MMAW NC I cheat sheet

Oxy-fuel gas cylinders: acetylene is never stored or used above 15 psi gauge (103 kPa); free acetylene is unstable above about 15 psig and can decompose explosively.

Acetylene cylinders must be stored and used upright; if laid down, stand upright at least 30 minutes before use to let acetone settle.

Oxygen cylinders and fittings must be kept free of oil and grease; hydrocarbon contact with high-pressure oxygen can ignite spontaneously.

Fuel-gas hose fittings are left-hand thread; oxygen fittings are right-hand thread, so hoses cannot be cross-connected.

Flashback arresters are required at the regulator outlet (or torch inlet) on both oxygen and fuel-gas lines; a flashback is a flame traveling back up the hose.

Arc welding fume: manganese and hexavalent chromium are the key exposure concerns for steel and stainless respectively; local exhaust ventilation controls exposure at the source.

Eye and face protection shade numbers scale with current: typical SMAW at 100–200 A uses shade 10–11; GTAW at low current can use shade 8–9.

Electrical: open-circuit voltage for constant-current SMAW machines is typically 50–80 V; wet conditions, damaged electrode holders, and missing work-lead grounds raise shock risk.

Hot work permits and fire watch: a fire watch is maintained during hot work and for at least 30 minutes after completion (longer where smoldering risk exists).

Heat input formula: HI (J/mm) = (arc voltage × current × 60) / (travel speed in mm/min × 1000); equivalently kJ/mm = (V × A × 60)/(1000 × mm/min).

Carbon equivalent (CE, IIW) = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15; higher CE raises hardenability and cold-cracking risk.

Preheat slows the cooling rate, lowering martensite formation and hydrogen-assisted cracking in high-CE steels.

The iron–carbon diagram: eutectoid at 0.76 % C and 727 °C; austenite transforms to pearlite (ferrite + cementite) on slow cooling.

Fast cooling of austenite forms martensite, a hard, brittle, body-centered-tetragonal phase; tempering restores toughness.

Heat-affected zone (HAZ) is base metal not melted but thermally altered; grain growth near the fusion line lowers toughness.

Sensitization in austenitic stainless: chromium carbide precipitation at 425–870 °C depletes grain-boundary chromium, promoting intergranular corrosion; low-carbon (L) grades or stabilized grades resist it.

Hydrogen cracking requires three conditions: susceptible microstructure, hydrogen, and tensile stress; low-hydrogen electrodes and preheat address the first two.

Distortion control: balanced welding sequence, backstep technique, minimizing passes, and fixturing reduce residual stress and angular distortion.

Electrode classification E7018: E = electrode, 70 = 70,000 psi minimum tensile strength, 1 = all-position, 8 = low-hydrogen iron-powder coating, usable on AC or DCEP.

E6010 has a cellulosic coating designed for DCEP only and deep penetration; E6011 is the AC-capable cellulosic equivalent.

Low-hydrogen electrodes (E7018) must be stored in a holding oven at about 120–150 °C (250–300 °F) and rebaked per manufacturer instructions if exposed.

Electrode polarity: DCEP puts more heat at the workpiece (deeper penetration); DCEN puts more heat at the electrode (shallower penetration, higher deposition).

Arc length control: a long arc raises voltage, widens the bead, increases spatter, and raises the risk of porosity; a short arc concentrates heat.

Current selection follows electrode diameter: roughly 30–40 A per mm (about 1 A per 0.001 in) of electrode diameter is a starting point.

Duty cycle: a 60 % duty-cycle machine at 200 A can run 6 of every 10 minutes at 200 A; excess current or duty reduces output or trips thermal protection.

Slag must be fully removed between passes; trapped slag is a common planar discontinuity in multi-pass SMAW.

Transfer modes: short-circuiting (low current, fine wire, low heat input), globular (transitional, spattery), spray (high current above transition, argon-rich gas), and pulsed spray.

Short-circuit transfer requires CO2 or CO2-rich shielding because the high surface tension supports the bridging droplet; pure argon short-circuit is unstable.

Spray transfer requires argon-rich shielding (≥80 % Ar) and current above the spray transition; it is not practical in the vertical or overhead position.

Shielding gas: CO2 gives deep penetration and low cost but more spatter; argon mixes improve arc stability and reduce spatter.

Wire feed speed sets current in constant-voltage (CV) GMAW; arc voltage sets arc length, so the two are adjusted together.

Contact-tip-to-work distance (CTWD) changes current: increasing CTWD raises resistance and lowers current, reducing penetration.

Polarity is DCEP for standard solid-wire GMAW; self-shielded FCAW uses DCEN.

Porosity causes: inadequate gas flow, drafts, dirty base metal, clogged nozzle, or excessive CTWD.

FCAW wires are classified E71T-1C (C = CO2 shielding) or E71T-1M (M = mixed Ar/CO2 shielding); the T indicates a tubular flux-cored electrode.

Self-shielded FCAW (e.g., E71T-11) uses no external gas and runs on DCEN; the flux provides shielding and slag.

Gas-shielded FCAW typically runs DCEP and uses CO2 or Ar/CO2 blends; it delivers high deposition rates and good penetration.

FCAW is more tolerant of rust, mill scale, and wind than GMAW because the flux supplements shielding; self-shielded FCAW tolerates higher wind.

Flux-cored wires are stored dry; rutile and basic fluxes absorb moisture, which raises hydrogen and porosity risk.

Slag removal is required between passes; basic (low-hydrogen) flux-cored wires give better toughness and crack resistance than rutile types.

Typical FCAW current range is high (200–400 A) with large-diameter wires (1.2–2.4 mm), giving high deposition and deep penetration.

FCAW produces more fume than solid-wire GMAW; ventilation and respiratory protection are more critical.

GTAW uses a non-consumable tungsten electrode; the arc is established between the electrode and the work, with filler added separately.

Polarity: DCEN is standard for steel, stainless, and titanium; DCEP concentrates heat at the electrode and is used only for AC or special cases.

AC GTAW is used for aluminum and magnesium because the electrode-positive half-cycle breaks the surface oxide; the electrode-negative half-cycle limits electrode heating.

Tungsten electrode color codes: green = pure tungsten (AC), red = 2 % thoriated (DCEN), gray = 2 % ceriated, gold = 1.5 % lanthanated.

Shielding gas is argon or helium; helium gives higher heat input and deeper penetration, useful on thick aluminum.

Tungsten electrode tip shape controls the arc: a pointed tip concentrates the arc; a balled tip (AC) spreads it.

Contamination of the tungsten (touching the puddle or filler) requires stopping, removing the contaminated tip, and re-sharpening.

GTAW produces the cleanest welds and is used for root passes and thin material; travel speed and current control bead width and penetration.

Oxy-fuel cutting cuts steel by oxidizing it: a preheat flame raises the steel to its ignition temperature (about 870 °C), then a high-velocity oxygen jet burns the iron.

Cutting oxygen purity matters: a drop from 99.5 % to 99 % oxygen reduces cutting speed and increases kerf width.

Neutral flame has equal oxygen and acetylene; an oxidizing flame (excess O2) is hotter and used for cutting, while a carburizing flame (excess acetylene) is used for some brazing.

Acetylene maximum working pressure is 15 psi gauge; oxygen is regulated to the pressure needed for the tip size, typically 20–40 psi for cutting.

Kerf is the width of the cut; drag lines lean in the direction of travel, and excessive drag indicates too fast a travel speed or too little oxygen.

Piercing: hold the torch above the plate, preheat, then slowly lower and begin the cut; piercing thick plate requires a larger tip and more preheat.

Oxy-fuel cutting works on carbon steel but not effectively on stainless or aluminum, because their oxides have melting points above the base metal.

Flashback arresters and reverse-flow check valves protect against flame traveling back into the hose or regulator.

Joint types: butt, lap, T, corner, and edge; groove types: square, V, bevel, U, J, and flare.

Root opening, root face, and bevel angle together define the groove; a typical single-V groove uses 60–75° included angle with a 1.5–3 mm root face.

Welding symbol: the arrow points to the joint; the reference line carries the symbol, with the tail for process or specification and the flag for field welds.

Symbol placement: the symbol below the reference line means the weld is on the arrow side; above the line means the other side.

Dimensions left of the weld symbol are groove or weld size; dimensions right are length and pitch (for intermittent welds).

Fit-up errors-excessive root gap, misalignment, or high-low-increase burn-through, distortion, and lack of fusion risk.

Tack welds must be of weldable quality and are consumed by the root pass; undersized or cracked tacks are a common source of root defects.

Backing, back-gouging, and root gaps are used to ensure complete joint penetration for full-penetration groove welds.

Discontinuity vs. defect: a discontinuity is any interruption in the weld; it becomes a defect only when it exceeds the acceptance criteria of the applicable code.

Porosity is gas entrapped in the weld; causes include moisture, dirt, low gas flow, and long arc. It is rounded and appears as dark spots on radiographs.

Slag inclusion is a solid nonmetallic discontinuity from trapped flux; it appears as irregular dark shapes on radiographs and is common in multi-pass SMAW/FCAW.

Lack of fusion is a planar discontinuity between weld metal and base metal or between passes; it is more harmful than rounded porosity because it concentrates stress.

Undercut is a groove melted into the base metal at the weld toe; it reduces cross-section and acts as a stress riser.

Visual inspection (VT) is the first method and covers bead shape, undercut, overlap, spatter, and surface cracks; it requires good lighting and, often, a weld gauge.

Liquid penetrant (PT) detects surface-breaking discontinuities in nonporous materials; magnetic particle (MT) detects surface and near-surface discontinuities in ferromagnetic materials only.

Ultrasonic testing (UT) detects internal planar discontinuities (lack of fusion, cracks) and is good for thick sections; radiography (RT) detects volumetric discontinuities and shows a 2-D image.

Weld procedure qualification (PQR) supports a written WPS; welders are qualified by performance tests to the WPS, and qualification is specific to process and position.