Mississippi Welding

Key Concepts

Equipment Components and Assembly

• The oxyacetylene system consists of: oxygen cylinder, acetylene cylinder, pressure regulators, hoses, torch body, mixing chamber, and tip.
• Oxygen hose is green, right-hand threads. Acetylene hose is red, left-hand threads (left-hand fittings have a notch on the nut — this prevents cross-connection).
• Always attach the regulator before opening cylinder valves. Open oxygen cylinder valve fully. Open acetylene cylinder valve no more than 1.5 turns (so it can be shut quickly in an emergency).
• Purge each hose separately before lighting — this removes air and prevents mixed-gas ignition inside the hose.

Flame Types and Adjustment

• Neutral flame: Equal oxygen and acetylene. Two zones (inner cone + outer envelope). Used for most welding — steel, cast iron, copper, stainless. The standard starting point.
• Carburizing (reducing) flame: Excess acetylene. Three zones — inner cone, white "feather" (acetite feather), outer envelope. Used for high-carbon steel, hard surfacing, aluminum (with some alloys). Adds carbon to the weld pool.
• Oxidizing flame: Excess oxygen. Shorter, pointed…

Key Concepts

AWS Welding Symbol Components Every welding symbol has a reference line (horizontal baseline), an arrow pointing to the joint, and a weld symbol placed on the reference line. The tail (at the opposite end of the arrow) contains specification, process, or procedure information — it's omitted when no special instruction is needed. Weld symbols placed below the reference line apply to the arrow side of the joint; symbols placed above apply to the other side.

Five Basic Weld Joint Types - Butt joint – two members in the same plane, edge to edge - Lap joint – two overlapping members (fillet welds typical) - Corner joint – two members at approximately 90°, forming an L-shape - T-joint – one member perpendicular to the face of another - Edge joint – edges of two parallel members joined

Key Concepts

Brazing vs. Braze Welding (AWS Definitions) - Brazing: Filler metal liquidus above 840°F (450°C), below base metal solidus. Filler metal distributed by capillary action into closely fitted joints. Base metal is not melted. - Braze Welding (BW): Same temperature range, but filler metal is not distributed by capillary action. Forms a fillet or groove weld bead. Fit-up is less critical than brazing. - Both processes are distinguished from soldering, which operates below 840°F (450°C).

Joint Design & Clearance - Brazing requires tight, closely fitted joints for capillary action to work. Poor fit interrupts capillary pull and creates weak, leaky joints. - Accepted brazing joint types: butt, lap, modified butt, modified lap, butt-lap, pierced. - Braze welding tolerates looser fit-up; standard arc/oxyfuel groove and fillet joint designs are acceptable. - Dissimilar metals require consideration of differential thermal expansion — joint design must accommodate movement during heating and cooling.

Key Concepts

Pipe Joint Preparation - Bevel angle: Standard groove bevel is typically 37.5° per side (75° included angle) for butt joints on pipe. - Root face (land): The flat portion at the bottom of the bevel, typically 1/16" (1.6 mm). Too large = incomplete fusion; too small = burnthrough. - Root opening: Gap between pipe ends, typically 1/16"–1/8" (1.6–3.2 mm) depending on process and wall thickness. Consistent root opening is critical for full penetration.

Welding Positions for Pipe - 1G: Pipe horizontal, rotated — flat position welding throughout. - 2G: Pipe vertical axis, fixed — welder works in horizontal position around the pipe. - 5G: Pipe horizontal axis, fixed — welder must weld in all positions (flat, vertical, overhead) without rotating the pipe. - 6G: Pipe at 45° fixed angle — requires all-position welding; most difficult and most comprehensive qualification. Passing 6G qualifies a welder for all positions. - Uphill vs. Downhill: Uphill (3G/5G vertical up) provides better penetration and mechanical properties; preferred for structural/pressure applications. Downhill is faster but produces shallower…

Key Concepts

Ferrous Metal Classification by Carbon Content

• Wrought iron: ~0.2% carbon – nearly pure iron, easily formed
• Low-carbon (mild) steel: up to 0.30% carbon – most weldable
• Medium-carbon steel: 0.30%–0.50% carbon – preheat required at 0.45%+; preheat to 200°F–400°F (95°C–200°C)
• High-carbon steel: 0.50%–1.0% carbon – preheat to 400°F (200°C) or higher; postweld heat treatment required
• Cast iron: 2.1%–4.5% carbon – brittle, requires special welding procedures

Physical and Mechanical Properties

• Tensile strength: resistance to pulling/stretching forces
• Ductility: ability to deform plastically without fracturing (drawn into wire)
• Hardness: resistance to surface indentation or scratching
• Toughness: ability to absorb energy and resist fracture (combines strength + ductility)
• Brittleness: tendency to fracture without plastic deformation (opposite of ductility) — cast iron is a prime example

Key Concepts

Equipment & Power Sources

• SMAW station components: Constant current power source, electrode lead, electrode holder, workpiece lead, workpiece connector, electrodes, welding helmet, chipping hammer, wire brush.
• AC power sources: Transformer, engine-driven generator, inverter.
• DC power sources: Transformer + DC rectifier, engine-driven generator + DC rectifier, inverter.
• Inverter machines can output AC, DC, or both — most portable and energy-efficient option.
• SMAW always uses a constant current (CC) power source ("drooper"), not constant voltage.
• Engine-driven generators are the go-to for field/pipeline work where grid power is unavailable.

AWS Electrode Classification System

• Format: E XXXX (e.g., E6010, E7018)
• E = electrode
• First two digits = minimum tensile strength in ksi (e.g., 60 = 60,000 psi; 70 = 70,000 psi)
• Third digit = welding position (1 = all positions; 2 = flat and horizontal only; 4 = flat, horizontal, vertical-down, overhead)
• Fourth digit = flux/covering type and usable current/polarity
• Common electrodes:
• E6010: DC+ (DCEP), deep penetration, cellulosic covering — standard…

(9 questions, 15% of exam — high priority topic)

Key Concepts

Current Polarity & Effects

• DCEN (Direct Current Electrode Negative) — electrode is negative, workpiece is positive. Produces deep, narrow penetration. No cleaning action. Used for most GTAW applications (steel, stainless, titanium).
• DCEP (Direct Current Electrode Positive) — electrode is positive, workpiece is negative. Produces shallow penetration but strong cathodic cleaning action (removes oxides). Rarely used in GTAW because it overheats the tungsten.
• AC — alternates between DCEN and DCEP each half-cycle. Provides moderate penetration + cleaning action. Standard choice for aluminum and magnesium welding. High-frequency current is used to stabilize the AC arc and prevent rectification.

Key Concepts

Nondestructive Examination (NDE) Methods

• Visual Inspection (VT) — Most common NDE method. Checks size, contour, location, and surface defects (cracks, undercut, lack of penetration, spatter). Limited to external surfaces only. Performed before, during, and after welding.
• Magnetic Particle (MT) — Detects surface and near-surface discontinuities in ferromagnetic materials only. Cannot be used on austenitic stainless steel or aluminum.
• Liquid Penetrant (PT) — Detects surface-breaking defects on any non-porous material. Works on non-ferrous metals where MT cannot. Requires dwell time, developer application, and inspection under proper lighting.
• Ultrasonic (UT) — Uses high-frequency sound waves; detects internal and surface defects. Highly sensitive; good for thick sections and detecting laminations or lack of fusion.
• Radiographic (RT) — X-ray or gamma-ray imaging. Creates a permanent film record. Excellent for internal volumetric defects (porosity, slag). Radiation safety protocols required.
• Eddy Current (ET) — Uses electromagnetic induction. Effective on conductive materials for surface and near-surface defects; commonly used on tubing.
• NDE symbols governed by AWS A2.4 — symbol placed below…