456A Metallurgy & Heat Treatment

Question 1

What is the carbon equivalent (CE) formula used in Canada (IIW formula) for assessing the weldability of carbon-manganese steels?

Accepted Answer

CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15

Answer

The IIW (International Institute of Welding) carbon equivalent formula used in Canada is CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. A CE above approximately 0.40–0.45% indicates increased susceptibility to hydrogen-induced cracking, requiring preheat, low-hydrogen electrodes, and potentially PWHT. This formula is referenced in CSA W59 and used to determine preheat requirements.

Question 2

What microstructural phase forms in carbon steel when austenite is cooled extremely rapidly (quenched)?

Accepted Answer

Martensite

Answer

Martensite forms when austenite is cooled faster than the critical rate (quenched), preventing carbon from diffusing out of the iron lattice. The carbon becomes trapped, distorting the crystal structure into a hard, brittle body-centred tetragonal (BCT) structure. Martensite in the HAZ of welded joints is a primary cause of hydrogen-induced cracking.

Question 3

What is the purpose of 'tempering' a hardened steel weld or HAZ?

Accepted Answer

Reheating the hardened zone to a temperature below the lower critical temperature to reduce brittleness while maintaining hardness

Answer

Tempering involves reheating hardened (martensitic) steel to a temperature below Ac1 (typically 150–650°C depending on the application). This allows some carbon to diffuse, relieving internal stresses, increasing toughness and ductility, and slightly reducing hardness. PWHT of carbon steels typically serves as a stress relief and temper.

Question 4

What is the 'heat-affected zone' (HAZ) in a weld and why is it often the weakest area of a welded joint?

Accepted Answer

The area adjacent to the fusion line that experienced elevated temperatures during welding; it can contain coarse grains, martensite, or soft zones depending on the steel and cooling rate

Answer

The HAZ is the base metal zone that was heated by welding but did not melt. Temperatures in the HAZ range from near the melting point (at the fusion line) to just above ambient (at the outer edge). Depending on the steel's composition and the thermal cycle, the HAZ can contain coarse-grained austenite (reducing toughness), hard martensite (causing HIC susceptibility), or over-tempered soft zones in previously heat-treated steels.

Question 5

What is 'hot cracking' (solidification cracking) in welds and which elements in steel promote it?

Accepted Answer

Cracking during weld metal solidification; promoted by high sulfur, phosphorus, and low melting point impurities that form liquid films at grain boundaries

Answer

Solidification cracking (hot cracking) occurs as the weld metal solidifies and shrinks. Low-melting-point liquid films (formed by sulfur, phosphorus, and other impurities) persist at grain boundaries after the surrounding metal has solidified, and weld shrinkage tears these films open. High S and P content, along with high restraint and convex bead profiles, increase susceptibility.

Red Seal Welder 456A Practice Test

Red Seal Welder 456A Practice Test

456A Metallurgy & Heat Treatment