FAILURE ANALYSISOF WELDED COMPONENTS 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.

polygonization cracks, which are formed in the lower temperature zone (800 – 1100°C), in the heat affected zone and the weld metal The surfaces of polygonization cracks are pure, and without secondary phases. Closed hot cracks which are not opened to air are characterized by thermal faceting of their surface (Fig. 4), which is present at above 900°C as a result of metal ion evaporation into vacuum and this unambiguously differentiates hot cracks from lower temperature ones. Figure 4 – Thermal faceting of liquation

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).

Lamellar cracks (tearing) Lamellar cracks are typical defects in rolled steels with clearly anisotropic properties. They are mainly formed in fillet welds and thick through thickness loaded plates.

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).

Analysis Of A Fractured Crane Frame Weldment A fractured pipe/flange weldment from a crane frame assembly was received for analysis to determine the cause of fracturing along the circumferential fillet weld. Results indicate the fracture occurred by cyclic fatigue crack growth that initiated at the root of the fillet weld due to incomplete penetration by the root pass and a lack of fusion to the pipe or flange at several locations around the weldment. The lack of penetration was noted around 60% of the circumferential fillet weld. Lack of fusion to the pipe and flange were noted around approximately 50% of the weld. The flange and pipe were manufactured from weldable grades of high strength, low alloy structural steels.

A photograph of the flange and pipe sections in the as-received condition. The weldment was a circumferential single bevel groove weld with a reinforcing fillet weld.

A side view of the flange and pipe in the as-received condition A side view of the flange and pipe in the as-received condition. Fracture occurred along the circumferential fillet weld.

Close-up view of the reassembled fracture Close-up view of the reassembled fracture. The fracture on the OD is mainly along the toe of the filler weld on the pipe side. A smooth uniform reinforcing fillet weld is observed along the OD

A photograph of the fracture surface on the pipe side A photograph of the fracture surface on the pipe side. Areas exhibiting a lack of penetration of the root pass (blue) and lack of fusion to the pipe (green) and flange (red) are indicated

A close-up photograph of the fracture on the pipe end exhibiting a lack of penetration to the root of the weld and a lack of fusion to the flange and pipe

A closer view of the fracture surface on the flange side A closer view of the fracture surface on the flange side. Thumbnail-shaped fatigue progression zones are noted along the weld root

A closer view of one of the thumbnail-shaped fatigue zones along the weld root in the fracture surface on the flange side. Multiple ratchet marks (fine radial features) are visible indicating multiple fatigue initiation sites

A closer view of a typical lack of weld fusion site along the weld fracture. A fatigue fracture initiated in the reinforcing weld above the lack of fusion site. The green dash lines illustrate the location of a cross-section taken through the weld for subsequent metallographic examination

A low magnification SEM photomicrograph of the fatigue fracture surface on the flange side previously. The fatigue fracture occurred at multiple initiation sites. A dash line delineates the initial high cycle fatigue zone from the lower cycle fatigue progression region and subsequent shearing overload.

A higher magnification SEM photomicrograph of the high cycle fatigue zone. Porosity cavities are noted along the weld/flange interface where fatigue initiated. Ratchet marks arealso observed between the pores further confirming fracture from multiple initiation sites

A high magnification SEM photomicrograph of ill-defined fatigue striations (arrows) in the fracture surface

A high magnification SEM photomicrograph of the "elongated dimpled” surface of shearing overload region along the OD.

A photograph of the mounted, polished, and etched weldment cross-section at a site exhibiting a lack of root pass penetration and fusion. The black dash line outlines the reinforcing fillet weld. The white dash box encloses the approximate location of subsequent microhardness testing.

A magnified optical photomicrograph of the weldment cross-section presented in Figure. The boxed areas are further magnified in subsequent figures

A higher magnification photomicrograph unetched view of the indicated boxed area in Figure . High temperature oxides line the separation indicating a lack of fusion at this location

A higher magnification photomicrograph of the indicated boxed area in Figure . The fatigue crack (arrow) has propagated from the nonfused weld/pipe interface, into the reinforcing fillet weld.

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.