Millwright Certification Welding and Cutting Processes 2

Question 1

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

Accepted Answer

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

Answer

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.

Question 2

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

Accepted Answer

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

Answer

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.

Question 3

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

Accepted Answer

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

Answer

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.

Question 4

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

Accepted Answer

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

Answer

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.

Question 5

What is undercut in welding, and what causes it?

Accepted 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

Answer

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.

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Millwright Certification Welding and Cutting Processes 2