Millwright Certification Welding and Cutting Processes 2 — Questions and Answers

Question 1: What is the primary purpose of a shielding gas in GMAW (MIG) welding?

To heat the base metal faster
To protect the molten weld pool from atmospheric contamination (oxygen, nitrogen, hydrogen) (Correct answer)
To improve the visibility of the weld
To cool the weld joint more quickly

Correct answer: To protect the molten weld pool from atmospheric contamination (oxygen, nitrogen, hydrogen)

The shielding gas creates an inert or reactive atmosphere around the molten weld pool, preventing oxygen, nitrogen, and hydrogen from the atmosphere from causing porosity, embrittlement, and other defects.

GMAW shielding gases serve multiple functions: primary shielding from atmospheric contamination, arc stabilization, and influence on weld penetration and bead profile. Common gases: 100% CO2 (deep penetration, high spatter, carbon steel only), 75% Ar/25% CO2 (general purpose for carbon steel), 90% Ar/10% CO2 (spray transfer, low spatter), 98% Ar/2% O2 (stainless steel), and 100% Argon (aluminum and non-ferrous metals). Shielding gas flow rate (typically 15-25 L/min) must be adequate to protect the pool but not so high that turbulence draws in air.

Question 2: When making an oxy-fuel cut on carbon steel, what causes the actual cutting action?

The flame melts through the steel
A high-pressure oxygen stream rapidly oxidizes (burns) the preheated steel, and the force of the oxygen blows the molten oxide away (Correct answer)
The acetylene pressure forces the metal apart
Thermal shock cracks the steel along the cut line

Correct answer: A high-pressure oxygen stream rapidly oxidizes (burns) the preheated steel, and the force of the oxygen blows the molten oxide away

In oxy-fuel cutting, the preheat flames bring the steel to its ignition temperature (about 870 degrees C), then a stream of pure oxygen rapidly oxidizes (burns) the steel. The oxygen jet's force blows the molten iron oxide out of the kerf.

Oxy-fuel cutting relies on the exothermic reaction of iron with oxygen. The process: preheat flames heat the starting point to about 870 degrees C, then cutting oxygen is directed onto the heated steel. The iron oxidizes rapidly, generating significant additional heat, and the oxygen stream blows the molten oxide out the bottom. The process is chemical, not thermal: the preheat flames alone cannot cut through steel. This is why oxy-fuel cutting works only on ferrous metals that form fluid oxides. Stainless steel, aluminum, and copper cannot be oxy-fuel cut.

Question 3: What is the main advantage of plasma cutting over oxy-fuel cutting?

Plasma torches are cheaper to purchase
Plasma cuts faster, works on any electrically conductive metal, and produces a narrower kerf with less heat distortion (Correct answer)
Plasma cutting produces no sparks
Oxy-fuel cuts are always cleaner

Correct answer: Plasma cuts faster, works on any electrically conductive metal, and produces a narrower kerf with less heat distortion

Plasma cutting is significantly faster (up to 5x on thin material), cuts any conductive metal (stainless, aluminum, copper, not just carbon steel), and its concentrated arc produces a narrower kerf with a smaller heat-affected zone.

Plasma cutting uses a constricted electric arc (temperatures up to 30,000 degrees C) and high-velocity gas to melt and blow away metal. Advantages over oxy-fuel: 2-5x faster on material under 25 mm, cuts any electrically conductive material, narrower kerf (1-3 mm vs 2-5 mm), smaller heat-affected zone, no preheat time, and easily automated. Disadvantages: higher equipment cost, requires electrical power, and limited maximum thickness (typically 50 mm handheld, 150 mm mechanized).

Question 4: What does preheat accomplish before welding on thick or high-carbon steel?

It makes the weld look better
It slows the cooling rate to prevent hydrogen cracking, reduces residual stress, and allows hydrogen to diffuse out of the weld zone (Correct answer)
It increases the welding speed
Preheat is only for aesthetic purposes

Correct answer: It slows the cooling rate to prevent hydrogen cracking, reduces residual stress, and allows hydrogen to diffuse out of the weld zone

Preheating raises the base metal temperature to slow post-weld cooling rates, which reduces the risk of hydrogen-induced cracking in the heat-affected zone, lowers residual stresses, and allows trapped hydrogen to escape.

Preheat is critical for high-carbon steels, low-alloy steels, thick sections, and dissimilar metal joints. Benefits: reduces cooling rate (avoiding martensite formation), allows hydrogen diffusion (preventing delayed cracking), and reduces residual stress. Preheat temperature depends on material composition, thickness, restraint level, and hydrogen content of the welding process. Typical ranges: 50-200 degrees C. Temperature is measured with contact pyrometers or tempil sticks at a distance of 75 mm from the weld joint.

Question 5: What is undercut in welding, and what causes it?

Extra weld metal deposited at the toe
A groove melted into the base metal at the weld toe that is not filled by weld metal, caused by excessive current, speed, or improper technique (Correct answer)
A crack in the center of the weld
Excess spatter around the weld

Correct answer: A groove melted into the base metal at the weld toe that is not filled by weld metal, caused by excessive current, speed, or improper technique

Undercut is a groove or channel melted into the base metal adjacent to the weld toe (or root) that was not filled with weld metal, creating a stress concentration that weakens the joint and can initiate fatigue cracking.

Undercut is one of the most common weld defects. Causes include: excessive welding current, excessive travel speed, incorrect electrode angle, excessive weaving width, and incorrect arc length. Undercut creates a sharp stress concentration at the most highly stressed point of the weld joint. Under cyclic loading, fatigue cracks initiate at undercut. Acceptance criteria vary by code but typically limit undercut depth to 0.8-1.6 mm for structural applications. Correction involves grinding and depositing additional weld metal with lower current.

Question 6: Why must acetylene gas never be used at pressures exceeding 15 PSI (103 kPa)?

It becomes less flammable at high pressure
Acetylene becomes unstable and can spontaneously decompose explosively above 15 PSI gauge pressure (Correct answer)
Higher pressure wastes gas
The regulator cannot handle higher pressure

Correct answer: Acetylene becomes unstable and can spontaneously decompose explosively above 15 PSI gauge pressure

Acetylene is chemically unstable, and at pressures above 15 PSI (103 kPa) gauge, it can spontaneously decompose in a self-accelerating exothermic reaction (explosion) even without a spark or flame present.

Acetylene has a positive heat of formation, meaning it contains more energy than its constituent elements. Above 15 PSI gauge pressure, acetylene can spontaneously decompose explosively. This is why acetylene regulators are limited to 15 PSI maximum, cylinders contain the gas dissolved in acetone within a porous filler, flashback arrestors are required on all oxy-acetylene equipment, and cylinders must be stored upright. If an acetylene cylinder is involved in a fire, evacuate and cool with water from a safe distance, as internal decomposition can continue even after the external fire is extinguished.

Millwright Certification Welding and Cutting Processes 2

What is the primary purpose of a shielding gas in GMAW (MIG) welding?