Welding Metallurgy Questions and Answers

Answer: C

Preheating is essential when welding cast iron.

Cast iron is brittle and has a high carbon content. If the weld area heats and cools quickly, the uneven contraction cracks it, and the rapid cooling forms hard, brittle white iron and martensite near the weld. Preheating the casting (often to a few hundred degrees) and cooling it slowly reduces these thermal stresses.

Some tool steels such as high speed steel are also preheated, but cast iron is the standard case where preheating is essential. Stainless steel and German silver (a copper-nickel-zinc alloy) are normally joined without it.

Answer: C

Cast iron is hard to weld because of its hardness and brittleness.

Cast iron has a high carbon content (about 2% to 4%), which makes it hard and brittle with very little ductility. When the weld heats and cools part of the casting, the uneven expansion and contraction cannot be absorbed by stretching, so it cracks, and fast cooling near the weld forms even harder, brittle structures. That is why cast iron needs preheating, special electrodes and slow cooling.

Cast iron actually has good fluidity and its low melting point helps casting rather than hindering welding. High compressive strength is a useful property, not a welding problem.

Answer: B

The gas that causes cracking is hydrogen.

Hydrogen dissolves in the molten weld metal, coming mainly from moisture in electrode coatings, damp flux or grease on the work. As the weld cools, the hydrogen diffuses into the hard heat-affected zone and causes hydrogen-induced cracking, also called cold or delayed cracking, which can appear hours after welding.

Oxygen and nitrogen are harmful too, causing oxides, porosity and brittleness, but cracking is the characteristic effect of hydrogen. That is why low hydrogen electrodes are baked before use.