Laser Weldability of High Nitrogen Austenitic Stainless Steel

Conclusions

Approach and Objectives

Motivation

Results

Impact

Weld aspect ratio versus travel

speed: aspect ratio approaches

asymptote limited by laser power.

•Examine weldability of type 21-6-9 stainless with LBW, compare to EBW •Verify feasibility and variability of commercial 21-6-9 material to deliver consistent welding

•Bead morphology •Defect formation •Solidification behavior •Alloying element loss •Solidification crack susceptibility

•Provide experimental data for computer modeling effort

Increase in primary Austenite solidification

mode as travel speed increases.

•Laser welding of 21-6-9 with no defects is obtainable

•Processing parameters must be carefully selected to avoid

defects:

•High travel speeds cause porosity

•No solidification cracking issues in current study

•δ-Ferrite content can be varied with processing parameters

and chemical composition

•Espy predictive model provides best δ-Ferrite content

prediction

•Nitrogen loss occurs, particularly at weld surface

•With proper processing parameters,

defect free laser beam welding of type

21-6-9 stainless is possible

•Processing parameters can also be

used to tailor the bead morphology and

microstructure desired

•Further work is needed to examine

the effect of chemical composition

variation on weldability

•Hot crack susceptibility testing is in

progress

Laser welding enclosure with 21-6-9

specimen

Decreasing ferrite content as

travel speed is increased. Gap in

δ-Ferrite content attributed to chemistry differences.

21-6-9 Composition

(wt. pct.)

Centerline section showing typical void at end of weld where

keyhole freezes.

•Travel Speed and Laser Power Matrix

•15-195 ipm

•500-1100 W

•2 heats of 21-6-9

Colorado School of Mines: Stephen Tate, Dr. Stephen Liu Los Alamos National Laboratory: Dr. Pat Hochanadel

1100 W, 75 ipm

1100 W, 135 ipm 1100 W, 195 ipm

Element Range

Cr 19.00-21.50

Ni 5.50-7.50

N 0.2-0.4

Mn 8.00-10.00

C 0.08 max

Si 1.00 max

P 0.06 max

S 0.03 max

O N/A

Fe Balance

•Type 21-6-9 stainless named after nominal composition of 21Cr-6Ni-9Mn

•Has weldability issues of porosity and hot cracking •Type 21-6-9 stainless steel: potential for use in energy industries •High equipment and maintenance cost of electron beam welding (EBW) typically used with type 21-6-9 •Desire to replace EBW with Laser Beam Welding

Bead Morphology

Defects: Porosity Solidification Behavior Alloying

Element Loss

0

1

2

3

4

5

6

7

8

10 40 70 100 130 160 190

D/W

Rat

io

Travel Speed (ipm)

Heat 2, 500W

Heat 2, 700W

Heat 2, 900W

Heat 2, 1100W

Heat 1, 500W

Heat 1, 700W

Heat 1, 900W

Heat 1, 1100W

0

2

4

6

8

10

12

14

0 50 100 150 200 250

Ferr

itre

(%

)

Travel Speed (ipm)

Initial Matrix

High Speed Matrix

Polished and etched section with no apparent voids.

Void observed during light optical microscope

examination.

Heat 1 21-6-9

Heat 2 21-6-9

WR

C-9

2

Cr Eq. 19.7 20.0

Ni Eq. 13.4 15.4

Cr Eq./ Ni Eq. 1.47 1.30

FN 3 1

Mode FA AF

Esp

y

Cr Eq. 20.6 21.1

Ni Eq. 13.2 15.2

Cr Eq./ Ni Eq. 1.56 1.39

FN 6 0

Content (weight percent)

Element

Low Speed Matrix 21-6-9

(Heat 1)

High Speed Matrix 21-6-9

(Heat 2)

Cr 19.7 19.9

Ni 6.8 7.2

N 0.27 0.34

Mn 9.5 8.6

C 0.035 0.04

Si 0.58 0.54

P 0.013 0.016

S 0.001 0.001

O 0.0017 0.0013

N loss at weld

surface likely cause

of primary ferrite cap

in all welds.

Laser Varestraint Test

setup

Quantity of voids observed:

Sudden porosity formation at

high travel speeds

Travel

Speed

(ipm)

Power

(W)

Number

of Voids

Aspect

Ratio

45 500 0 3.0

45 700 0 3.0

45 900 0 3.5

45 1100 0 4.9

75 500 0 2.8

75 700 0 4.0

75 900 1 4.7

75 1100 1 6.7

105 700 19 3.5

105 900 18 5.4

105 1100 21 7.0

135 900 24 5.5

135 1100 19 6.8

165 900 26 5.2

165 1100 23 6.6

195 1100 21 7.1

Location of electron

microprobe scans to

determine N content.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Base Metal Top Middle Root

N (

weig

ht

%)

45 ipm, Heat 2

105 ipm, Heat 2

165 ipm, Heat 2

55 ipm, Heat 1

SEM micrograph of

porosity interior surface.

Research from an NSF I/UCRC: Center for Integrative Materials Joining Science for Energy Applications

Acknowledgements: Drs. Matt Johnson, Paul Burgardt, and Dan Javernick of Los Alamos National Laboratory