CWI Metallurgy and Weld Discontinuities Questions and Answers

Question 1

In the heat-affected zone (HAZ) of a single-pass carbon steel weld, which region is characterized by the largest grain size and is typically located immediately adjacent to the fusion boundary?

Accepted Answer

Grain-coarsened region (GCHAZ)

Answer

The grain-coarsened region (GCHAZ) experiences the highest temperatures in the HAZ, just below the melting point of the material. This high heat input allows for significant austenitic grain growth, which results in a coarse grain structure upon cooling. This region is located directly next to the fusion line and often exhibits the lowest toughness within the HAZ.

Question 2

A welder completes a groove weld on a thick section of high-strength, low-alloy (HSLA) steel using an SMAW electrode that was not stored in a proper rod oven. An inspection performed 48 hours later reveals a crack in the heat-affected zone. What is the most likely type and cause of this discontinuity?

Accepted Answer

Hydrogen-induced crack caused by moisture in the electrode.

Answer

The combination of three factors points to hydrogen-induced cracking (also called delayed or cold cracking): a susceptible microstructure (the HAZ of HSLA steel), sufficient residual stress (from welding), and the presence of diffusible hydrogen (from moisture in the improperly stored electrode). The delayed nature of the crack appearing 48 hours later is also a classic indicator of this mechanism.

Question 3

During fabrication, a T-joint is made by welding a plate perpendicularly onto the surface of another, thicker plate. After welding, a crack is found in the base material of the thicker plate, running parallel to the plate surface and having a step-like appearance. This discontinuity is best described as:

Accepted Answer

A lamellar tear.

Answer

Lamellar tearing is a specific type of cracking that occurs in the base material, beneath the weld. It is caused by high through-thickness strains acting on a material with poor ductility in that direction, often due to the presence of flattened, non-metallic inclusions. The T-joint configuration, which induces high stress perpendicular to the plate surface, and the crack's location and step-like appearance are classic indicators of lamellar tearing.

Question 4

A welding procedure for a thick-walled chrome-moly pressure vessel requires post-weld heat treatment (PWHT). What is the primary metallurgical reason for performing this step?

Accepted Answer

To temper brittle microstructures and reduce residual stresses.

Answer

PWHT is primarily used to relieve the high residual stresses induced by welding and to temper any hard, brittle microstructures (like martensite) that may have formed in the weld and HAZ during cooling. This process improves ductility, toughness, and resistance to brittle fracture. It does not typically increase tensile strength (it often slightly reduces it) or refine the grain structure.

Question 5

Solidification cracking, also known as hot cracking, is most likely to occur in a weld metal that has:

Accepted Answer

A wide solidification temperature range and high levels of impurities like sulfur and phosphorus.

Answer

Hot cracking occurs at high temperatures as the weld metal solidifies. It is promoted by a wide solidification temperature range, which creates a prolonged period where weak, low-melting-point liquid films exist between solidifying grains. Tensile stresses from shrinkage can then easily pull these grains apart. Impurities like sulfur and phosphorus exacerbate this by forming these weak liquid films.

CWI Practice Test

CWI Practice Test

CWI Metallurgy and Weld Discontinuities Questions and Answers