Welding (Steel Joining)

Welding is the process of joining two metal pieces by application of heat, with or without filler material, to produce a metallurgical bond. In Indian structural steel construction, welding is the dominant method of fabrication and field erection joining. The relevant Indian standards: IS 816:1969 (manual metal arc welding code of practice — main reference), IS 9595:1996 (manual metal arc welding of carbon and carbon-manganese steels), IS 7307 (welder qualification testing), and IS 800:2007 Section 10 (design of welded connections).

The two principal weld geometries in structural steel: (1) Fillet welds — triangular cross-section weld joining two surfaces at right angles or oblique. Used for angle-to-plate, beam-to-column, and stiffener connections. The throat thickness (perpendicular distance from corner to hypotenuse) is the design parameter; throat = 0.7 × leg size for equal-leg fillets. (2) Butt welds — full or partial penetration welds joining two pieces in the same plane. V-groove, U-groove, square, and double-V are common preparations. Butt welds in tension members require 100% radiographic inspection per IS 800 Cl. 10.7.5 for important structures.

Design capacity per IS 800:2007 Section 10: fillet weld design strength = 0.7 × leg size × length × (fu/√3) ÷ γmw, where γmw = 1.25 (shop weld) or 1.5 (site weld). For Fe-410 steel with fu = 410 MPa, fillet weld design shear strength ≈ 132 N/mm² of weld throat area for shop welds. The typical Indian fabrication weld defects: undercut (groove at weld toe), porosity (gas bubbles), incomplete fusion (insufficient melting of base metal), cracks (longitudinal or transverse). IS 9595 specifies acceptance criteria for each defect type. Site welds are routinely lower quality than shop welds because of weather exposure, non-flat positions (overhead/vertical), and welder fatigue — design code accounts for this with γmw = 1.5 for site welds.