24-08-2013, 03:54 PM
FAILURE ANALYSISOF WELDED COMPONENTS
ANALYSISOF WELDED.pptx (Size: 3.14 MB / Downloads: 49)
The welded joints are most critical places of steel structures due to high residual stresses and stress concentration (constructional and structural notches) and due to possible cracking originated from welding process.
In service, cracks can easily propagate, either suddenly (brittle fracture) or gradually (fatigue, creep or corrosion). It is very important to distinguish the crack types, as different measures are often taken to eliminate the various cracks
CRACK TYPES IN WELDED JOINTS
There are four basic crack types which occur in the welded joint of steels, namely:
hot cracks,
cold cracks,
lamellar tearing and
reheat cracks.
Hot cracks
Three types of hot cracks occur in welded joints, namely:
solidification cracks, which are formed during solidification in the weld metal, and are most often orientated towards the weld axis, in the direction of columnar crystals, they are of typical interdendritic character (Fig. 1)
liquation cracks, which are formed in the underbead zone of the base metal, or in multi-pass weld of the weld metal
The residues both of solidified liquid film in the form of eutectic secondary phases (Fig. 2) and of round grains with typical soliditied bridges between them in the case of soluble lower temperature elements (Fig. 3) can be detected on the surfaces of solidification and liquation cracks.
Cold cracks
Cold cracks are longer, less laminated and generally more open than hot cracks . This is due to higher contraction stresses in the time of their formation. Their open surface is metallic lustrous or has a blue tinge. The oxidation layer is comparatively thin. The initial fractured areas are predominantly of intercrystalline cleavage type (Fig. 5).
Reheat cracks
Reheat cracks are formed mainly during stress-relief annealing of welded joints. They are a serious problem in the huge structures of low-alloyed Cr, Ni, Mo, V steels. These are mostly microcracks situated in the coarse-grained underbead zone of the heat affected zone normal to the fusion line. Reheat cracks are of intercrystalline character with smooth , or more frequently intercrystalline ductile facets with carbide particles in the dimples (Fig. 8).
CONCLUSIONS:
Fracture of the pipe/flange weldment was due to fatigue crack progression, which initiated at the root of the weld due to a severe stress raiser effect by incomplete root pass weld penetration and fusion to the pipe and flange.
These weld flaws resulted in a lowering of the weldment load-bearing capacity. Fatigue cracks initiated along the weld root and lack of fusion interfaces where the weldment cross-section was reduced.
No unusual conditions were noted in the pipe or flange base steel compositions or microstructures. No over softening or embrittlement was found associated with the weld and pipe HAZ.
To distinguish the crack types and to know their causes is very important for adoption of proper measures at their remedy.