NOMENCLATURE
CHAPTER 2 Literature Review
2.6 Mechanical and metallurgical characterization of weld joint
Characterization of mechanical properties is performed to check if the weld is sound enough for the specific application. Generally tensile, hardness, impact and fatigue test of the welds along with some microstructure are popular. Submerged arc welding of thick ferritic martensitic 12Cr stainless steel was performed by Taban et al. [133]. They varied the consumable electrodes and compared the joint strength for different cases. The strength of the weld joints for different cases were characterized by measuring hardness and toughness.
Microshear test is a characterization method to estimate the mechanical properties and fracture toughness of pressure vessels. Zhang et al. [134] performed narrow gap submerged arc welding of nuclear pressure vessel steel, A508CL3 and the joints were characterized by the microshear test method. To know the weld joint strength comparison of the yield strength (YS) and ultimate tensile strength (UTS) with the base metal is very much essential, researchers [135]
estimates the local tensile strength, YS and UTS of welded joints. Study on double sided single pass submerged arc welding is also reported for high thickness metals [136]. SAW welded 2205 duplex stainless steel and the joint strength was characterized by determining the micro- hardness of the different weld zones. The data was recorded at distance perpendicular to the weld line to know the hardness distribution over fusion zone, heat affect zone and the base metal. It will generally increase towards to the weld center i.e. the fusion zone due to the melting of the metal. Impact test shows the ductile mode fracture in the weld zone, a mixture fracture feature appears with a shear lip and tears in the fusion zone near the fusion line.
Ramakrishnan et al. [137] studied fracture toughness of cold wire addition in narrow groove submerged arc welding process. The improvement in toughness in cold wire narrow gap SAW is demonstrated through different tests such as impact energy test, fracture toughness tests, plane strain fracture toughness test, and crack tip opening displacement test.
Over the years researchers has worked to study the mechanical properties as well as the microstructural developments of submerged arc welded work pieces. Welding microstructure can provide adequate information about the effect of welding parameters on the mechanical and tribological behaviour so it is very essential to investigate the welding microstructure. A schematic diagram of various microstructure zones of a butt bead-on-plate weld is presented in Figure 2.15, where BH is the bead height, BW is the bead width, DP is depth of penetration, DH is depth of HAZ and HW is the HAZ width.
Figure 2.15 Microstructure zones of a butt bead-on-plate
Kolhe and Datta [138] have investigated and correlate the relationship between the various parameters; mechanical properties and microstructure of single “V” butt joint of mild steel plate, and performed the phase analysis of the multipass welded joint to get defect free welded structures. They presented the experimental results of hardness, fracture toughness, HAZ width from top and bottom of the weld joint and the theoretical analysis of phases of the steel at different heat input of multipass submerged arc welding from the top of welded joint by using image analysis software and correlated the same to the microstructures of fusion zone as well as the heat-affected zone of mild steel. Lu et al. [139] has determined the microstructure and wear property of Fe-Mn-Cr-Mo-V alloy cladding by submerged arc welding.
Figure 2.16 (a) Macrograph, (b) micrograph of HAZ, (c) micrograph of fusion zone [133]
Figure 2.17 SEM fractograph [133]
Microstructural characterization was performed by Taban et al. [133], Figure 2.16 (a) shows the cross section of the weld joint, welded by 21 multi-passes using submerged arc welding.
Figures 2.12 (b and c) shows the micrographs of the heat affected zone (HAZ) and the fusion zone. Few samples of the face and root bend testing were failed, which implies decent joint strength of the welds. The fractograph using scanning electron microscope (SEM) is shown in the Figure 2.17. Prasad and Dwivedi [140] investigates the influence of the submerged arc welding process parameters (welding current and welding speed) on the microstructure, hardness, and toughness of HSLA steel weld joints. Attempts have also been made to analyse the results on the basis of the heat input. High heat input increases the deposition rate and lowers the welding time. Paniagua-Mercado et al. [141] presents a study of the effect of TiO2
additions in fluxes on the mechanical properties and microstructure of the weld metal formed during Submerged-Arc Welding (SAW) of ASTM A-36 steel plates.
Figure 2.18 TEM micrographs showing (a) acicular ferrite plates and inclusion distribution, (b) irregular MA constituent distributed along the acicular ferrite plate boundary [142]
In depth study of the microstructural variation in high strength bainitic low carbon steel weld was carried by Lan et al. [142] by means of optical microscope, transmission electron microscope (TEM) and scanning electron microscope (SEM). The results displayed (Figure 2.18) confirmed the presence of acicular ferrite, coarse granular ferrite and fine polygonal ferrite in the weld microstructure. The martensite-austenite (MA) constituent has a variable structure in each sub-zone, which includes fully martensite and fully retained austenite.
Meanwhile, the fine grained heat affected zone has higher content of retained austenite than the welded metal (WM) and coarse grained heat affected zone (CGHAZ).