ITI/NCVT Welder (Fab & Fit) (AITT Theory) cheat sheet

Arc eye (photokeratitis) is caused by UV radiation from the arc; a welding helmet with shade 10-13 (higher for high current) plus side shields is required, and bystanders need screens or goggles.

Electric shock risk rises with damp conditions and sweat: dry leather gloves, insulating mats, and an open-circuit voltage under 80 V AC (or reduced-voltage device) are standard safeguards for SMAW.

Fume extraction: welding fume contains iron oxide, manganese and hexavalent chromium when welding stainless; local exhaust ventilation at the arc and a respirator are used where extraction is inadequate.

Acetylene is flammable over 2.5-80% in air and forms explosive acetylene-copper acetylide with copper; never use copper fittings or pure copper pipe for acetylene.

Oxygen is not flammable but supports combustion vigorously: keep cylinders 3-5 m from fuel gas cylinders and 10 m from open flames or hot work; oil or grease on oxygen fittings can ignite.

Fire watch and hot work permit: combustible material within 10-11 m must be removed or protected; a fire extinguisher (dry powder or CO2 for electrical) must be at hand.

Cylinder colour/identification: in India oxygen is black, acetylene maroon, and LPG/DA red; always identify by label and valve type, never by colour alone.

Noise, spatter burns and slips are common welder injuries: leather apron, gauntlets, safety boots, and clear goggles under the helmet for chipping slag reduce them.

First-angle projection: the view below the front view is the top view and the view to the right is the left-side view; third-angle reverses this (top above front).

Scale = drawing length / actual length; representative fraction (RF) is expressed as 1:n, and a diagonal scale can measure to three units of precision.

Line types: continuous thick = visible outline; dashed = hidden edge; chain thin = centre line or pitch circle; continuous thin = dimension, hatching, and leader lines.

Dimensioning rules: dimension lines are thin and parallel to the measured edge, arrowheads touch the extension lines, and the smallest dimension is placed nearest the outline.

Limits and fits: clearance fit always has positive allowance, interference fit always negative; H7/g6 is a common clearance fit, H7/p6 an interference fit.

Geometric tolerancing symbols: straightness (-), flatness (parallelogram), circularity (circle), cylindricity, parallelism (//), perpendicularity (⊥), and position (crosshair in circle).

Section views: hatching lines at 45 degrees show cut material; adjacent parts are hatched in opposite directions, and a half section shows one quarter removed.

Isometric projection uses axes at 30 degrees to the horizontal with true lengths (isometric drawing) or 0.816 times true length (true isometric projection).

ISO 2553 / AWS A2.4 symbol: reference line with arrow; weld on the arrow side is drawn below the reference line, on the other side above it.

A fillet weld is shown as a right triangle on the reference line; the leg size is written to the left of the symbol and the length to the right.

A supplementary circle at the junction of reference line and arrow means weld all around; a tail carries process or specification notes.

A V-groove symbol is drawn as a V; bevel angle, root gap and root face are shown on the WPS or detail drawing, not by the symbol alone.

A flag at the junction of reference line and arrow indicates field (site) weld; absence of a flag means shop weld.

Fit-up layout: mark centre lines and datum edges with a scriber or chalk line, then work from a single datum to avoid cumulative error; check squareness with a try square or 3-4-5 method.

For a butt joint, root gap is typically 1.5-3 mm for plate up to 6 mm and increases with thickness; root face is usually 1.5-2 mm.

Weld length and pitch: a staggered intermittent fillet shows symbols on both sides offset; the pitch is measured from the start of one weld to the start of the next.

Plain carbon steel: up to about 0.30% C is readily weldable; 0.30-0.55% needs preheat and low-hydrogen practice; above 0.55% C welding becomes difficult and brittle martensite forms.

Carbon equivalent CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15; CE above about 0.40% calls for preheat to avoid cold cracking.

Austenitic stainless steel (304, 316) is welded with low-carbon (L) or stabilised (Ti, Nb) grades to prevent sensitisation and intergranular corrosion in the 450-850 C range.

Aluminium: oxide film melts at about 2050 C versus 660 C for the metal, so AC TIG with the cleaning half-cycle or a suitable flux is needed; high thermal conductivity causes distortion.

Copper and its alloys conduct heat rapidly, requiring preheat and high current; brass welding releases zinc fume (metal fume fever), so ventilation is essential.

Heat-affected zone (HAZ) softening occurs in cold-worked or heat-treatable alloys; grain growth near the fusion line lowers toughness in high-heat-input welds.

Cooling rate controls microstructure: fast cooling in the 800-500 C range hardens steel and raises cracking risk; slow cooling from above the upper critical temperature can cause grain growth.

Weldability tests and preheat: preheat temperature is often 100-200 C for medium-carbon steels; interpass temperature is controlled to limit distortion and loss of properties.

Neutral flame: oxygen to acetylene ratio about 1:1, giving a bright inner cone and a slightly reducing outer envelope; used for steel welding.

Oxidising flame (excess oxygen) has a shorter, sharper cone and is used for brass; carburising flame (excess acetylene) has a feathery secondary cone and is used for hard facing and some aluminium work.

Maximum flame temperature is about 3200 C for oxy-acetylene, about 2800 C for oxy-hydrogen, and about 2500 C for oxy-LPG.

Backfire is a pop with the flame going out at the tip; flashback is the flame travelling back into the hose or regulator, requiring immediate closure of the torch valves then cylinder valves.

Cutting is oxidation, not melting: preheat the steel to ignition temperature (about 870 C), then open the cutting oxygen jet; cutting is practical for steel up to several hundred millimetres with the right tip.

Cutting tip size is chosen by plate thickness; too large a tip overheats and rounds the top edge, too small a tip causes drag and incomplete cut.

Brazing filler melts above 450 C but below the base metal; capillary action fills the joint, and a flux (borax-based) removes oxide and prevents re-oxidation.

Leftward (forehand) welding suits plate up to about 5 mm; rightward (backhand) gives better penetration and speed for thicker plate; filler rod diameter is roughly half the plate thickness.

SMAW power sources: constant-current (drooping) characteristic is used because arc length variation must not cause large current change; OCV is typically 50-90 V and arc voltage 20-35 V.

AC transformers are cheap and simple; DC generators/rectifiers give smoother arcs and allow electrode polarity choice; inverter sets give high efficiency and adjustable arc force.

Duty cycle: a 300 A set at 60% duty cycle delivers 300 A for 6 out of 10 minutes; at 100% duty the allowable current is lower (I100 = I60 x sqrt(0.6)).

Electrode classification E6013 (AWS): E = electrode, 60 = 60 ksi minimum tensile strength, 1 = all-position, 3 = rutile/acid coating, AC or DC either polarity.

E7018 is a low-hydrogen, iron-powder, basic-coated electrode; it must be stored dry (baking 250-300 C for 1-2 h after exposure) and used on DC+ or AC.

Cellulosic electrodes (E6010, E6011) have a thin coating giving deep penetration and are used for pipeline root passes; E6011 works on AC.

Arc length is kept about equal to the electrode core diameter; too long an arc gives spatter and lack of penetration, too short causes sticking and slag entrapment.

Electrode angle: about 70-80 degrees to the plate in the direction of travel for downhand; for vertical-up, a short arc with slight upward push and a weave controls the puddle.

MIG uses inert gas (argon or argon-helium) for aluminium and stainless; MAG uses active CO2 or Ar/CO2 mixtures for carbon steel and gives deeper penetration.

Short-circuiting transfer uses low current and voltage and is suited to thin sheet and positional work; spray transfer needs higher current and argon-rich gas and is for flat thick work.

DCEP (electrode positive) is standard for MIG/MAG: it gives stable arc, good penetration and cleaning action; DCEN is used for some hard facing and self-shielded FCAW.

Wire feed speed sets current; voltage sets arc length. Too high a voltage gives a wide flat bead and spatter; too low gives a stubbing arc and poor fusion.

Typical shielding gas flows are 10-20 L/min; excessive flow causes turbulence and porosity, while too little leaves oxidation and porosity.

FCAW: flux-cored wires can be self-shielded (no gas) or gas-shielded; self-shielded wires are used outdoors in wind, and gas-shielded wires give better toughness.

Contact tip to work distance (CTWD) should be about 10-15 mm; increasing it reduces current and penetration and increases spatter.

Common GMAW defects: porosity from gas or moisture, lack of fusion from low current or fast travel, and burn-through on thin sheet from excess current.

TIG uses a non-consumable tungsten electrode; DCEN concentrates heat at the work and is used for steel, stainless and titanium; AC is used for aluminium and magnesium.

AC TIG gives cleaning action during the electrode-positive half-cycle, which breaks up the aluminium oxide; the balance control adjusts cleaning versus penetration.

Electrode colour codes: green = pure tungsten (AC, aluminium), thoriated (red/yellow) for DC, ceriated/lanthanated for general and AC/DC use; the tip is ground to a point (DC) or balled (AC, pure tungsten).

Filler rod is fed at a shallow angle ahead of the puddle; the tungsten must never touch the puddle or filler, or it contaminates the weld.

Argon is the usual shielding gas at 8-15 L/min; argon-helium mixes increase heat input for thick aluminium and copper.

Plasma arc welding uses a constricted arc through a nozzle, giving higher energy density and keyhole capability than TIG; plasma cutting uses a transferred arc with a high-velocity gas jet.

TIG is used for root passes in pipe welding because it gives clean, controlled penetration without slag.

Other allied processes: submerged arc welding (SAW) uses granular flux and gives high deposition for long straight welds; electroslag and electrogas are for thick vertical joints.

Five basic joints: butt, lap, tee, corner and edge; five positions per AWS: 1G/1F flat, 2G/2F horizontal, 3G/3F vertical, 4G/4F overhead, and 6G for pipe at 45 degrees fixed.

Butt joint with a V-groove needs a root gap and root face; a single-V is economical up to about 12 mm, a double-V above that to reduce weld volume.

Distortion types: longitudinal shrinkage, transverse shrinkage, angular distortion, bowing and buckling; the main cause is non-uniform heating and cooling.

Control methods: back-step welding, balanced (back-to-back) sequence, skip sequence, pre-setting (pre-camber), rigid fixturing, and peening of the root pass (not the cover).

Residual stress is reduced by stress-relief heat treatment (about 600-650 C for carbon steel, held 1 h per 25 mm of thickness) or by thermal/mechanical methods.

Neutral axis rule: welds placed near the neutral axis of a section cause less bending distortion; welding is done alternately on both sides of the neutral axis.

Shrinkage allowance: butt welds shrink about 1-2 mm per metre of length; fillet welds cause less transverse shrinkage but more angular distortion.

Fixture and tack sequence: tacks should be at least as strong as the weld and placed at the ends and centre; remove or consume tacks in the final weld.

Porosity is caused by gas trapped in the solidifying metal: moisture, oil, paint, too long an arc, or inadequate shielding gas flow.

Slag inclusion results from poor inter-run cleaning, incorrect electrode angle, or too low a current; it is detected by radiography or ultrasonic testing.

Lack of penetration (incomplete root fusion) arises from too small a root gap, too large a root face, low current, or fast travel; it is a serious planar defect.

Undercut is a groove melted into the base metal at the toe and left unfilled: caused by excessive current, long arc, or wrong electrode angle.

Cracks: hot (solidification) cracks occur at high temperature with high sulphur/phosphorus and deep narrow beads; cold (hydrogen) cracks occur after cooling and need hydrogen control and preheat.

Spatter, arc strikes and weld spatter are surface defects; arc strikes cause local hardening and can initiate cracks, so they must be ground out.

Burn-through occurs on thin material from excessive current or slow travel; it is corrected by reducing current, increasing travel speed, or using a backing.

Distortion and wrong weld size (excess reinforcement, insufficient throat) are dimensional defects checked against the drawing or WPS acceptance criteria.

Non-destructive testing (NDT): visual (VT), liquid penetrant (PT), magnetic particle (MT), ultrasonic (UT) and radiographic (RT); PT and MT detect surface/near-surface defects only.

Magnetic particle testing applies only to ferromagnetic materials; it cannot inspect austenitic stainless steel or aluminium.

Ultrasonic testing detects planar defects (cracks, lack of fusion) deep in thick sections and needs a couplant; radiographic testing detects volumetric defects (porosity, slag) and shows a permanent film/image.

Destructive tests: tensile, bend (face and root), nick-break, impact (Charpy), hardness and macro/micro examination; bend tests reveal ductility and fusion defects.

Visual inspection before, during and after welding covers fit-up, root gap, cleaning, preheat, bead profile and final dimensions.

Acceptance standards: ISO 5817 (quality levels B, C, D) and AWS D1.1 define allowable defect size; the WPS and drawing specify which level applies.

Pressure and leak tests: hydrostatic test at 1.5 times design pressure, pneumatic test at 1.1 times with a safety factor, and vacuum box or bubble test for tanks.

Weld gauges: a fillet weld gauge checks leg length and throat; a V-WAC gauge measures undercut and cap height; a bridge cam gauge measures reinforcement and pit depth.

Marking: use a scriber on steel, chalk or soapstone on rough plate, and a centre punch for hole locations; allow for kerf and bend allowance before cutting.

Shearing: blade clearance is about 5-10% of plate thickness; too much clearance gives a rough, burred edge, too little gives a ragged edge and overload.

Bend allowance BA = (pi/180) x bend angle x (R + K x T), where R is inside radius, T thickness and K the neutral-axis factor (about 0.33-0.5).

Bend deduction = 2 x outside setback minus bend allowance; the flat length = sum of outside dimensions minus bend deductions.

Minimum bend radius depends on material and thickness: roughly 1T for mild steel, 2T for aluminium, and higher for high-strength steel.

Pipe fitting: a 45-degree elbow offset = offset x 1.414 minus fitting take-outs; a 90-degree elbow take-out equals the centre-to-end dimension.

Pipe/tube cutting and bevelling: use a pipe cutter for small bore, a bevelling machine or grinder for weld prep; the bevel angle for pipe is typically 30-37.5 degrees.

Rolling and forming: plate rolls need edge pre-bending to avoid flat spots; check roundness with a template and diameter with a pi tape.

Area of a circle = pi x r^2; circumference = pi x d; volume of a cylinder = pi x r^2 x h.

Pythagoras: in a right triangle, c^2 = a^2 + b^2; the 3-4-5 method sets out a right angle on the shop floor.

Trigonometry: sin = opposite/hypotenuse, cos = adjacent/hypotenuse, tan = opposite/adjacent; used for bevel and offset calculations.

Mass = volume x density; densities: mild steel 7.85 g/cm3, aluminium 2.7, copper 8.9, stainless steel about 7.9 g/cm3.

Ohm's law V = I x R; power P = V x I = I^2 R; used to check welding cable and fuse ratings.

Heat input (kJ/mm) = (voltage x current x 60) / (travel speed in mm/min x 1000); it controls cooling rate and HAZ properties.

Percentage elongation and tensile strength: stress = load/area; a 10 mm diameter specimen with 50 kN load carries 637 MPa.

Deposition rate and electrode consumption: weight of weld metal = area of cross-section x length x density; add 10-20% for spatter and stub loss.

5S workplace organisation: Sort, Set in order, Shine, Standardise, Sustain; it underpins shop floor safety and efficiency.

Kaizen means continuous improvement; Poka-yoke is mistake-proofing; Kanban is a pull signal for material replenishment.

A WPS (welding procedure specification) states the process, base metal, filler, current range, voltage, gas flow, preheat and pass sequence; a PQR supports it with test results.

A welder qualification test (e.g. ASME IX 6G or AWS D1.1) qualifies a welder for a range of thickness, position and process; it is not the same as a WPS.

Communication and documentation: fill in job cards, material traceability records, and inspection reports accurately; report defects rather than hiding them.

Cost awareness: minimise spatter, stub loss, rework and gas waste; correct electrode storage and machine settings reduce consumable consumption.

Rights at work: PPE must be provided free, unsafe work can be refused, and accidents must be reported; first aid and emergency exits must be known.

Teamwork and housekeeping: clean work area, segregated scrap, correct storage of gas cylinders upright and secured, and clear walkways reduce accidents.