tent is a bit higher when using nitrogen as shielding gas.

According to figs. 42-44 the solidification structures of austenitic stainless steels are very fine due to Austenite-ferrite or Ferrite-austenite-ferrite solidification sequence /3,4/. The HAZ is almost undetected. The solidifica-tion structure depends on cooling rate so that the solidification becomes more austenitic when the cooling rate increases/4,5/. The Hammar Svensson idex of the AISI 201 LN and EN 1.4318 steels were over 1,7 and the impurity content (S+P



45(45)

6 Summary

-The Cr ekv/Ni ekv shold be over 1,7 to protect the solidification cracking in keyhole laser welding without filler material. On the other hand the impurity content of steel should be under 0,04 %.

-The fatigue strength of laser weld depends on the geometry of the weld. The influence of microstructure of the weld on the fatigue strength seems to be quite slight (fig.18.).

-It is very important to use a careful argon shielding when welding of Ti-stabilized ferritic stainless steel in order to achieve a ductile weld (Figs. 33-36)

-The energy input should be relatively low when welding of ferritic stainless steels.

-The energy input should not be too low when welding of LDX 2101 stainless steel

-The use of nitrogen as shielding gas is advantageous in order to ahcieve the ductile welds of LDX 2101 steel (fig.37)

-The 50 % austenite content of LDX 2101 laser weld is difficult to reach in laser welding without the post heat treatment of the weld

7 References

1. Verhagen, J.G., denOuden, G.,Liefkens, A.,Tichelaar, G.W., Nitrogen absorption by Ferritic weld met-al during arc welding, Metal construction, 1970, 2. S. 135.

2. Kyröläinen A., Lukkari J., Ruostumattomat teräkset ja niiden hitsaus, Metalliteollisuuden keskusliitto

3. Hammar, Ö., Svensson, U., Solidification and casting of metals, The metals society, London, 1979, 401-410

4. Suutala, N., Solidification studies on austenitic stainless steels, Doctoral thesis, University of Oulu, 1982

5. David, S.A., Welding journal, 1981, 60, pp. 63-s … 71-s.

6. Takalo, T.,Moisio, T., Austenitic solidification mode in austenitic stainless steel welds, Metallurgical transactions A, Vol 10A, August 1979, 1173

7. Outokumpu Oyj, Welding handbook, First edition, 2010.

8. Westin, E.M. Microstructure and properties of welds in the lean duplex stainless steel LDX 2101®, KTH

international engineering and management, Doctoral thesis in material science, Stockholm, Sweden, 2010

9. Ogawa K. et al, Hydrogen embrittlement cracking in duplex stainless steel weld metal, IIW Doc., IX-1461-87

Date post:	03-Feb-2021
Category:	Documents
Upload:	others
View:	3 times
Download:	0 times

PROLAS - Luleå tekniska universitet, LTU/file/PROLAS tekninen...PROLAS Process Optimization of...

Documents