•  Porosity •  Loss of

Nitrogen • Nitrides • Hot cracks Problems in

fusion welding

•  Solid welding •  Low heat input •  Large strain and

high strain rate deformation

Characteristics of FSW

•  Integral correlation

•  Local correlation

Research items

Fe Cr Mn Mo N C Cu Si 62.69 18.24 15.71 2.26 0.86 0.02 0.02 0.28

NZ HAZ BM

Local tensile Integral tensile

Welding conditions

Rotational / Travelspeed：800r/min stpeed：50mm/min Tool material：W-La alloy Tool size: shoulder diameter/16mm，Pin length/2.3mm

Post-weld heat treatment 1100°C × 1h, quenched in water Tensile specimens

(a)

Advancing side Retreating side

NZ BM

1mm TMAZ HAZ

As-

wel

ded

Afte

r PW

HT

Hardness distributions

-20 -15 -10 -5 0 5 10 15 20

280

300

320

340

360

380

400

420

Vic

kers

Har

dnes

s/H

v

Distance from welding center/mm

Top layer Bottom layer

Stir Zone

TMAZ TMAZ

(a)

-20 -15 -10 -5 0 5 10 15 20

280

300

320

340

360

380

400

420

Vic

kers

Har

dnes

s/H

v

Distance from welding center/mm

Top layer Bottom layerStir Zone

TMAZ TMAZ

(b)

0 5 10 15 20 25 30 35 40 450

200

400

600

800

1000

1200

1400

1600

Engi

neer

ing

Stre

ss/M

Pa

Engineering Strain/%

BM

FSW-TopFSW-Bottom

NZ-BottomNZ-Top(a)

0 5 10 15 20 25 30 35 40 450

200

400

600

800

1000

1200

1400

1600HTNZ-Bottom

HTNZ-Top

HTFSW-Bottom

Engi

neer

ing

Stre

ss/M

Pa

Engineering Strain/%

BMHTFSW-Top

(b)

Tensile properties Discussions

Position d/µm kRX kD k′ Deff /µm

FSP-Top 14.1 0.62579 1.306733 0.32382 4.6 FSP-Middle 12.8 0.756768 0.829173 0.477173 6.1 FSP-Bottom 8.7 0.734564 0.565874 0.56486 4.9

BM 54.7 0.25318 0.00011 0.999566 54.7 HT-Top 28.0 0.317841 0.006304 0.980553 27.5

HT-Middle 32.4 0.307069 0.006447 0.979437 31.7 HT-Bottom 17.7 0.319658 0.004726 0.98543 17.4

kH ：Holl-Petch slop

d ：Grain Size kD : Deformation factor kRX:：Recrystallization factor

0.1 0.2 0.3 0.4 0.5 0.6280

300

320

340

360

380

400

Vick

ers

Har

dnes

s/H

v

original average grain size d effective grain size Deff

BM

HT-MiddleHT-Top

HT-Bottom

FSP-Middle

FSP-BottomFSP-Top

Inverse of the square root of grain size/µm-1/2

0.93

0.77 1mm

Distribution of Nitrogen element

f

1. FSW can effectively mitigate the loss of nitrogen and avoid other metallurgical problems associated with fusion welding of high

nitrogen austenitic steel;

2. The as-welded FSW joint exhibited high strength and low elongation due to grain refinement in the NZ.

3. PWHT effectively tailored the microstructure in the as-welded joint and consequently the mechanical property of the joint was

restored to the level of BM;

4.  It was revealed that there existed a quantity of sub-grain boundaries in the NZ of as-welded joints. These sub-structures has

significant effect on the properties of the NZ. A modified Holl-Pecth relation between the hardness and effective grain size in the NZ

was established.

This work not only verified the feasibility to weld high nitrogen steels by using FSW, but also identified the key factors

including the internal restrain effects among different zones and sub-grain boundary strengthening that determine the

property of the joints. Acknowledgements This work was sponsored by NSF of Hebei province of China. Also thanks for the helps from CWJCR of CSM

Reduced property gradient after PWHT

Hv = H0 + kH Deff -1/2 = H0 + kH[(kRX + kD)/kRX]1/2d-1/2

There are no loss of nitrogen and weld defects in the as-welded joint of this high nitrogen steel . The gradients of microstructure and hardness distribution in the as-welded joint result in internal restrain effect between different zones of the joint. As a result, the strength is increased but the elongation of the integral joint is decreased. PWHT effectively reduced this gradient feature in the joint. Consequently, the property of the integral joint was restored to the level of BM.

Ruidong Fu1,2, Dongxu Du1, Yijun Li1, Zhenzhen Yu2 1. Yanshan University, China 2. Metallurgical and Materials Engineering Department, Colorado School of Mines, Golden, CO