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University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Microstructures in (High Strength) Steel Welds
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Aim
To determine – A relationship between microstructure and
crack path– The effect of chemical composition on
propensity to crack
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Procedure
• Two parallel strands:– Experimental – Examination of Weld
Microstructures– Simulation - Aachen.
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Microstructure – Literature Review
• HAZ cracking can be reasonably predicted by calculation of hardness using CE, Pcm etc. values
• CE, Pcm values are not a good guide for the calculation of weld metal susceptibility because hardenability appears to be an important parameter
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Microstructure - Literature Review
• High strength weld metals contain localised variations in hardness of 50– 80HV5
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
250x
Experimental Result
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Micro hardness
0
100
200
300
400
500
2kJFCAW 3kJFCAW 4kJFCAW GMAW2kJ GMAW3kJ GMAW4kJ
Welding Parameters
Micr
ohar
dnes
s
interCellular-dendriticboundary
cellular dendrite
Experimental Result
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Experimental Result
• Local variation of microstructures controls location of cracks in multi-pass welds
• Most of the cracks are densely populated in the microstructural region of higher hardness.
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Experimental Result
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Microstructure - Literature Review
• In most cases cracking originates at a point between 0.1 and 0.3 of the plate thickness when measured from the top of the plate, regardless of the plate thickness.
• This is usually immediately below the second or third to last runs.
• Region of the highest HD levels
• Region of highest residual stress
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Experimental Result
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Microstructure – Literature Review
• The appearance of hydrogen cracked fracture surfaces may be either brittle (cleavage) or ductile (dimpled).
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Experimental Result
Region close to the initiation point
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Experimental Result
Region some distance from the initiation point
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Microstructure – Literature Review
• Cracking may have a relationship with grain boundary ferrite and former austenite grain boundaries but the correlation is uncertain.
• More detailed metallographic research required to relate microstructure and crack path
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Simulation - Aachen
• Use directional solidification of the alloy compositions of interest.
• Identify the compositions of the phases formed.
• Determine how the elements are partitioned.• Determine the segregation ratios of the
elements.• Compare the outcomes with micro-hardness
and x-ray analysis results from weld samples.
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Summary• The appearance of the fracture surface
close to the point of initiation has been found to be consistent for all cases studied i.e.:– Hydrogen charged weld sample form DSTO– Weld sample from ???– FCAW and GTAWsamples from two MSc
students– Multi-run sample from D. Nolan/M. Pitrun
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Modelling of Hydrogen Methodology
• Jointly, with the Foundry Institute of RWTH Aachen endeavor to integrate the software packages;
MICRESS – simulation software which can predict crystal growth and phase transformations from thermodynamic data whilst taking into account micro-segregation
with CASTS – simulation software for the weld
morphology and macro-segregation
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Modeling of Hydrogen
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Modeling of Hydrogen – Literature Review
• Diffusible hydrogen is considered to be the cause of hydrogen-assisted cold cracking in welds and is associated with reversible traps.
• Residual hydrogen is term for permanently trapped hydrogen atoms in irreversible traps sites.
• Traps include the host lattice, vacancies, solute atoms, dislocations, grain boundaries, voids, and second phase particles (inclusions).
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Models of Hydrogen Cracking– Literature Review
• hydrogen enhanced decohesion (HEDE) which hereby describes the decrement in lattice interaction energy due to the accumulation of hydrogen with the aid of triaxial stress,
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Aim
• To model the distribution of hydrogen concentrations present in a double “v” butt weld.
• ANSYS will be used as the modelling package.
University of Adelaide -Cooperative Research Centre for Welded Structures
CRC-WS
Hydrogen Model• Diffusible hydrogen HD is a maximum at 0.75 –
0.9 of plate thickness regardless of plate thickness
• Repetitive thermal cycling and weld bead overlapping determine the final local HD
concentration• Crack density increases in the same manner
with cracks predominately located below the second or third weld beads from the surface.
• This point also coincides with maximum residual stress level
