PROCESS OF FUSION WELDING

In the weld material, whether melted from the edges to be joined (autogenous welding), or supplied separately, solidifies from the liquid state and usually below the recrystallization temperature without any applied deformation. Fusion welds are, therefore, essentially castings. Since the surrounding parts are good conductors of  heat the fusion weld may be called a chilled casting, and its structure will, therefore, usually be columnar (dendritic). The actual crystalline structure presents, however, depends primarily on the number of  "runs" made to deposit the weld metal. 

In a single - run weld, long columnar crystals will grow outwards from the sides of the weld. If the temperature is high enough these columnar crystals will meet at the center of the weld deposit forming a plane of weakness. This will eventually cause inter-crystalline cracking within the weld. If , on the other hand, the welding temperature is correct, equiaxed grains will form at the center resulting in an appreciably stronger joint. 

A multi-run welding exhibits a quite different structure. The first run, as before, shows the structure for a single - run weld. In ordinary steels, the second run normalizes the first layer, causing a considerable degree of  grain refinement. Each successive run thus normalizes the preceding one so that the final deposits exhibit the coarse cast structure typical of a single-run weld. However, in a multi-run weld the possibility of defects such as slag and gas inclusions will increase.

Slag inclusions are frequently trapped in fusion welds due to bead contour and the difficulty of melting the slag in subsequent runs. In metallic arc welds, in mild steel, microscopic inclusions are also present. Controlled amounts of nitrogen and manganese together with a dispersion of fine non metallic inclusions and especially a fine grain size with high dislocation density provide of 43 to 57 kg-f/mm3 (430 to 570 N/mm2) with adequate ductility to mild steels arc welds. 

The contour of welds by forming 'notches' can affect both fatigue and low temperature properties of a structure.

Gas solubility in liquid and solid weld metals, and gas reaction, are important in controlling the porosity of a weld. Metallic arc welds are made with a bare wire are liable to contamination by gases from the atmosphere. The nitrogen of the atmosphere frequently appears as needles on certain planes in the crystals in the long run. This causes low impact strength especially after water quench. 

The stresses set up in the weld by shrinkage are often of considerable importance as they are found to be responsible for weld metal cracking. These stresses may be relieved by annealing the entire object after welding is finished. 

In brief, the action of the atmosphere on the melted metal, its fluidity and surface tension, the solidification process including segregation and shrinkage, all play their anticipated important roles as in castings.