Mississippi Welding

Key Concepts

Acetylene pressure limit. Acetylene is unsafe above 15 psig (103 kPa); storing acetylene in gaseous form under pressure is not safe above that. Most acetylene low-pressure gauge scales are graduated only to 15 psig, with higher pressures shown by a solid red arc (Ch. 12, Sections 12.3.1 and 12.3.3; also Section 13.8.4: "Never use acetylene pressure in excess of 15 psig (103 kPa)").

Cylinders. Oxygen cylinders are forged in one piece with no part less than 1/4" thick, from the same high-carbon steel used in military grade armor plating (Section 12.2.1). The oxygen cylinder valve is a back-seating valve — turn it all the way open in use; never use it partly open (Section 12.2.1). New empty acetylene cylinders are filled with an inert porous filler that cures to 85% porosity as required by federal safety regulations, enabling absorption of acetylene with acetone solvent (Section 12.3.1).

Mississippi Welding Exam — 6 questions (10%)

Key Concepts

Welding symbol layout - Arrow-side weld information is always shown below the reference line; other-side information is always shown above the reference line (Ch. 3, Sec. 3.5.3). - The root opening size appears inside the weld symbol; the groove angle (included/total angle) is shown beyond/outside the weld symbol (Ch. 3, Sec. 3.5.4). - For bevel-groove or J-groove welds, only one piece is cut or ground. The arrow is bent to point to the piece to be cut or ground (Ch. 3, Sec. 3.5.4). - The S position gives depth of bevel or depth of preparation; groove weld size is given in parentheses in the E position and is the depth the weld penetrates into the base metal (Ch. 3, Sec. 3.5.6). - If no groove weld size is shown for a single- or double-groove weld, complete penetration is required (Ch. 3, Sec. 3.5.6). - Contour and finish: a straight contour line = flat weld face; a curved line = convex (or concave). Finish letters: **C…

Modern Welding — Chapter 17, Brazing and Braze Welding

Key Concepts

Definitions and the 840°F line (Section 17.1) - Both brazing and braze welding are performed above 840°F (450°C); soldering is performed below 840°F (450°C). This temperature is the dividing line. - AWS brazing: a group of joining processes producing coalescence by heating to brazing temperature with a filler metal having a liquidus above 840°F (450°C) and below the solidus of the base metal, with filler "distributed between the closely fitted faying surfaces of the joint by capillary action." - AWS braze welding (BW): same liquidus/solidus condition, base metal is not melted, and unlike brazing, the filler metal is not distributed in the joint by capillary action. - Vocabulary: coalescence = the joining or uniting of materials; faying surface = a mating surface; brazement = an assembly of brazed or braze welded parts; adhesion = the molecular attraction exerted between mating surfaces of the brazement. Brazing is an adhesion process. - Brazing filler metals are nonferrous — mixtures of copper, zinc, tin, aluminum, beryllium,…

Key Concepts

Joint preparation and geometry - Uphill pipe welds use a 75° included groove angle, which is a 37.5° bevel on each pipe end; downhill welding uses a narrower 60° groove angle (Modern Welding, p. 23-6; Pipe Welding Procedures, Ch. 4). - The bevel angle always equals one-half the included angle (Pipe Welding Procedures, Ch. 4, Fig. 4-1, 4-2, 4-4). - Standard thick-wall joint specification: included angle 75°, root face 3/32″ to 1/8″ (Pipe Welding Procedures, Ch. 4, Fig. 4-2). - A "nickel" root face is 3/32″ (2.4 mm); a "dime" root face is 1/16″ (1.6 mm) (Modern Welding, p. 23-11). - The root face (land) is the flat left at the bottom of the bevel: too thin burns through, too thick prevents full penetration (Pipe Welding Procedures, Ch. 4). - Fit-up: bend a wire equal in diameter to the required root opening into a vee as a spacer. Maximum allowable misalignment is 1/16″, specified by the ASME Code; inside surfaces must blend smoothly (Pipe Welding Procedures, Ch. 4, Fig. 4-4). - Four…

Key Concepts

Carbon content ranges (Modern Welding, Ch. 28) - Low-carbon steel: less than 0.30% carbon. - Medium-carbon steel: 0.30% to 0.55%. - High-carbon steel: 0.55% to 0.80%. - Very high-carbon steel: 0.80% to 1.70%. - Cast iron: 1.8% to 4%. - Wrought iron: 0.003% carbon ("three-thousandths of 1%"). Note that Ch. 21 states wrought iron contains about 0.2% carbon; the verified answer follows Ch. 28.

Mechanical properties (Pipe Welding Procedures, Ch. 11) - Yield strength: the stress to which a metal can be subjected without permanent deformation (distinct from ultimate tensile strength, the maximum stress before rupture). - Toughness: ability to withstand a sudden shock, determined by energy absorbed when a notched specimen is struck by a pendulum hammer blow. - Hardenability: the depth below the surface to which a metal can be hardened — not the maximum hardness achievable. - Brittleness: less than 5% elongation in a tensile test indicates a brittle metal (Ch. 28); brittleness is the opposite of ductility. - Fatigue: failures always occur at a stress level below the…

Key Concepts

Power sources and current characteristics - Under a no-load (open-circuit) condition, when welding is not being performed, the voltage of a DC arc welding machine is about 60V to 80V. When the arc is struck, voltage decreases to between 15V and 40V (Modern Welding, Ch. 6, p. 128). - A traditional transformer makes adjustments to the welding output every 1/60th or 1/120th of a second; an inverter can make small changes every 1/20,000th of a second, producing a more constant current or voltage output (Modern Welding, Ch. 5, p. 106). - On a constant current (drooper) power source, a large percentage change in arc voltage produces only a small change in current: voltage rising from 20V to 25V (25% increase) drops current from 135A to 126A — about 6.7% (Modern Welding, Ch. 5, pp. 104–105). - There is no arc blow in AC welding. The current's constantly changing direction virtually cancels the magnetic blow effects in the AC circuit (Modern Welding, Ch. 6, pp. 132 and 145).

Key Concepts

Heat distribution and polarity (Modern Welding, Ch. 10, Sec. 10.1) - DCEN: approximately 70% of heat at the workpiece, 30% at the electrode. DCEN produces the best penetration. - DCEP: approximately 70% at the electrode, 30% at the workpiece. Because of this, an electrode's current-carrying capacity on DCEP is only about 1/10 of its DCEN capacity, and a larger electrode is required. - AC: about 50% to base metal, 50% to electrode (balanced wave). Penetration falls between DCEN and DCEP. - AC or DCEP is required to break up refractory oxides on aluminum, magnesium, and some other nonferrous metals — positively charged shielding gas ions strike the workpiece and break up the oxides. DCEN gives no cleaning action.

AC waveform and rectification (Ch. 9, Sec. 9.2 and 9.2.1) - Rectification occurs when current is reduced or absent during the electrode-positive half cycle, making the arc unstable. A high-frequency voltage generator built into the machine produces several thousand volts at several megahertz with only a fraction of an ampere, keeping the shielding gas…

Key Concepts

Discontinuities vs. defects. All welds contain flaws (discontinuities). A discontinuity becomes a defect only when it makes the weld unsuitable for its intended service; the size limit at which this happens is the critical defect size, written into codes or company standards (Modern Welding, Ch. 30, Sec. 30.1).

NDE vs. destructive testing. Nondestructive examination (NDE) checks weld quality without damaging it — the part is still usable. Destructive testing determines physical properties and destroys the weld (Modern Welding, Ch. 30, intro and Sec. 30.1).

NDE methods (Modern Welding, Ch. 30, Sec. 30.1): visual (VT), magnetic particle (MT), liquid penetrant (PT), ultrasonic (UT), X-ray (RT), eddy current (ET), leak (LT). VT is the most common; VT is limited to external surfaces. - MT detects surface or near-surface flaws only in materials that can be magnetized; PT works on any material with surface-breaking flaws (Sec. 30.1.2). - UT requires a couplant (water, oil, or glycerol) because ultrasonic waves do not travel through air. The only specification for a couplant is that it not be…