Study of Solidification Cracking under FCB Welding of Butt Welding.
〇Shintaro MAEDA*, Yui OKIMI*, Noboru Kiji**, Masakazu SHIBAHARA**Graduated school of engineering, Osaka Prefecture University, ** Japan Marine United Corporation

Background

Idealized Explicit FEM

Concept of occurrence of hot crack

Solidification cracking

Concept of IEFEM
IEFEM
In a load step, displacement is calculated using Dynamic Explicit FEM until the entire
domain converges to static equilibrium state.
After convergence isobtained, the next loadincrement is given.

Hot cracking may decrease the strength of the structure.
Repair welding necessary if it is occurred.
・Increase of production cost・Decrease of production efficiency

By using these procedures,
it is not necessary
to solve the large
simultaneous equations.

Objective of this study•Prediction of occurrence of hot crack in analysis•Consider hot crack prevention measures on analysis

Evaluation of strain on weld metal
during BTR by FEM analysis.

BTR plastic strain increment

Smaller memory and Faster computation

Prediction of crystal growth direction and hot crack occurrence position

Strain in FEM

Crystal growth direction≒ Maximum temperature gradient∆𝜀 = ∆𝜀 𝑒 + ∆𝜀 𝑝 + ∆𝜀 𝑇

𝜃: 𝑆𝑚𝑎𝑙𝑙∆𝜀 𝑒 : Negligibly small becauseof high temperature

Hot Crackingevaluation position

𝜃: 𝐿𝑎𝑟𝑔𝑒∆𝜀 𝑇 : Negative value proportional
to BTR in cooling process.

𝜃: 𝑆𝑚𝑎𝑙𝑙∆𝜀 𝑒 and ∆𝜀 𝑇 are not considered
to themselves cause cracking incooling process.

BTR Temperature Gradient Vector :Temperature gradient in the BTR during cooling

BTR plastic strain increment:
Plastic strain increment ∆𝜀 𝑝in the BTR during cooling𝑝BTR plastic strain increment ∆𝜀𝐵𝑇𝑅 is used

BTR Temperature gradient vector :𝝏𝑻𝝏𝑻𝝏𝑻
𝛁𝑻𝑩𝑻𝑹 = 𝒙𝒊+ 𝒚𝒋+ 𝒛𝒌𝝏𝒙𝝏𝒚𝝏𝒛

as an generation index of hot cracking

Prediction of solidification cracking position from 𝛁𝑻𝑩𝑻𝑹 on association part

Thermal elastic plastic analysis results Temperature distribution
Applied to 4-electrodes
Backside (Welding End)
Temperature
One-side submerged arc welding

Analysis model

Columnar Association Angle θθ ∶ 𝑆𝑚𝑎𝑙𝑙 → 𝐻𝑜𝑡 𝐶𝑟𝑎𝑐𝑘 𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 ∶ 𝑆𝑚𝑎𝑙𝑙θ ∶ 𝐿𝑎𝑟𝑔𝑒 → 𝐻𝑜𝑡 𝐶𝑟𝑎𝑐𝑘 𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 ∶ 𝐿𝑎𝑟𝑔𝑒

Tensile strain generation timing
The front molten poolreaches the tab plateTop surface
Temperature distribution

300mm
360mm

Central section
BTR plasticstrain increment

300mm

Tensile BTR plastic strain increment isgenerating behind the first and second torch.

Plate thickness
25mm

Distribution of BTR plastic strain increment

Displacement yBasePlate

Nodes: 514,552
Elements:477,850z
yTabPlate•Open deformationis generated.

x4-electrodes One-side
submerged arc welding

Total heat input

Physical properties (SM490)

BTR plastic strain
increment becomeslarger at weld end
Smaller welding
speed generatesmaller BTR plasticstrain increment

Distribution of BTR plastic strain increment

12.1 kJ/mm
Temperature dependent material constants•Sequential analysis with moving heat source is performed.

v = 7.66 mm/sec

v = 15.33 mm/sec

Crystal association
angle : Large⇒ Low possibility to crack

Crystal association
angle : Small⇒ High possibility to crack

Thermal conduction analysis resultsv = 7.66 mm/sec

10v = 15.33 mm/sec

864200500
1000
Temperature [℃]

BTR:1350-1450℃••

The larger the welding speed, the longer the BTR length.
Penetration shape of analysis result is in good agreement withexperimental results.

Small welding speeds may reduce hot cracking.

Conclusion

In this research, a new crack generation evaluation method based on mechanics and metallurgy has been proposed. From the metallurgical
viewpoint, the solidification growth direction is predicted by using the BTR temperature gradient vector. And the hot cracking resistance is evaluated fromcolumnar association angle. From the mechanical side, strain generated in the molten and solidified part is evaluated using the BTR plastic strain increment.
Then, the proposed method was applied to 4-electrode One-side submerged arc welding. The following results were obtained:• It was confirmed that the penetration shape and crystal association angle changes with the welding speed.
• In the small welding speed condition, BTR plastic strain increment become small and the crystal association angle become large.
• This indicates that changing the welding speed may be effective to prevent solidification cracking.