Shielded Metal and Gas Metal Arc Welding Questions and Answers

Answer: D

Helium or argon is used to act as a shielding medium.

In metal inert gas (MIG) welding, the inert gas flows from the nozzle around the arc and the molten pool, keeping out oxygen and nitrogen from the air, which would otherwise cause oxidation and porosity. Being inert, argon and helium do not react with the weld metal.

MIG welding uses no flux; the gas does the job flux does in stick welding. Any cooling is incidental, and the electrode wire is consumed rather than protected for its own sake.

Answer: A

Metal active gas (MAG) welding uses carbon dioxide.

The word 'active' means the shielding gas takes part in the arc chemistry. Carbon dioxide, alone or mixed with argon, is the usual active gas: it is cheap and gives deep penetration on steel. Using CO₂ is also why MAG is often called CO₂ welding.

Argon and helium are inert gases, which define metal inert gas (MIG) welding, not MAG. Nitrogen is not used as a shielding gas for steel, since it causes porosity and brittleness.

Answer: C

The highest deposition rate comes from the iron powder electrode.

Iron powder electrodes carry a large amount of iron powder in their coating. As the coating melts, that iron joins the weld pool along with the core wire, so much more weld metal is laid down per unit time, and the arc runs smoothly with little spatter. They are used for long, flat fillet and butt welds where speed matters.

Low hydrogen electrodes are chosen to avoid hydrogen cracking, heavy coated electrodes for better shielding and slag cover, and deep penetration electrodes for penetration; none is defined by a high deposition rate.

Answer: C

Low hydrogen electrodes are baked to make welding free from moisture pick up.

Their basic (lime-fluoride) coatings absorb moisture from the air. In the arc, that water breaks down and releases hydrogen into the weld, which can cause hydrogen cracking, the very problem these electrodes are meant to prevent. Baking (typically at a few hundred degrees) drives off the moisture, and the electrodes are then kept in a holding oven until use.

Baking does not set the weld's strength, prevent arc blow (a magnetic effect) or reduce the current needed.

Answer: D

The rod melts first and then the coating melts.

The core wire carries the current and is heated directly by the arc, so it melts slightly ahead of the flux coating around it. The coating is left projecting a little beyond the wire, forming a small cup or sleeve at the tip. This cup concentrates and directs the arc, and as the coating melts it releases shielding gas and forms slag over the weld.

If the coating melted first, the bare wire would lose its shielding and the arc would wander; and the order does not depend on polarity. So the other options are wrong.

Answer: C

The third digit '2' shows the welding position.

The classification is the Indian Standard (IS 815) code for covered electrodes. After the letter E, the next letter gives the type of coating (B is basic). Then come four digits: the first for tensile and yield strength, the second for elongation and impact strength, the third for the welding positions the electrode can be used in, and the fourth for the welding current and voltage conditions. Letters at the end mark special properties, such as H for hydrogen-controlled electrodes.

So in EB5426, the 2 is the position digit. Polarity and welding current are covered by the fourth digit, 6, and the type of flux coating by the letter B.

Answer: A

The electrode usable on both AC and DC is the mild steel coated electrode.

General-purpose mild steel electrodes have coatings containing arc stabilisers, such as potassium and sodium compounds, which keep the arc burning as an alternating current passes through zero a hundred times a second. They therefore run on AC transformers as well as DC machines.

A bare electrode has no stabilisers, so its arc keeps going out on AC and it is used on DC. Carbon arc electrodes and aluminium electrodes are likewise normally run on DC.