welding conditions - american welding society … k′ d eff /µm fsp-top 14.1 0.62579 1 ... fsw can...
TRANSCRIPT
• 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